Tree rings as an indicator of atmospheric pollutant deposition to subalpine spruce forests in the Sudetes (Southern Poland)

Atmospheric Research 151 (2015) 259–268

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Tree rings as an indicator of atmospheric pollutant deposition to subalpine spruce forests in the Sudetes (Southern Poland) Michał Godek a,⁎, Mieczysław Sobik a, Marek Błaś a, Żaneta Polkowska b, Piotr Owczarek c, Anita Bokwa d a

Department of Climatology and Atmosphere Protection, Institute of Geography and Regional Development, University of Wroclaw, Kosiby Street 8, 51-621 Wroclaw, Poland Department of Analytical Chemistry, Chemical Faculty, Gdansk University of Technology, Gabriela Narutowicza Street 11/12, 80-233 Gdansk, Poland c Department of Physical Geography, Institute of Geography and Regional Development, University of Wroclaw, Plac Uniwersytecki 1, 50-137 Wroclaw, Poland d Department of Climatology, Institute of Geography and Spatial Management, Jagiellonian University, Gronostajowa Street 7, 30-387 Krakow, Poland b

a r t i c l e

i n f o

Article history: Received 31 October 2013 Received in revised form 28 August 2014 Accepted 1 September 2014 Available online 6 September 2014 Keywords: Fog deposition The Sudety Mountains Slope aspect Dendrochronology Norway spruce Forest degradation

a b s t r a c t In spite of their moderate altitude (1000–1600 m a.s.l.), the Western Sudety Mountains belong to areas with the most efficient fog precipitation in Europe. Intense industrial activity in the area of windward western foothills caused an exceptional intensification of atmospheric pollutant deposition via precipitation and fog to take place since the 1950s. In the second half of the 1970s a massive spruce forest dieback began affecting around 42% of coniferous forest in the Polish part of the Sudety Mountains. As the result of emission abatement in the region, gradual improvement of forest health status has been observed in the last decade. In October 2010 there were 70 dendrochronological samples collected from Norway spruce (Picea abies) stems at 7 different locations using an increment borer. It was documented for six sites that lowest annual growth rates took place between the early eighties and the early nineties which coincides with the highest pollutant deposition rates. Only one site representing the lowest parts of leeward slope showed gradual decrease of tree rings as a result of increasing tree age rather than due to an increase in ecological stress conditions. Tree ring widths were then compared with spatial distribution of fog frequency in the Western Sudety Mountains. The achieved results document a strongly negative dependence of tree ring widths on fog deposition rates. Spruce forest ecosystems have an ability to respond quickly to both negative and positive stimuli, related to increasing and decreasing environmental contamination. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In various mountain areas in Central Europe developing coniferous forest degradation was observed since the 1950s. This process intensified in the late 1970s and in general has been continuing until today (Grodzińska and Szarek-Łukaszewska, 1997; Modrzyński, 2003; Stachurski and Zimka, 2002). Environmental effects of large-scale forest dieback will be noticeable at ⁎ Corresponding author. Tel.: +48 694 351 642; fax: +48 71 348 54 41. E-mail addresses: [email protected] (M. Godek), [email protected] (M. Sobik), [email protected] (M. Błaś), [email protected] (Ż. Polkowska), [email protected] (P. Owczarek), [email protected] (A. Bokwa).

http://dx.doi.org/10.1016/j.atmosres.2014.09.001 0169-8095/© 2014 Elsevier B.V. All rights reserved.

least for the next several decades. Although this process is also taking place in some parts of the Carpathians and the Alps, still the most strongly affected areas are the relatively low elevation hercynian mountains within the German Uplands and the Bohemian Massif (particularly the Ore Mountains and the Western Sudety Mountains; Kandler and Innes, 1995; Lorenz et al., 2008). Intensive power industry development in the vicinity of this area has resulted in a strong increase of pollutant deposition with special relevance of fog deposition (Baranowski and Liebersbach, 1978; Błaś et al., 2008; Lovett, 1984; Migała et al., 2002; Zimmermann and Zimmermann, 2002). For that reason the territory straddling the contact boundaries of Poland, Germany and the Czech Republic (former

