Silver fir growth responses to drought depend on interactions between tree characteristics, soil and neighbourhood features

Mixed forests are expected to be more productive and resilient against disturbances than pure forests. However, tree to tree interactions are complex and vary depending on tree characteristics and multiple site features including soil properties. Such complexity is not normally accounted for in studies of mixing effects on tree response to drought. Here, we evaluate if neighbourhood tree identity influence soil biotic and abiotic characteristics in three mature, mixed Silver fir (Abies alba) forests. Further, we investigate the relationships between radial growth response to drought, tree characteristics (growth rate, size and competition pressure), tree neighbourhood, and soil physico-chemical and biological properties. Patterns of covariation between tree size, competition pressure, soil microbial composition, and growth recovery after drought were observed among different neighbourhood types. Slow-growing, small Silver fir trees experiencing high competitive pressure were associated with higher proportions of Scots pine (Pinus sylvestris) in the neighbourhood, high soil C:N ratios, and showed a faster growth recovery after drought. A neighbourhood dominated by broadleaf species (mainly European beech, Fagus sylvatica) was associated with high biomass of arbuscular mycorrhizal fungi, and low biomass of Gram-positive bacteria and Eukaryote, together with lower tree growth recovery. Therefore, coexistence with other Silver fir, Scots pine or European beech trees is associated to different covariation patterns of tree, soil and drought performance variables of the target Silver fir trees. Finally, our study remarks that combining tree, soil and neighbour variables at individual level helps to understand patterns of tree growth and growth response to drought under different stand mixtures