Legacy effects of deer activity on the growth of tree seedlings in symbiosis with different types of mycorrhizal fungi are opposite due to changes in soil microbial properties
Ungulate activity can influence forest vegetation by causing legacy effects. Soil microbes should play important roles in this indirect effect, but their contributions are understudied. Mycorrhizal fungi and their associated tree seedlings differ in their preferences for soil N economy. This suggests that tree seedlings with different mycorrhizal associations may receive opposite legacy effects from changes in N cycling-related soil microbial activities caused by ungulate activities.  We tested this idea by conducting seedling surveys, pot experiments, and soil RNA analyses in fenced and control plots in a cool-temperate forest in Japan, where arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) trees are co-dominant and high densities of sika deer (Cervus nippon) are observed.  We found that indirect legacy effects of sika deer on tree seedling performances were indeed opposite, with effects positive for ECM seedlings (increased survival) but negative for AM seedlings (decreased growth). The former was likely driven by increased ECM fungal dominance in soils with sika deer legacies.  Our results reveal the benefit of considering mycorrhizal fungal characteristics when studying legacy effects in ecosystems comprising plants associated with differing groups of mycorrhizal fungi, and the potential role of soil microbes in the legacy effects of ungulate activity.