Duration of wood formation phases in Scots pine and Norway spruce lengthens towards the south in the Nordic countries

Environmental conditions such as temperature and light affect the wood formation of trees in complex ways, as cell division and each subsequent phase of wood formation may respond differently. We analyzed the influence of latitude and annual temperature sum on the dynamics of wood formation phases in the boreal conifers Picea abies (L.) Karst. and Pinus sylvestris L. Furthermore, we investigated the relationships of these dynamics with the number of conduits produced in a year. We found that wood formation phases started earlier and ended later at lower latitudes and years with higher temperature sum. However, latitude had a relatively weak explanatory power on the dynamics of the cell enlargement phase (i.e. the phase that starts earliest in the growing season) in P. abies, indicating a higher sensitivity to annually changing conditions like the temperature sum. Although the dynamics of the wood formation phases cannot directly affect the number of conduits produced, as this is determined by the preceding cell division, longer phases were associated with a greater number of conduits. This indicates that both cell division and the duration of the subsequent wood formation phases responded positively to the same conditions. Thus, in P. abies and P. sylvestris growing at latitudes from 60 ° to 67.5 °, warmer conditions, as expected under climate change, will enable a longer wood formation period and all its phases, and a higher wood production, at least until other factors become limiting.