Overyielding in young tree plantations is driven by local complementarity and selection effects related to shade tolerance

Abstract

Overyielding in mixed‐species forests has been demonstrated in a vast body of literature, and the focus of functional biodiversity research is now shifting towards a mechanistic understanding of these observations. We explored diversity–productivity relationships at two sites of a large‐scale tree diversity experiment, with harsh (Ged) and benign (Zed) environmental conditions for plantation establishment. Additive partitioning methodologies were adopted to detect phenomenological patterns in the productivity data, and the trait structure of mixed communities was used to advance insights into compositional effects. After 6 years of plantation development, biomass productivity was significantly higher in mixtures compared to the monocultures of component species. We observed that processes operated through direct tree–tree interactions, as the diversity signal disappeared where trees in mixed stands were surrounded by conspecific neighbours only. This result is particularly relevant for mixed‐species plantation systems, as trees are commonly planted in monospecific patches to simplify management.Partitioning unveiled strong selection effects at both plantation sites. However, at the harsh Ged‐site this was caused by competitive dominance of species with fast young growth, whereas at the benign Zed‐site, species with slow young growth improved their performances but not at the expense of others (i.e. trait‐dependent complementarity). Species tolerance to shading is an influential trait for predicting biodiversity effects, with community‐weighted means in shade tolerance mediating dominance effects (Ged) and functional diversity in shade tolerance mediating (trait‐dependent) complementarity effects (Zed). Synthesis. This study highlights that biodiversity effects in young tree plantations could be explained by the functional composition of mixed communities, with a key role for species levels of shade tolerance. As contrasting results between plantation sites were observed, future research should target the context‐dependency of diversity–productivity relationships.

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

“…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. Because the planted tree species performed differently under the environmental conditions of the oil palm plantation, fast‐growing trees species with high establishment success ( P. canescens ) might have benefited at the expense of species with difficulties to establish ( S. leprosula and D. zibethinus ).…”

“…Because the planted tree species performed differently under the environmental conditions of the oil palm plantation, fast‐growing trees species with high establishment success ( P. canescens ) might have benefited at the expense of species with difficulties to establish ( S. leprosula and D. zibethinus ). They are, however, exceptions as less adapted species D. polyphylla grew better in multi‐species plots and well‐performing species A. pauciflorum grew better in single‐species plots, which might be related to specific traits such as shade tolerance (Van der Peer et al, ). 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, ).…”

“…They are, however, exceptions as less adapted species D. polyphylla grew better in multi‐species plots and well‐performing species A. pauciflorum grew better in single‐species plots, which might be related to specific traits such as shade tolerance (Van der Peer et al, ). 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, ). Besides the influence of site conditions, the predominance of selection over complementarity effects is common in young tree plantations (Grossman et al, ; Mori, ; Potvin & Dutilleul, ; Tobner et al, ; Van der Peer et al, ). 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.…”

“…In contrast to grassland experiments, however, tree‐based BEF‐experiments are still scarce, particularly in the tropics (Paquette et al, ). Results from these tree diversity experiments suggest that tree diversity often positively affects tree growth (Fichtner et al, ; Grossman et al, ; Huang et al, ; Paquette & Messier, ; Potvin & Gotelli, ; Van der Peer, Verheyen, Ponette, Setiawan, & Muys, ), and temporal stability of primary productivity (Hutchison, Gravel, Guichard, & Potvin, ; Morin, Fahse, Mazancourt, Scherer‐Lorenzen, & Bugmann, ). 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, ).…”