Larger investment in roots in southern range-edge populations of Scots pine is associated with increased growth and seedling resistance to extreme drought in response to simulated climate change

Extreme climate events such as heat waves or severe drought have the potential to fundamentally alter species dynamics, and are expected to increase in frequency in the coming decades. Early life stages of plant species are highly sensitive to these variations, with the potential for profound consequences for plant communities. In order to explore the response of Scots pine (Pinus sylvestris L.) seedlings to predicted increases in temperature and extreme drought, we performed a controlled-conditions experiment using growth chambers to simulate (1) current and future temperatures expected at the southernmost edge of the range, (2) current and drier growing seasons before an acute summer drought, and (3) drought alleviation by sporadic rainy events during summer. We analysed the response of seedlings by assessing survival, growth, biomass allocation and isotopic discrimination. Southern range-edge seedlings were compared under same conditions with those from the northern range edge, which experience cooler and wetter conditions throughout the year. The combination of extreme drought and the predicted temperature rise severely reduced overall survival for both provenances. A reduction in precipitation before the onset of drought diminished the survival and final biomass of seedlings, while the interruption of summer drought drastically increased survival probabilities. Southern seedlings invested a higher proportion of their biomass in roots, which conferred on them higher growth, higher survival probabilities, better nutritional status and lower drought stress. These results help us to understand the mechanisms of local adaptation at the southern range edge and indicate southern populations as a valuable genetic resource to buffer the response of Scots pine against such extreme climatic events