Long-term thinning effects on tree growth, drought response and water use efficiency at two Aleppo pine plantations in Spain.

In Mediterranean areas where drought-induced forest dieback and tree mortality have been widely reported, it is still under debate how the likely risks of climate change will affect tree growth and consequently forest productivity. Increasing tree mortality has been associated not only to increased drought, but also to a lack of management in many dense pine forests and plantations, where warming may intensify tree-to-tree competition for soil water. This emphasizes the need of using silviculture to adapt dense stands of Mediterranean pine reforestations to warmer and drier conditions. Here we combined dendrochronology and C and O isotope analyses of wood in two Aleppo pine (Pinus halepensis) plantations, growing under semiarid conditions and experimentally thinned at high and moderate intensities along with control. The main aim was to understand the responses of radial growth and water use efficiency (WUEi) to different thinning intensities, and to analyze the effectiveness of thinning to enhance post-drought growth resilience. Thinning had a positive effect on growth, produced an increase of δO, reduced growth sensitivity to drought and decreased WUEi, suggesting a reduction of drought stress. These results were consistent across sites, and were significant even 20 years after the intervention took place. Considering the climate effects on growth through the SPEI drought index to calculate resistance and recovery indices, an increase of resistance after thinning was observed. We conclude that high thinning intensity (50% of basal area removed) is a useful silviculture intervention on Mediterranean Aleppo pine plantations that enhances their growth, and makes them less dependent on harsh climatic conditions, improving their resilience against drought and consequently making them better adapted to more unfavourable conditions.