Drought induced decline could portend widespread pine mortality at the xeric ecotone in managed mediterranean pine-oak woodlands

• Predictive mortality models of one Mediterranean oak and one pine species.
• Describe variability in the pattern of mortality and decline along climatic gradients.
• Pine mortality and decline are partly related to enhanced drought.
• Climate and management interact to cause oak-pine decline.
• Pines at their xeric ecotone are threatened by global change.

There is a need to better understand how different biotic and abiotic factors interact to determine climate change enhanced tree mortality. Here, we investigated whether rising water stress determined enhanced Pinus sylvestris L. mortality at the species low-elevation limit in Central Spain. We analyzed the factors determining the health status of pines and compared with co-occurring and more drought-tolerant Quercus pyrenaica Willd. along one transect following an elevation gradient. We used ordinal logistic regression to model the susceptibility of a tree to decline in relation to variability in stand competition and individual growth-patterns. The mortality pattern differed with local site conditions. Pine growth was faster but life-span shorter at drier and warmer low-elevations than at high-elevations. However, within stands, healthy trees exhibited less abrupt growth reductions and higher growth-rates but not as a consequence of lower competition, which under present stand conditions did not seem to increase adult mortality risk. Low moisture availability reduced tree-growth and, although P. sylvestris is less tolerant to drought, Q. pyrenaica was more sensitive to year-to-year moisture variability. Previous growth of dead trees from both species declined with rising water stress after the 1970s at low-elevations, which suggests that water stress intensity limited particularly tree-growth of dead trees in the long-term. For pines, widespread symptoms of crown decline (expressed by mistletoe infestation and defoliation) were only observed at low-elevation stands where, in opposition to oaks, weakened and healthy pines also exhibited recent negative growth-trends parallel to those of dead trees. The pervasive growth decline with enhanced water stress in pines from all health status at the species sampled xeric ecotone combined with the abundant crown decline symptoms observed, suggest pine vulnerability and could portend widespread mortality at its current low-elevation limit.