Xavier Morin, CEFE, CNRS, University of Montpellier, EPHE, IRD, Montpellier, France, discusses his article: More species, more trees: The role of tree packing in promoting forest productivity
The effect of tree species diversity on forest productivity is now well-known. On average, plots with more species (in mixed forests) have a greater net productivity than plots with a single species (monospecific forests). Such a pattern has been found through both empirical data from forest inventories and experimental designs. As a postdoc more than 10 years ago, I led a study in which we showed the same pattern with an original approach, by simulating diversity-productivity relationships (DPRs) in silico, i.e. relying on a forest dynamics model.
When we looked for the mechanisms underpinning our results, we found a relevant explanation in line with what was further identified as the canopy packing, a more efficient use of total canopy volume in mixed forests than in monospecific forests that leads to increased light interception at the ecosystem level. Yet, we were not completely satisfied because there seemed to be a relationship between the number of tree species in the plots and the number in the simulated communities. Did such a trend correspond to an actual pattern in real forests, or was it an artefact of the modelling approach? In fact, most hypotheses proposed to explain DPRs in forests had primarily focused on how the individual growth of trees is impacted by their immediate neighbourhood, whether it consists of one or multiple species. In doing so, the potential role of processes observable at the whole-stand level—such as changes in the maximum tree density within forests—has generally been overlooked.
First, we needed to check whether the link between species richness and stand density (i.e. the number of trees in a forest stand) was actually true. To do so, we assessed self-thinning lines with data from nearly 200,000 forest plots (containing 2,367,776 trees) from forest inventories in six European countries. The results clearly showed a strong positive effect of species richness on stand density.
Second, we needed to adapt a forest model to independently control stand density and species richness. We recently developed such a model and used it to simulate DPRs with or without density control across 1,015 sites in Europe – conducting over 7 million simulations. The analysis of these simulations revealed that the effect of the number of species on productivity could be up to 10 times stronger when stand density was allowed to vary according to species composition, compared to simulations with fixed density. Notably, this effect was more pronounced in the most extreme climates. Finally, we had the more comprehensive answer we were looking for more than a decade ago.
What are the implications of this study? First, it calls for new kinds of BEF experiments in forest ecosystems because most past experiments have compared monospecific and mixed plots with the same density. Second, while many European forest stands are monospecific with controlled densities (as is the case for half of the forest plots in France), increasing the cover of mixed forests is emerging as an effective strategy to reduce forest vulnerability to climate change. This study confirms that promoting species mixtures enhances forest productivity, and thus carbon sequestration capacity, to a magnitude far beyond previous estimates. Therefore, these findings open new perspectives for forest policies and strategies to combat climate change.
