Effects of tree canopy composition on earthworms and other macro-invertebrates in beech forests of Upper Normandy (France)

The effect of canopy composition on soil macro-invertebrate communities in two deciduous temperate forests: a pure beech (Fagus sylvatica L.) and a mixed beech-hornbeam (Carpinus betulus L.) stand was investigated. The initial hypothesis was that heterogeneity of trophic resources within the mixed stand might increase diversity and heterogeneity of soil macro-invertebrate communities at stand level. The macrofauna was sampled in autumn and spring by hand sorting 121 (25 x 25 x 30 cm) soil monoliths regularly distributed within each stand. Earthworms were identified to species level while the remaining macro-invertebrates were identified to family level. Results were analysed by univariate and multivariate (PCA) statistical tools as well as a geostatistical tool. Few differences were observed when comparing the total macro-invertebrate density between sites and dates. In contrast, significant patterns were found for several taxonomic or trophic groups taken separately (e.g. Lumbricidae, detritivore and predator densities were significantly higher in the pure beech stand (PS). The first and the third PCA axes respectively revealed a site and a season effect while the second axis revealed a spatial segregation within the detritivore group as it distinguished high densities of Lumbricidae from those of Isopoda and Diplopoda. The variance of record scores on this axis measured for each site and date revealed that spatial variability of soil macrofauna communities was greater for PS on both dates than for MS. Semi-variance analysis performed on record coordinates on the second axis of the PCA revealed that only macro-invertebrate communities sampled under the pure stand in autumn were spatially structured (autocorrelation range about 32 m). These results do not support the general agreement that resource diversity and patchiness increases soil fauna biodiversity and heterogeneity.