BOREAL ENVIRONMENT RESEARCH 11: 451–461 ISSN 1239-6095 Helsinki 29 November 2006 © 2006 Trends in sulphate deposition on the forests and forest floor and defoliation degree in 16 intensively studied forest stands in Finland during 1996–2003 Antti-Jussi Lindroos1), John Derome2), Kirsti Derome2) and Martti Lindgren1) 1) Vantaa Research Unit, Finnish Forest Research Institute, P.O. Box 18, FI-01301 Vantaa, Finland (e- mail: antti.lindroos@metla.fi) 2) Rovaniemi Research Unit, Finnish Forest Research Institute, P.O. Box 16, FI-96301 Rovaniemi, Finland Received 6 Oct. 2005, accepted 6 June 2006 (Editor in charge of this article: Raija Laiho) Lindroos, A.-J., Derome, J., Derome, K. & Lindgren, M. 2006: Trends in sulphate deposition on the for- ests and forest floor and defoliation degree in 16 intensively studied forest stands in Finland during 1996–2003. Boreal Env. Res. 11: 451–461. Deposition samples (bulk deposition, stand throughfall) were collected in 8 Norway spruce and 8 Scots pine stands throughout the year in Finland during the period 1996–2003. Defoliation was also estimated annually in the same stands. Sulphate deposition in Finland during the period 1996–2003 decreased, the decrease being most evident at the beginning of the period. Sulphate deposition in the northern part of the country has been considerably lower than that in the southern part of the country, and therefore the decrease during the period 1996–2003 in the northern part of the country has been relatively insignificant. In the eastern part of Finland, which receives a considerable proportion of the sulphate depo- sition from the St. Petersburg area and the shale-oil power stations in NE Estonia, there were no clear decreasing trends in sulphate deposition. The decrease in sulphate deposition elsewhere in the country was mainly due to the decrease in the sulphate concentrations since the amount of precipitation did not decrease during the monitoring period.There was a clear decreasing gradient running from southern to northern Finland in the net-through- fall of sulphate from the tree canopies in both the spruce and pine stands. No clear decrease was observed in defoliation on any of the plots during the study period, although sulphate deposition decreased significantly during 1996–2003. Sulphate deposition alone is not, at current levels in Finland, likely to cause changes in forest health in terms of defoliation in the short term at least. Introduction The threat posed by acidifying deposition (both nitrogen and sulphur) to forest ecosystems has been widely recognized during the past two dec- ades, and the monitoring of sulphur and nitrogen deposition on the forests has played an important role in determining and predicting the effects of acidifying deposition on the functioning of forest ecosystems. During the past decade, the Pan-European ICP Forests Level I and II net- works have made a major contribution to this work. Now, 25 years after the ratification of the Convention on the Long-Range Transboundary 452 Lindroos et al. • BOREAL ENV. RES. Vol. 11 of Air Pollutants (CLRTAP), we have the oppor- tunity to estimate the effects of reductions in pollutant emissions, especially of sulphur com- pounds, on the annual sulphur deposition on forests during the past decade. Monitoring the level of sulphur deposition on forests ecosystems cannot be performed merely by monitoring sulphur deposition in the open (i.e. bulk deposition, which is the sum of wet and dry deposition), because coniferous forests are known to be relatively efficient at intercept- ing dry deposition. This dry deposition is sub- sequently washed-off as rainfall passes down through the crown canopy, and the amount of e.g. sulphate in stand throughfall is therefore normally considerably greater than that meas- ured in the open (Lindberg and Lovett 1992). Determining the actual deposition sulphur load on the forest floor therefore means that so-called stand throughfall must also be monitored. There is also a certain amount of interaction between the chemical compounds