Jukuri, open repository of the Natural Resources Institute Finland (Luke) All material supplied via Jukuri is protected by copyright and other intellectual property rights. Duplication or sale, in electronic or print form, of any part of the repository collections is prohibited. Making electronic or print copies of the material is permitted only for your own personal use or for educational purposes. For other purposes, this article may be used in accordance with the publisher’s terms. There may be differences between this version and the publisher’s version. You are advised to cite the publisher’s version. This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Signe Lett, Ingibjörg Svala Jónsdóttir, Antoine Becker-Scarpitta, Casper T. Christiansen, Heinjo During, Flemming Ekelund, Greg H.R. Henry, , Simone Lang, Anders Michelsen, Kathrin Rousk, Juha Alatalo, Katlyn Rose Betway, Masters in Biology, Sara Busca, Terry Callaghan, Michele Carbognani, Elisabeth J. Cooper, Johannes H. C. Cornelissen, Ellen Dorrepaal, Dagmar Egelkraut, PhD, Tatiana G. Elumeeva, Siri V. Haugum, Robert D. Hollister, Annika K. Jägerbrand, Frida Keuper, Kari Klanderud, Esther Lévesque, Xin Liu, Jeremy May, Pascale Michel, Martin A Mörsdorf, Alessandro Petraglia, Christian Rixen, Bjorn Robroek, Agnieszka M. Rzepczynska, Nadia Soudzilovskaia, Anne Tolvanen, Vigdis Vandvik, Igor Volkov, Irina Volkova, and Kristel van Zuijlen Title: Can bryophyte groups increase functional resolution in tundra ecosystems? Year: 2021 Version: Final draft Copyright: The Author(s) 2021 Rights: CC BY 4.0 Rights url: http://creativecommons.org/licenses/by/4.0/ Please cite the original version: Dr. Signe Lett, Dr. Ingibjörg Svala Jónsdóttir, Dr. Antoine Becker-Scarpitta, Dr. Casper T. Christiansen, Dr. Heinjo During, Dr. Flemming Ekelund, Prof. Greg H.R. Henry, Simone Lang, Dr. Anders Michelsen, Dr. Kathrin Rousk, Dr. Juha Alatalo, Ms. Katlyn Rose Betway, Ms. Sara Busca, Dr. Terry Callaghan, Dr. Michele Carbognani, Dr. Elisabeth J. Cooper, Prof. Johannes H. C. Cornelissen, Ellen Dorrepaal, Dr. Dagmar Egelkraut, Dr. Tatiana G. Elumeeva, Ms. Siri V. Haugum, Dr. Robert D. Jukuri, open repository of the Natural Resources Institute Finland (Luke) All material supplied via Jukuri is protected by copyright and other intellectual property rights. Duplication or sale, in electronic or print form, of any part of the repository collections is prohibited. Making electronic or print copies of the material is permitted only for your own personal use or for educational purposes. For other purposes, this article may be used in accordance with the publisher’s terms. There may be differences between this version and the publisher’s version. You are advised to cite the publisher’s version. Hollister, Dr. Annika K. Jägerbrand, Ms. Frida Keuper, Dr. Kari Klanderud, Dr. Esther Lévesque, Dr. Xin Liu, Dr. Jeremy May, Dr. Pascale Michel, Dr. Martin A Mörsdorf, Dr. Alessandro Petraglia, Dr. Christian Rixen, Dr. Bjorn Robroek, Ms. Agnieszka M. Rzepczynska, Dr. Nadia Soudzilovskaia, Dr. Anne Tolvanen, Prof. Vigdis Vandvik, Dr. Igor Volkov, Dr. Irina Volkova, and Dr. Kristel van Zuijlen. Can bryophyte groups increase functional resolution in tundra ecosystems?. Arctic Science. Just-IN https://doi.org/10.1139/AS-2020-0057 1Can bryophyte groups increase functional resolution in tundra ecosystems? Signe Lett1*, Ingibjörg S. Jónsdóttir2, Antoine Becker-Scarpitta3, Casper T. Christiansen1,4, Heinjo During5, Flemming Ekelund1, Gregory H. R. Henry6, Simone I. Lang7, Anders Michelsen1, Kathrin Rousk1, Juha M. Alatalo8, Katlyn R. Betway9, Sara Busca10, Terry Callaghan11,12, Michele Carbognani13, Elisabeth J. Cooper14, J. Hans C. Cornelissen15, Ellen Dorrepaal16, Dagmar Egelkraut10, Tatiana G. Elumeeva17, Siri V. Haugum10, Robert D. Hollister9, Annika K. Jägerbrand18, Frida Keuper19, Kari Klanderud20, Esther Lévesque21, Xin Liu22, Jeremy May23, Pascale Michel10, Martin Mörsdorf24, Alessandro Petraglia13, Christian Rixen25, Bjorn J.M. Robroek26, Agnieszka M. Rzepczynska1, Nadejda A. Soudzilovskaia27, Anne Tolvanen,28 Vigdis Vandvik10, Igor Volkov29, Irina Volkova30, Kristel van Zuijlen20 1Terrestrial Ecology Section, Department of Biology, University of Copenhagen, Universitetsparken 15, 2100 Copenhagen, Denmark 2Institute of Life and Environmental Sciences, University of Iceland, Sturlugata 7, 102 Reykjavik, Iceland 3Spatial Food Web Ecology Group & Research Centre for Ecological Change, Faculty of Agriculture and Forestry, University of Helsinki, PO Box 27 (Viikinkaari), 00014, Helsinki, Finland. 4Center for Permafrost (CENPERM), University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark 5Ecology & Biodiversity, Department of Biology, Utrecht University, 3584 CH Utrecht, The Netherlands 6Department of Geography, University of British Columbia, Vancouver, Canada Page 1 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 27Department of Arctic Biology, The University Centre in Svalbard (UNIS), 9171 Longyearbyen, Svalbard, Norway 8Environmental Science Center, Qatar University, Doha, Qatar 9Biology Department, Grand Valley State University, 1 Campus Dr., Allendale, MI 49401 USA 10Department of Biological Sciences, University of Bergen, PO Box 7801, N-5020 Bergen, Norway 11Department of Animal and Plant Sciences, University of Sheffield UK 12Department of Botany, Tomsk State University, Russia 13Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Parco Area delle Scienze 11/A, I-43124, Parma, Italy 14Department of Arctic and Marine Biology, Biosciences Fisheries and Economics, UiT-The Arctic University of Norway, N-9037 Tromsø, Norway 15Department of Ecological Sciences, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands 16Climate Impacts Research Centre, Department of Ecology and Environmental Science, Umeå University, Sweden 17Department of Ecology and Plant Geography, Biological Faculty, Lomonosov Moscow State University, Leninskie Gory, 1/12, 119234 Moscow, Russia 18Ecology and Environmental Science, RLAS, Halmstad University, Halmstad, Sweden 19BioEcoAgro Joint Research Unit, INRAE, F-02000 Barenton-Bugny, France 20Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, 1432, Ås, Norway 21Département des Sciences de l'environnement et Centre d'études nordiques, Université du Québec à Trois-Rivières, Qc, Canada, G8Z 4M3 Page 2 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 322Chinese Academy of Sciences Key Laboratory of Mountain Ecological Restoration and Bioresource Utilization & Ecological Restoration and Biodiversity Conservation Key Laboratory of Sichuan Province, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China 23Department of Biology, Florida International University, 11200 SW 8th Street, Miami, Florida USA 24Department of Geobotany, University of Freiburg, Schänzlestrasse 1, 79104 Freiburg, Germany 25WSL Institute for Snow and Avalanche Research SLF, Flüelastr. 11, 7260 Davos Dorf 26Aquatic Ecology & Environmental Biology, Institute for Water and Wetland Research, Faculty of Science, Radboud University Nijmegen, AJ 6525 Nijmegen, The Netherlands. 27Centre for environmental Sciences, Hasselt University, Martelarenlaan42, 3500 Hasselt, Belgium 28Natural Resources Institute Finland, B.O. Box 413, 90014 Oulu, Finland 29Laboratory of Biodiversity and Ecology, Tomsk State University, Lenin pr., 36, 634050 Tomsk, Russia 30Department of Botany and Laboratory of Ecosystem and Climate Change Studies, Tomsk State University, Lenin pr., 36, 634050 Tomsk, Russia *Corresponding author: signe.lett@bio.ku.dk Page 3 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 4Abstract The relative contribution of bryophytes to plant diversity, primary productivity, and ecosystem functioning increases towards colder climates. Bryophytes respond to environmental changes at the species level, but because bryophyte species are relatively difficult to identify, they are often lumped into one functional group. Consequently, bryophyte function remains poorly resolved. Here, we explore how higher resolution of bryophyte functional diversity can be encouraged and implemented in tundra ecological studies. We briefly review previous bryophyte functional classifications and the roles of bryophytes in tundra ecosystems and their susceptibility to environmental change. Based on shoot morphology and colony organization, we then propose twelve easily distinguishable bryophyte functional groups. To illustrate how bryophyte functional groups can help elucidate variation in bryophyte effects and responses, we compiled existing data on water holding capacity, a key bryophyte trait. Although plant functional groups, can mask potentially high inter- and intraspecific variability, we found better separation of bryophyte functional group means compared to previous grouping systems regarding water holding capacity. This suggests that our bryophyte functional groups truly represent variation in the functional roles of bryophytes in tundra ecosystems. Lastly, we provide recommendations to improve monitoring of bryophyte community changes in tundra study sites. Page 4 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 5Keywords: Mosses, Arctic-Alpine, environmental change, functional traits, water holding capacity Page 5 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 61 Introduction In the Arctic, bryophytes represent 30% of all plant species (Walker and Raynolds 2011). Unlike the general trend for vascular plants, regional bryophyte species richness does not decline when moving from the equator towards the poles (Geffert et al. 2013; Mateo et al. 2016). In many tundra ecosystem types, bryophytes contribute significantly (>50 %) to primary production and standing biomass (Wielgolaski 1971; Huemmrich et al. 2010) and play important roles for soil moisture, biogeochemical cycling, surface energy balance, and species diversity (e.g. Lindo and Gonzalez 2010; Turetsky et al. 2012). Tundra ecosystems are facing dramatic shifts in structure and function due to