Using stem diameter variations to detect and quantify growth and relationships with climatic variables on a gradient of thinned Aleppo pines

Thinning is a forestry technique that can improve growth in the remaining trees and make them less vulnerable to water stress, buffering the imminent climatic changes. However, thinning effects have not been thoroughly explored using high-resolution measurements of stem-diameter variation in natural ecosystems, which can provide valuable information on secondary tree growth, relations to climatic variables, and possible water-stress events. In this study, a gradient of thinning intensity was applied on a Pinus halepensis Mill. afforestation in the semiarid SE of the Iberian Peninsula to ascertain the effects on the daily, monthly, annual, and total cumulative growth as well as the relationships between climatic variables and tree-growth parameters (climate-growth sensitivity). The thinning treatments, applied in 2005, consisted of reductions of 75% in the mean basal area (175), 60% (T60), 48% (T48), and an unthinned control (T0). Digital dendrometers were installed in 2010, 5 years after thinning, to characterize secondary tree growth-i.e. daily-stem variation (DSV), cumulative growth (CG), and the maximum daily shrinkage (MDS), but also the number of days of growth over a 3-year period. Thinning treatments affected the onset and end of growth, making the growing season longer after more intense thinning treatments. Also, the total cumulative growth of the stem diameter increased over the study period following the gradient of thinning intensity. From correlations among climatic variables and DSV, MDS, and CG, we conclude that thinned trees showed better climatic sensitivity and efficiency in the resources use and therefore a stronger adaptation to the environment and greater resilience to perturbations, despite that tree monitoring started 5 years after thinning. In view of these results, we recommend thinning as a tool to adapt forest ecosystems to climate change in Mediterranean areas.