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

Abstract

AimTree mortality is increasing worldwide, leading to changes in forest composition and altering global biodiversity. Yet, due to the multi-faceted stochastic nature of tree mortality, large-scale spatial patterns of mortality across species ranges and their underlying drivers remain difficult to understand. Our main goal is to describe the geographical patterns and drivers of the occurrence and intensity of tree mortality in Europe. We hypothesize that the occurrence of mortality represents background mortality and is higher in the margin than the core populations, whereas the intensity of mortality could have a more even distribution according to the spatial and temporal stochasticity of die-off events. LocationEurope (Spain, France, Germany, Belgium, Sweden and Finland) Time period1981 to 2014. Major taxa studiedMore than 1.5 million trees belonging to 20 major forest tree species MethodsWe develop hurdle models to tease apart the occurrence and intensity of tree mortality in National Forest Inventory plots at range-wide scale. The occurrence of mortality indicates that at least one tree has died in the plot and the intensity of mortality refers to the number of trees dead per plot. ResultsThe highest mortality occurrence 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 uniformly scattered across species ranges. Main conclusionsOur findings provide a new perspective in our understanding of tree mortality across species ranges. We show that tree background mortality but not die-off is generally higher in the trailing edge populations, but whether other demographic traits such as growth, reproduction and regeneration would also decrease at the trailing edge of European tree populations needs to be explored.