environmental change, which affects the abundance of bryophytes (Elmendorf et al. 2012a, 2012b; Lang et al. 2012; Olofsson et al. 2014; Cooper et al. 2019). However, although both functionality of bryophytes and their responses to environmental change differ considerably among species (Cornelissen et al. 2007; Hudson and Henry 2010; Lang et al. 2012) very few field studies include bryophytes at the species or other subgroup level. Therefore, studies are largely inconclusive and speculative in predicting responses of tundra bryophyte communities to environmental changes (Elmendorf et al. 2012a, 2012b) and the potential consequences of these changes for ecosystem functioning. The resolution of the bryophyte component in tundra vegetation and ecosystem studies could be increased considerably by applying relevant bryophyte functional groups. Traditionally, functional classification has been used in the opposite manner as an effort to reduce complexity in e.g. vascular plant ecology using ‘plant functional types’. Such a priori functional grouping has been challenged because effect and response traits do not necessarily match. Therefore, a good starting point for establishing fine-resolution linkages between bryophyte abundance, environmental changes, and ecosystem functioning could be to assign bryophyte species to functional groups through post hoc trait-based aggregation. This is done increasingly for other primary producer groups (Thomas et al. Page 6 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 72019; Mauffrey et al. 2020), because it directly provides ecologically meaningful functional groups. Such groups translate and aggregate species responses to more general functional responses and allow cross-site comparisons of responses regardless of species (Lavorel et al. 1997). However, for bryophytes the challenge is not limited to translating species into function. Bryophyte species identification as such is challenging, especially in the field. It is time- consuming and requires identification skills that few ecologists possess (Grace 1995). In practice, this causes most field ecologists to lump bryophytes into one group (e.g., bryophytes or even as ‘non- vascular plants’, with lichens as ‘cryptogams’), or two or more bryophyte groups (e.g., ‘Sphagnum’ and ‘other bryophyte species’). Consequently, important ecological information is lost, comparison between different studies is not straightforward and opportunities for addressing functional responses of bryophytes across sites and at larger scales are hampered. Therefore, for bryophytes, using a priori defined functional groups, based on coarse morphological characteristics that can be identified in the field, may be a more promising approach. Previous work on bryophyte classifications that has been based on life history traits (During 1979), position of sexual reproductive organs (La Farge-England 1996) and bryophyte colony structure (Mägdefrau 1982) offer useful insights about bryophyte ecology, but none of them focus primarily on functional diversity. Here we propose a priori defined, field-identifiable ‘bryophyte functional groups’ (BFGs) as a cost- and time-efficient, and meaningful way to increase bryophyte data resolution, allow measurement of change in bryophyte communities in response to environmental change, obtain comparable bryophyte data across tundra habitats and sites, and enhance understanding of bryophyte ecosystem effects and responses. To this end, we 1) provide an overview of the role of bryophytes in tundra ecosystems and their susceptibility to environmental change; 2) review previous efforts to group bryophytes and 3) build on these efforts to propose twelve field identifiable BFGs. 4) We evaluate the relevance of these BFGs in relation to water holding capacity (WHC), a functionally Page 7 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 8important and commonly measured bryophyte trait, in a case-study where we re-analyze existing data. As such, if BFGs separate into more than one cluster based on water holding capacity, the groups improve the functional resolution compared to the commonly used single 'bryophytes' group for ecosystem function governed by this trait. Finally, we discuss how BFGs may differ in regard to other key bryophyte functions and provide recommendations on how to apply BFGs in tundra ecological studies. 2. Bryophytes in tundra ecosystems 2.1 Ecosystem functions and functional diversity of tundra bryophytes An important feature of tundra bryophytes is that they often grow in dense carpets or colonies in many habitats. It is in the colony form that bryophytes most strongly affect the environment through their physical presence, as well as biogeochemically and biotically through interactions with other organisms in the ecosystem (Fig 1). The physical properties of a dense and deep bryophyte layer may significantly control the soil environment by buffering substrate moisture and insulate soil from diurnal and annual air temperature variation with consequences for biogeochemical processes (Gornall et al. 2007; Soudzilovskaia et al. 2013; Jaroszynska 2019), active layer development and permafrost ice content (Jorgenson et al. 2010). Through their effects on water balance bryophytes affect energy partitioning and decrease ecosystem ground heat flux (Blok et al. 2011) and affect surface albedo (May et al. 2018). Biogeochemically, bryophytes are important as they contribute to the ecosystem carbon (C) balance through their great abundance, high C use efficiency and because they are active beyond the short vascular plant growing season (Douma et al. 2007; Woodin et al. 2009; Street et al. 2012, 2013). Bryophytes control the input of nitrogen (N) to the ecosystem through associations with N2 fixing bacteria and by efficiently immobilizing N from deposition within the bryophyte layer (Jónsdóttir et al. 1995). Both C and N fixation rates are highly dependent on moisture Page 8 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 9conditions within the bryophyte tissue (Solheim and Zielke 2003; Turetsky 2003; Gavazov et al. 2010; Lett and Michelsen 2014; Rousk et al. 2015, 2017). Last, their recalcitrant litter and effects on pH are an important feature, which slows the release of C and N cycling in the ecosystem (Russell 1990; Lang et al. 2009; Soudzilovskaia et al. 2010). Through a combination of these physical and biochemical effects, bryophytes interact with the biotic environment. As such, they affect vascular plant growth and establishment through competition and facilitation (Gornall et al. 2011; Soudzilovskaia et al. 2011; Keuper et al. 2011; Lett et al. 2017, 2018, 2020). For instance, bryophytes can grow in places where vascular plants cannot root, such as rocks and glacial forelands, where they over time form an organic substrate which can later be colonized by plants (Jones and Henry 2003; Gavini et al. 2019). Their colonies comprise a matrix for unique food webs of microfauna and microbes (Lindo and Gonzalez 2010; Glime 2012; Jonsson et al. 2015). Although their dietary value is low (Prop and Vulink 1992; Hübner 2007), bryophytes are also consumed by vertebrate herbivores such as rodents, geese, reindeer/caribou, and muskox, (Glime 2006; Ihl and Barboza 2007; Bjørkvoll et al. 2009; Soininen et al. 2013). Bryophyte ecosystem functional trait data are still scarce in comparison to such data for vascular plants (St. Martin and Mallik 2017). Morphological shoot traits, and colony traits such as moss layer depth, colony density and surface texture and color, are important for determining the physical effects of bryophytes on tundra ecosystems. For instance, decomposition rates can vary more than 10-fold between the extremely recalcitrant Sphagnum mosses and more nitrogen rich species such as Ptilidium ciliare and Pleurozium schreberi (Lang et al. 2009; van Zuijlen et al. 2020) and water holding capacity may vary five-fold (Elumeeva et al. 2011). Such information is obtained by systematically screening species for important effect traits. 2.2 Responses in bryophyte cover to environmental change in tundra ecosystems Page 9 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 10 As for any tundra plant, climate warming is likely to promote bryophyte growth if water and nutrients are not limiting (Douma et al. 2007). Wetter and warmer climates, which are now occurring and expected in much of the tundra biome (Bintanja and Andry 2017; Thomas et al. 2018), should, in theory, promote bryophyte growth and thus abundance. Data on bryophyte abundance responses to climate change are scarce compared to those on vascular plants, but available data from the North American and European Arctic show an overall decline in bryophyte abundance in responses to climate warming across tundra ecosystems (Elmendorf et al. 2012a, 2012b). This decline seems more pronounced in moister sites and has been attributed to indirect effects of warming through competition from vascular plants (shading). However, bryophyte responses to warming vary substantially across species and sites, and habitats within sites. For example, no effect of experimental warming on bryophyte covers was observed in a sub-Arctic Racomitrium lanuginosum heath (Jónsdóttir et al. 2005), positive effects of warming were observed for common boreal bryophyte species in Arctic and subarctic alpine tundra plant communities (Lang et al. 2012) and for bryophytes in various habitats within a high Arctic tundra site (Hudson and Henry 2010; Edwards and Henry 2016). Changes in water availability under warming, or susceptibility to other global change drivers may also contribute to the observed general negative trends. Unlike vascular plants, most bryophytes are poikilohydric and cannot actively control their water balance. They have no or only thin leaf cuticles and do not have leaf stomates. Most species do not have efficient vascular systems (but see Brodribb et al. 2020) nor true roots, and access water and nutrients passively through their leaves. Bryophytes can, to varying degrees, tolerate desiccation during dry periods after which they return to normal physiological activity (Proctor and Tuba 2002; Proctor et al. 2007). Increased herbivore pressure may disturb the bryophyte layer in the tundra, as for example through spring grubbing by the increasing goose populations in the Arctic (Kotanen and Jefferies 1997; Wal et al. 2007). Page 10 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 11 Exclusion of lemmings and reindeer (Olofsson et al. 2014) and sheep (Jónsdóttir 1991) in subarctic alpine heath tundra increases bryophyte cover and colony depth through promotion of tall stature bryophytes. Furthermore, goose and sheep grazing can increase small scale bryophyte diversity at the species level (Jónsdóttir 1984; Jasmin et al. 2008). Increased snow depth may promote bryophyte biomass production and cover (Dorrepaal et al. 2004; Paradis et al. 2016; Cooper et al. 2019). Importantly, bryophytes show species-specific responses to multiple environmental factors operating at different spatial scales, with variable consequences for both community composition and total bryophyte cover across alpine and Arctic tundra regions. 