in deposition and the tree canopies but, in the case of sulphate, this is relatively limited (Lindberg et al. 1986, Brede- meier 1988, Draaijers and Erisman 1995). Deposition loads of a wide range of ions from the atmosphere to the forest floor below the tree canopies (stand throughfall) have been investigated in a number of studies in Finland in order to determine the contribution and effects of these fluxes on the biogeochemical cycling of chemical elements and compounds within forest ecosystems (e.g. Hyvärinen 1990, Ukonmaanaho et al. 1998, Lindroos et al. 2000, Piirainen et al. 2002). Ukonmaanaho et al. (1998) and Lin- droos et al. (2000) reported a decreasing trend in sulphur deposition to the forest floor in stand throughfall during 1989–1997. The proportion of sulphate washed off from the tree canopies (cor- responding to dry deposition) has been found to make an important contribution to the total sul- phur deposition in stand throughfall in Finnish conditions (Lindroos et al. 2000). Sulphur deposition was as its highest during the 1980s in Finland but, since then, the dep- osition load has decreased considerably due to reductions in sulphur emissions (Nordlund 2000). Acidifying sulphur compounds are con- sidered to have negative effects on forest health either directly (e.g. sulphur dioxide) or indirectly by increasing soil acidification (e.g. Berdén et al. 1987). In Finland, the effects of acidifying depo- sition on forest health have been investigated in a number of studies since the 1980s, and the defoliation degree of trees has been used to indi- cate their overall state of health (Strand 1997, Forest condition in Europe 1999, Lindgren et al. 2000, Derome et al. 2001). In studies carried out in Finland, no direct overall connection has been found between acidifying sulphur deposition and forest health expressed as the degree of defolia- tion (Lindgren et al. 2000, Derome et al. 2001). On the Kola Peninsula close to Finland’s north- eastern border, however, the defoliation degree has been relatively high in areas affected by high sulphur and heavy metal deposition derived from local industrial point sources (Lindroos et al. 1995, Salemaa et al. 1995). In this area the degree of defoliation is also strongly affected by the direct effect of toxic levels of sulphur dioxide in the atmosphere on tree foliage. Many of these studies have been based on comparison of the defoliation degree and e.g. the sulphur depo- sition at different monitoring plots. However, relatively little is known about plotwise changes in the deposition load of acidifying compounds and its possible relationship to the defoliation degree. The first of the two aims of this study was to determine whether the reductions in sulphur emissions, which started during the late 1980s and early 1990s, continue to be reflected in sul- phur deposition measured both in the open and inside the stand during the period 1996–2003. The second aim was to estimate the propor- tion of sulphate deposition washed off from the tree canopies (net-throughfall) out of the total deposition in coniferous stands. We also hypoth- esized that possible changes in the deposition of acidifying compounds would be better reflected in the plotwise level of forest condition than in between-plot comparisons. This is because, in between-plot comparisons, many factors can vary considerably between the plots and sub- sequently also have a significant effect on the defoliation degree. We therefore investigated the plotwise relationships between the trends in sul- phate deposition in bulk deposition and stand throughfall and the degree of defoliation during the period 1996–2003. BOREAL ENV. RES. Vol. 11 • Trends in sulphate deposition and defoliation 453 Material and methods Deposition samples were collected throughout the year and the defoliation degree estimated annu- ally in 8 Norway spruce and 8 Scots pine stands in the Finnish intensive monitoring (Level II) network (see Fig. 1 and Table 1) of the EU-Forest Focus/UN-ECE Pan-European Level II Forest Condition Monitoring