Drivers of nitrogen oligotrophication in temperate deciduous forests
Date
2025
DOI
Authors
Version
Embargo Date
2026-03-05
OA Version
Citation
Abstract
Declining nitrogen (N) availability relative to plant demand, known as N oligotrophication, is a widespread phenomenon in rural terrestrial ecosystems around the globe, and has been particularly well documented in the temperate forests of the northeastern U.S. My dissertation research provides insight into the drivers of N oligotrophication across urban and rural temperate deciduous forests. First, I examined the combined effects of urbanization and forest fragmentation on mixed temperate forests by measuring soil N cycling rates and foliar N concentrations along urban to rural and edge to interior gradients over two years. To accomplish this work, I worked in eight forested sites along an urbanization gradient from Boston, MA in eastern MA to Harvard Forest in central MA. I found that urban forests had higher net rates of ammonification and mineralization, as well as higher foliar N concentrations than rural forests, but that these differences were canceled out at the forest edge. Together these results indicate that urban forests are experiencing less N oligotrophication than nearby rural forests, but that differences in N supply are diminished by forest fragmentation. In my next chapter, I carried out a snowpack manipulation experiment to accelerate or delay spring snowmelt in the White Mountains of New Hampshire to reveal how shifting snowmelt timing affects soil N cycling, fine root production, and N utilization by trees. I found that fine root biomass, soil solution nitrate (NO3-), and net nitrification were lower under shallower snowpack and earlier snowmelt compared to ambient or delayed snowmelt conditions. In Acer saccharum trees, foliar N as well as natural abundance 15N values, a key indicator of N supply relative to demand, were lower when snow melted early snowmelt under shallow snowpack conditions, suggesting that accelerated snowmelt induced by shrinking snowpack decreases N availability relative to plant demand in snow adapted forests. Finally, I used a controlled litter manipulation experiment in a northern hardwood forest at Hubbard Brook to isolate the effects of litter quality (C:N ratio) on soil N cycling rates and fine root production and test the hypothesis that low quality litter drives a positive feedback loop that ultimately reduces available N. I found that rates of net ammonification and mineralization were positively associated with litter N content and negatively associated with litter C:N, particularly after two years under low quality litter. Through my research, I demonstrated that forest fragmentation and changing seasonality decrease N cycling rates, available N pools, and N uptake by trees, thereby widening the gap between N supply and demand in temperate forest ecosystems.
Description
2025
License
Attribution 4.0 International