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Czechoslovakia) received its name “Black Triangle”. Large areas of coniferous (mainly spruce) forest have been destroyed or at least partially damaged (Ciołkosz and Zawiła-Niedźwiecki, 1990; Ekstrand, 1994; Gregory et al., 1996; Kandler and Innes, 1995; Slovik et al., 1995; Vacek and Leps, 1987). Strong anthropogenic environmental transformations and limited buffering capacity of the granite bedrock made the situation even worse. The observed ecological disaster was the initial reason for radical reduction of emissions in this area, which took place since the beginning of the 1990s (Błaś et al., 2008; Stjern, 2011; Vestreng et al., 2007). It was followed by a significant decrease of pollutant deposition, evidenced by numerous hydro-chemical studies. Presence of the non-climatic factors of environmental stress, e.g. atmospheric pollution, may significantly reduce the size of the annual growth of tree rings, to a degree that depends on the micro-scale diversity of the pollutant deposition loads. The observed variability of annual tree-ring widths responds to different environmental stimuli. This response is fast enough to be an important indicator of the forest ecosystem degradation and is accompanied by the changes of other dendrological factors, like: height increments, needle production and needle density (Ferretti et al., 2002). It should be recalled that the size of the annual tree rings is varying from year to year, under the influence of the specific annual meteorological conditions (Fritts, 1976). However, the temporal anomalies in weather conditions are similar within the same small area, thus the course of the dendroscales for the specified tree population of the same species is very similar. Different important stressinducing factors affecting the trees, the long-term influence of which should be excluded, comprise the mechanic impact of the snow cover, mass movements of the ground, feeding of herbivores, habitat competition, presence of dangerous fungi and insect pests, and more rarely seismic and volcanic activity, cyclic forest fires and glacier movement (Schweingruber, 1996). This research project on spatial distribution of both pollutant deposition and the pace of tree ring growth was undertaken in the Western Sudetes with the aim to characterize mountainous forest response to existing ecological stress. The secondary goal was to show the role of local climatic conditions as the crucial factor controlling deposition of pollutants via fog.

2. Study area 2.1. Location The Western Sudety Mountains are the highest mountains in the north-eastern border part of the Bohemian Massif which belongs to Hercynian European Highlands. In spite of relatively low altitudes (1000–1600 m a.s.l.), the Western Sudety Mountains are a significant orographic barrier for moist polar maritime air masses coming from the Atlantic Ocean. The highest range of the Western Sudety Mountains is the Karkonosze Mountains (with the highest summit of Śnieżka Mt., 1602 m a.s.l.), located at the border between Poland and the Czech Republic. The field research was carried out in the western part of the range, close to Szrenica Mt. (1362 m a.s.l.) (Fig. 1).

The axis of the main chain in the Karkonosze Mountains goes, more or less, from the north-west to the south-east. Its characteristic features are high coherence and little differences in altitude (1200–1400 m a.s.l.). Deep and long valleys separating mountain chains and passes make a specific structure which favors the occurrence of local air circulation. The Karkonosze Mountains are located at about 600–650 km distance from the North Sea, which is the main source area of the moist polar maritime air masses advected to the study area due to the dominating zonal air circulation. An important factor is the absence of higher mountain ranges, i.e., the lack of important orographic barriers, in the NW–N–NE sector. 2.2. Geology and plant cover The western part of the Karkonosze Mountains belongs to the granite massif which intruded into older metamorphic layers at the end of the Hercynian orogenesis, about 328–310 million years ago. Lower parts of the slopes and Karkonosze Foothills are built mainly of porphyrous granite while the highest parts of the Karkonosze Mountains consist of the fine-grained granite which is more resistant to denudation. However, that type of rock is characterized with high acidity which strongly decreases its natural buffer capacity as a bedrock, estimated to reach about 200–300 mol H+ ha1 a− 1 (Nilsson and Grennfelt, 1988). Therefore, mountain ecosystems are endangered with particularly strong pressure due to acid deposition. This is also common in other areas within the Bohemian Massif and the German Highlands (Acker et al., 1995; Eliáš et al., 1995; Oulehle et al., 2007). In the Western Sudety Mountains, there is a clear pattern of the vertical climatic zones. It is followed by the zonal pattern of the plant cover. The areas up to 500 m a.s.l. belong to the foothills zone, dominated by deciduous and mixed tree stands, consisting mainly of pedunculate oak (Quercus robur), hornbeam (Carpinus betulus), small-leaved lime (Tilia cordata), large-leaved lime (Tilia platyphyllos) and with addition of European beech (Fagus sylvatica), sycamore (Acer pseudoplatanus) and Norway maple (Acer platanoides). In the altitudes of 500–950 m a.s.l., there is the zone of lower subalpine forest where in natural conditions the dominating forest community is acidic Sudety beech (less often: fertile Sudety beech), with addition of Norway spruce (Picea abies), European silver fir (Abies alba) and sycamore (A. pseudoplatanus). Above 950 m a.s.l., there is the zone of upper subalpine spruce forests with the addition of European mountain ash (Sorbus aucuparia). In the upper subalpine zone, a dominating forest community is Sudety spruce forest (Calamagrostio villosae-Piceetum), in its regional variety typical for the Western Sudety Mountains. It is the only forest community growing in the zone 950–1250 m a.s.l. where it can be seen that the higher the altitude, the more dwarfed the trees and the more deformed tree crowns are. The upper border of the upper subalpine forest zone is also a natural tree line in the Western Sudety Mountains which runs at 1250–1350 m a.s.l. Above that there is an alpine zone with dwarf mountain pine (Pinus mugo) brushwood, meadows, and isolated spruce trees. However, due to a few hundred years of human activity in that area, the potential plant cover was strongly anthropogenically transformed and its present state is significantly different from the potential one. Foothill forest areas with mild climatic conditions

