Did stand opening 60 years ago predispose a European beech population to death?

Abstract

Widespread increases in tree mortality have fueled the debate on the mechanisms of heat- and drought-related tree death. While much research focused on the immediate causes of tree death, the role of predisposing factors for death is not well understood. We employed tree-ring and climate sensitivity analysis of growth to study the importance of legacy effects of past disturbances for growth patterns and a recent mortality burst in a protected European beech (Fagus sylvatica) forest, comparing groups of live and recently died trees in the same stand. Live and dead trees showed a decade-long decrease in radial growth since the severe 1976 drought, which indicates that the high mortality in the last two decades is not solely caused by recent drought spells, but that trees were apparently predisposed by previous events to death decades later. An even more distant event than the 1976 drought that has imprinted on the chronologies is the severe 1947 drought in combination with extensive logging at that time. It appears that stand opening 60 and 40 years ago due to high immediate drought mortality in conjunction with heavy logging resulted in much higher inter-annual ring width variability with more extreme negative and positive pointer years in the subsequent years, and led to permanent reduction in tree health. We identified the first derivative of the ring width curve as the most reliable early-warning signal of a predisposition to drought-induced death, while inter-annual growth variability and growth resistance and resilience to drought were less suited. We suggest that the causes of recent climate warming-related dieback of beech and other temperate hardwood forests may only be fully grasped, when the imprint of past stress events on growth and vitality is understood. The physiological mechanisms causing legacy effects with long delay of death require further study.