Effects of Urbanization and Fragmentation on Atmospheric Nitrogen Deposition, Nitrogen Oxides, and Ozone in New England Forests

Jenna Rindy1, Erin Pierce2, Sarah Garvey3, Jonathan Gewirtzman4, Lucy Hutyra5 and Pamela Templer6

In the northeastern U.S., rates of atmospheric nitrogen deposition are almost an order of magnitude greater than pre-industrial levels and are twice as high in urban compared to rural areas. Fragmentation of forests in this region is increasing as a result of urbanization. Previous studies show greater rates of atmospheric deposition at forest edges compared to forest interiors — a consequence of increased exposure of edge trees to atmospheric inputs, known as an edge effect. As the prevalence of fragmented forests increase throughout this region, it is important to understand how fluxes of atmospherically deposited and ambient pollutants affect nutrient cycling in forested ecosystems. In order to understand if edge effects are intensified due to urbanization, we quantified rates of nitrogen deposition and ambient concentrations of nitrogen oxides (NOx) and ozone (O3) across the forest edge to interior along an urbanization gradient that spanned 120 km from Boston to central Massachusetts. A transect established at each site extended from the forest edge (0 m) to 90 m into the forest interior, where three throughfall collectors and two Ogawa passive samplers for gasses were placed at each distance (n=6 throughfall and n=4 passive samplers per site). Additionally, three bulk throughfall collectors were placed in open fields nearby to each forest site. Across the years 2018 and 2019, we found a significant increase in the amount of nitrogen deposited at urban forest edges compared to interiors, while the pattern was not displayed in rural forests. Specifically, total nitrogen deposition at urban forest edges was 1.5-times that of urban interiors (5.0 ± 0.3 and 3.4 ± 0.3 kg N/ha/yr, respectively), while nitrogen deposition at rural forest edges was not significantly different from rural interiors — a finding which indicates an exacerbation of edge effects in urban areas. Additionally, concentrations of both NOx and O3 were two times higher in urban forests than rural forests, while edge and interior concentrations were only found to be significantly different in the urban sites. Results of this study provide insight into how forest fragmentation promotes increased atmospheric deposition and concentrations of ambient pollutants in both urban and rural forests and contributes to knowledge of how these pollutants are affected by both urbanization and fragmentation.

 

1Boston University, jer0275@bu.edu
2Boston University
3Boston University
4Boston University
5Boston University
6Boston University