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Fig. 1. Geographical setting of the study area. Contour lines representing 500 and 1000 m a.s.l. are shown.

were largely converted into arable lands. In the lower subalpine forest zone, primeval forests were cut down in the 17th and 18th centuries and replaced with spruce monocultures. Spruce cultivation was supposed to bring large economic profits and therefore its share in all forested areas increased dramatically, with many forest communities of the same age. Unfortunately, the trees were planted too dense, in inadequate ecological habitats, and using the seeds of the lowland ecotype. An opposite situation occurs in the upper subalpine zone where much of the primeval forest communities survived until present in their original state. 2.3. Climate The Sudety Mountains connect the SW part of the Polish border with the Czech Republic and are located in the range of zonal, western air circulation. The summits of the Karkonosze Mountains belong to the most windy areas in the continental part of Europe. Mean annual wind speed at the Śnieżka Mt. exceeds 12 m s−1 and at the Szrenica Mt. 9.4 m s−1 (www. ncdc.noaa.gov). Annual sums of precipitation at a chosen altitude decrease toward south-east. It is caused by the gradual weakening of the atmospheric fronts' activity and gradual drying of moist, polar maritime air masses which move mainly from SW–W to NE–E. Annual sums of precipitation in the Karkonosze Mountains increase with altitude from 900 mm at the foothills to about 1500 mm at the summits. In the highest parts of the Karkonosze Mountains the share of snow precipitation in the annual precipitation total reaches 74%. In the higher parts of the Karkonosze Mountains there are favorable conditions for the development of the orographic cloudiness. When the cloud base touches the ground, an effective water intake occurs connected with a significant air pollution deposition. Those processes are the most intensive on the windward side of the orographic barrier (slopes of SW–W– NW aspects). Therefore, the most favorable conditions for frequent formation of fog and orographic clouds (of advection-

orographic origin) occur in the western part of the Karkonosze Mountains. The annual number of days with fog at Szrenica Mt. is 274 and at the nearby Śnieżka Mt. — 296 (Błaś et al., 2002; Migała et al., 2002). The total time of fog duration at Szrenica Mt. in the period 1961–2000 reached 3900 h yearly, which corresponds to 44.5% of the year. In the warmest months V–VIII the frequency of fog reaches about 30% of total time. The highest frequencies of 62 and 59% are typical for XI and XII, respectively, and are connected with seasonal dynamic cyclonic circulation. Altitude is not the only important factor controlling the spatial pattern of fog frequency. Parameters such as slope aspect and position of a particular site in relation to local morphology are also of crucial importance. Thus, sites situated quite high but screened by prominent landforms show relatively low number of days with fog (Błaś et al., 2002). Fog deposits in the Karkonosze Mountains are an important element of the water balance and air pollution deposition (Błaś et al., 2008; Migała et al., 2002; Sobik and Migała, 1993). In Poland, the share of fog deposits in the total atmospheric water income varies from a few percent in the lowlands to over 70% in the Western Sudety Mountains. Locally the water income from fog may be larger than from precipitation, especially in the areas close to the tree line or at the edges of dense forests, exposed to the advection of moist air (Błaś et al., 2012). 2.4. Emission background The region located at the junction of the borders between Poland, the Czech Republic and Germany (including the Western Sudety Mountains) has been one of the most polluted ones in Europe for several decades. This is mainly the result of large emissions of: SO2 from burning coal and lignite with high contents of sulfur (energy production and municipal emissions); NOx from energy production and transportation; NH3 from agricultural activities in the vicinity of this area (Kryza et al., 2010). For several decades acid deposition to a large territory of Western and Central Europe exceeded 1500 mol H+ ha−1 a−1 (National Institute for Public Health and the Environment,

