Effects of neighbourhood identity and diversity on the foliar nutrition of sessile oak and beech

Nutrient imbalances, caused by changing environmental conditions or site conditions, can negatively affect productivity and tree vigour. Mixing could possibly mitigate these effects by influencing foliar nutrition. Therefore we investigated foliar nutrition of sessile oak and beech surrounded by one to three other tree species in broadleaved temperate forests in the Belgian Ardennes. Neighbourhood was determined by measuring basal area, crown projection area and litter proportions for 82 oak trees and 44 beech trees. Using linear mixed models, the hypothesis was tested whether neighbourhood identity (i.e. the presence of a particular species) or diversity (i.e. the number of species present) had a positive effect on foliar nutrition. We found that there was a general positive effect of mixing, although it proved difficult to completely separate the identity effect from the diversity effect. In oak, litter affected Ca, K and P the strongest. Aboveground interactions were also involved, especially for Mg and both ratios (N/P and N/Mg), and for Ca and P in beech. The effects on N nutrition turned out to be difficult to separate from other sources, which also influenced the observed effects for the ratios. Generally it can be concluded that mixing had a positive effect on foliar nutrition, indicating its potential as a measure to mitigate nutrient imbalances, especially in oak stands.