Effects of air quality, urbanization, and fragmentation on aboveground carbon storage of temperate forest ecosystems
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Abstract
Urbanization has diminished intact forest cover worldwide, leaving behind fragmented forests. Temperate forest fragments encounter unique stressors and enhancements at forest edges, including increased temperature, light, and nitrogen (N) emissions. Anthropogenic additions of reactive N to the atmosphere and enhanced atmospheric deposition to the biosphere have dramatically altered the N cycle. While rates of N emissions and deposition have recently decreased in the form of nitrate in rural areas across the U.S., they remain elevated in urban centers. High rates of oxidized N (NOx) emissions to the atmosphere can negatively impact vegetation through the production of tropospheric ozone, which damages plant tissues and reduces the capacity for carbon (C) sequestration. Although elevated N deposition and tropospheric ozone have been studied individually, much less is known about their combined effects on C storage in trees, especially in urban ecosystems. To examine the combined effects of urbanization and air quality on C storage in vegetation, we selected seven sites along a 120 km rural to urban gradient across Massachusetts. At all sites, we established a 90 meter transect from forest edge to interior to evaluate within-site forest edge effects and urbanization effects across the entire gradient on C and N dynamics. To assess air quality, we measured concentrations of ozone and NOx using Ogawa passive samplers and measured N deposition using mixed ion exchange resin collectors under the forest canopy. To characterize standing C stocks, we measured tree diameter and scaled these values to aboveground biomass using allometric equations. Results demonstrate that standing biomass C is higher at the forest edge than interior in rural areas, but that urban areas do not have differences between edge and interior biomass. Concentrations of ozone and NOx are higher at urban than rural sites and at the forest edge compared to forest interior. Rates of total atmospheric N inputs in throughfall are not significantly greater in urban than rural sites, but nitrate inputs in throughfall at forest edges are higher in urban areas. Our study suggests that edge enhancements of biomass C are present in rural areas, but that diminished air quality may suppress potential stimulatory effects of forest edges in urban areas. This work builds upon our understanding of the quantities and spatial heterogeneity of air pollutants in the greater Boston area to better understand their consequences for tree health and the terrestrial carbon sink. As carbon dioxide concentrations continue to rise, plants will continue to play an important role in removing carbon dioxide from the atmosphere through photosynthesis and plant growth; investigating their response to additional environmental factors such as elevated N deposition and tropospheric ozone is essential to understanding vegetation and global carbon dynamics.