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2002). In 1980 this area included central and eastern part of the United Kingdom, most of the area of the Benelux, Germany and western Czechoslovakia as well as around half of Poland (Fig. 2). The top period of the acid pollution emission, particularly SO2, took place in the late 1980s. Since the early 1990s, a significant decrease of air pollution emissions and concentrations has been observed in the study area, due to the restructuring and modernization of the energy production sector. The initial phase of pollutant emission reductions contributed to contraction of these areas to the western part of Germany and the “Black Triangle” region. Five years later only in some higher parts of the Ore Mountains (north-west edge of the Bohemian Massif) such efficient deposition was observed, while in the Western Sudety Mountains it decreased to approximately 1000 mol H+ ha− 1 a− 1 (Fig. 2). The total emission of SO2 in Poland reached 4039 Gg in 1980 and later decreased by 72% to 2007 (1131 Gg). In Germany and in the Czech Republic the reduction was even larger and reached 86% and 87%, respectively (Olendrzyński et al., 2009). This caused a significant decrease in the concentration and deposition of SOx and NOx, followed by the decrease in acidification of particular categories of atmospheric water in the Western Sudety Mountains. Acidity of precipitation decreased by factor of 5 to 6 while pH increased more than half a unit (to 4.5 in 2008) (Błaś et al., 2008). Nevertheless, in the forest ecosystems of the Western Sudety Mountains, the critical loads of acidic deposition are still exceeded (Kryza et al., 2013). At the same time the reduction of NO2 emission was definitely smaller. Although in Germany it reached over 55%, however in Poland and the Czech Republic did not exceed 50%. In contrast to Germany no persistently decreasing tendency is visible in those countries (Vestreng et al., 2007). Although exhaust emission from the internal-combustion engines is systematically reduced, in the former Eastern European

countries the number of cars continues to increase since the beginning of the 1990s, thereby compensating the expected decrease of NO2 emission in general. 3. Material and methods In the present study, the dendrochronological research was carried out in the western part of the Karkonosze Mountains, the highest mountain range of the Sudety Mountains. The samples were collected mainly in the areas around the Szrenica Mt. (1362 m a.s.l.) on which the meteorological station of the Wrocław University is located. Additionally, since several years, the air pollution deposition from different categories of atmospheric water (e.g. precipitation, fog, dew) is monitored at the station. Seven sample sites were chosen according to the following criteria (Table 1, Fig. 3): • five sites represent the ecosystem of upper subalpine forest zone, on the slopes of various aspect and altitude (K1, K2, K3, K4, K5); • one site represents the ecosystem of lower subalpine forest zone with limited influence of fog deposition (K6); • one site represents the ecosystem of the foothills in a place with the lowest influence of fog deposition (K7). At each site 10 samples were collected at the height of 1.3 m above the ground using a MORA CORETAX increment borer with a 400 mm long auger. One sample was collected from each tree. The trees were chosen following the criteria of maximum site representativeness concerning: trunk circumference, tree height, tree crown habit and injury degree, tree stand density and type of substratum around a tree. All sampled trees reached the age of over 80 years, measurable at the height of 1.3 m above the ground. Only the site K6 is represented by younger trees (about 50 years old) because it was difficult to

Fig. 2. Annual acid deposition [mol H + ha−1 a−1] in Europe in the years: A) 1980 and B) 1995 (National Institute for Public Health and the Environment, 2002).

