Seeing the wood for the trees, irregular silviculture supports bat populations in conifer plantations

Abstract

Conifer plantations are a major land use globally and provide a range of social, economic and environmental benefits, particularly the provision of timber. There is a growing interest in alternative forestry techniques, such as irregular silviculture, to create more sustainable and resilient plantations in response to climate change. Plantations are often considered poor for bats due to limited structural diversity. Irregular forestry increases structural diversity and reduces growing stocks, with potentially positive effects on bats, but this remains poorly understood in conifer plantations. At two estates in south-west Britain, we specifically tested whether bat species richness and activity respond to (1) the surrounding landscape cover types (2) habitat structure within irregular stands and (3) increasing progress along the transformation continuum in three continuous cover forestry stands undergoing transformation including irregular high forest. Stands further progressed along the transformation continuum had lower basal area, higher mean tree diameter, higher levels of fallen deadwood and greater canopy cover of broadleaf trees. A total of 13 species of bat, equivalent to 76% of the UK resident species were recorded. The activity of six species was significantly influenced by the proportion of four land use types (two positive and two negative associations with conifer woodland, one positive and one negative association with broadleaved woodland, two positive associations with improved grassland and two negative associations with arable) in the surrounding landscape at three spatial scales (500 m, 1500 m and 3000 m around each survey plot). Four species showed significant positive associations with habitat structural features including larger mean tree diameter, greater canopy openness, higher vertical structural complexity, higher quantities of standing and fallen deadwood, and higher canopy cover of broadleaved trees. Many of these habitat features were more prevalent in stands further progressed towards irregular high forest. We did not detect any significant differences in bat species richness or activity levels between the three stand stages, except for Plecotus auritus/austriacus which had higher activity in the Stage 3 stand compared to Stage 2, as the limited replication of our study may have precluded detection of any differences. Our study helps inform us on how bats populations respond to novel management of conifer plantations. Further research to determine thresholds for deadwood and broadleaved trees to further support bats in plantations would be beneficial.