Drought mitigation by thinning: Benefits from the stem to the stand along 15 years of experimental rainfall exclusion in a holm oak coppice

In northern Mediterranean forests, increasing drought stress due to the on-going climate change is combined with stand ageing due to the lack of management. Management by thinning may alleviate drought stress by reducing competition, but its application is challenging in coppices of resprouting species where its long-term consequences for tree demography and stand dynamics are difficult to evaluate. In this study, we investigate the long-term (15 years) demographic responses of holm oak (Quercus ilex L.) to a combination of thinning from below (- 30% basal area) and experimental rainfall exclusion (- 27% precipitation). Stem growth, survival and resistance to an extreme drought event were positively linked to both stem size and local competition release after thinning. Thinning improvement of growth and survival were thus due to both a selection of the biggest, most vigorous, trees and to a release of competition for water. Rainfall exclusion, on the other hand, led to a shift of the tree size-mortality relationship, which resulted in the death of bigger trees, in a faster loss of stool density and in a slower evolution of the stand basal area compared to the control. Thinning was beneficial by cancelling the rainfall exclusion effects on growth and mortality, and by doubling the stand basal area increment compared to unthinned control. The initial loss of stools due to thinning was compensated by a lower mortality, suggesting that thinning do not reduce further the amount of unique genotypes on the long-term. Positive thinning effects on stem growth decreased over time but remained significant 15 years after thinning, while resprouting dynamics strongly decreased with time. These results indicate that moderate thinning from below is a relevant strategy to increase stem vitality and stand production in old coppices, particularly in a context of a chronic rise in drought stress and more frequent extreme drought episodes.