Background/Question/Methods Recent studies have demonstrated the importance of coarse woody debris in maintaining forest arthropod diversity. Several Red-Listed arthropods in Europe are saproxylic species associated directly or indirectly with decaying wood. Most work to date has focused on beetles (Coleoptera) but, in many cases, flies (Diptera) are at least as abundant and species-rich in the same habitats. Despite that diversity, there has been little empirical study of host or microhabitat preferences in saproxylic Diptera in North America, especially at finer taxonomic scales. The objective of this study was to determine the effect of host tree species and decay stage on the abundance, species-richness and community composition of saproxylic Diptera associated with decaying deciduous logs. The study was carried out at the Mont Saint-Hilaire Biosphere Reserve in southern Quebec, Canada. Emergence traps were placed over cut logs of two species (Sugar Maple, American Beech) in two decay stages (two and six years after tree death) and insects were collected for 16 weeks from May to September 2004. All Diptera were identified to species level, where possible, for subsequent quantitative analyses.

Results/Conclusions More than 200 species were identified but almost 100 rare species (1-2 specimens) were omitted in analyses. Rarefaction-estimated species richness and Simpson diversity were highest in young maple and old beech, and did not differ significantly between young maple and old beech, or between old maple and young beech. In 2-factor ANOVA, there were significant effects of tree species and decay stage on species richness, but only decay stage had a significant effect on abundance. Community composition in young maple differed significantly from old maple and old beech. The number of specimens collected between replicates within young maple and young beech logs was significantly different. Correspondence analysis also showed tighter clustering of old logs; outliers were all young logs, suggesting that assemblages in young logs are more variable, but become more similar in older logs. These differences may be dictated by ecological interactions through the decay cycle, or by a random colonization sequence of young logs. Most Diptera do not appear to differentiate between host species and decay stage appears to have a stronger influence, so the quantity and age of coarse woody debris may be more important than tree species in maintaining saproxylic Diptera diversity in these forests.