Complementarity effects on tree growth are contingent on tree size and climatic conditions across Europe

Abstract

Neglecting tree size and stand structure dynamics might bias the interpretation of the diversityproductivityrelationship in forests. Here we show evidence that complementarity is contingent ontree size across large-scale climatic gradients in Europe. We compiled growth data of the 14 mostdominant tree species in 32,628 permanent plots covering boreal, temperate and Mediterraneanforest biomes. Niche complementarity is expected to result in significant growth increments of treessurrounded by a larger proportion of functionally dissimilar neighbours. Functional dissimilarity atthe tree level was assessed using four functional types: i.e. broad-leaved deciduous, broad-leavedevergreen, needle-leaved deciduous and needle-leaved evergreen. Using Linear Mixed Models we showthat, complementarity effects depend on tree size along an energy availability gradient across Europe.Specifically: (i) complementarity effects at low and intermediate positions of the gradient (coldesttemperateareas) were stronger for small than for large trees; (ii) in contrast, at the upper end of thegradient (warmer regions), complementarity is more widespread in larger than smaller trees, whichin turn showed negative growth responses to increased functional dissimilarity. Our findings suggestthat the outcome of species mixing on stand productivity might critically depend on individual sizedistribution structure along gradients of environmental variation.