Many forested areas in industrial countries are exposed to excess deposition of airborne nitrogen (N) and sulfur (S). Biodiversity decline is one of many observed effects. Unlike N deposition, S loads have been decreasing continuously over the last 25 years. In this paper, we evaluated the use of species diversity in 4 taxonomic groups (vascular plants, lichens, bryophytes, birds) as indicators of N and S deposition. Long-term monitoring data from 1992 to 2005 were taken from an intensively monitored site in Austria. Both temporal changes and determinants of diversity were explored. On average, diversity declined from the beginning of the 1990s until the year 2005, but there was a considerable variation among the organisms and the diversity indicators. Few changes in diversity were statistically significant. Strongest changes occurred at the level of single species, as an increase or decrease of their abundance. Factors other than N and S deposition—particularly historical forest management and natural disturbances—were significant, complicating interpretation of the observed diversity changes. We conclude that species diversity alone is not a reliable indicator of N and S impacts on forests, particularly if few indicator species groups are used and the observation period is short.
Nitrogen (N) and sulfur (S) emissions increased dramatically during the second half of the 20th century, causing excess deposition of N and S in natural and seminatural ecosystems, particularly in industrialized countries (Bouwman et al 2002; Galloway et al 2004). Owing to the canopy characteristics of forests, deposition of airborne pollutants is significantly elevated in these areas. Although direct pollution sources are less abundant in mountain areas, mountain forests are exposed to high precipitation and thus higher deposition loads (Lovett 1984; Erisman and de Vries 2000).
Chronic N deposition may cause soil acidification, disrupt the soil nutrient balance, increase susceptibility to parasite attack as well as emissions of nitrogenous greenhouse gases from the soil, and elevate nitrate loss to groundwater (Fenn et al 1998; Erisman and de Vries 2000; Aber et al 2001; Bobbink et al 2003). Significant changes of lichen, bryophyte, and vascular plant species composition in response to chronic N deposition have been reported for many ecosystems (Bobbink et al 2003). Since N emissions are predicted to increase globally during the decades to come, N excess will be among the major threats to biodiversity in future (Bouwman et al 2002; Galloway et al 2004; MEA 2005). Elevated S deposition, causing soil acidification, the depletion of base cations from the soil, and the leaching of aluminum and heavy metals into groundwater, has been recognized as a major environmental problem since the 1970s. As for N, detrimental effects on living organisms have been found in freshwater and terrestrial ecosystems (van Dobben et al 1999; Legge and Krupa 2002), including biodiversity loss (Zvereva et al 2008).
Unlike N emissions, S emissions have been successfully abated in Europe through internationally ratified protocols (United Nations Economic Commission of Europe/Convention on Long-Range Transboundary Air Pollution [UN-ECE/CLRTAP]) and related measures. Loads of S have been decreasing continuously during the last 25 years (EMEP 2006; Rogora et al 2006; Figure 1).
After the World Summit on Sustainable Development and the Convention on Biological Diversity (CBD), which aims at significantly reducing the rate of biodiversity loss by the year 2010 (CBD 2003), the effects of air pollutants on biodiversity have become a central issue. Although the term biodiversity takes into account “variation within species, between species, and of ecosystems” (Article 2 of the Convention on Biological Diversity), species diversity is most often used as the key indicator. A change in the diversity of species involves complex population processes such as migration, extinction, and colonization. In most organisms, these processes are not well studied, often involve response lags, and determinants vary with different spatial and temporal scales (Rosenzweig 1995; Crawley and Harral 2001; Ibáñez et al 2006).
In this paper, we evaluate the use of diversity as an indicator of the effects of N and S deposition in an intensively monitored site in the Northern Limestone Alps in Austria. Long-term measurements of deposition, climate, and forest management have been combined with the monitoring of several taxonomic groups and biodiversity. Our first question is therefore whether the diversity of different taxonomic groups exhibits the same temporal changes. As diversity indices, we used species numbers (SN) and Shannon Index of Diversity (SH) (Margurran 1988). The second purpose of this paper was to quantify the major determinants of diversity in the study area by including impacts through airborne N and S deposition (Figure 2).
Material and methods
The size of the site is 90 ha, and it is situated in the Northern Limestone Alps National Park (47°50′30″N, 14°26′30″E) (Figure 3). The altitude ranges from 550 m to 956 m above sea level (masl). The main type of rock is Norian dolomite, partly overlaid by limestone. The catchment area is divided into a steep (30–70°) slope from 550–850 masl and an almost flat plateau (850–956 masl). The long-term average annual temperature is 7.2°C. The coldest monthly temperature at 900 masl is −1°C (January), and the highest monthly temperature is 15.5°C (August). Annual rainfall ranges from 1500 to 1800 mm. Monthly precipitation ranges from 75 mm (February) to 182 mm (July). Snowfall occurs between October and May, and the average duration of snow cover is about 4 months. The slope is mainly covered by mixed mountain forest with beech (Fagus sylvatica) as the dominant species, Norway spruce (Picea abies), maple (Acer pseudoplatanus), and ash (Fraxinus excelsior), whereas Picea abies predominates on the plateau following plantation after a clear-cut around the year 1910. Long-term trends of