Oak Decline as Illustrated Through Plant-Climate Interactions Near the Northern Edge of Species Range

This paper investigates historical growth and climate records among the oak sites representing the northern edge of species range in northernmost Europe (Finland). This is to characterize plant-climate interactions for a multitude of sites where oak decline has recently been observed and understand this most recent decline in the context of the past decline studies elsewhere. Further, our paper demonstrates the procedures the tree-ring data can be used in isolating those factors significantly contributing to decline. Our findings point towards complex tree mortality dynamics. Compared to oaks that remain healthy, the declining and dead oaks represent the trees clearly having suffered from competition and edaphic position within their site. This was indicated by their reduced growth rates and more drastic growth disturbance, with indications of reduced resilience. Growth of these trees was also deteriorated by cold soil temperatures during the dormancy in addition to summer droughts. By contrast, the growth of healthy oaks has been notably ameliorated by springtime soil warming over the past decades. The results demonstrate the climatic determinants for observed decline in the northern oak sites, which may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, although their habitats are not normally considered water-limited.