Estimating species loss of saproxylic insects under scenarios of reduced coarse woody material in eastern boreal forests

Abstract

Increased exploitation of forests and residual biomass remaining after harvest has the potential to reduce biodiversity particularly of saproxylic organisms. We compare incidence-based species accumulation curves based on saproxylic flies collected using in situ emergence cages in 2006 and 2007 under two biomass harvesting scenarios. In these scenarios volume-based biomass targets would be achieved by either 1) selective removal of the largest individual pieces of coarse woody material (CWM) or alternatively 2) selective removal of smaller pieces of CWM with preferential retention of larger pieces of CWM with presumably greater conservation value. We then extrapolated a species accumulation curve to estimate thresholds of potential species loss as a function of CWM volume left within a forest stand using binomial mixing. More species would be maintained under the scenario that selectively targets larger individual pieces of CWM because of extremely large species turnover between individual sample logs and little relation between species richness and diameter of CWM. Given these circumstances, the number of individual pieces of CWM present in a stand may be more important in determining species richness than total volume of CWM for flies. When the species accumulation curve was extrapolated to stand-level volumes, species richness began to decline when CWM volumes were reduced below 40 m3/ha. We suggest that intensive management strategies aimed at recovering additional woody biomass should not exploit CWM volumes below these thresholds without mitigative measures aimed at replenishing dead wood volumes such as standing retention if maintaining biodiversity is an objective.