Potential of forest thinning to mitigate drought stress: A meta-analysis

Increasing frequency of extremely dry and hot summers in some regions emphasise the need for silvicultural approaches to increase the drought tolerance of existing forests in the short term, before long-term adaptation through species changes may be possible. The aim of this meta-analysis was to assess the potential of thinning for improving tree performance during and after drought. We used results from 23 experiments that employed different thinning intensities including an unthinned control and focused on the response variables: radial growth, carbon- and oxygen-isotopes in tree-rings and pre-dawn leaf-water potential. We found that thinning effects on the growth response to drought differed between broadleaves and conifers, although these findings are based on few studies only in broadleaved forests. Thinning helped to mitigate growth reductions during drought in broadleaves, most likely via increases of soil water availability. In contrast, in conifers, comparable drought-related growth reductions and increases of water-use efficiency were observed in all treatments but thinning improved the post-drought recovery and resilience of radial growth. Results of meta-regression analysis indicate that benefits of both moderate and heavy thinning for growth performance following drought (recovery and resilience) decrease with time since the last intervention. Further, growth resistance during drought became smaller with stand age while the rate of growth recovery following drought increased over time irrespective of treatment. Heavy but not moderate thinning helped to avoid an age-related decline in medium-term growth resilience to drought. For both closed and very open stands, growth performance during drought improved with increasing site aridity but for the same stands growth recovery and resilience following drought was reduced with increasing site aridity. This synthesis of experiments from a wide geographical range has demonstrated that thinning, in particular heavy thinning, is a suitable approach to improve the growth response of remaining trees to drought in both conifers and broadleaves but the underlying processes differ and need to be considered.