network (ICP Forests). Thirteen of the monitoring plots were located in commercially exploited forests that were rela- tively even aged and had a homogeneous forest structure. The remaining three plots (numbers 19, 20 and 21) were located in relatively pristine areas where forestry management had not been carried out for a considerable period of time. The mean age of the tree stands on these three plots was high (Table 1). The Scots pine plots were located on soils composed of sorted sand, and the Norway spruce stands on till soils. The deposition samples were collected inside the stand (stand throughfall) and in an adjacent open area (bulk deposition) at 4-week intervals during the winter and spring, and at 2-week intervals (bulked to give one sample per 4-week interval) during summer and autumn during the period 1996–2003. The open area was located at Table 1. The amount of precipitation and some characteristics of the stands on the monitoring plots. BD = bulk deposition, TF = stand throughfall, SP = Scots pine, NS = Norway spruce. Monitoring No.1 Lat. N Precipitation (BD) Precipitation (TF) Tree Stand Basal area plot (mm2) (mm2) species age3 with bark3 (years) (m2 ha–1) Sevettijärvi 1 69° 410 (70) 367 (63) SP 200 13.4 Kivalo 6 66° 587 (113) 469 (88) SP 55 21.3 Ylikiiminki 9 64° 551 (93) 467 (81) SP 90 12.5 Lieksa 20 63° 580 (85) 522 (67) SP 130 28.8 Juupajoki 10 61° 629 (76) 521 (63) SP 80 17.9 Punkaharju 16 61° 526 (76) 363 (56) SP 80 29.4 Tammela 13 60° 619 (97) 471 (77) SP 60 21.9 Miehikkälä 18 60° 600 (82) 476 (71) SP 120 16.9 Pallasjärvi 3 67° 545 (81) 458 (62) NS 140 13.0 Kivalo 5 66° 585 (100) 552 (97) NS 70 21.6 Oulanka 21 66° 431 (65) 469 (80) NS 170 25.6 Uusikaarlepyy 23 63° 462 (138) 269 (62) NS 55 34.8 Juupajoki 11 61° 629 (76) 464 (64) NS 80 33.2 Punkaharju 17 61° 526 (76) 342 (62) NS 70 28.5 Evo 19 61° 616 (79) 445 (60) NS 170 54.1 Tammela 12 60° 584 (82) 430 (54) NS 60 27.6 1 Site code. 2 Mean (S.D.), 1996–2003. 3 Raitio et al. (2002). about 300 m distance from the tree stand. There were 20 systematically located precipitation col- lectors (Ø = 20 cm, h = 0.4 m) within the stand during the snow-free period, and 6–10 snow collectors (Ø = 36 cm, h = 1.8 m) during winter. The corresponding number of collectors for the adjacent open area was 3 and 2, respectively. The design of the deposition monitoring plots is given in Raitio et al. (2002). The samples were filtered in the laboratory through 0.45 µm mem- brane filters, and the sulphate (SO 4 -S) concentra- tions determined by ion chromatography (IC). Plot-specific linear regressions were calculated in order to determine whether there were statis- tically significant temporal trends in sulphate deposition. These regression equations were sta- tistically tested using the F- and t-tests. The defoliation degree was estimated using 5% classes (0% = 0%, 1%–5% = 5%, 6%–10% = 10%, 11%–15% = 15%, etc.) and expressed as the relative needle loss. The tree to be estimated was compared with a completely non-defoliated tree (an imaginary tree with a defoliation degree of 0%). A defoliation degree of 100% means that the tree has lost all its leaves or needles. The defoliation degree of 25% is generally considered to represent the limit value above which the tree 454 Lindroos et al. • BOREAL ENV. RES. Vol. 11 is damaged (Forest condition in Europe 1999). Defoliation was estimated on the upper half of the living crown in the case of Norway spruce and the upper two thirds in the case of Scots pine. The mean defoliation degree for the plot was calculated from 60 trees. Pearson’s correla- tion coefficients were calculated for each of the plots separately in order to study whether there is a relationship between the sulphur deposition values in bulk deposition and stand throughfall and the defoliation degree. Results Precipitation The mean annual precipitation in the open areas varied between 410–629 mm at the different monitoring sites (Table 1). The spruce stand can- opies intercepted, on average, 26% of the pre- cipitation measured in the open (excluding the Oulanka plot, no. 21). No interception occurred on the Oulanka plot in NE Finland, and through- fall precipitation was even higher than that in the open area. In the pine stands the interception value was clearly lower (mean 19%) than that in the spruce stands. No clear trends were found for the annual amount of precipitation in the open area and in stand throughfall during the period 1996–2003 (Figs. 2 and 3). 