2.3 Functional trait responses to the environment and intraspecific variation By combining data for total bryophyte cover and species composition with data for bryophyte functional traits we can understand how environmental changes affect ecosystem functionality (Díaz and Cabido 2001). Some functional traits of bryophytes may, however, themselves be responsive to environmental change causing considerable intraspecific trait variation in addition to interspecific trait variation that is caused by species turnover. For example, bryophyte tissue P content and shoot water holding capacity and growth showed high variation within species, while traits like pH, N content and litter decomposability showed less intra- than interspecific variation in alpine ecosystems (Jägerbrand et al. 2014; Roos et al. 2019; van Zuijlen et al. 2020). However, Roos et al. (2019) concluded that bryophyte species turnover rather than intraspecific variation drove changes in community abundance-weighted means of all six measured traits (N and P concentration and ratio, pH, specific leaf area and water holding capacity) across an elevational gradient. This supports the possibility to assign bryophyte species to groups, which could represent certain ecosystem functions. 3. Grouping of bryophytes Page 11 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 12 3. 1 Previous grouping of bryophytes During (1979, 1992) identified life history types to classify bryophytes according to life strategies (e.g., life span and reproductive strategy, -age and -effort). Life history types may depend on the environment and life history traits are likely a key to understanding bryophyte population dynamics (Austrheim et al. 2005); however, they do not provide full insights into bryophyte functional roles in the ecosystem. Currently, the majority of trait data for bryophytes occurring in the TRY database are on life history traits (Kattge et al. 2020). The growth form classification has often been used in combination with life form and/or perichaetial position in the literature. La Farge-England (1996) distinguishes growth form, life form and perichaetial position and indicates which are environmentally modified versus genetically fixed. This provided a comprehensive and unambiguous way to assess the structure of moss (Bryophyta) individuals. They refer to growth form as the structure of individual shoots, including direction of growth and branch form. Here, growth form (modified by the environment) is differentiated from the perichaetial position (La Farge-England 1996). Perichaetial position, which classifies acrocarpy, cladocarpy, and pleurocarpy, is analyzed and reviewed with an evolutionary perspective within major Bryophyta lineages. Huttunen et al. (2018) mapped “carpy” phylogenetically across the lineages in an extensive review on bryophyte functional traits. They show that perichaetial position alone does not determine the ecosystem function of bryophytes or how populations respond to environmental change. Growth form, on the other hand, seems to influence how shoots are organised in colonies, which is thought to be important for ecosystem functioning (Bates 1998). The life form classification of bryophytes was developed by Gimingham and Robertson (1950) and later refined and modified by Gimingham & Birse (1957), Mägdefrau (1982), Longton (1988), Grace (1995), Bates (1998), Hill et al. (2007) and Vanderpoorten and Goffinet (2009). The classification is based on the organization of the colony (group of shoots) although exact groups differ Page 12 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 13 between authors. The life form classification integrates shoot morphology, such as branching pattern, growth direction and colony structure. Grace (1995) showed that life forms are easily identifiable in the field across different levels of bryophyte identification skill. Importantly, it is convenient and meaningful to view bryophytes in terms of colonies rather than individuals in order to understand their effects on ecosystems (Bates 1998; Huttunen et al. 2018). Some physical colony properties, such as density and thickness, are directly related to ecosystem function, e.g., insulation capacity and water holding capacity (Gornall et al. 2007; Elumeeva et al. 2011; Soudzilovskaia et al. 2013), whereas it is unknown whether chemical properties such as N content are linked to colony structure. Through their colony features bryophytes also affect the biotic environment, e.g., by hosting specific micro- and mesofauna communities and through competition with or facilitation of other plants. Colonies in tundra ecosystems may be a mixture of several species (see below), which is a limitation to the life form classification. In conclusion, with their identifiability and ecological relevance, life forms integrate many of the desired features for a priori defined bryophyte functional groups. 3.2 Modified ‘life form’ as bryophyte functional groups Our primary aim is to improve the representation of functionally different bryophytes in studies of tundra ecosystems by (1) focusing on the specific context of their responses to environmental changes and their effects on key ecosystem functions, and to encourage this by (2) proposing field-identifiable bryophyte functional groups as an alternative to determination at either the highest level, ‘species’, and often not feasible level or the lowest level, ‘bryophyte’, of resolution, which is too coarse to be useful. Therefore, our bryophyte functional groups (BFGs) are chosen to be as morphologically distinct as possible to aid field identification. The BFGs are organized as a key (Fig 2). The first steps follow the British Field Flora for Mosses and Liverworts (Atherton et al. 2010) and rely on morphological differences in macro-characteristics of the shoots and thalli, including branching Page 13 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 14 pattern, and later steps divide groups based on colony structure, i.e. life forms (Fig 2). By combining the growth form and life form concept, we optimize the possibility to have functionally and morphological distinct groups, which are also taxonomically distinct. The first split divides bryophytes into those with thallus and those with leaves (Fig 2a). The ‘Thalloid’ group contains liverworts and hornworts (Marchantiophyta and Anthocerotophyta, Fig A1). Leafy bryophytes are divided based on the characteristics and placement of the leaves (Fig 2b). Leaves can be arranged either in 2-3 ranks, mostly rounded or 2-lobed, and always without nerves, ‘Leafy liverworts’, or have leaves which are arranged in a spiral and often with a nerve and acute tip, mosses (Bryophyta). Mosses are further divided into those with a capitulum, i.e. ‘Sphagnum’ and those without capitulum, i.e. Non-Sphagnum (Fig 2c). The group and genus Sphagnum is easily recognized in the field as no other bryophytes have a capitulum. Non-Sphagnum mosses are divided into those with branched shoots and those with shoots not or infrequently branching (Fig 2d) roughly corresponding to pleurocarps and acrocarps; cladocarps fall into both groups. Colonies with shoots not or- infrequently branching are divided into those with thick, non-transparent leaves and those with thin, more transparent leaves (Fig 2e). Non-transparent leaves are a feature of ‘Polytrichales’ (Fig A1) and are caused by lamellae on the surface of the leaves. These lamellae are usually visible with a hand lens but common for this group is that stem and leaves tend to be sturdier than in individuals in the contrasting group. The contrasting group, mosses with thin, more transparent leaves, form a large group, which is divided into ‘Cushions’ and ‘Unbranched turfs’ (Fig 2f). ‘Unbranched turfs’ correspond to Bates’ turfs (Bates 1998), except our group includes only acrocarps. All shoots grow vertically from the substrate and, depending on the length of the shoots, ‘Unbranched turfs’ are divided into ‘Short unbranched turfs’ (<5 cm) and ‘Tall unbranched turfs’ (>5 cm, Fig 2g). The ‘Cushions’ have dome shaped colonies (as in 'cushion plants' such as Silene acaulis). Page 14 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 15 ‘Cushions’ are divided into ‘Small cushions’ and ‘Large cushions’. Small cushions have shoots emerging from a shared, central origin so that shoots grow centrifugally and are less than 5 cm deep, Fig 2h), e.g., genus Grimmia and Andreaea. ‘Large cushions’ are more than 5 cm deep and may or may not have shoots growing from a central point. Species of this group also appear in other BFGs e.g., Racomitrium lanuginosum in branched turf (see below) or Leucobryum glaucum, Dicranum elongatum and Anoectangium aestivum, in tall unbranched turfs. Colonies with branched shoots are divided into ‘Dendroid’, ‘Weft’, ‘Mat’ and ‘Branched turf’ (Fig 2i). The dendroid classification is technically a growth form. Dendroids have shoots that extend from horizontal stem and have branches placed towards the tip of the shoot making them resemble miniature trees. This is a small group and the most common species in tundra ecosystems is Climacium dendroides, but also e.g. Thamnobryum alopecurum and Isothecium alopecuroides are found in subarctic areas. Wefts also have strongly branched shoots, but branches are distributed throughout the entire stem giving rise to the colloquial name, feather moss. Colonies appear loose and chaotic with large heavily branched shoots growing both vertically and horizontally. The emblematic boreal species Pleurozium schreberi and Hylocomium splendens belong to this group. Shoots of the Mat group grow horizontal to the substrate. Tips of shoots can become erect, giving the mat a rougher surface as described in Bates (1998), but generally the branched shoots lie flat on the surface and therefore have a rather compact appearance. Mats are often found on solid substrates like logs or stones. Branched turf forms a new group containing pleuro- and cladocarp forming turfs. Like the unbranched turf, they have erect shoots but differ in that their shoots are branched, although usually not as branched as the Wefts. The abundant tundra species Tomentypnum nitens and species from the Racomitrium genus belong to this group. The 12 BFGs (Fig 2) do not encompass all tundra bryophyte species but focus on the perennial bryophytes, which constitute the vast majority of species (in the British bryophyte flora, 90 Page 15 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 16 % of species are perennial, Hill et al. 2007). Most species can be ascribed to only one BFG but some variable species will have a primary and a secondary life form, as recognized in Hill et al. (2007). For example, the cladocarpous Racomitrium lanuginosum is often quite branched and can form continuous and often deep layers, which would place it in the ‘branched turf’ group. In more exposed sites with less vegetation cover, it can form dense cushions and would therefore be better placed within the ‘Large cushion’ group. Furthermore, although species within each BFG do not necessarily share all characteristics (Dormann and Woodin 2002; Dorrepaal 2007) we argue that our bryophyte functional groups will increase resolution in tundra ecosystem studies compared to the frequent lumping of all bryophytes into one or very few functional groups. 3.3 Ecosystem functions of the BFGs We assessed whether our choice of field-identifiable bryophyte functional groups can lead to a more meaningful representation of tundra bryophytes in the study of their ecosystem function by investigating how the groups separate for one key trait that is frequently measured, the water holding capacity (WHC). As such, if BFGs separate into more than one cluster, we can conclude that the resolution for that trait is improved compared to the commonly used 'bryophytes'. Currently bryophyte trait data are poorly represented in global and regional trait databases such as the TRY (Kattge et al. 2020) and the tundra plant specific Tundra Trait Team database (Bjorkman et al. 2018), which limits the possibility to fully test the BFGs. However, a wide set of ecosystem functions are ultimately linked to bryophyte water balance. Water content, in turn, depends on habitat, seasonal climate and the species-specific ability of bryophytes to retain and hold water. The trait WHC is determined by a set of other traits, such as colony density and leaf and shoot morphology (Elumeeva et al. 2011).Water content is important for key bryophyte traits such as insulation capacity, albedo, Page 16 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 17 flammability, growth and association with N2-fixing bacteria (Cornelissen et al. 2007; May et al. 2018). To assess if our groups indeed perform better than those previously identified, we also compared our groups to three previous grouping systems. We chose the life form classification by Grace (1995) as this has been tested on non-expert people and found to be user-friendly, and it is most similar to our groups with Sphagnum defined as a separate group, ‘whorled branched turf’. We included the primary life forms as defined by Bernhardt-Römermann et al. (2018), because these are the most recent of the life form classifications. As a third grouping system, we included perichaetial position (La Farge-England 1996), with the only adaptation that we included liverworts as a separate group). 3.3.1 Water holding capacity data collection and analysis We collated existing data on water holding capacity (WHC) defined as maximum water held per gram of dry mass of bryophyte shoots or monospecific bryophyte colonies. The full dataset included 1360 observations of 59 species from both published and unpublished studies. All data were from tundra ecosystems, except one Norwegian coastal heathland study (Rui, Vandvik, Haugum unpubl.). Although the method across studies did not follow any standardized protocol the studies could be grouped into three methods of measuring WHC: as “internally” (shootint) and “internal and externally” (shootint+ext) held water in shoots and for whole bryophyte colonies (see Appendix Table A1 for the descriptions of methods for the individual studies). While colonies represent the most realistic field situation, measurements at the shoot level are less destructive and therefore possible to conduct in long-term experimental plots and often data will therefore exist in this form. Internally held water is likely what directly links to physiological processes taking place inside bryophyte cells but is likely ultimately dependent on colony WHC. Water holding capacity shootint data included 36 Page 17 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 18 species representing 7 BFGs (Elumeeva et al. 2011; Michel et al. 2012; Roos et al. 2019; van Zuijlen et al. 2021). WHC Shootint+ext, data included 28 species representing 8 BFGs (Busca, Vandvik, Haugum, unpubl.; Elumeeva et al. 2011; Rzepczynska, Lett, Michelsen unpubl.) and WHC colony data included 33 species representing 7 BFGs (Elumeeva et al. 2011; Jónsdóttir, unpubl.; Lett et al. 2017, Liu and Rousk unpubl.; May et al. 2018; Michel et al. 2012; Rzepczynska, Lett, Michelsen, unpubl.). Correlation analyses with averaged species WHC values showed that WHC of whole colonies and WHC of shootsint+ext were well correlated (Fig A2a) whereas WHC shootint did not correlate with other ways of measuring WHC (Fig A2, b, c). Data for WHC shootint was therefore excluded from further analyses. The remaining dataset included 963 observations of 37 bryophyte species, which we assigned to eight different BFGs (Fig 3, Fig A3) and to existing grouping schemes, namely ‘perichaetial position’ (Liverworts grouped separately) (La Farge-England 1996), life form according to Grace (1995) and life form according to the BryForTrait database (Bernhardt‐Römermann et al. 2018). Differences in WHC between groups within the different grouping schemes were analyzed with a mixed-effect model followed by Tukey's HSD test (see Appendix). All data were handled and analyzed in R version 4.0.3 (R Core Team 2020). 3.3.2 Tundra bryophyte functional groups in relation to water holding capacity Three clusters with distinct WHC (shootsint+ext and colony) materialized from our analysis (Fig 3). ‘Sphagnum’ had the highest WHC, with an average of 17 g water per g dw for colonies and shootsint+ext. The high WHC of Sphagnum species is primarily attributed to their specialized hyaline cells, which greatly increase their water holding capacity. With 2 g/g, ‘Polytrichales’ had the lowest WHC and there was little variation between species within the group. Polytrichales are unique in several ways as they have relatively well-developed water conducting tissue and root-like structures, Page 18 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 19 rhizoids and waxy leaves, a feature that reduces water evaporation rather than increasing water storage. The four groups, ‘Weft’, ‘Mats’, ‘Tall and Short unbranched turf’ and ‘Branched turf' had intermediate WHC and did not differ from each other. Large variation between species within those groups (Fig A3) shows that not all our groups distinguish themselves from each other in terms of WHC. Particularly the groups ‘Short unbranched turf’ and 'Tall unbranched turf’ displayed almost as much variation between species within groups as the entire spectrum of the dataset. A better separation between those groups might have been achieved by use of standardized protocols or a larger number of species representing each group. However, the groups possibly differ in other functional traits and thus represent functionally distinct species clusters, and this should be tested in future work. Four of our groups were not represented in the analyses. Of these, ‘Thalloid’ is, with its absence of leaves, the morphologically most distinct. From this group Marchantia foliacea and Monoclea forsteri from New Zealand forests had WHC of 20 and 10 g/g, respectively (Green and Snelgar 1982), which is within the upper end the spectrum covered in our study. ‘Cushions’, large and small, were also not represented by our data. Cushion growth is considered an adaptation to water conservation and low temperatures (Rice and Schneider 2004; Sand-Jensen and Hammer 2012). The WHC of ‘Dendroid’ bryophytes has not been studied but the dendroid life form is associated with habitats of relatively high moisture or humidity (Atherton et al. 2010). Dendroids grow in loose patches, often intermingled with other species, and with limited branches at the lower stem, which could suggest that their WHC is not improved by colony structure. 