Mycorrhization, root tip vitality and biomass of Fagus sylvatica, Picea abies and Pseudotsuga menziesii in monospecific and mixed combinations under water reduction and nitrogen addition

In mixed combinations, the negative impact of water reduction and N addition is mitigated for F. sylvatica, but not for conifers. Insight into the responses of trees in mixed and monospecific cultivation to water scarcity and nitrogen (N) excess is necessary to recommend suitable tree mixtures for future European forests. Our aim was to investigate the impact of water reduction, N addition or water reduction + N addition in comparison with control (well-watered, no excess N) on mycorrhizal roots and biomass of three temperate forest species (Fagus sylvatica Fs, Picea abies Pa, and Pseudotsuga menziesii Pm) grown in monospecific (FsFs, PaPa, PmPm) and mixed combinations (PaFs, PmFs). We predicted the probability of the responses with a Bayesian approach. Mycorrhizal colonization declined marginally for P. menziesii in PmFs under water reduction. Under water reduction and N addition combined, we found a decline in root tip vitality of F. sylvatica in PaFs and of P. abies in PaPa. F. sylvatica shoot biomass declined under water reduction + N addition in the monospecific combination. P. abies and P. menziesii had lower root biomass in mixture with F. sylvatica under water reduction + N addition and under water reduction as single factor, respectively. With water reduction and N addition, P. menziesii performed better than P. abies in terms of biomass production. F. sylvatica had a growth advantage in mixture with conifers instead of conspecifics. These findings suggest that young trees show rapid and interspecific responses to species mixing and water + N availability. The selection of suitable tree species should, therefore, consider their interactive responses to changing abiotic factors.