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M. Scherer-Lorenzen, H. Olde Venterink, and H. Buschmann
leaching into groundwater (Schulze 2000; Nadelhoffer 2001). Ecosystems that
are characterized by low natural levels of nitrogen availability are most vulnerable
– for example, ombrothrophic bogs, heathlands, temperate and boreal
forests, or nutrient-poor grasslands. Critical loads for acidifying and eutrophying
nitrogen for these ecosystems lie in the range of 5–20 kg N ha –1 year –1 ,
currently exceeded in many parts of the world (EEA 2005). These alterations
of growing conditions also have direct consequences for the competitive success
of species, associated with often dramatic changes in community composition
and decrease of species richness, especially at N-limited oligo- and
mesotrophic sites such as in northern temperate forests (Bobbink et al. 1998;
Sala et al. 2000): slow-growing plants adapted to nutrient-poor soils are outcompeted
by faster-growing species originating from nutrient-rich sites.
Responsiveness of species to N deposition is thus highly related to plant traits
typical for high-nutrient environments.
Since invasive species are often associated with a particular suite of traits
that are characteristic for plants of nutrient-rich sites (see Sect. 10.1), one
might assume that N deposition could also influence the abundance patterns
of native vs. invasive plant species. In addition, the invasibility of ecosystems
is most often also related to the degree of resource availability (Davis et al.
2000), and invasive species generally occur more frequently at nutrient-rich
sites, as shown for the floras of Germany (Scherer-Lorenzen et al. 2000), and
the Czech Republic (Pyšek et al. 1995). One could therefore hypothesize that
invasive species are more successful, and therefore more abundant in areas of
high atmospheric N deposition than in areas of low deposition.
10.3.2 A Short Note on Mechanisms
As Scherer-Lorenzen et al. (2000) have discussed, the mechanism of competitive
exclusion of species adapted to low nutrient levels by faster-growing,
more nitrophilic species is independent of whether the invading species is
either alien or native. There are numerous examples of successful native
species invading species-rich communities under high N deposition (Bobbink
et al. 1998). Particularly well documented is the case of ombrotrophic
bogs, an ecosystem type among the most sensitive to N enrichment because
these bogs receive most of their nutrients from the atmosphere (e.g.,
Tomassen et al. 2004). Other examples include community changes in wet
heathlands (e.g.,Aerts and Berendse 1988), or after acidic deposition-induced
forest dieback. In the latter case, native grasses responsive to nitrogen invade
the understorey of the forests, replacing species adapted to less fertile soils.
Examples from mountain spruce forests in Europe include the replacement of
the dwarf shrubs Vaccinium myrtillus and Calluna vulgaris or the grass
Deschampsia flexuosa by the grass Calamagrostis villosa (Scherer-Lorenzen
et al. 2000). In all cases, combinations of specific traits, such as high relative