0 200 400 600 800 1994 Precipitation (mm) 3 5 23 11 12 17 21 19 1996 1998 2000 2002 2004 0 200 400 600 800 1994 Precipitation (mm) 1 6 9 10 16 13 18 20 1996 1998 2000 2002 2004 Fig. 2. Annual precipitation in the open area for the Norway spruce stands during 1996–2003. The num- bers in the legend refer to the plot number (see Fig. 1 and Table 1 for their location). Fig. 3. Annual precipitation in the open area for the Scots pine stands during 1996–2003. The numbers in the legend refer to the plot number (see Fig. 1 and Table 1 for their location). Fig. 1. Map of Finland showing the location of the monitoring plots in the Norway spruce and Scots pine stands. BOREAL ENV. RES. Vol. 11 • Trends in sulphate deposition and defoliation 455 Deposition Mean sulphate deposition in the open area (BD) varied between 111–326 mg S m–2 yr–1 during 1996–2003. The deposition values were higher on the plots located in southern Finland than those in the north (Figs. 4 and 5). The high- est plotwise mean sulphate deposition in stand throughfall (TF) during 1996–2003 was 557 mg S m–2 yr–1, and was measured on the Tam- mela spruce plot (no. 12) located in southern Finland. The corresponding lowest value was 138 mg S m–2 yr–1 on the northernmost spruce plot (Pallasjärvi, no. 3). The sulphate deposition values in stand throughfall were also generally higher in southern Finland than in the northern part of the country (Figs. 4 and 5). The spruce canopies clearly caused a stronger increase in the sulphate flux on the forest floor than the pine canopies, and net-throughfall of sulphate (i.e. TF–BD, sulphate wash-off) from the canopy was higher in the southern parts of the country than in northern Finland (Figs. 4 and 5). The net-throughfall of sulphate varied between 141 ± 86–296 ± 117 mg S m–2 yr–1 (mean ± S.D.) on the spruce plots in southern and central Finland, while the corresponding range for the plots in northern Finland was 27 ± 11–49 ± 18 mg S m–2 yr–1. The relative increase in sulphate deposition as precipitation passed down through the canopy layer on the spruce plots was, on average, 36%–56% in southern and central Finland, and 19%–29% in northern Finland. The corresponding values (i.e. TF–BD) for the pine stands in southern and central Fin- land were 24 ± 18–76 ± 32 mg S m–2 yr–1 (9%– 22%), and in northern Finland 14 ± 20–15 ± 13 mg S m–2 yr–1 (8%). The Sevettijärvi plot (no. 1) located in the NE corner of northern Finland has been excluded from these ranges for the pine stands because the values measured on this plot clearly differed from the general pattern of net throughfall of sulphate. The mean net-through- fall of sulphate for the monitoring period on this plot was 52 ± 18 mg S m–2 yr–1, and the propor- tion of sulphate deposition washed-off from the canopy was 31% despite the fact that this plot is the northernmost plot in the whole monitoring network. Trends in sulphate deposition Sulphate deposition in the open area and in stand throughfall during 1996–2003 on many of the spruce and pine plots located in southern Fin- land showed a significant or almost significant decrease over time. This was reflected in the linear regressions ( p < 0.05 or < 0.10) (Table 2 and Figs. 6 and 7). On the two pine plots in SE Finland (nos. 16 and 18), there was no signifi- 0 200 400 600 3 (67) 5 (66) 21 (66) 23 (63) 11 (61) 17 (61) 19 (61) 12 (60) SO 4-S (mg m –2 a –1 ) BD TF Plot number (latitude, °N) 1 (69) 6 (66) 9 (64) 20 (63) 10 (61) 16 (61) 13 (60) 18 (60) BD TF Plot number (latitude, °N) 0 200 400 600 SO 4-S (mg m –2 a –1 ) Fig. 4. The mean annual SO4-S deposition in bulk dep- osition (BD) and in stand throughfall (TF) during 1996– 2003 in 8 Norway spruce stands. The plot number and latitude of the plots is given below the x-axis (see Fig. 1 and Table 1 for their location). Fig. 5. The mean annual SO4-S deposition in bulk deposition (BD) and in stand throughfall (TF) during 1996–2003 in 8 Scots pine stands. The plot number and latitude of the plots is given below the x-axis (see Fig. 1 and Table 1 for their location). 