Assistant Professor Harvard University, United States
Abstract: Nearly 85% of plants depend on root-colonizing mycorrhizal fungi for key functions such as nutrient and water acquisition, making mycorrhizal partnerships important drivers of plant productivity and nutrient cycles. Through these functions, mycorrhizal fungi mediate forest ecosystem dynamics and can drive patterns of forest composition and plant growth. There are two dominant mycorrhizal functional types- arbuscular (AM) and ectomycorrhizal (EcM)- that greatly influence plant-plant interactions and provide us insight into the soil microbial processes that regulate aboveground tree growth. Neighborhood interactions provide us a framework to examine how tree density and proximity regulates plant growth through competition for common resources (i.e., light and soil nutrients). Often, the competitive dynamics between trees are assessed without regard for the mycorrhizae with which the trees associate, but there is strong reason to believe that these mycorrhizae may impact tree-tree neighborhood interactions. There are two contrasting possibilities for how mycorrhizal associations might mediate neighborhood interactions. The Niche Similarity hypothesis predicts that species with overlapping niches (e.g. having similar mycorrhizal partners) should have stronger negative effects on each other. The Mycorrhizal Associated Nutrient Economy (MANE) hypothesis suggests that each mycorrhizal functional type will create self-reinforcing nutrient economies, which may lead to stronger negative effects between trees of contrasting mycorrhizal types. We tested these hypotheses using two censuses of data from a 16-ha ForestGEO plot located at Harvard Forest in Petersham, MA, USA. We examined the growth responses of trees to neighborhood crowding (NCI) from conspecific, heterospecific, con-mycorrhizal (AM-AM and EcM-EcM interactions), and hetero-mycorrhizal neighboring trees. Growth of AM trees was more negatively affected by the total amount of crowding than growth of EcM trees. After controlling for the total amount of crowding, AM trees were unaffected by how much of their crowding came from conspecific or con-mycorrhizal species. However, growth of EcM tree species was significantly reduced by crowing from conspecific trees and even more negatively affected by crowding from con-mycorrhizal trees. These results show that AM species are particularly sensitive to crowding from neighboring trees and that EcM trees grow fastest when surrounded by AM neighbors. Together these data suggest that the competitive dynamics of EcM trees, in particular, are more regulated by Niche Similarity than by self-reinforcing mycorrhizal associated nutrient economies (MANE hypothesis).
