Abstract

Abstract Single species forest systems often suffer from low resistance and resilience to perturbations. Consequently, fostering tree species diversity is discussed as an important management approach to address the impacts of changing climate and disturbance regimes. Yet, the effect of the spatial grain of tree species mixtures remains unknown. We asked whether increasing tree species diversity between stands (beta diversity) has the same effect as increasing tree species diversity within stands (alpha diversity) at similar overall levels of richness (gamma diversity). We conducted a multi‐model simulation experiment under climate change, applying two forest landscape models (iLand and LandClim) across two contrasting landscapes of Central Europe. We analysed the effect of different levels and configurations of diversity on the disturbance impact and the temporal stability of biomass stocks and forest structure. In general, increasing levels of diversity decreased disturbance impacts. Positive diversity effects increased with increasing severity of climate change. Beta diversity buffered disturbance impacts on landscape‐level biomass stocks more strongly than alpha diversity. The effects of the spatial configuration on forest structure were more variable. Diversity effects on temporal stability were less pronounced compared to disturbance impacts, and mixture within and between stands had comparable effects on temporal stability. Diversity effects were context‐dependent, with patterns varying between landscapes and indicators. Furthermore, we found a strong species identity effect, with increasing diversity being particularly beneficial in conifer‐dominated systems of the European Alps. The two models agreed on the effects of different levels and configurations of tree species diversity, underlining the robustness of our findings. Synthesis and application. Enhancing tree species diversity can buffer forest ecosystems against increasing levels of perturbation. Mixing tree species between stands is at least as effective as mixing tree species within stands. Given the managerial advantages of between‐stand mixtures (e.g. reduced need to control competition to maintain diversity, higher timber quality, lower logistic effort), we conclude that forest management should consider enhancing diversity at multiple spatial scales. ​

Highlights

  • Global change increases the pressure on forest ecosystems through changed environmental conditions, which, in turn, alter ecological processes substantially (Trumbore et al, 2015)

  • We focused on the response of above-­ground forest biomass and the abundance of large trees, two indicators tightly linked to ecosystem service supply in the two study regions

  • We investigated the temporal stability in these indicators, as this aspect of ecosystem service provisioning is gaining importance in practical forest management (Albrich et al, 2018)

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Summary

| INTRODUCTION

Global change increases the pressure on forest ecosystems through changed environmental conditions, which, in turn, alter ecological processes substantially (Trumbore et al, 2015). Diverse traits increase the probability of a positive response to disturbances, thereby enhancing the speed of recovery and rendering the ecosystem more resilient (Mori et al, 2013) Building on these theoretical considerations, a number of quantitative studies have shown that tree species diversity increases the resistance of forest ecosystems to disturbances (see the reviews of Jactel et al, 2017; Knoke et al, 2008). We conducted a simulation experiment applying two well-­established forest landscape models (i.e. iLand and LandClim) in two contrasting forest landscapes of Central Europe, aiming to study the effects of tree species diversity at different spatial scales across a wide environmental gradient. Does tree species diversity at the landscape scale (gamma diversity) reduce disturbance impacts and increase the temporal stability of biomass stocks and the abundance of large trees under climate change in Central Europe? If local processes are the main driver of positive diversity effects, we would expect alpha diversity to yield higher positive effects than beta diversity at a given level of gamma diversity

| MATERIALS AND METHODS
Findings
| DISCUSSION
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