3.3.3 Tundra bryophyte functional groups and other functional traits Data at the species level for more than one functional trait are required to fully understand the functional roles of each BFG. Water holding capacity is only one of many important traits which Page 19 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 20 relate bryophytes to key ecosystem functions (Cornelissen et al. 2007) and the BFGs are likely to cluster in unique ways for different functional traits. Here we discuss how additional functional traits, which are presumed to be of importance for ecosystem functioning, likely differ between the BFGs i.e., colony density, bryophyte layer depth, relative growth rate, decomposability and nutrient content and identify the need for further research (Fig 4). Colony density together with bryophyte layer depth affect soil insulation efficiency, which in turn affects soil temperature, organic matter decomposition, nutrient cycling, active layer depth and permafrost (Gornall et al. 2007; Soudzilovskaia et al. 2013). Colony density is relatively low in ‘Sphagnum’, ‘Weft’ and ‘Tall unbranched turf’ whereas ‘Branched turf’ and ‘Polytrichales’ are BFGs that often have relatively higher density values (Fig 4). Bryophyte layer depth or mat thickness is here defined as the distance from bryophyte layer surface to the point where bryophyte shoots or thalli begin to disintegrate. Mat thickness differs between some BFGs partly because size is an explicit character defining some groups (‘Unbranched turf’ and ‘Cushion’). Along with ‘Sphagnum’, ‘Tall unbranched turf’ and ‘Large cushion’ create relatively deep bryophyte layers (Fig 4). ‘Thalloid’ bryophytes and ‘Mats’ are never deep as they grow in close contact with the substrate. The contribution of bryophytes to ecosystem C balance is manifested by the bryophyte layer depth as the balance between their net primary production and litter decomposability leads to variation in accumulation of bryophyte-derived organic matter. Both growth rate and decomposability show high variation between species and potentially between the BFGs. Studies comparing growth rates between multiple bryophyte species are relatively sparse and likely very sensitive to the method used. In growth chambers, length increment of ‘Sphagnum’ was high compared to intermediate ‘Weft’ and ‘Branched-’ and ‘Unbranched turfs’ and slow growing ‘Leafy liverworts’ (Fig 4, Rzepzynska, Lett, Michelsen unpubl.), whereas maximum biomass gain under highly standardized conditions was the highest in ‘Short unbranched turf’ and ‘Leafy liverworts’ (Furness and Grime Page 20 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 21 1982a). While both length and biomass gain may be relevant performance indicators, they are two different functional traits (Furness and Grime 1982b). This highlights the need for standardized bryophyte trait protocols. Bryophyte functional groups seem to have a relatively high variability in decomposability as ‘Sphagnum’ had the lowest decomposition rates, followed by ‘Thalloid’ bryophytes and ‘Branched turfs’ (Fig 4, Lang et al. 2009). ‘Unbranched turf’, ‘Weft’ and ‘Polytrichales' had intermediate, and ‘Leafy liverworts’ had the highest decomposition rates. The rates of these two processes are likely influenced mostly by the contents of C-rich recalcitrant compounds, but also by tissue nutrient contents. Both bryophyte growth and decomposability are likely to be strongly affected by tissue nutrient content (Lang et al. 2009), and N content can be regarded as a separate functional trait. Although there are indications from previous studies that BFGs are likely to differ in important functional traits (Fig 4), there are big gaps in available data and the BFGs are not evenly represented. ‘Dendroids’, ‘Mats’, ‘Small and large cushion’ and ‘Thalloids’ are heavily understudied and focused efforts to include species representing these groups is crucial. In addition, there is a range of other important functional bryophyte traits which are less studied. For example, flammability has large impacts on ecosystem C and N balance and could become more important under future warmer and drier climate conditions. As water content greatly influences bryophyte flammability (Blauw et al. 2015), flammability could be linked to WHC and thus predicted by the BFGs. Bryophytes constitute an important substrate in many tundra ecosystems for N2 fixing bacteria with substantial inter species variation (Gavazov et al. 2010; Stuart et al. 2020). The mechanisms controlling N2 fixation in bryophytes are poorly understood but traits like WHC and perhaps specific leaf area are potential important predictors of species differences (Rousk et al 2018; Liu and Rousk unpubl.). In conclusion, the BFGs are likely to have unique combinations of trait values for a range of functional traits and the application of BFG could improve functional resolution in ecosystem studies. Page 21 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 22 While BFGs may differ in one or many functional traits, many traits are plastic and may cause substantial intraspecific (or intra-BFG) variation. This variation needs to be further explored and it is possible that for some traits, the extent of the intraspecific variation may be predicted by the BFGs. For example, the trait ‘bryophyte albedo’ is highly plastic for some species e.g. within the genera of Sphagnum and Racomitrium, which turn whiteish upon drought (May et al. 2018). In addition, trait plasticity could be important for understanding changes in bryophyte community composition in relation to environmental change as species or BFGs with higher trait plasticity may be less susceptible to environmental changes (Henn et al. 2018; Roos et al. 2019). 4 Using BFGs in vegetation surveys The inclusion of bryophyte functional groups in vegetation assessments using standard methods such as the point-intercept method, a standard within the ITEX network (Molau and Mølgaard 1996), and visual cover estimates could improve vegetation analyses in tundra ecosystems by providing greater resolution of the bryophyte component in plant communities. The BFGs are partly defined by the type of colony they appear in, but bryophytes do not always grow in mono-specific patches. Often, they grow in complex assemblages of multiple species, which may or may not belong to the same functional group. If species growing in the same colony do not belong to the same BFG, how should a BFG then be determined based on colony type? For the point intercept method, this is partly solved in our classification system which combines shoot and colony characteristics. The hierarchical organization (Fig 2) of the BFGs allows group determination at a ‘lower’ level in cases where colony type cannot be determined (Broad functional groups, Fig 2). For visual cover estimates of bryophyte colonies, the BFG that the dominating species belongs to may be recorded. In relation to these issues, functional properties of mixed and single species bryophyte colonies can differ beyond the additive effect of the combination of species (Mulder et al. 2001; Rixen and Mulder 2005; Michel et al. 2012). Page 22 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 23 In this way, the function of a given colony may not be the weighted mean of the species present in the colony. This is especially an issue when assessing bryophyte community function based on the species present in the ecosystem. Functional groups, like the ones suggested here, do not eliminate this issue. Despite these unresolved situations, we believe that the benefits of using BFGs will exceed the drawbacks. In practice, we suggest using the proposed bryophyte functional group classification as a complement to the species approach. Thus, when a species cannot be determined due to issues such as time or skill limitation, hits are assigned to the group. It may also be advisable for field researchers to learn the two to three most common species in their plots and go to species level here. Importantly, the BFGs could also be used the “other way around” as a means to combine species allowing comparison of bryophyte-cover data across experiments and field sites. This type of aggregation is essential for enabling comparison when sites do not have the same species and has been done successfully for vascular plants within the ITEX network (Walker et al. 2006; Elmendorf et al. 2012a, 2012b). This would provide important insights into the responses of bryophyte communities to climate and environmental change and their ecosystem impacts and consequences. 5 Directions for future research The BFGs suggested here are a first step to facilitate inclusion of bryophytes in vegetation surveys at a higher functional resolution than simply 'bryophytes', while still accessible to non-experts and a means to lump bryophytes into meaningful groups that share important functional traits. However, the suggested BFGs need to be further evaluated for their usefulness by statistical testing for additional traits measured at the species level. With the WHC data, we provide an example of how this evaluation can be done and show that for WHC bryophyte functional resolution is increased from one to three by using the BFGs rather than the generic ‘bryophytes’ and that the BFGs explain WHC Page 23 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 24 better than previous grouping systems. The outcome might either support the grouping suggested here or require adjustments. Further testing of the BFGs requires accessible functional trait data, which is currently limited. To allow robust analyses, these traits must be gathered using standardized protocols building on previous efforts (Jónsdóttir et al. 1999; Cornelissen et al. 2007; Hill et al. 2007). In order to improve or include bryophyte representation in current trait databases such as TRY and Tundra Trait Team (Kattge et al. 2020; Björkman et al. 2018) bryophyte functional trait data need to be geographically and taxonomically diverse for good representation of species. Unlike vascular plants, bryophyte functional traits are rarely recorded in field experiments, as