The Effect of Urbanization on the Mycorrhizal Associations and Survival of Three Species of Eastern Hardwoods

Abstract

Mycorrhizal fungi colonization can be a significant determinant of plant health and establishment success. By protecting roots from pathogens and increasing plant uptake of nutrients and water, mycorrhizal colonization can determine the outcome of competitive interactions between plants, thereby shaping plant community composition. Currently, in remnant forest patches, plants and their fungal symbionts are exposed to varied anthropomorphic effects related to the encroachment of metropolitan areas into rural landscapes. However, little is known about the impact of urbanization on the plant-mycorrhizal fungi association. To assess the effect of urbanization on mycorrhizal fungi root colonization and their role on seedling establishment, we investigated the relationship between mycorrhizal colonization of tree seedlings and seedling survival along an urbanization gradient typical of the mid-western region of the USA. We planted three species of temperate tree seedlings (Acer rubrum, Carya ovata, and Quercus rubra) in each of three landscape types: urban, suburban, and rural forests. We measured the percent of root length of the seedlings colonized by ectomycorrhizal (ECM) and arbuscular mycorrhizal fungi (AMF) and monitored seedling survival during their first growing season. We analyzed the percent root length colonized by mycorrhizae as a function of landscape type (urban-rural) and additional variables known to contribute mycorrhizal colonization (soil phosphorus, soil nitrogen, and initial plant height). We then analyzed seedling survival as a function of the degree of mycorrhizal fungi colonization associated with the landscape gradient and of additional environmental factors (available light and soil moisture). Within a species, we found no changes in levels of mycorrhizal fungi colonization across the urban landscape gradient. Environmental variables (light, soil moisture, soil nutrients) did not significantly vary along the urban gradient, and differences in these variables did not have a measureable effect upon mycorrhizal colonization or survival. Each seedling species had markedly different levels of colonization and responded differently to increasing levels of mycorrhizal colonization. For A. rubrum, survival was independent of mycorrhizal colonization, Q. rubra had a statistically non-significant rise in survival as colonization increased, and C. ovata had a significant positive survival response to more than 60 % colonization. These findings highlight the resilience of mycorrhizal communities across the rural-urban gradient and the potential sensitivity of some species to lower levels of mycorrhizal colonization.