Long-term effects of forest management on post-drought growth resilience: An analytical framework

There is great interest in determining the effects of forest thinning as a tool to improve growth recovery from drought in different tree species and climatic conditions. However, we lack a robust framework to determine how transient are post-drought growth resilience and enhancement, and if such growth improvement involves an uncoupling with climate conditions. We used regression analysis to determine differences in growth, sensitivity to drought and previous-year growth, and long-term growth in five plantations of three pine species (Pinus halepensis Mill., Pinus nigra Arn. and Pinus sylvestris L.) under different thinning intensities. Then, we simulated post-drought and post-thinning growth trajectories based on fitted models, and we computed drought resistance, resilience and recovery indices based on these trajectories. Moreover, the simulation allowed us to calculate the time to recovery after a drought. Using this analytical framework, we found that thinning enhanced radial growth (between 85 and 150%, significant in all sites with p < 0.05), and reduced previous-year growth dependence (between -13 and -26%, significant in two out of five sites) and climatic dependence of growth (-23 to -49%, significant in two sites). We interpret these effects as a result of competition reduction by thinning and a transitory alleviation of growth climatic constraints. Thinning consistently improved drought resistance (+4 to +20%) and resilience (+1 to +4%). Recovery, on the contrary, was reduced (-1 to -15%). Since the growth loss during the drought was reduced due to higher drought resistance, the recovery was proportionally lower. Thinning reduced the time to recovery by one to two years. The thinning legacy effect persisted up to 15 to 20 years after thinning. Taken together, these findings enhance the benefits of adaptive silviculture in making pine plantations less vulnerable to unfavourable extreme climate events such as droughts. We present a novel and robust analytical framework to assess drought-thinning interactive effects on tree growth.