456 Lindroos et al. • BOREAL ENV. RES. Vol. 11 Table 2. Linear regression equations for the SO4-S deposition (mg m –2 a–1) and time (years 1996–2003). Deposition = intercept + X variable ¥ year. BD = bulk deposition, TF = stand throughfall, NS = Norway spruce, SP = Scots pine, Lat. = latitude. Only statistically significant equations are presented. Tree Plot Lat. Type R 2 Intercept (S.E.) p X variable (S.E.) p n species NS 03 67 TF 0.52 17175.9 (6672.2) 0.042 –8.52 (3.34) 0.043 8 05 66 TF 0.57 28033.1 (9916.6) 0.030 –13.92 (4.96) 0.031 8 21 66 BD 0.80 26405.3 (5901.9) 0.007 –13.14 (2.95) 0.007 7 21 66 TF 0.52 27883.0 (11888.6) 0.066 –13.86 (5.94) 0.067 7 11 61 BD 0.56 27878.5 (10043.8) 0.032 –13.83 (5.02) 0.033 8 11 61 TF 0.56 72253.3 (25817.6) 0.031 –35.95 (12.91) 0.032 8 17 61 TF 0.38 63376.5 (32603.8) 0.100 –31.47 (16.31) 0.102 8 19 61 BD 0.47 22528.8 (9601.6) 0.057 –11.15 (4.80) 0.059 8 19 61 TF 0.39 49490.4 (25274.5) 0.098 –24.52 (12.64) 0.100 8 12 60 BD 0.47 32369.6 (13824.9) 0.058 –16.06 (6.91) 0.059 8 12 60 TF 0.49 96620.6 (40057.9) 0.052 –48.04 (20.03) 0.053 8 SP 10 61 BD 0.56 27890.0 (10035.8) 0.032 –13.83 (5.02) 0.033 8 10 61 TF 0.53 28270.9 (10802.1) 0.040 –14.01 (5.40) 0.041 8 13 60 BD 0.61 38496.8 (12389.2) 0.021 –19.12 (6.20) 0.022 8 13 60 TF 0.66 79144.0 (22939.2) 0.014 –39.42 (11.47) 0.014 8 0 200 400 600 800 1000 1200 1996 3, BD 3, TF 5, BD 5, TF 21, BD 21, TF 23, BD 23, TF a 0 200 400 600 800 1000 1200 11, BD 11, TF 12, BD 12, TF 17, BD 17, TF 19, BD 19, TF b SO 4-S (mg m –2 a –1 ) SO 4-S (mg m –2 a –1 ) 1998 2000 2002 2004 1996 1998 2000 2002 2004 1, BD 1, TF 6, BD 6, TF 9, BD 9, TF 20, BD 20, TF a 10, BD 10, TF 16, BD 16, TF 13, BD 13, TF 18, BD 18, TF b 0 200 400 600 800 1996 SO 4-S (mg m –2 a –1 ) 1998 2000 2002 2004 0 200 400 600 800 1996 SO 4-S (mg m –2 a –1 ) 1998 2000 2002 2004 Fig. 6. Trends in SO4-S deposition in bulk deposition (BD) and in stand throughfall (TF) during 1996–2003 in 8 Norway spruce stands in (a) northern and (b) south- ern Finland. The numbers in the legend refer to the plot number (see Fig. 1 and Table 1 for their location). Fig. 7. Trends in SO4-S deposition in bulk deposition (BD) and in stand throughfall (TF) during 1996–2003 in 8 Scots pine stands in (a) northern and (b) southern Finland. The numbers in the legend refer to the plot number (see Fig. 1 and Table 1 for their location). BOREAL ENV. RES. Vol. 11 • Trends in sulphate deposition and defoliation 457 cant decrease during the monitoring period even though the highest sulphate deposition values were recorded in 1996 (Fig. 7). No decreasing trends during the monitor- ing period were found for sulphate deposition in the open area and in stand throughfall in the pine stands located in northern Finland (Fig. 7). The trend was statistically significant for some of the spruce stands in northern Finland, but the decrease in sulphate deposition was limited by the fact that the deposition level in 1996 was already very low (Fig. 6). Defoliation degree The mean defoliation degree of the Scots pine stands varied by 5%–23% during the period 1996–2003 (Fig. 8). There was no clear geo- graphical trend in the defoliation values, and the highest mean value was determined on the plot in eastern Finland (Lieksa, no. 20). The age of the trees on this plot is relatively high (mean 130 yrs) and the plot is located in an area where forestry practises have not been carried out for many decades. The yearly mean defoliation values were relatively stable for most of the pine stands during the period 1996–2003 (Fig. 9). The only clear change (i.e. increase) in the defoliation degree during the study period was