Table 1 Detailed characteristics of the dendrochronological measurements sites (see also Fig. 3). Coordinates

Location

Forest type and degree of forest injury

Altitudinal zone

Slope aspect/ inclination [°]

Number of trees

Average age Correlation coefficient of trees with Master series (generated in COFECHA)

K1

λ 15° 27′ 53″ E φ 50° 48′ 03″ N h 1180–1195 m a.s.l.

Western slope below the peak of the Mumlawski Wierch Mt.

Upper subalpine forest zone

W/5

10

147.6

0.549

K2

λ 15° 29′ 08″ E φ 50° 47′ 26″ N h 1250–1260 m a.s.l.

Western slope below the peak of the Kamiennik Mt.

Upper subalpine forest zone

W/5

10

123.9

0.526

K3

λ 15° 30′ 27″ E φ 50° 47′ 51″ N h 1210–1230 m a.s.l.

The Szrenica Mt. slope above the Świąteczny Rock, close to skiing facilities

Upper subalpine forest zone

NW/10–15

10

129.2

0.597

K4

λ 15° 29′ 48″ E φ 50° 47′ 39″ N h 1190–1200 m a.s.l. λ 15° 31′ 09″ E φ 50° 47′ 37″ N h 1200–1210 m a.s.l.

The Kamiennik Mt. slope above the upper part of the Kamieńczyk Stream Eastern slope of the Szrenica Mt. above the Szrenicki Kocioł cirque

Upper subalpine forest zone Upper subalpine forest zone

NE/10

10

137.6

0.550

E/15

10

110.3

0.518

K6

λ 15° 30′ 23″ E φ 50° 48′ 45″ N h 820–840 m a.s.l.

Lower subalpine forest zone

NNW/12–15

10

51.3

0.614

K7

λ 15° 35′ 08″ E φ 50° 50′ 49″ N h 500–530 m a.s.l.

Lower parts of the Szrenica Mt. slopes between the Ciekotka and Świetlik streams Northern slopes of the Piechowicka Góra Mt. above the center of the town of Piechowice, east from the road tunnel

Extremely thinned out Norway spruce forest, only individual alive trees, about 90% of dead tree trunks Coppice, thinned out Norway spruce forest, numerous injuries in the tree crowns, about 40% of dead tree trunks Strongly thinned out Norway spruce forest, almost all trees with injured crowns, over 60% of dead tree trunks Dense Norway spruce forest, rare tree injuries, below 10% of dead tree trunks Dense Norway spruce forest with addition of the European mountain ash, in selected places slightly thinned out, rare tree crown injuries, single dead tree trunks — about 15% Dense, monoculture Norway spruce forest with trees of the same age, rare tree crown injuries, no dead tree trunks Medium dense mixed forest with Norway spruce, beech, sycamore and birch, no dead tree trunks, Norway spruce trees have well-developed crowns without visible injuries

Foothills

NNE/10–15

10

76.6

0.569

K5

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Site code

263

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Fig. 3. Location of sampling sites and vertical vegetation zones (WGS84 coordinates used, see also Table 1). Site K3 is identical with the meteorological observatory.

find older ones. It is worth mentioning that in the cool climate of the upper subalpine forest zone, trees are usually much older than indicated by tree rings in a certain sample. This is caused by the long time that it takes young seedlings to grow to the height of breast height diameter (i.e. 1.3 m) which can last from 20 years in the central part of the upper subalpine forest zone to even 30 years close to the tree line, as shown for the Beskid Żywiecki Mountains. All samples used in the present study were collected in October 2010 when the vegetation period already had ended. Obtained oval cores with a diameter of 5 mm were inserted into wooden slats and then polished. Prepared samples were then scanned with high resolution – 1200 dots per inch (472 pixels per cm) – and analyzed using the WinDENDRO software (Regent Instruments Canada Inc.). In the mountain climate of the middle latitudes the spruce latewood show a clear edge in almost all years. Therefore, the uncertainty of the data is relatively small. All the data were transformed with the standard procedure: the trend line was removed with the Arstan program (Cook and Holmes, 1986) and the chronology was based on the autoregressive modeling calculating the chronology with the least square method. Further crossdating was computed with the COFECHA program (Grissino-Mayer et al., 1996). Data synchronicity, limited by an influence of local mountain climate and atmospheric pollution, e.g. potentially missing rings, is shown in Table 1 (correlation coefficient with Master series).

