Occurrence but not intensity of mortality rises towards the climatic trailing edge of tree species ranges in European forests

Abstract

Aim: Tree mortality is increasing world-wide, leading to changes in forest composi-tion and altering global biodiversity. Nonetheless, owing to the multifaceted stochas-tic nature of tree mortality, large-scale spatial patterns of mortality across species ranges and their underlying drivers remain difficult to understand. Our main goal was to describe the geographical patterns and drivers of the occurrence of mortality (presence of a mortality event) and the intensity of tree mortality (amount of mortal-ity related to that mortality event) in Europe. We hypothesized that the occurrence of mortality represents background mortality and is higher in the margin than in core populations, whereas the intensity of mortality could have a more even distribution according to the spatial and temporal stochasticity of die-off events.Location: Europe (Spain, France, Germany, Belgium, Sweden and Finland).Major taxa studied: More than 1.5 million trees belonging to 20 major forest tree species.Methods: We developed binomial and truncated negative binomial models to tease apart the occurrence and intensity of tree mortality in National Forest Inventory plots at the range-wide scale. The occurrence of mortality indicated that at least one tree had died in the plot, whereas the intensity of mortality referred to the number of dead trees per plot.Results: The highest occurrence of mortality was found in peripheral regions and the climatic trailing edge linked with drought, whereas the intensity of mortality was driven by competition, drought and high temperatures and was scattered uniformly across species ranges.Main conclusions: We show that tree background mortality, but not die-off, is gener-ally higher in the trailing-edge populations. It remains to be explored whether other demographic traits, such as growth, reproduction and regeneration, also decrease at the trailing edge of European tree populations.