The nutrition of oak and beech trees along a tree diversity gradient

Louvain-La-Neuve

September 25, 2019

16:30

Louvain-la-Neuve

Salle Jean-Baptiste Carnoy

Tree nutrition is deteriorating in many parts of Europe. Nutrient imbalances, caused by changing environmental conditions or site conditions, can negatively affect productivity and tree vigour. Mixing species forests could possibly mitigate these effects by influencing tree nutrition. We therefore investigate foliar nutrition of sessile oak and beech, surrounded by one to three other tree species in mature broadleaved temperate forests in the Belgian Ardennes. Neighbourhood was determined by measuring basal area, crown projection area and litter proportions of all trees in a fixed radius around selected target trees. Using mixed model comparison, we observed that diversity had a positive effect on foliar nutrient concentrations in oak. Compared to oak, beech showed limited neighbourhood effects but some effects of tree size and stand density. The strongest diversity effects were observed when diversity was expressed through the litter composition around the target tree. We tested the same hypothesis in a plantation of beech and sycamore maple, planted along a double gradient of density and species proportion. Positive effects of tree size on foliar nutrient content were observed, mostly stronger in beech. The presence of beech reduced intraspecific competition in maple, resulting in a positive effect on the foliar nutrient content.

Due to the strong effects of litter composition on foliar nutrient concentrations in the observational network, we further investigated the role of litter in this process. We used a leaf dispersal model to determine leaf shedding parameters and litter production of the four species present in the mixture, in order to see how they affect litter composition and nutrient return. Using ion-exchange resins, we then measured the integrated nutrient availability in the soil originating from different sources (not only nutrient return). We used mixed model comparison to look at the relation between these available nutrients, litter quality and composition and foliar nutrient concentrations.

Neighbourhood will probably have an effect on the nutrition of target trees, but these effects are obscured by the large number of processes and pathways potentially involved. In order to isolate the driving mechanisms, we developed a piecewise structural equation modelling. This approach proved useful since it could study the interactions between the different variables simultaneously with their effects on foliar nutrient concentrations. Direct effects of neighbourhood remained limited, indicating that its effect is probably distributed over different pathways. Although dependent on target species and the studied nutrient, neighbourhood mostly showed indirect effects through litter diversity, nutrient return and their combined effect on nutrient availability in the soil.

Generally it can be concluded that mixing had a positive effect on foliar nutrition, indicating its potential as a measure to mitigate nutrient imbalances. For practical purposes, tree size will be of key importance for tree nutrition in the early stages of stand development. The addition of accompanying species, with beneficial traits regarding nutrient return, could be of use to improve the nutrient balance in nutrient-poor forest ecosystems.