detected on the Lieksa plot (no. 20). No statistically signifi- cant ( p < 0.05) correlations were found between the defoliation degree and sulphate deposition in bulk deposition or in stand throughfall on any of the pine sites (Table 3). The mean defoliation degree of the Norway spruce stands varied by 9%–28% during the period 1996–2003 (Fig. 10). The highest mean Table 3. Correlations between SO4-S deposition in bulk deposition (BD) and stand throughfall (TF) and defolia- tion degree (DEF, %) in Norway spruce (NS) and Scots pine (SP) stands. Statistically significant correlations (p < 0.05) are set in boldface. Tree species Plot BD TF n NS 3, DEF –0.58 –0.71 8 5, DEF –0.07 –0.60 8 21, DEF 0.04 –0.14 7 23, DEF –0.38 0.23 7 11, DEF –0.83 –0.71 8 12, DEF –0.60 –0.59 8 17, DEF 0.43 0.22 8 19, DEF –0.72 –0.79 8 SP 1, DEF 0.05 0.13 8 6, DEF 0.22 0.42 8 9, DEF 0.30 0.07 8 20, DEF 0.42 –0.51 7 10, DEF –0.57 –0.53 8 16, DEF 0.14 0.55 8 13, DEF –0.21 0.06 8 18, DEF –0.47 –0.47 8 0 5 10 15 20 25 1 (69) 6 (66) 9 (64) 20 (63) 10 (61) 16 (61) 13 (60) 18 (60) Plot number (latitude, °N) Mean defoliation (%) 0 5 10 15 20 25 30 1996 Defoliation (%) 1 (69) 6 (66) 9 (64) 20 (63) 10 (61) 16 (61) 13 (60) 18 (60) 1998 2000 2002 2004 Fig. 8. Mean defoliation degree of the Scots pine stands in 1996–2003. The plot number and latitude of the plots is given below the x-axis (see Fig. 1 and Table 1 for their location). Fig. 9. Annual defoliation degree of the Scots pine stands during 1996–2003. The numbers in the legend refer to the plot number and latitude (in parentheses) (see Fig. 1 and Table 1 for their location). 458 Lindroos et al. • BOREAL ENV. RES. Vol. 11 value was determined on the Evo plot (no. 19) in southern Finland. The stand age is relatively high on this plot (mean 170 years), and the plot is located in a pristine area with no for- estry management for a long period of time. The clearest change in the yearly mean defoliation degree during the study period was detected on this plot, where the defoliation value increased sharply between the years 1996 and 1997 (Fig. 11). For some of the Norway spruce stands there was a slight increase in the defoliation degree during 1996–2003, although the absolute change was very small. In general, the values remained very stable during the study period. No signifi- cant ( p < 0.05) positive correlations were found between the sulphate deposition in bulk deposi- tion and stand throughfall and the degree of defo- liation; i.e. although sulphate deposition showed a decreasing trend during 1996–2003 on some of the plots, this was not associated with any decreasing trend in defoliation on any of the plots (Table 3). In fact, an opposite trend was found in some of the spruce stands (nos. 3, 11, 19). Discussion Precipitation Although stand structure usually has an effect on the amount of precipitation reaching the forest floor in stand throughfall, the most important factor in Finnish conditions is the amount of precipitation falling in the open (Starr 1995, Lindroos et al. 2000). This was clearly evident from the statistically significant correlations ( p < 0.05) between the amount of precipitation in the open (bulk deposition) and in stand throughfall (pine stands: r = 0.81, spruce stands: r = 0.78). However, the situation at one of the spruce plots (Oulanka, no. 21) was different to that at the other plots. On this plot the amount of stand throughfall is high presumably because of the windy conditions during wintertime: large amounts of snow accumulate in the tree cano- pies, but considerably less on the ground in the open area. The interception of precipitation as it passes down through the canopy layer was lower in the pine stands than in the spruce stands due to the differences in crown structure between the two tree species (Hyvärinen 1990). The mean proportion of precipitation retained by the cano- pies (pine 19%, spruce 26%) was very close to the values reported by Lindroos et al. (2000) for a different monitoring network in Finland, in operation at the beginning of the 1990s, where