Tamm review: Drought-induced Scots pine mortality – trends, contributing factors, and mechanisms

A recent increase in the intensity and frequency of climate extremes under global warming is likely to continue to cause unprecedented rates of forest dieback in different habitats around the world. As one of the most widely distributed tree species, Scots pine (Pinus sylvestris L.) has experienced widespread mortality over the past two decades and many of those forests transitioned to broadleaved dominance inducing massive changes in ecosystem functioning and services. Here, we synthesize the factors and processes underlying drought-induced Scots pine mortality. Our review identifies agreement across studies on the impact of drought, prevalence of crown defoliation prior to mortality, influence of stand density and ecological growth memory, as well as the presence of biotic agents, such as insects and mistletoes. Studies generally agree that drought triggered self-thinning plays an important role. The post-mortality stand density seems far below the comparable pre-drought numbers of trees per hectare, which indicates a significant reduction in the productivity of forest stands. Most surprisingly, we show while Scots pine mortality in the early-2000 s occurred at the species' arid distribution limits, high mortality rates are now also reported from the species' climatic optimum where growth conditions used to be more beneficial. Extreme droughts such as 2003, 2015 and 2018 are causing this observed pattern, which may indicate that an increase in frequency of extreme drought could challenge Scots pine trees growing in climatically favorable areas. Our review indicates that tree level acclimation strategies such as lowering leaf area and enhancing water-use efficiency as well as stand-level adjustments including self-thinning are apparently not sufficient to prevent Scots pine mortality induced by frequent extreme droughts and associated impact of biotic agents (insects and mistletoes). However, we still lack clear understanding in linking functional strategies of the species to local climatic variation to fully understand the capabilities of the species to grow and survive in the future climate.