Diversity and forest productivity in a changing climate

Abstract

Contents Summary 50 I. Introduction 50 II. Drivers of the diversity-productivity relationship 51 III. Patterns of the diversity-productivity relationship 55 IV. Responses of mixed stands to climate change 57 V. Conclusions 60 Acknowledgements 61 References 61 SUMMARY: Although the relationship between species diversity and biomass productivity has been extensively studied in grasslands, the impact of tree species diversity on forest productivity, as well as the main drivers of this relationship, are still under discussion. It is widely accepted that the magnitude of the relationship between tree diversity and forest stand productivity is context specific and depends on environmental conditions, but the underlying mechanisms of this relationship are still not fully understood. Competition reduction and facilitation have been identified as key mechanisms driving the diversity-productivity relationship. However, contrasting results have been reported with respect to the extent to which competition reduction and facilitation determine the diversity-productivity relationship. They appear to depend on regional climate, soil fertility, functional diversity of the tree species involved, and developmental stage of the forest. The purpose of this review is to summarize current knowledge and to suggest a conceptual framework to explain the various processes leading to higher productivity of species-rich forests compared with average yields of their respective monocultures. This framework provides three pathways for possible development of the diversity-productivity relationship under a changing climate.

Tree performance in a biodiversity enrichment experiment in an oil palm landscape

“…The positive effect of tree diversity was, apart from the six‐species plots, not permanent, an indication that the effect was due to a selection rather than a complementarity effect (Loreau & Hector, ). Furthermore, inter‐specific competition was not lower than con‐specific competition, which contrasts previous studies in a Bornean dipterocarp rainforest (Stoll & Newbery, ) and in a temperate forest (Zhao et al, ) where local neighbourhood diversity reduced competitive pressure in line with classical niche differentiation theory (see review in Ammer, ). Here, it is more likely that competitive dominance (Potvin & Dutilleul, ; Van der Peer et al, ) related to selection effects was the main mechanism at play.…”

“…The slight interacting effect of tree diversity with elevation on tree growth is in accordance with previous findings stating that BEF relationships may vary with site conditions (Mina, Huber, Forrester, Thürig, & Rohner, ; Van der Peer et al, ; Zhang, Chen, & Taylor, ). However, the mechanisms driving these interactions remain poorly understood (Ammer, ). One potential explanation is that complementarity in water resource utilization by different tree species is more pronounced under limited soil water availability (Schwendenmann, Pendall, Sanchez‐bragado, Kunert, & Hölscher, ).…”

“…The planting density was relatively low in our experimental plots (compared to 1.5 m × 1.5 m in Van der Peer et al, , for example) and canopy closure did not fully occur. Therefore, competition reduction and facilitation driven by above‐ and below‐ground interactions among trees (see review in Ammer, ) might take longer to arise.…”

“…Plot‐level analyses are considered useful to assess overyielding (and underyielding), a widely used concept in BEF research to compare productivity across stands (Ammer, ). However, local neighbourhoods predominantly drive the response of tree productivity to diversity and can inform about the BEF relationship at plot level (Chen et al, ; Fichtner et al, ; Potvin & Dutilleul, ).…”

“…Such BEF‐effects have been linked to mechanisms such as niche differentiation, increased resource use efficiency due to interactions between species (“complementarity effects”) or selection and dominance of species with particular traits in mixtures (“selection effects”; Loreau & Hector, ). Analysing the effects of tree diversity on growth and survival at different levels (tree individual, species, neighbourhood, plot) and over time, accounting for site‐specific conditions, is needed to understand mechanisms underlying BEF relationships and thereby advance forest landscape restoration (Ammer, ; Paquette et al, ).…”