Impacts of droughts on the growth resilience of Northern Hemisphere forests

AimThe intensity and frequency of drought have increased considerably during recent decades in some Northern Hemisphere forested areas, and future climate warming could further magnify drought stress. We quantify how forests resist drought events and recover after them, i.e. we determine their growth resilience. LocationNorth America and Europe. MethodsWe use a large tree-ring database to study how drought influences forest growth resilience. We selected 775 tree-ring width chronologies and studied the occurrence of years with extremely dry conditions (low soil moisture and/or high evaporative stress; hereafter drought') in these forests. For each drought in each forest we calculated three indices that represent different components of growth resilience to drought: resistance (Rt), recovery (Rc) and resilience (Rs). We related the variation in these indices with geographical, topographic, climatic and ecological conditions from each region. ResultsThe three components of forest growth resilience were interrelated. Resistance and recovery were negatively related, and both were positively and nonlinearly related to resilience. Drought resistance increased with latitude, soil moisture and slope, whereas drought recovery decreased with latitude, soil moisture and summer normalized difference vegetation index. Drought resilience increased with elevation and decreased with the variation in soil moisture. Temperate broadleaf forests from wet regions showed a greater growth resistance (e.g. north-eastern USA, central Europe) while conifer forests from dry to semi-arid regions (e.g. south-western USA, southern Europe) presented a greater growth recovery. Main conclusionsThe geographical patterns of growth resilience indices confirm the existence of different strategies among forests to cope with droughts, depending on the biome, the tree species and the prevailing climatic conditions. Geographical patterns in soil moisture availability tend to override species-specific responses to drought.