4. Results and discussion Intensive acidification of the whole environment observed in the Western Sudety Mountains, caused mainly by the wet and fog pollutant deposition, was an impulse which started unfavorable changes in soils and plants of the ecosystems. The main effect was the degradation of subalpine spruce forests (Błaś et al., 2002; Igawa et al., 2014; Lovett, 1984; Pahl et al., 1994; Weathers et al., 1995; Zimmermann and Zimmermann, 2002). The significance of fog deposition increases in areas where two elements occur at the same time: the areas are covered with coniferous forests and experience high frequency of strongly polluted orographic clouds (summit areas, windward slopes). Coniferous trees have the highest ability to intercept water droplets from the air among all the trees, and at the same time they are most susceptible to the effects of the acid deposition. The forest ecosystem dieback in the Western Sudety Mountains took place mainly in the upper subalpine forest zone. The threshold of maximum resistance for the Norway spruce was exceeded in the second half of the 1970s. Around 1985, at the Szrenica Mt., mean annual values of deposits and precipitation pH decreased below 3.8 (Błaś et al., 2008). Large-scale dieback of Norway spruce forests first began in the Izerskie Mountains (reaching 900–1000 m a.s.l.), located in the NW of the Karkonosze Mountains. The dieback occurred mainly in the anthropogenic Norway spruce monocultures, but in the

Fig. 4. Forest dieback in the Western Sudety Mountains, at NW slope of Szrenica Mt. (left) and Wysoka Kopa Massif (right). Photos were taken in 1992–1993.

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265

Fig. 5. Temporal variations of spruce tree rings width at selected sites in the Western Sudety Mountains [% of the 1938–1942 reference value, 5-year moving averages], see also Table 1 for site description.

Karkonosze Mountains it already reached ecosystems of natural Norway spruce forests of limited spatial extent within the upper subalpine forest zone. The final and direct factor which caused the forest dieback was an infestation by Larch Tortrix (Zeiraphera grisenana), a moth species of which the larvae also feed on Norway spruce needles. The invasion of 1978 was of unprecedented scale and lasted for the next 4–5 years. Iracka et al. (2000) used aerial infrared photos to produce a map of forest degradation in the Polish part of the Izerskie Mountains and of the Karkonosze Mountains. This map showed vast deforested areas and areas with completely dead forests, especially in the summit part of the Izerskie Mountains and the Western Karkonosze Mountains, e.g. at the hills surrounding the Mumlava valley (Fig. 4). After 1984, the forest dieback slowed down but was still proceeding, as well as in the Czech part of the Karkonosze Mountains. Up till now at least 42% of Norway spruce forests on the Polish part of the Sudety Mountains have been injured or destroyed. The forest dieback mainly concerns the mature trees, leaving suckers and seedlings unaffected. Since the mid-1980s, a clear decrease in air pollution emissions and deposition has been observed. Mean annual pH values in precipitation increased by more than 0.5 unit, up to a pH of about 4.5 in 2008 (Błaś et al., 2008). The decrease of acid deposition was also confirmed by results from earlier dendrochronological research. Changing the width of the annual tree rings is a distinct form of the ecosystem's response to the changing environmental stimuli (Schweingruber, 1996). Contemporary ecological studies show more and more often that the Norway spruce forests in the upper subalpine forest zone of the Sudety Mountains have a satisfactory regeneration potential and are able to revive naturally (Feliksik and Wilczyński, 2003). New tree stands show relatively good structure with respect to tree age and biosocial variety. However, further studies are necessary for a better understanding of the efficiency of wet deposition, the relevance of different deposition mechanisms, and spatial variability of deposition in the study area. In order to show the changes in tree health conditions, the changes in 5-year moving average of tree ring widths are presented for the period 1940–2008. The changes are shown in relative values, using the period 1938–1942 as a reference (Fig. 5).

