Thinning enhances stool resistance to an extreme drought in a Mediterranean Quercus ilex L. coppice: insights for adaptation

Abstract

Mediterranean holm oak forests are subjected to chronic seasonal droughts coinciding with the warmest conditions during the summer. Importantly, climate change projections support increased frequency and intensity of droughts in the future. In order to evaluate whether thinning practices can be used as efficient adaptation strategies to climate change, we measured canopy cover as a surrogate of defoliation in 49 Quercus ilex L. subsp. ilex individual genets subjected to experimental ramet thinning. Canopy cover measurements were obtained before, during and after the exceptionally intense and prolonged drought of 2016. Our results show that intensity of basal area reduction determined higher resistance, but not resilience, to this intense-long drought period. Independently of shoot thinning intensity, every holm oak exhibited crown defoliation during the dry period but most of them recovered their leaves the next spring. Holm oaks subjected to thinning with more than over 50% of basal area removed, were less vulnerable to severe drought, however, and supported more leaves later on. Additionally, our results highlight the positive influence of canopy shading from neighbouring trees preventing defoliation after drought events. A greater canopy shading per stool led to a higher resistance in holm oak trees exposed to severe drought. These findings suggest that, in order to cope with concomitant climate change and to optimize genet-level stress tolerance, coppice selection should be done by removing shoots with fewer leaves per unit area, preferably those unexposed to shading from neighbouring trees until a reduction around 50% of basal area per stool is achieved.