this requires skills in species identification. For instance, in the database TRY, moss shoot length only has 716 entries and none of the observations are georeferenced, whereas vascular plant height has 249,551 observations (Kattge et al. 2020) and Arctic plant traits are generally highly under-represented (Bjorkman et al. 2018). Future challenges therefore lie in identifying and measuring bryophyte traits that underpin key ecological functions and to add these to existing trait databases. Importantly, species level identification cannot be circumvented for these trait studies. Bryophyte species identification will likely remain a struggle for many ecologists. This is further challenged by the lack of a comprehensive flora covering the Arctic region and poor representation of bryophytes in plant-identification mobile phone applications such as SEEK- iNaturalist or PictureMe (pers. obs., the authors). To date, there is not a comprehensive bryophyte flora that covers Greenland and North America. Fennoscandia and the Russian Arctic are covered by several regional floras (Table A2). As the Arctic biome consists of many countries, language can be another obstacle for identification. Comprehensive floras are needed to facilitate further focus on tundra ecosystem functioning where bryophytes are a major component both in terms of biomass, primary production and diversity. In the longer term, novel field-based genetic profiling technology Page 24 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 25 (Parker et al. 2017) and plant identification applications may develop to become powerful tools for aiding field identification. Until then, our contribution seeks to minimize the loss of data in the long- term monitoring of the Arctic vegetation and elucidate the functional role and importance of bryophytes in tundra ecosystems. This may in turn stimulate further focus on species identification as well as facilitate openings for innovative research projects. 6 Final remarks Today, bryophytes constitute a missing functional and evolutionary dimension in most tundra ecosystem studies, hindering our ability to understand ecosystem functionality and responses to environmental change. Using BFGs could be a means to include functional diversity of bryophytes in ecological studies while bypassing difficulties with species identification. Our example with bryophyte WHC shows some of the potentials and challenges of using BFGs and the groups can likely be improved through further studies at the species level. If proven robust, the groups could likely be expanded to include the boreal zone, another region where bryophytes play a major role (Turetsky et al. 2012). The hierarchical organization of the BFGs allows functional resolution to be adjusted to the scientific question in mind. Importantly, our suggestion to use the BFGs in ecological studies is not a suggestion to abandon studies of bryophyte functionality and responses at the species level. Rather it should be seen as an encouragement to include bryophytes at a higher functional resolution than simply ‘bryophyte’ in more studies. 7 Acknowledgements This study was supported by a grant to SL from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie, Grant No. 797446 and by the Independent Research Fund Denmark, Grant no. 0135-00140B. Funding from the Academy of Finland (grant Page 25 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 26 322266), National Science Foundation (1504224, 1836839, PLR-1504381 and PLR-1836898), Independent Research Fund Denmark (9040-00314B), Moscow State University, (project № 121032500089-1), Natural Sciences and Engineering Research Council of Canada, Arctic Net, Polar Continental Shelf Program, Northern Science Training Program, Polar Knowledge Canada, Royal Canadian Mounted Police, Tomsk State University competitiveness improvement program and the Russian Science Foundation (grant № 20-67-46018) is gratefully acknowledged. Matthias Ahrens provided valuable insights on the cushion growth form, and we are most thankful. We thank Gaius Shaver and two anonymous reviewers for providing valuable critique and input to earlier versions of this manuscript. Author contributions SL and ISJ conceived the original idea for the paper, which was developed with input from ABS, CTC, HD, FE, GHRH, SIL, AM, KR. 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It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 38 Figure 1 Schematic overview of some of the structural and functional roles of the bryophyte layer (Bryosphere) in tundra ecosystems. The bryosphere (green) does not substantially penetrate the soil but creates a zone between the active layer (brown) belowground and the atmosphere (light blue) aboveground. Blue-framed boxes are physical properties (solid) and conditions (dashed), yellow- framed boxes are biogeochemical pools (solid) and processes (dashed) and pink-framed boxes are the biota. Dashed arrow indicates an uncertain connection between boxes. The diagram does not include all environmental factors acting on bryophyte functioning. Figure 2 Bryophyte functional groups (BFGs) building on shoot morphology and Life forms (Mägdefrau 1982). The left diagram functions as a key, which splits bryophytes into 12 BFGs listed in the right panel with their abbreviations (Abb.), short descriptions and examples of species. The key starts at the grey bubble and dichotomies leading from each step are indicated with a number and same color arrow. The 12 BFGs are placed at the periphery. Red crossed circles mark “not Sphagnum” or “not Polytrichales”. Groups originating from orange bubbles can be collated to form broader functional groups. Note, in nature bryophytes often occur as a mixture of species, usually individuals occupy the same BFG, but if they do not, the most abundant BFG should be recorded. Figure 3 Water holding capacity (WHC, g water per g dry mass bryophyte, g/g) analyzed across each of four grouping schemes. Black dots are species means and grey diamonds are group means. The dataset includes 963 observations of 37 species. Each species was assigned a bryophyte functional groups (BFGs), perichaetial position (La Farge-England 1996) and life form according to Bernhardt- Römermann (2018) and life form following Grace (1995). Bryophytes functional groups are represented in the dataset by Po, Polytrichales; BT, Branched turf; LL, Leafy liverworts; We, Weft; SU, Short unbranched turf; TU, Tall unbranched turf; Ma, Mat; Sp, Sphagnum). BryForTrait Life form following Bernhardt-Römermann et al (2018) are represented by Weft, Mat, Turf, cushion and species not assigned to groups (NA). Life form following Grace (1995) are represented by Large (L) cushion, Smooth mat, Tall (T) turf, Short (S) turf, Weft, Rough (R) mat, Whorled branch (WB) turf. Page 38 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 39 Groups with different lower-case letters are significantly different, N.S. indicate where groups that are not different (Tukey’s test, p<0.05). Full variation within species for each BFG can be seen in the Appendix Fig A3. Figure 4 Selection of six bryophyte traits important for ecosystem functioning and their estimated relative value across the 12 bryophyte functional groups (BFGs). Three shades of green, light to dark indicate relative trait values (low, intermediate, high) assessed from the referenced sources. Diagonally split cells reflect that for a given BFG trait values range across the full spectrum. Striped cells are not covered by the given reference but are hypothesized based on authors’ expert knowledge, no propositions are made for white cells; question marks indicate lack of data. Traits are water holding capacity (WHC), colony density, bryophyte colony layer depth/ shoot length, growth rate, tissue decomposability and nitrogen (N) content. References for a given trait do not necessarily use common protocols or units. No study covers all traits or all BFGs. Bryophyte functional groups are Sp, Sphagnum; De, Dendroid; We, Weft; Ma, Mat; BT, Branched turf; SC, Small cushion; LC, Large cushion; SU, Short unbranched turf; TU, Tall unbranched turf; Po, Polytrichales; LL, Leafy liverworts; Th, Thalloid. Page 39 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . Page 40 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . SC Abb BFG Short description Examples of common/ typical species Th Thallose liverwort and hornwort Have undifferentiated vegetative tissue (no stems and shoots). Marchantia foliacea, Aneura pinguis LL Leafy liverwort Shoots with leaves in 2-3 ranks, mostly rounded or 2-lobed and always without veins. Ptilidium ciliare, Lophozia floerkii Sp Sphagnum Have capitulum. All Sphagnum species. Sphagnum fuscum, S. girgensohnii, S. warnstorfii Po Polytrichales Unbranched shoots. Lanceolate leaves appear thick due to lamellae parallel to the vein (visible with hand lens). All Polytrichales Polytrichastrum alpinum, Polytrichum commune SC Small cushion Dome-shaped colonies < 5 cm deep. Shoots grow from a central point with limited growth. Andreaea rupestris, Schistidium apocarpum, Grimmia spp. Sh oo ts n ot o r s pa rs el y br an ch ed LC Large cushion Dome-shaped colonies > 5 cm deep. Colonies may reach a radius of several decimeters. Racomitrium lanuginosum, Anoectangium aestivum, Dicranum elongatum SU Short unbranched turf Colonies of unbranched (or sparsely branched) erect shoots. Shoots < 5 cm. Aulacomnium turgidum, Dicranum acutifolium, Cinclidium stygium TU Tall unbranched turf Colonies of unbranched (or sparsely branched) erect shoots. Shoots > 5 cm. Dicranum majus, D. scoparium, Plagiomnium ellipticum, Paludella squarrosa De Dendroids Main stem creeping and becomes erect. Branching stems from apex of main stem. Climacium dendroides, Thamnobryum alopecurum Sh oo ts b ra nc he d We Weft Shoots grow erect and horizontally. Multiple branching stems distributed throughout the main stem. Sometimes appearance of a feather. Hylocomium splendens, Pleurozium schreberi Ma Mat Branched shoots grow horizontal to substrate. Sometimes shoots possess erect lateral branches. Hypnum cupressiforme, Plagiothecium denticulatum BT Branched turf Colonies of erect shoots with some branching. Tomentypnum nitens, Racomitrium spp., Drepanocladus revolvens Th Mosses LL Sp Po c We DeMa Bryophytes a e TU SU LC b BT d g i f h Page 41 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 207x166mm (600 x 600 DPI) Page 42 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . Trait Sp De We Ma BT SC LC SU TU Po LL Th Reference WHC ? ? ? ? This study Colony density ? ? ? ? Soudzilovkaia et al 2013, Lett et al 2017, Elumeeva et al. 2011 Depth/ shoot length ? * ? Soudzilovkaia et al 2013 Growth rate ? ? ? Furness 1982, Rzepczynska, Lett, Michelsen unpubl. Decom- posability ? ? ? ? Lang et al 2009 N content ? ? ? ? ? Roos et al 2019, Rzepczynska, Lett, Michelsen unpubl. Bryophyte functional groups - BFGs * Mats will always be relatively shallow while their shoots may grow to an extensive length along the substrate ? No data Page 43 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 1Appendix for ‘Can bryophyte groups increase functional resolution in tundra ecosystems?’ Methodology on water holding capacity: Water holding capacity (WHC) in all studies was measured as maximum held water per gram dry weight bryophyte. No recognized standardized protocol exists for measuring WHC and methodology therefore differed between studies (Table A2). However, across the WHC studies examined, three different general approaches were followed: WHC for shootint+ex, WHC for shootext and WHC for colonies. Shootint+ext WHC was the weight difference of shoots at full water saturation and after complete drying. Shootint WHC was measured in a similar manner except external water was removed before weighing by blotting shoots dry on a paper towel (Elumeeva et al. 2011). Bryophyte colony WHC was measured by weighing colonies at fully saturated conditions and after complete drying. For bryophyte colonies, volume varied between 20 and 3200 cm3 between studies, though one study had volumes down to 2.5 cm3 for some small statured bryophytes Neoorthocaulis floerkii, and Dicranum elongatum (Rzepczynska, Michelsen, Lett unpubl.). To test if WHC measurements of the three approaches were correlated, we averaged species values and where species were represented for at least two approaches, these were fitted using linear models (Fig A1). Colony WHC and shootint+ext WHC were significantly correlated (p <0.001, R2 = 0.70), whereas shootint WHC did not correlate with colony or shootext WHC (Figure A1). This suggests that WHC can be measured on single shoots (WHC of shootsint+ext) in permanent plots where destructive measurement must be kept to a minimum and still represent colony WHC reasonably well. Water holding capacity of shootint on the other hand should perhaps be considered an entirely separate trait. Because WHC of shootint+ext and colony were well-correlated (Fig A2), differences between tundra bryophyte functional groups (BFGs) were analysed for WHC of shootint+ext and colony together with a mixed effects model followed by Tukey’s HSD test. To take into account potential structural biases across studies, study ID was included as a random factor. Species was nested inside study ID to take into account the expected smaller variation within- compared to between bryophyte species. Page 44 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 2Table A1 Methods of individual water holding capacity (WHC) studies. Full references for published studies can be found in the reference list for the main text. Water holding capacity was measured either for whole colonies, for shoots with internally and externally held water (WHC shootint + ext) and for shoot only with internally held water (WHC shootint). See above for overall description of methodology. Study Description Dimensions WHC colony Elumeeva et al. 2011 Colonies kept in plastic containers. Shoots of non-target species removed to <1%. Colonies remoistened and surplus water drainage allowed. Samples weighed and full moisture and after oven-drying at 90°C until constant mass. N: 5, area: 7.5 x 7.5 cm2, Depth: similar height as in field, green and basal parts included Jónsdóttir, unpubl. Colonies kept in plastic containers. Shoots of non-target species removed to <1% (usually not needed). Colonies sprayed with water until saturated and allowed to drain surplus water. Weighed at full saturation and after drying at 70°C. N: 10, area 19.6 cm2 (circle, 5 cm in diameter), trimmed to 5 cm depth Lett et al. 2017 Colonies kept in plastic containers. Shoots of non-target species removed to <1%. Colonies sprayed with water until field saturation and allowed to drain surplus water. Weighed at full water and after oven-drying at 85°C until constant mass N: 4, area: 11 x 11 cm2, depth: 2.8 -7.3 cm depending on species, green and basal parts included Liu and Rousk unpubl. Colonies kept in plastic containers. Shoots of non-target species removed to <1%. Placed in a tray of distilled water for 12 h to saturate and allowed to drain surplus water. Weighed at full water and after oven-drying at 65°C until constant mass. N: 3, area: 10.75 cm2 (circle, 3.7 cm Ø), depth: 2.1-7.8 cm depending on species, green and basal parts included May et al. 2018 Vertical faces of colonies wrapped in cellophane and placed in trays. Colonies had vascular plants and soil removed and contained 95% target species moss. Placed in a tray of distilled water (3 cm depth) to hydrate. Soaked for 2 h until full saturation, then drained for 1 h. Colonies weighed after draining and at 0% water content after dried at 50°C. N: 4, Area: 20 x 20 cm2, depth: fixed depth, 8cm Michel et al. 2012 Colonies kept in plastic containers. Shoots of non-target species removed to <1%. Colonies sprayed with water until field saturation and allowed to drain surplus water. Weighed at full water and after oven-drying at 60°C until constant mass. N: 6 for each colony, 16 colonies, area 19.6 cm2 (circle 5 cm in diameter), fixed depth, 5cm depth Rzepczynska, Lett, Michelsen, unpubl Colonies kept in plastic containers. Non-target species removed (~95%). Sprayed with distilled water until full saturation, allowing the excess water to drain. Samples were weighed and then dried at 85°C for 48h. N: 20; dimensions varied between species (vol. 3 to 248 cm3) WHC shootint + ext Busca, Vandvik, Haugum, unpubl. Soil removed. Soaked in water for 30 min. Suspended in sealed container with water at the bottom for 24 hours at 22 to allow excess of water drops and prevent evaporation. Then, weighed, oven-dried for 72h at 70°C and weighed again N: 30, Sample size: 1g dw Elumeeva et al. 2011 Separate shoots remoistened in deionized water for min 12 h before weighing at full turgor. Less than 30 s before initial weighing every shoot was taken out of the water, shaken and lightly blotted to remove the extra external water not well connected with the shoot structures. Shoots dried at 90°C until costant mass and weighed again. N:10, Sample size: 1 shoot Rzepczynska, Lett, Michelsen unpubl. Shoots collected from each sample placed in a glass vial and sprayed with distilled water until full turgor. Vials sealed with perforated parafilm for 24h, shoots then weighed before and after drying at 50°C for 48h. N: 20 , number of shoots differed between species, always covering area of 1 cm2 WHC shootint Roos et al 2019 Shoots submersed in demineralized water for 30 min. Placed on moistened filter paper in sealed Petri dishes for ~ 24 hr. Shoots blotted dry and weighed, air‐dried and weighed again. For each batch of samples, one replicate was oven‐dried at 40°C for 6 hr and weighed to provide a conversion factor between air‐ and oven‐dry mass. N: 10, Sample size :1 shoot, (i.e. the top part of the shoot with green leaves) van Zuijlen et al. 2021 Same as Roos et al. 2019, except one replicate per batch was oven-dried at 70 °C for 24 hours (to provide a conversion factor between air- and oven-dry mass. Same as Roos et al. 2019 Elumeeva et al. 2011 Shoots moistened in deionized water and shaken to remove water against gravity, then blotted to remove as much external water as possible and weighed. Shoots oven-dried at 90°C. N: 12, Sample size: 1 shoot. Michel et al. 2012 Shoots moistened in deionized water and shaken to remove water, then blotted to remove as much external water as possible and weighed. Shoots oven-dried at 60°C for 48h. N: 6 shoots for each species, 16 colonies, 8 species Page 45 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 3Sphagnum, Sp Dendroid, De Weft, We Mat, Ma Small cushion, SC Large cushion, LC Short unbranched turf, SU Tall unbranched turf, TU Branched turf, BT Leafy liverwort, LL nn A B C D E F G H I J K L Polytrichales, Po 0.5 cm 2 cm 2 cm 1 cm 2 cm2 mm0.5 cm 1 m 5 cm 1 cm 1 cm1 cm Thallose liverwort + hornwort, Th Figure A1 The twelve bryophyte functional groups (BFGs) and their abbreviations illustrated with photos of characteristic species A–L. A: Sphagnum fuscum, B: Climacium dendroides, C: Hylocomium splendens, D: Hypnum cupressiforme, E: Tomentypnum nitens, F: Grimmia pulvinata, G: Racomitrium lanuginosum, H: Aulacomnium turgidum, I: Dicranum flexicaule, J: Polytrichum commune, K: Ptilidium ciliare and L: Marchantia foliacea. White bars indicate approximate scale. Photos by Signe Lett and through Creative commons Page 46 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 4Figure A2 Relationships between water holding capacity (WHC) in bryophyte colonies, shoots external and internal and in shoots only internal. Dots represent species means and are colored according to tundra bryophyte functional groups (TBFGs, We, Weft; Ma, Mat; BT, Branched turf; SU, Short unbranched turf; TU, Tall unbranched turf; Po, Polytrichales; Sp, Sphagnum, LL, Leafy liverworts). Colony WHC and shootint+ext WHC were significantly correlated, whereas shootint WHC did not correlate to colony or shootext WHC (b,c). Page 47 