At all sites in the upper subalpine forest zone, a gradual reduction of the tree ring widths can be seen starting in the 1950ies. At some sites (mainly K1 and K7) a temporal improvement of tree health can be seen at the end of the 1960s and beginning of the 1970s (K1), or at the end of the 1950s and beginning of the 1960s (K7). In the case of K1, the increase in tree ring widths by 30% was connected with the early phase of the ecological disaster which occurred on the western slopes. It caused the thinning out of tree crowns in dense tree stands which increased the level of incoming solar radiation on those trees which survived. Therefore, for some time the tree ring widths increased, too. A similar effect, but of smaller magnitude, could have occurred already in the 1950s at site K2. However, at site K7, representing lower and the most sheltered habitat in the foothills zone, the increase of tree vitality by about 20% was the effect of changes in local meso- and microclimatic conditions. Apart from the cases mentioned above, a stable decreasing tendency in the trees vitality was observed at all sites. The process began in the Karkonosze Mountains in the 1950s and reached its maximum intensity at the end of the 1970s and beginning of the 1980s. Such a strong decreasing tendency could lead to a stop of tree ring formation within a short time, which would as a consequence mark the beginning of the dying of a tree. Besides, the samples in the present study were collected in 2010 and only from alive trees which survived the ecological disaster. So probably in case of other trees which could not be probed because they did not survive the disaster, a complete reduction of tree rings formation must be assumed. The most intensive phase of forest destruction at most sites, but excluding the Karkonosze Foothills, occurred between 1985 and 1995. At the site K2 it was the second half of the 1980s, while at K1 it rather was in the first half of the 1990s. On the western slope of the Kamiennik Mountain (K2), mean tree ring widths decreased in comparison to 1938–1942 by as much as 80%, which documents a very strong worsening of the ecosystem health status and reaching the state close to the complete degradation. At two other sites, K3 and K5, the decrease in tree ring widths exceeded 70%, but K3 is located on the north-west, windward slope of the Szrenica Mountain. The site K5 represents the eastern slope, which may be regarded as a leeward one. However, both points are located on the

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northern macro-slope of the Karkonosze Mountains, at similar altitude, with a relatively short distance between them. Therefore, total air pollution deposition is similar at both locations and so is the reaction of the tree stands. The minimum tree ring width at the site K5 is a bit smaller (by 1.5%) than at K3. But the forest is more thinned out at the site K3 (over 60% of dead tree trunks) than at K5 (about 15% of dead tree trunks). Additionally, at the site K5 there are much less tree crown deformations and injuries observed than at the site K3. The site K3 represents a slope of NE aspect and is located at a similar altitude as site K5. In this case, on a leeward slope, the air pollution deposition from fog is much less efficient, which can be seen in smaller reduction in tree ring widths (only by 50.1%). The tree stand degradation is similar at both sites (less than 10% of dead tree trunks). The site K7, representing the lowermost part of the study area in the Karkonosze Foothills, shows significantly different features than other points mentioned above. Wet deposition plays a much more significant role there than deposition from fog. Therefore, the changes in tree ring widths were much smaller than at other sites. First, a gradual decrease in the annual tree rings could be seen, and then there were the fluctuations of that index with a weak decreasing tendency. The reason was the relative worsening of climatic and habitat conditions for the Norway spruce, as that species does not naturally grow at such altitudes. After the period of a large decrease in tree vitality, a significant improvement was recorded, shown in the gradual increase of tree ring widths. This concerns mainly the sites where the vitality decreased the most, i.e. sites K1, K3 and K4. At the same time, data from the site K2 show that the most damaged ecosystem (even 90% of dead tree trunks) reacted to the improvement of aerosanitary conditions in the slowest way. At the sites K1 and K3 the width of the tree rings recently reached 123% and 103%, respectively, in comparison with the value from 1938–1942. In case of K4 it was about 92%. In other habitats the increase was significant but smaller. Only at the site K7 in the foothill zone the increase reached a low value of only 3% above the minimum value which occurred as late as 2006. It is worth mentioning that recently at the sites K6 and K7 an unusual decrease of tree ring widths occurred (by about 18–21%) which can be linked to climatic changes. Norway spruce trees at the sites K6 and K7 grow in discordant habitats, in a zone of potentially too warm and too dry climate. Consequently, the observed air temperature increased in summer,

