The effect of urban proximity on mycorrhizal associations of three eastern hardwood tree species

Background/Question/Methods

Mycorrhizal 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 fungi are key in determining the outcome of competitive interactions between plants and shaping the plant community structure aboveground. One of the most common changes surrounding remnant vegetation is urbanization. Urbanization alters abiotic and biotic conditions but little is known about the impact of these factors on the mycorrhizal community.  In order to assess the effect of urbanization on the plant-mycorrhizal fungi relationship we investigated this interaction along an urban-to-rural gradient. We planted seedlings of three temperate tree species (Quercus rubra, Acer rubrum, and Carya ovata) in each of three landscape types: urban, suburban, and rural forest.  We measured the percent of root length of the seedlings colonized by ectomycorrhizal (ECM) and arbuscular mycorrhizal fungus (AMF) during the 2014 growing season. We analyzed the percent root length colonized by mycorrhizae, seedling survival, and growth as a function of landscape type (urban-rural) and explanatory variables known to contribute to seedling establishment success: soil moisture, soil nutrients, available light, and species of canopy tree under which we placed our plot. 

Results/Conclusions

All seedling species had higher survival in rural forests. Out of 810 seedlings planted, rural survival was 40-80% and urban survival was 10-48%. The large urban mortality was largely influenced by the increased herbivory in these areas. Plants tend to form associations with particular groups or species of mycorrhizal fungus.  Two of our seedling species, Carya ovata and Quercus rubra, are ECM associated species while Acer rubrum, is an AMF associated species.  For C. ovata and Q. rubra, ECM colonization remained high throughout the urban-rural gradient at nearly 80%.  Unexpectedly, C. ovata and Q. rubra seedlings also formed relationships with AMF, with the highest colonization under conspecific trees in rural areas and significant decreases in colonization percentages in suburban and urban forests under all canopy tree types.  A. rubrum was only colonized by AMF and showed a significant decrease in mycorrhizal colonization in urban areas.  Both groups of mycorrhizae were positively correlated with increasing soil moisture and iron.  Additionally, EMF colonization was negatively correlated with seedling height, suggesting that these fungal species tax the seedlings carbon resources significantly. These findings highlight the resilience of EMF communities across the rural-urban gradient and the sensitivity of AMF fungal species to urban environments.