most of the 43 monitoring plots were located in eastern and northern Finland. Deposition Sulphate deposition is generally higher inside the stand than in the open due to the wash-off of 0 5 10 15 20 25 30 3 (67) 5 (66) 21 (66) 23 (63) 11 (61) 17 (61) 19 (61) 12 (60) Plot number (latitude, °N) Mean defoliation (%) 0 5 10 15 20 25 30 35 1996 Defoliation (%) 3 (67) 5 (66) 21 (66) 23 (63) 11 (61) 17 (61) 19 (61) 12 (60) 1998 2000 2002 2004 Fig. 10. Mean defoliation degree of the Norway spruce stands in 1996–2003. The plot number and latitude of the plots is given below the x-axis (see Fig. 1 and Table 1 for their location). Fig. 11. Annual defoliation degree of the Norway spruce stands during 1996–2003. The numbers in the legend refer to the plot number and latitude (in parentheses) (see Fig. 1 and Table 1 for their location). BOREAL ENV. RES. Vol. 11 • Trends in sulphate deposition and defoliation 459 dry deposition accumulated in the tree canopies (Lindberg and Lovett 1992). As a result, sulphate deposition in stand throughfall is generally con- sidered to represent rather well total sulphate deposition from the atmosphere into the forests (ICP Forests Manual 1998). The sulphate deposi- tion values measured in this study in the open areas, as well as inside the stands, were very low compared to the deposition situation in the southern parts of Scandinavia and central Europe (De Vries et al. 2001). Higher sulphur deposition in southern Finland than in the northern parts of the country has also earlier been reported in many other studies (e.g. Järvinen and Vänni 1990, Ruoho-Airola et al. 1998, Ukonmaanaho et al. 1998, Nordlund 2000). There was a clear decreasing gradient in sulphate deposition from south to north throughout Finland in both the open and in stand throughfall. Sulphate deposition in stand throughfall was higher in the spruce stands than in the pine stands although deposition in the open was at a similar level. This is undoubtedly due to differences in the canopy structure of the two tree species. Norway spruce canopies intercept dry deposi- tion from the atmosphere more effectively than pine canopies (Bredemeier 1988). There was a clear decreasing gradient running from southern to northern Finland in net-throughfall of sul- phate in both the spruce and pine stands. How- ever, net-throughfall of sulphate varied consider- ably between the stands. The highest sulphate net-throughfall values were over 50% of stand throughfall deposition, while the lowest values were less than 10%. The proportion of net- throughfall of sulphate out of the total sulphate load to the forest floor has obviously been con- siderable on many of the plots during the study period, which means that even though the gen- eral deposition load has clearly decreased during the last decades and years, the net-throughfall of sulphate still makes an important contribution to the total deposition load on the forests and forest floor in Finnish conditions. However, an interest- ing result was also the fact that, on some of the plots, the difference between the annual sulphate deposition in stand throughfall and bulk deposi- tion in the open was very small, i.e. the propor- tion of net-throughfall of sulphate was almost insignificant. The situation on the northernmost plot (Sevet- tijärvi pine plot, no. 1) differed from that at the other plots in that there was no decreasing gradi- ent in net-throughfall of sulphate. On this plot, 31% of the sulphate deposition in stand through- fall was derived from wash-off from the canopy layer. This value is high compared to that at the other plots in northern Finland. However, the plot is located only 70 km to the west from the major sulphur emission sources on the Kola Peninsula (Cu-Ni smelters), NW Russia, as well as being located close to the Barents Sea (part of the Arctic Ocean) and therefore receives sulphate from marine sources. Both of these factors undoubt- edly considerably increase the dry deposition of sulphate, and subsequently sulphate wash-off from the canopy layer. It has