Ecological outcomes for multiple taxa from silvicultural thinning of regrowth forest

Abstract

Management of forest regrowth is a key issue globally, particularly where it occurs as high densities of small-sized stems and where there is little to no self-thinning. Thinning can reduce stem density to accelerate tree growth and potentially restore forest-structural complexity. Yet, in some forest types, it remains unclear how biodiversity responds to thinning, especially over longer periods. White cypress pine Callitris glaucophylla is geographically widespread in Australia and has history of silvicultural thinning in parts of its distribution. We used a chronosequence approach to assess the short- and longer-term responses of multiple taxa (bats, birds, invertebrates, reptiles, non-volant mammals, and plants) to thinning of C. glaucophylla in the dry Pilliga forests of New South Wales. We recorded data from a total of 227 taxa. The short (<8 years)- and longer-term (>20 years) responses to thinning of C. glaucophylla regrowth were mostly positive or neutral for the taxa considered. Bat activity and diversity were comparable to levels in long-undisturbed reference forest, though this was influenced by time since thinning. Reptile diversity and abundance was positively associated with thinning at an intermediate time since thinning (8–20 years), with both metrics greater than unthinned regrowth and long-undisturbed reference forest. Bird diversity was greater in recent and old thinning treatments when compared to unthinned forest regrowth, although species composition was not affected by thinning. Though thinning did not affect volant invertebrate biomass, it was associated with a relatively more even distribution among size classes >8 years post-thinning, and a greater representation of beetles to overall insect biomass. The activity, diversity and composition of non-volant mammals did not differ among treatments, nor did understorey plant diversity or composition. Combining these multiple taxa into a composite ‘biodiversity’ index demonstrated a 16–22% increase with thinning relative to index values in long-undisturbed reference forests. Overall these results indicate that thinning has neutral to positive effects on biodiversity, but responses are species-specific and likely to be dependent on forest-type and the broader landscape of a site. Given unthinned forest represented habitat of similar value to thinned forest for some taxa, we recommend that that regrowth patches of varying size are retained across the landscape to provide a mosaic forest structure suitable for a diverse suite of flora and fauna.