How does deadwood structure temperate forest bat assemblages?

Thirty percent of forest species depend on deadwood. Some of them are now considered rare or at high risk of extinction mainly due to an insufficient quantity of deadwood substrates. Some bats roost in dead trees and snags. Because European bats are strictly insectivorous, we can wonder whether deadwood plays an important role by providing potential preys too. We conducted ultrasonic surveys in different deciduous French forests dominated by oaks ( Quercus spp.) and beech ( Fagus sylvatica). Our results showed a positive relationship between the volume of deadwood and the richness of bat species. Some species were positively related to deadwood volumes, either lying or standing, with detected thresholds. Species richness increased particularly from 25 m(3) per hectare of standing deadwood. This link can be explained by deadwood-dwelling preys or by changes in the forest structure, due to openings created by dead trees that are favorable for edge-habitat species. Other species negatively reacted to the presence of deadwood, either because bats were not able to forage there or because dead trees did not provide relevant preys. Contrary to our hypothesis, clutter by foliage and basal area of the living trees explained more the presence/occurrence of gleaning bats than deadwood. Whereas several species were considered as forest bats (Myotis bechsteinii, Plecotus auritus, Barbastella barbastellus), we did not find any relationship between their occurrence and deadwood. This result strengthens the need of further studies on the relationships between forest habitats and bat assemblages.