been estimated that marine-derived sulphate (SO 4 -S) accounts for about 10% of the total sulphate deposition in BD and 13% in TF on this plot (no. 1) (Lindroos et al. 2001). The effect of emissions from the smelters is reflected in somewhat elevated heavy metal concentrations in pine needles on this plot (Raitio 1999). The overall impact of sulphur and heavy metal emissions on forest ecosystems in Finnish Lapland has been found to be relatively limited (Tikkanen and Niemelä 1995). Time trends in sulphate deposition Sulphur deposition in Finland has decreased since the beginning of the 1980s (Nordlund 2000). A similar decrease in the amounts and concentra- tions of sulphate deposition in stand through- fall has also been reported during the period 1989–1997 (amounts) and 1989–1995 (concen- trations) on sites belonging to the ICP/Integrated Monitoring network in Finland (Ukonmaanaho et al. 1998, Lindroos et al. 2000). Three of these plots are now part of the ICP Forests monitoring network and were included in our study. During the 1996–2003 monitoring period, sulphate dep- osition in the open and in stand throughfall continued to decrease linearly on many of the plots located in southern Finland, despite the fact that the deposition values were already much lower in 1996 than earlier. The decreasing trend was evident on both the pine and spruce plots. Because there was no corresponding decrease 460 Lindroos et al. • BOREAL ENV. RES. Vol. 11 in the amount of precipitation during the same period, the reason for the decrease in sulphate deposition in the open and in stand throughfall on these plots was the decrease in the sulphate concentrations. On two pine sites in south-east- ern Finland, however, sulphate deposition in the open and in stand throughfall did not decrease linearly. The values were higher in 1996 than in any of the other years, but there was otherwise no clear decreasing trend. Deposition in this part of the country is considerably influenced by emission sources in the St. Petersburg area and the shale-oil power stations located in NE Estonia. There were no decreasing trends for sulphate deposition in the open and in stand throughfall on the pine plots in northern Finland. Although a decreasing trend was found for some of the spruce sites in northern Finland, the absolute deposition levels decreased only slightly because the deposition levels in northern Finland have earlier been already very low. Overall, it is clear that the reductions in sulphur dioxide emissions are reflected as lower deposition values in many parts of Finland, and the deposition levels are currently relatively low. Defoliation degree The mean defoliation degree was generally very low on the plots; < 15% for the Scots pine stands and < 20% for the Norway spruce stands. The only exception to these values for the Scots pine stands was the Lieksa plot (no. 20) located in eastern Finland, where the mean defoliation was 23%. For the Norway spruce stands, the excep- tions were the Oulanka plot (no. 21) and the Evo plot (no. 19) with values close to or above 25%. On all these plots, the stand age is relatively high and no forestry management has been performed for many decades, i.e. the plots are located in semi-natural, relatively pristine forest. Stand age is known to be an important natural factor causing defoliation (Lindgren et al. 2000), and this was also clearly reflected in our study. The annual defoliation values were stable or indi- cated a slight increase in defoliation on many of the plots during 1996–2003. However, the changes were in many cases very limited in absolute values. No clear decrease was observed in defoliation on any of the plots during the study period, although there were statistically significant decreasing trends in sulphate depo- sition values during 1996–2003. This clearly demonstrates that sulphate deposition alone is not, at current levels in Finland, likely to cause changes in forest health in terms of defoliation in the short term at least. 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