Climate, but not trait, effects on plant-soil feedback depend on mycorrhizal type in temperate forests

Plants interact with a variety of soil biota. The accumulation of beneficial or antagonistic biota over time can have strong positive or negative effects on the recruitment of new individuals into the population (i.e., plant-soil feedback, PSF). Despite the potential influence of PSF on plant diversity in terrestrial ecosystems, our understanding of its determinants remains incomplete. Within forests, recent evidence indicates that mycorrhizal type can regulate PSF: Arbuscular mycorrhizal trees typically experience negative PSF, while ectomycorrhizal trees experience positive PSF. Other studies have found that plant traits and the local environment also affect PSF; yet, these studies are based on arbuscular mycorrhizal plants in herbaceous systems. Consequently, the relationship between PSF and plant traits or the environment may depend on the mycorrhizal type of the tree. Using PSF measurements for 520 populations belonging to 52 North American temperate forest tree species, we tested whether mycorrhizal type influences how plant traits and climate relate to PSF. For both mycorrhizal types, species that produced larger seeds and were less drought tolerant had more positive (or less negative) PSF. Precipitation during the dry season was negatively related to PSF, but only for arbuscular mycorrhizal trees. These results suggest that investing more resources in seeds may minimize PSF for tree species, irrespective of mycorrhizal type. Species that have historically been exposed to greater pathogen abundances in wetter environments may also be better defended against attack, minimizing negative PSF. However, for species currently found in wetter environments, PSF may be strongly negative if their mycorrhizas do not defend them against attack, as is common among arbuscular mycorrhizal trees. Combined, these results highlight the importance of mycorrhizal type in determining the relationship between PSF and climate, but indicate that the relationship between PSF and plant traits may be independent of mycorrhizal type. Nevertheless, mycorrhizal type remains an important consideration when studying PSF, and forest dynamics in general.