The diversity of saproxylic insects (Coleoptera, Heteroptera) on four tree species of the Hyrcanian forest in Iran

In Central European temperate forests, the host tree identity is one factor that determines the diversity of saproxylic organisms. These forests have been affected by humans for millennia, in contrast to the Hyrcanian forests south of the Caspian Sea, with their numerous old-growth features and endemic species. How the tree host species in this temperate relict of Tertiary forests affects saproxylic biodiversity is unknown, as an inventory of the saproxylic fauna present is incomplete, and new species are still being described. To analyze the importance of four dominant tree species in this region (Fagus orientalis, Quercus castaneifolia, Alnus subcordata, and Carpinus betulus), we sampled saproxylic beetles and true bugs with flight-interception traps at five dead trees per host species. We identified 361 beetle and 7 true bug species; of these, 268 were saproxylic, 56 were endemic, 11 were Central European primeval-forest relicts, and at least 16 species were undescribed. Rarefaction-extrapolation curves indicated higher species richness of saproxylic beetles and true bugs on F. orientalis and C. betulus than on A. subcordata and Q. castaneifolia, but ordination showed that communities were most distinct on Q. castaneifolia. Higher species richness on C. betulus and F. orientalis compared to Q. castaneifolia was unexpected based on literature but is in line with recent findings of experimental studies from Central Europe. Our results underline previous findings on the importance of moribund trees and dead wood in the Hyrcanian forests, and thereby support current efforts of Iranian authorities to stop the removal of veteran trees. The complementary fauna on oak trees compared to the other three tree species illustrates the importance of fragments of the almost destroyed oak forests along the coastline for forest biodiversity conservation in the Hyrcanian region.