Abstract
Impacts of tree diversity on forest soil fauna remain poorly known. Two groups of functionally similar invertebrates, millipedes (Diplopoda) and woodlice (Isopoda, Oniscidea), were sampled at 61 forest plots in four European countries (Finland, Poland, Romania, Italy) in order to (1) compare the abundance and species richness of these saprophagous macroarthropods in monospecific and mixed, three-species forest stands, and (2) determine whether the influence of tree species richness depends or not on tree identity. The results showed that, in boreal forests of Finland, regional conditions (temperature, soil) proved to be very unfavourable for millipedes and woodlice, which prevented further comparisons. In the other three regions, tree species richness had no consistent effect on macroarthropod abundance but there was a clear trend towards a higher macroarthropod species richness in mixed stands compared to monospecific stands. This resulted mainly from a significantly positive response of millipedes to tree mixtures in hemiboreal forests of Poland and mountainous forests of Romania, and from a slightly positive response of woodlice in thermophilous forests of Italy. However, not all tree mixtures were equally favourable for saprophagous macroarthropods. An examination of tree identity in mixed stands showed that mixtures including hornbeam and conifers in Poland and those including beech and spruce in Romania had significantly higher macroarthropod species richness than the other mixed stands. Moreover, macroarthropod species richness was higher in these broadleaf-conifer mixtures than in monospecific stands of either component tree species, which supports the hypothesis of a greater diversity of resources for macroarthropods in these mixed stands. This hypothesis was not supported in Italy, where macroarthropod species richness was highest in monospecific chestnut stands. The positive influence of conifer presence on the species richness of saprophagous macroarthropods in Eastern European forests contrasts with results of previous studies, which calls for further research to explain those differences.