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 5P o B T LL W e S U TU M a S p 0 10 20 Polytrichum juniperinum Polytrichum commune Polytrichum strictum Polytrichastrum sexangulare Racomitrium fasciculare Racomitrium lanuginosum Tomentypnum nitens Lophozia floerkii Ptilidium ciliare Lophozia lycopodioides Scorpidium scorpioides Hylocomium splendens Pleurozium schreber i Rhytidiadelphus squarrosus Limprichtia cossonii Warnstorfia pseudostraminea Dicranum elongatum Oncophorus wahlenbergii Aulacomnium turgidum Philonotis caespitosa Dicranum fuscescens Aulacomnium palustre Cinclidium stygium Dicranum drummondii Dicranum scoparium Paludella squarrosa Plagiomnium ellipticum Hypnum jutlandicum Plagiothecium undulatum Sphagnum angustifolium Sphagnum capillifolium Sphagnum fuscum Sphagnum compactum Sphagnum riparium Sphagnum lindbergii Sphagnum palustre Sphagnum russowii WHC, g/g study Busca, unpubl. Elumeeva 2011 Jonsdottir, unpubl. Lett 2017 Liu, unpubl. May 2018 Michel 2009 Rzepczynska, unpubl. WHC_trait WHC_colony WHC_shoot_ext Figure A3 Water holding capacity of bryophyte colonies (open circles) and shoot including external moisture (closed circles) measured in gram water per gram dry weight bryophyte. Bryophyte species are ordered according to tundra bryophyte functional groups (TBFGs, Po, Polytricales; BT, Branched turf; LL, Leafy liverworts; We, Weft; SU, Short unbranched turf; TU, Tall unbranched turf; Ma, Mat; Sp, Sphagnum). Color of dots mark study ID. Boxes contain 1st and 3rd quartile and show median, dots outside whiskers mark values more than 1.5 times the length of the box. Page 48 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 6Table A2 List of recommended bryophyte floras and species lists (*) covering Arctic areas. The list is not comprehensive. Where available, number of species is given. Arctic region Flora/species list* Language Authors (ref) Year Comment North America Flora of North America, Vol. 27 and 28 English Flora of North America Committee (1) 2007, 2014 Mosses. Available as e-book, 621 + 698 species USA The mosses of Arctic Alaska* English W. C. Steere (2) 1978 Mosses. 415 species Out of print A Bryophyte Species List for Denali National Park and Preserve, Alaska, with Comments on Several New and Noteworthy Records* English S. E. Stehn, J. K. Walton, C. A. Roland (3) 2013 Bryophytes, Covers Denali National Park, 499 species Canada Flore des bryophytes du Québec et du Labrador, Vol. 1-3 French J. Faubert (4) 2012 Bryophytes, 892 species A key and annotated synopsis of the mosses of the northern lowlands of Devon Island, N.W.T., Canada English V. D. Vitt (5) 1975 Mosses. Covers Devon Island. 131 species The Mosses of Northern Ellesmere Island, Arctic Canada. II. Annotated List of the Taxa* English G. R. Brassard (6) 1971 Mosses. Covers N Ellesmere Island. 151 species Greenland Illustrated Moss Flora of Arctic North America and Greenland vol. 1-3 English D. Long, H. Crum, B. Murray, G. Mogensen, ed. (7–9) 1985 1. Polytrichaceae, 2. Sphagnaceae, 3. Andreaeobryaceae – Tetraphidaceae. Out of print Liverworts of Greenland English K. Damsholt (10) 2013 Liverworts, 178 species Mosses (Bryophyta) and liverworts (Marchantiophyta) of the Zackenberg valley, northeast Greenland* English K. Hassel, H. Zechmeister, T. Prestø (11) 2014 Mosses and liverworts, 212 species Fennoscandia Illustrated Moss Flora of Fennoscandia. II. Musci. Vol. 1-6 English E. Nyholm (12) 1954- 1969 Bryophytes. Out of print Illustrated flora of Nordic Mosses, Vol. 1-4 English E. Nyholm (13–16) 1987- 1998 Mosses; vol 4 out of print Illustrated moss flora of Nordic liverworts and hornworts English K Damsholt (17) 2009 Liverworts and hornworts. Out of print Iceland Íslenskir mosar Icelandic B. Jóhannsson (18) 1989- 2003 Bryophytes. Available as reports. 604 species, detailed descriptions and distribution maps for Iceland Mosar á Íslandi Icelandic Á. H. Bjarnsson (19) 2018 Bryophytes. Key to all species in Iceland Norway Norges torvmoser Norwegian K.I. Flatberg (20) 2014 Sphagnaceae. 55 species Bryophytes of the Longyearbyen area* English T. Prestø, M. Lüth, K. Hassel (21) 2014 Bryophytes Page 49 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 7Reference list for Table A2 1. Committee F of NAE, editor. Flora of North America: Volume 27: Bryophytes: Mosses, Part 1: North of Mexico. New York, NY: Oxford University Press; 2007. 734 p. 2. Steere WC. The mosses of Arctic Alaska. Vaduz: J. Cramer; 1978. 3. Stehn SE, Walton JK, Roland CA. A Bryophyte Species List for Denali National Park and Preserve, Alaska, with Comments on Several New and Noteworthy Records. Evansia. 2013 Mar;30(1):31–45. 4. Faubert J. Flore des Bryophytes du Quebec-Labrador. Vols. 1–3. Société Québécoise de Bryologie; 2012. 5. Vitt DH. A key and annotated synopsis of the mosses of the northern lowlands of Devon Island, N.W.T., Canada. Canadian Journal of Botany. 1975;53(19):2158–97. 6. Brassard GR. The Mosses of Northern Ellesmere Island, Arctic Canada. II. Annotated List of the Taxa. The Bryologist. 1971;74(3):282–311. 7. Long DG. Illustrated Moss Flora of Arctic North America and Greenland. Mogensen GS, editor. Museum Tusculanum Press; 1985. 64 p. (Monographs on Greenland; vol. 1. Polytrichaceae). 8. Crum HA. Illustrated moss flora of Arctic North America and Greenlans. Mogensen GS, editor. Museum Tusculanum Press; 1986. (Monographs on Greenland; vol. 2. Sphagnaceae). 9. Murray BM. Illustrated Moss Flora of Arctic North America and Greenland. Mogensen GS, editor. Museum Tusculanum Press; 1987. 40 p. (Monographs on Greenland; vol. 3. Andreaeobryaceae- Tetraphidaceae). 10. Damsholt K. Liverworts of Greenland. Lund, Sweden: Oikos editorial office; 2013. 11. Hassel K, Zechmeister H, Prestø T. Mosses (Bryophyta) and liverworts (Marchantiophyta) of the Zackenberg valley, northeast Greenland. lnbg. 2014 Jan;37(2):66–84. Bryophytes, Lichens and cyanoprocaryotes in surrounding of pyramiden (Svalbard): A consise handbook English M. Dodd, I. Tatarenko, N. Koroleva (22) 2015 Bryophytes, subset of Svalbard species, 87 species Sweden National Nyckeln, 4 volumes Swedish + English T. Hallingbäck, N. Lönnel, H. Weibull, L. Hedenås (23–26) 2005- 2019 All mosses, 852 species Mossor Swedish T. Hallingbäck (27) All bryophytes Bryophytes of the Tornetraesk area, northern Swedish Lapland* English O. Mårtensson (28–30) 1956 Bryophytes Russia Moss Flora of Russia, Vol. 2, 4 and 5 Russian, English M.S. Ignatov et al. (31–33) 2017, 2018, 2020 Oedipodiales - Grimmiales; Bartramiales - Aulacomniales; Hypopterygiales - Hypnales (Plagiotheciaceae - Brachytheciaceae) Other useful resources Britain and Ireland Mosses and Liverworts of Britain and Ireland – a field guide English I. Atherton, S. Bosanquet, M. Lawley (34) 2010 Mosses, Liverworts and Hornworts. 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Illustrated flora of Nordic Mosses. Vol. 4. Aulacomniaceae-Orthotrichaceae. Lund, Sweden: Oikos editorial office; 1998. 17. Damsholt K. Illustrated moss flora of Nordic liverworts and hornworts. Lund, Sweden: Oikos editorial office; 2009. 18. Jóhansson B. Íslenskir mosar (Icelandic bryophytes). Iceland: Fjörlrit Náttúrufræðistofnunar; 1989 2003. Report No.: 12, 13, 15, 16, 19–22, 24, 26, 29, 30, 33, 36, 38 and 41–44. 19. Bjarnason ÁH. Mosar á Íslandi. Forlagid; 2018. 20. Flatberg KI. Norges Torvmoser. Norway: Akademiska Forlag; 2014. 21. Prestø T, Lüth M, Hassel K. Bryophytes of the Longyearbyen area. Trondheim: NTNU Vitenskapsmuseet, Sektion for naturhistorie; 2014 p. 1–68. (NTNU Vitenskapsmuseet naturhistorisk notat). 22. Dodd M, Tatarenko I, Koroleva N. Bryophytes, lichens and cyanoprocaryotes in surroundings of Pyramiden (Svalbard): a concise guide-book. Dodd M, Tatarenko I, Koroleva N, editors. Apatity, Murmansk: Polar-Alpine Botanical Garden-Institute; 2015. 253 p. 23. Hallingbäck T, Lönnell N, Weibull H, Hedenäs L. Bladmossor: Sköldmossor - blåmossor: Bryophyta: Buxbaumia - Leucobryum klot. SLU Artdatabanken; 2005. 416 p. 24. Hallingbäck T, Lönnell N, Weibull H. Bladmossor : kompaktmossor - kapmossor. Bryophyta : anoectangium - orthodontium. Vol. AJ 24-36. SLU Artdatabanken; 2008. 25. Hedenäs L, Hallingbäck T. Bladmossor : skirmossor - baronmossor. Bryophyta : hookeria - anomodon. Vol. AJ 37-57. SLU Artdatabanken; 2014. 26. Lönnell N, Hallingbäck T. Bladmossor: Vitmossor – knappnålsmossor: Bryophyta: Sphagnum– Tetrodontium. Vol. AJ 1-5. SLU Artdatabanken; 2019. 27. Hallingbäck T. Mossor: En fältguide. Naturcentrum; 2016. 28. Mårtensson O. Bryophytes of the Torneträsk Area, Northern Swedish Lappland I - Hepaticae. Kungl. Sv. Vetensk. Akad. avhandlingar i naturskyddsärenden 12; 1955. 29. Mårtensson O. Bryophytes of the Torneträsk Area, Northern Swedish Lappland II - Musci. Kungl. Sv. Vetensk. Akad. avhandlingar i naturskyddsärenden 14; 1956. 30. Mårtensson O. Bryophytes of the Torneträsk Area, Northern Swedish Lappland III - General part. Sv. Vetensk. Akad. avhandlingar i naturskyddsärenden 15; 1956. 31. Ignatov MS, Ignatova EA, Fedosov VE. Moss Flora of Russia. Vol. 2. Oedipodiales-Grimmiales. Moscow, Russia: KMK; 2017. Page 51 of 52 Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd . 932. Ignatov MS, Ignatova EA, Fedosov VE. Moss Flora of Russia. Vol. 4. Bartramiales-Aulacomniales. Moscow, Russia: KMK; 2018. 33. Ignatov MS, Ignatova EA, Fedosov VE. Moss Flora of Russia. Vol. 5. Hypopterygiales-Hypnales (Plagiotheciaceae-Brachytheciaceae). Moscow, Russia: KMK; 2020. 34. Atherton I, Bosanquet SDS, Lawey M. Mosses and liverworts of Britain and Ireland: a field guide. British Bryological Society; 2010. Page 52 of 52Arctic Science (Author?s Accepted Manuscript) © The Author(s) or their Institution(s) A rc tic S ci en ce D ow nl oa de d fro m c dn sc ie nc ep ub .c om b y M ET LA /L EH TI SA LI o n 12 /2 0/ 21 Fo r p er so na l u se o nl y. T hi s J us t-I N m an us cr ip t i s t he a cc ep te d m an us cr ip t p rio r t o co py e di tin g an d pa ge c om po sit io n. It m ay d iff er fr om th e fin al o ffi ci al v er sio n of re co rd .