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

Neglecting tree size and stand structure dynamics might bias the interpretation of the diversity-productivity relationship in forests. Here, we show evidence that complementarity is contingent on tree size across large-scale climatic gradients in Europe.

We compiled growth data of the 14 most dominant tree species in 32,628 permanent plots covering boreal, temperate and Mediterranean forest biomes. We used data from the Inventory Platform of FunDiv EUROPE. Niche complementarity is expected to result in significant growth increments of trees surrounded by a larger proportion of functionally dissimilar neighbours. Functional dissimilarity at the tree level was assessed using four functional types: i.e. broad-leaved deciduous, broad-leaved evergreen, needle-leaved deciduous and needle-leaved evergreen.

Using Linear Mixed Models we show that, 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 (coldest-temperate areas) were stronger for small than for large trees; (ii) in contrast, at the upper end of the gradient (warmer regions), complementarity is more widespread in larger than smaller trees, which in turn showed negative growth responses to increased functional dissimilarity (see Figure below). Our findings suggest that the outcome of species mixing on stand productivity might critically depend on individual size distribution structure along gradients of environmental variation.

Figure. Predicted tree growth as function of potential evapotranspiration and the proportion of other functional types for small (a) and large trees (b) (basal area 0.008 m2 and 0.35 m2 respectively) keeping stand basal area in the mean value (25 m2 ha−1). Plots on the right represent predicted growth relative to growth in monoculture (%) for small (c) and large trees (d) keeping stand basal area in the mean value (25 m2 ha−1). Red arrows are indicative of the main trends of tree growth along low, intermediate and high PET values (Figure 2 in Madrigal-González et al. 2016)

Reference

Madrigal-González, J., Ruiz-Benito, P., Ratcliffe, S., Calatayud, J., Kändler, G., Lehtonen, A., Dahlgren, J., Wirth, C. & Zavala, M.A. (2016) Complementarity effects on tree growth are contingent on tree size and climatic conditions across Europe. Scientific Reports, 6, 32233.

 

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