Competition, tree age and size drive the productivity of mixed forests of pedunculate oak, beech and red oak

In order to understand the relationship between tree diversity and forest productivity, it is crucial to understand what is going on at the level of individual trees, since each tree responds to the prevailing environmental conditions and interacts with his neighbours. We explored tree interactions in 53 forest plots in northern Belgium (TREEWEB platform) containing all possible combinations of pedunculate oak, beech and red oak. We cored 307 trees and determined the average annual basal area increment for the last 10 years for each sampled tree. We investigated the importance of neighbourhood competition, tree age an size for tree-level growth and then scaled up the species-specific tree growth to stand-level productivity. By exploring how competition, tree age and size varied along the studied gradient of tree species richness, we were able to explain the observed species-specific tree level and stand-level growth along the tree species richness gradient. We found no relationship between tree species richness and forest productivity, due to contrasting growth patterns of the three species along the richness gradient: the growth of pedunculate oak decreased, growth of beech was constant and growth of red oak increased along the tree species richness gradient. Increased heterospecific competition was the main reason for the observed lower growth of pedunculate oak in mixtures, whereas variation in tree age and size along the studied tree species richness gradient were the main drivers for the observed growth patterns of beech and red oak. Species-specific age differences along the tree species richness gradient could be linked with the species composition. We observed a multi-aged stand structure with the light-demanding pedunculate oak being older than the more shade-tolerant beech and red oak, probably a consequence of management interventions needed to enable long-term coexistence of light-demanding and shade-tolerant species. Our study highlights that the management history of mature forests may influence diversity-productivity relationships and should therefore be taken into account during the analysis.