Background
Forests are a cornerstone of the Earth system, playing a vital role in regulating climate and sustaining ecosystems by storing carbon, cycling nutrients, and controlling water availability. In the northeastern United States, forests have long functioned as an important regional carbon sink while supporting biodiversity and water resources. However, centuries of human activity—particularly deforestation, air pollution, agriculture, and rapid urban expansion—have profoundly altered forest structure and function. Furthermore, urbanization causes a variety of environmental changes, including higher air temperatures, elevated atmospheric carbon dioxide concentrations, and increased air pollution, all of which can strongly influence forest carbon, nitrogen, and water dynamics. These urban effects do not occur in isolation; rather, they interact with ongoing global climate change and regional air pollution, compound stress on forest ecosystems and potentially reshaping their capacity to store carbon, retain nutrients, and regulate hydrology. Despite growing recognition of these interacting pressures, their combined long-term impacts on regional forest biogeochemistry remain poorly understood.

Research Progress
In this study, we developed a regional modeling framework to characterize and quantify how forests in the northeastern United States may respond to ongoing environmental change by the mid-21st century, with particular emphasis on the complex interactions occurring in urbanized landscapes. We examined multiple future land-cover scenarios representing different urbanization pathways.

To represent future climate conditions, we constructed a robust climate projection based on a weighted ensemble of 32 global climate models. We also simulated urban environmental conditions associated with different levels of urbanization, building on our previous work that quantified changes in air temperature and air pollutant concentrations along urbanization gradients in New England.

We then applied the PnET-CN-daily model, an advanced version of the widely used PnET ecosystem model, to simulate forest ecosystem responses under these combined scenarios. Using this framework, we projected future changes in carbon, nitrogen, and water dynamics across New England and quantified the relative contributions of climate change, land-cover change, and atmospheric chemistry to these ecosystem responses.

Our study indicates that forests across New England will continue to function as a regional carbon sink through the mid-21st century, but the strength and spatial distribution of this sink will vary substantially with future urbanization and climate change. While a warmer climate and elevated carbon dioxide concentrations generally enhance forest growth, expanding urban land cover reduces forest area and increasingly offsets these gains, particularly in southern New England. At the same time, continuous vegetation nitrogen uptake and declining atmospheric nitrogen deposition drive progressive nutrient limitation, constraining long-term forest productivity. Urbanization and climate change also strongly alter regional water dynamics, leading to widespread reductions in snowpack and snow cover duration and shifting patterns of evapotranspiration and drainage. Together, these results highlight how interacting changes in land use, climate, and air quality reshape forest carbon, nitrogen, and water cycles across New England landscapes.

Future Perspectives
We aim to apply this modeling framework to other regions, but important uncertainties remain, and further research is needed to refine future projections. A primary limitation is that air quality responses to urbanization are highly dependent on energy type and urbanization patterns. For example, the Beijing–Tianjin–Hebei region of China may experience dynamics very different from those observed in New England, owing to higher population density and greater emissions of nitrogen oxides and volatile organic compounds from coal combustion, whereas New England more relies on natural gas. In such regions, urbanization may substantially intensify air pollution, leading to stronger ozone damage to vegetation and suppressing photosynthesis and forest growth. Extending similar studies to other regions will require careful stand-level calibration and validation for dominant vegetation types to ensure reliable projections.

The complete study is accessible via DOI:10.34133/research.1043