Timing and duration of drought modulate tree growth response in pure and mixed stands of Scots pine and Norway spruce

Climate change is increasing the severity and frequency of droughts around the globe, leading to tree mortality that reduces production and provision of other ecosystem services. Recent studies show that growth of mixed stands may be more resilient to drought than pure stands. The two most economically important and widely distributed tree species in Europe are Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris L.), but little is known about their susceptibility to drought when coexist. This paper analyses the resilience (resistance, recovery rate and recovery time) at individual-tree level using a network of tree-ring collections from 22 sites along a climatic gradient from central Europe to Scandinavia. We aimed to identify differences in growth following drought between the two species and between mixed and pure stands, and how environmental variables (climate, topography and site location) and tree characteristics influence them. We found that both the timing and duration of drought drive the different responses between species and compositions. Norway spruce showed higher vulnerability to summer drought, with both lower resistance and a longer recovery time than Scots pine. Mixtures provided higher drought resistance for both species compared to pure stands, but the benefit decreases with the duration of the drought. Especially climate sensitive and old trees in climatically marginal sites were more affected by drought stress. Synthesis. Promoting Scots pine and mixed forests is a promising strategy for adapting European forests to climate change. However, if future droughts become longer, the advantage of mixed stands could disappear which would be especially negative for Norway spruce.

Automated summary

Climate change is causing more severe and frequent droughts, leading to tree mortality and reduction in ecosystem services. Mixed stands are more resilient to drought compared to pure stands, and there is limited knowledge on the drought susceptibility of Norway spruce and Scots pine when coexisting. Norway spruce is more vulnerable to summer drought with lower resistance and longer recovery time than Scots pine. Mixed stands provide higher drought resistance for both species, but the advantage decreases with longer drought duration. Climate-sensitive and old trees in marginal sites are more affected by drought stress. Promoting Scots pine and mixed forests is a promising strategy for adapting to climate change.