Louvain-la-Neuve
Carnoy
Effects of tree species identity and mixing on below-ground resources availability and climate-growth relationships. A case study in mixtures of Scots pine and beech across Europe On average, increasing tree species diversity in forest stands results in higher productivity and stability of productivity, as well as in a larger range of ecosystem services; however, those effects are strongly context-dependent. In my PhD, I therefore explored some mechanisms possibly involved in species-mixing effects and looked at their dependency to spatial and temporal variations of environmental conditions. More specifically, I analyzed species-mixing effects on proxies of water and nutrient use, and on climate-growth relationships for Scots pine and European beech growing in pure and mixed-species stands (triplets) using a network of sites covering a large ecological gradient across Europe. This work was conducted along three axes. The first one aimed at analyzing the effects of species interactions on water related processes using wood carbon isotopic composition as an indicator of tree water use efficiency. I found that complementarity displayed a humped-shaped curve with maximal (positive) values for dry sites and close-to-zero values for extremely dry ones. In addition, species-mixing effects on non- or moderately constrained sites were strongly dependent on water- and possibly light-related species traits. No coherent species-mixing effects on drought resistance along the gradient was found for either species. The second axis aimed at analyzing pine and beech foliar nutrient composition in pure and mixed stands along a gradient of sites nutritional richness. Significant species-mixing effects on both foliar nutrient concentrations and stoichiometry were found. Interestingly, those effects tended to be higher on richer sites. Lastly, I analyzed the large-scale variability of species-mixing effects on average climate-growth relationships and drought resistance for pine and beech across Europe. The results highlighted the strong influence of regional climate on climate-growth relationship but coherent species-mixing effects were only found in case of overwhelming climatic constraint. While no species-mixing effects on drought response was found for beech, a significant effect was highlighted for pine, yet limited to situations where local climatic conditions allowed for compensatory growth to happen during the last part of the vegetation period. Building on our results, I was able to suggest improvements to an existing framework describing variations of species-mixing effects along gradient of environmental conditions, in particular by considering non-linear relationships. The results of this thesis also highlighted the importance of considering the interactions between processes, rather than considering them independently of each other