ÉVOLUTION SPATIO-TEMPORELLE DE L’ACIDIFICATION DES SOLS FORESTIERS FRANÇAIS

The acidity of many regions of the world has increased as a consequence of food and energy production to sustain a growing global population (Galloway, 1995). These factors have led to enhanced emissions of air pollutants which in turn lead to increased long-range transport and deposition of sulfur (S) and nitrogen (N). Both are carriers of soil acidity (Dentener et al., 2006a). By approximately the mid-20th century, global S and N emissions from anthropogenic processes overtook emissions from natural processes (Galloway, 2001). The effects of atmospheric pollution are widespread and appear in a number of ways including acidification of freshwater systems (Probst et al., 1999), changes in soil chemistry (De Schrijver et al., 2006), damage to vegetation (Bobbink et al., 2010), and changes in forest ecosystems leading to vegetation changes (Falkengren-Grerup, 1986). Most natural soils are acidic due to both organic matter production and decomposition, but human activities have contributed to making them more acidic (Galloway, 2001). Atmospheric deposition is a global concern and a current issue. After the peak of S in the 1970s and N in 1980s, and the demonstration of its negative consequences for the environment and human health, measures were implemented to control and reduce air pollution. As a result, S and N deposition within the European area decreased significantly from 1990s to our days, by approximately 60% and 38% , respectively (EMEP, 2011). In France, S and N deposition has followed the European tendency for reduction from their levels in the 1990s (1216 mg S.m-2 and 1530 mg N.m-2, computed from modeled data), decreasing by approximately 70% and 21 %, respectively, in 2010 (EMEP, 2011). While S deposition has declined sharply, returning to the deposition levels of the early 20th century, N deposition has remained elevated. Although acidifying deposition has substantially decreased in the last decades, few evidence of ecosystem recovery has been reported until now (see Skjelkvåle et al. (2005) for freshwater evidence). A likely recovery according to Driscoll et al.

The impact of increased acidifying inputs on soils and the species composition of forest ecosystems has been a major concern in North America and Europe (Aber et al., 1998). Since the soils in which forests grow change slowly and forests themselves grow slowly, the effects of acidic deposition on forests may not be manifested for years to decades (Galloway, 2001). As a consequence, detection of the long-term effects on vegetation and environment is needed to better understand how are they interact (Sebesta et al., 2011). Similarly, the detection of such effects over a large scale area is also necessary, because potential drivers and their ecological consequences operate at national and continental scales (Smart et al., 2003). However, such studies are scarce (e.g. Blake et al., 1999; Emmett et al., 2010; Kirk et al., 2010), and the majority of existing works have reported the magnitude of changes over time but covered only small regions or specific plots (De Schrijver et al., 2006; van der Heijden et al., 2011).

Tracking long-term environmental changes is particularly difficult due to the limited historical data with measurements of soil parameters, which in turn is related to the low number of measured sites or monitoring programs (Dengler et al., 2011). Without historical measurements, bioindicator values have become an option to determine the values of environmental parameters and to monitor their change (c.f. Braak et Dame, 1989; Birks et al., 1990). Bio-indication can be defined as making use of specific reactions of organisms to their environment (Diekmann, 2003). Because of the potential ability of plants to indicate the values of environmental variables (Bertrand et al., 2011b), significant insight into soil acidity changes can be identified using the available floristic data for any time period (Wamelink et al., 2005). Forest inventories (mainly started up in the 1980s) and phytosociological studies providing valuable ancient floristic information (e.g. Braun, 1915) represent an important background to long-term research for detecting the impacts of long-range air pollution on vegetation. As most of the plant biodiversity in temperate forest ecosystems is represented by the herb layer, which responds sensitively to disturbances across broad spatial and temporal scales, its dynamics reflect the evolution of forest status (Thimonier et al., 1992; Gilliam, 2007). Previous studies have shown significant shifts in the forest herb layer due to acidification (Diekmann et Dupré, 1997; Baeten et al., 2009; Van Den Berg et al., 2011). In France, while ample evidence from the northeastern region indicates that acidic deposition has deeply altered chemical soil properties, nutrient cycling and vegetation dynamics in forest ecosystems