Factors influencing soil macrofaunal communities in post-pastoral successions of western France
Introduction
Soil macroorganisms (i.e. living roots and macroinvertebrates) play a key role in the soil system in that they significantly regulate soil structure and nutrient cycling (Lavelle et al., 1994). In calcareous ecosystems, macroinvertebrates actively help to maintain a highly active mull by improving microbial activity (Scheu, 1990) and litter fragmentation and burial (van der Drift, 1963; Hirschenberger and Bauer, 1994a, Hirschenberger and Bauer, 1994b). Some taxonomic groups, such as earthworms, also affect the vegetation pattern by influencing the composition of soil seed banks (Thompson et al., 1994; Willems and Huijsmans, 1994).
For centuries, European chalk grasslands were traditionally used for sheep grazing. Since the abandonment of this practice in the 1950s, natural successional processes have led to woodland extension and/or species-poor coarse grasslands (Smith, 1980). In Upper Normandy, a study was carried out to identify the factors influencing biological diversity in these ecosystems and their response to different kinds of disturbances. As part of this study, we conducted a survey aimed at assessing changes in density, biomass and community structure of soil macrofauna during post-pastoral succession and identifying the main factors responsible for these changes. Soil macroinvertebrate communities were monitored in different stands of a potential succession from a permanent pasture to woody formations on chalky substrate.
Section snippets
The sites
We sampled the soil fauna on calcareous slopes of the Seine Valley in Upper Normandy, north-west France. Mean annual precipitation and temperature were, respectively, 804 mm and 9.9°C. Soils were calcareous rendzina with neutral pH, aggregated structure and highly active humus.
Successions in chalk grasslands are complex and can be described as multi-directional processes influenced by many biotic and abiotic factors (Dutoit and Alard, 1996). For this study, six plots were selected to represent
Results
Biomass and density were highest in the pasture and 2-year fallow plots; density was lowest in the 44 year fallow while biomass was lowest in the pine wood (Table 1). Earthworms were the major component of biomass in all the plots, their contribution to biomass being greatest in pasture (96%) and least under pine (73%). The highest taxonomic richness (N) was found in the 2 year fallow and the maple wood and the lowest in the pine wood. Diversity (H′) and evenness (E) were maximal in the wooded
Discussion
Macrofaunal biomass was very high (70.2–140.3 g m−2) as compared with other data from beech wood on limestone (12.7–25.5 g m−2, Schaefer and Schauermann, 1990; David et al., 1993). The high biomass can be related to the presence of suitable soil conditions (i.e. neutral pH, high Ca content and high organic matter content) that permit the presence of significant earthworm populations.
Spatial variability of the plant cover is known to increase the diversity of other functional groups (Babel et al.,
Conclusions
During secondary succession, macroinvertebrate communities first respond to a structural gradient that reflects the changes in the vegetation structure and resulting modification in microhabitat diversity and life conditions (i.e. diversification of the herbaceous stratum, appearance of a thin litter and/or moss layer). This agrees with Scheu (1992), who concluded that individual earthworm species react only to the availability of preferred microhabitats in the different successional stages.
Acknowledgements
We are grateful to the Regional Natural Conservatory of Upper Normandy and the Axa-Insurances Society for giving permission to work in the natural reserve of Saint Adrien. Financial support was provided by the French Ministry of Environment (DGAD-SRAE no 94220).
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