Reduction of stand density increases drought resistance in xeric Scots pine forests

In Valais, a dry Inner-Alpine valley in Switzerland, increasing tree mortality has been related to drought and reduced forest management, which have led to increased competition between trees. Since increasing drought and reduced forest management were co-occurring during the last decades, it is not clear if forest management alone could increase tree resistance to drought. To test whether thinning of Scots pine (Pinus sylvestris L.) stands could be used to mitigate the effects of drought, we analyzed a long-term thinning trial set up in xeric Pfynwald forest in 1965. The trial featured one control (unmanaged plots; basal area 40m²ha^-1) and three thinning intensities; light (33m²ha^-1), medium (22m²ha^-1) and heavy (11m²ha^-1). Thinning was repeated in 1971, 1978 and 2010. Individual tree radial growth responses and tree leaf area to sapwood area ratio were assessed and compared among treatment. Additionally, stand-level growth, tree mortality and the relationship between stand density and tree diameter (allometric slope r) were analyzed and compared between two periods (1978-1990; 1991-2009) where the last period was drier. Individual tree basal area increment increased significantly for up to three decades after heavy thinning and mortality rates decreased with decreasing basal area while stand-level growth did not significantly differ. The higher mortality rate and the more negative allometric slope r in the second period suggests that site conditions have become hotter and drier and can partially explain a decrease in the basal area in the control plots and in the light thinning treatment. Leaf area to sapwood area ratio increased with lower basal area and suggests that competition for water was reduced. Taken together, our results suggest that a reduction in basal area of 40-60% to ca. 15-25m²ha^-1 could mitigate drought effects on Scots pine on xeric sites for the coming decades prolonging the provision of important ecosystem services. After this time, sustainability of Scots pine forests at xeric sites will depend on the intensity of anticipated climate change, so that in the worst case, thinning might be applied to convert Scots pine forests to mixed forests with more drought-resistant species.