insufficient precipitation and less persistent snow cover caused the weakening of the spruce immunity system and increased the hazard of insect attacks. At the same time, air temperature increased and the decrease of cloudiness and precipitation contributed to the improvement of habitat conditions for spruce in the upper subalpine forest zone. These changes occur parallel with the contemporary decrease of acid air pollution deposition and thus can enhance the rate of the tree stand regeneration. The absolute tree ring widths (Fig. 6) show that during the whole study period the largest annual tree rings are found at the sites below the upper subalpine forest zone, i.e. at K6 and K7. This is in agreement with the assumed expectations that with decreasing altitude the role of fog deposition diminishes, and the growing season length increases. In the 1940s and 1950s the absolute widths of annual tree rings showed a very large spatial variation within the upper subalpine forest zone, and reached values in the range from about 0.5 mm at K1 to almost 2.0 mm at K3. During the first phase of the ecological disaster (1974–1983), these differences however practically disappeared and did not exceed 0.3 mm. At the same time a decreasing tendency was observed at all sites. Minimum tree ring widths and the reversal of the trend occurred at different times at the individual sites. These differences in timing of the trend reversal recently led to increases in the differences of tree ring widths among sites, reaching again up to 0.6–0.7 mm. Minimum tree ring widths at the sites K1 and K2 (i.e. the sites on the slopes most exposed toward the west) were as low as 0.25–0.30 mm during the most intensive phase of the ecological disaster. In the area of the Karkonosze Foothills, the tree ring widths were significantly larger than in the upper subalpine forest zone during the entire study period. A persistent tendency of reduction from 3.0 mm at the beginning of the 1970s to about 1.5 mm at present can be observed. Consequently, recently the tree ring widths at site K7 were approximately the same as at site K5 located in the upper subalpine forest zone. At the site K6 the tree ring widths significantly decreased from about 3.2 mm at the beginning of the 1970s to only 1.3 mm in 1983, and at present have recovered to values slightly greater than 2.0 mm. Paradoxically, absolute values of tree ring widths at the sites below the upper subalpine forest zone showed a larger rate of temporal changes than what was observed at the sites within the subalpine zone.

Fig. 6. Average tree ring widths at selected sites in the Western Sudety Mountains.

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5. Conclusions Spatial and temporal variability of the Norway spruce Picea abies tree ring widths in the Karkonosze Mountains is closely connected with air pollutant deposition. This concerns mainly the air pollutant deposition via fog which shows large spatial variations and depends on the frequency and degree of pollution of the orographic clouds base, wind speed, and surface roughness. The ecological disaster in the study area began at the end of the 1950s or the beginning of the 1960s, and was expressed by a significant decrease of tree vitality. It occurred at the same time when the huge power plants in Poland, Germany and the Czech Republic (at that time: Czechoslovakia) combusting lignite with a high sulfur content were established and enlarged. The ecological disaster was one of the impulses which initiated the actions aiming to decrease acidic air pollution emissions and deposition in the majority of the hercynian mountains in Central Europe. The Norway spruce forests in the Western Sudety Mountains reacted quickly to that change, which can be seen in the improvements of tree vitality. The emissions decreased too late to allow for the rescue and protection of spruce forests on summits and on the slopes exposed toward W–NW (sites K1, K2 and K3). It can be expected that if the present, reduced level of air pollution can be sustained, health conditions of mature trees will continue to improve. However, reduction of anthropogenic emissions is still recommended, so that the trees can permanently regain their condition from before the 1940s to 1950s (i.e. stabilization of the positive tendency at sites K2, K5, K6). The observed improvements of tree health status in the upper subalpine forest zone concerns matured individuals of the Norway spruce. It is not directly linked to the improvement of sanitary conditions within the forest renewal areas and open clearings. In these areas the following issues are still urgent problems to be addressed in the future: acidification and erosion of soils; strong winds blowing snow in the cold season exerting abrasive stress on young trees; and shoots damage due to herbivorous animals.

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Marek Błaś is a scientist in the Department of Climatology and Atmosphere Protection, Wrocław University, working on meteorological aspects of air pollution transport and deposition, mountain climatology. Manager of the mountain meteorological observatory at the Szrenica Mt. in the Karkonosze Mountains. Żaneta Polkowska is a scientist in the Department of Analytical Chemistry, Gdańsk University of Technology, specialist in modern analytical techniques applied for determination of atmospheric pollutants, working on the presence of pollutants and their concentration levels in atmospheric precipitation and runoff waters in relation to the prevailing meteorological conditions. Piotr Owczarek is a scientist in the Department of Physical Geography, Wrocław University, working on dendrochronology and dendroclimatology, geomorphology in regions of environmental contrasts. Anita Bokwa is a scientist in the Department of Climatology, Jagiellonian University, working on bio-meteorology, topo-climatology, urban climate, influence of the relief and land use on local climate.