Stand dieback and collapse in a temperate forest and its impact on forest structure and biodiversity

•There is global concern about forest dieback which may result in forest loss.•We examine a forest undergoing dieback using a dataset collected over 50 years.•Basal area declined, with a transition to a grass-dominated system in some areas.•The decline may have been driven by interactions between drought and overgrazing.•Resilience of the forest is low and management is needed to improve the situation.

Concern is increasing about large-scale dieback that is occurring in many forest ecosystems. However, understanding of the processes of dieback and its potential impacts is limited, partly owing to the lack of long-term monitoring data for forest stands in which dieback has occurred. Here we present monitoring data collected over 50 years along two transects in a temperate forest ecosystem, in which the canopy dominant beech (Fagus sylvatica L.) has demonstrated significant dieback. Our results show that basal area in the forest has declined by 33%, and juvenile tree densities have also been reduced by approximately 70%. Growing season temperatures have steadily increased and there have been a number of droughts causing climatic water deficits in recent decades, particularly in 1995. We hypothesise that these droughts may have interacted with novel pathogenic fungi to cause mortality of large trees. Curvilinear responses to BA loss were observed in tree community change, ground flora species richness, and percentage cover of grass, providing evidence of thresholds associated with stand dieback. Evidence also suggested that BA failed to recover once it declined. Critical values of basal area for a change in ground flora species richness and grass cover were around 40% decline from initial values. While these changes are dramatic, they cannot be considered a regime shift as the pressures that may have contributed to the ecosystem transition, drought, pathogenic fungi and overgrazing, are on-going. While managers might consider accepting forest dieback as part of an adaptive response of the system to novel environmental conditions, this would likely be associated with significant change in biodiversity and ecosystem service provision.