Species variables or environmental variables as indicators of forest biodiversity: a case study using saproxylic beetles in Maritime pine plantations

To assess the sustainability of plantation forest management we compare two types of biodiversity indicators. We used the species richness of saproxylic beetles as a case study to test the species and environmental indicator approaches. We compared single species abundance or occurrence and deadwood volume or diversity as predictor variables. Beetles were sampled with flight interception traps in 40 Maritime pine plantation stands. The volume and diversity of deadwood was estimated with line intersect and plot sampling in the same stands. Predictive models of species richness were built with simple linear or Partial Least Square regressions. Deadwood variables appeared to be good predictors of saproxylic beetle richness at the stand-scale with at least 75% of variance explained. Deadwood diversity variables consistently provided better predictive models than volume variables. The best environmental indicator was the diversity of deadwood elements larger than 15 cm in diameter. By contrast, the use of species variables appeared to be less relevant. To reach the quality of prediction obtained with environmental variables, the abundance or occurrence of 6 to 7 species some of which are difficult to identify - had to be used to build the indicator.