Chapter 1
Chapter 1
Regional Projections of the Impacts of Future Urbanization and Climate Change on Biogeochemical Cycles in New England Landscapes
Human activities have had complex, long-term impacts on forest function across New England-a trend expected to continue. To assess these impacts, we conducted a regional-scale modeling study using the PnET-CN-daily model, simulating multiple scenarios that reflect projected changes in land cover, climate, and air quality. The results suggest that while New England will continue to serve as a regional carbon sink, carbon accumulation in the southern portion of the region will slow and may shift to a net carbon source due to aggressive urban expansion. Carbon dioxide fertilization and carbon loss associated with urbanization are the dominant factors controlling future carbon dynamics. However, carbon dioxide fertilization may diminish over time due to nutrient limitations, while rising temperatures are expected to accelerate soil decomposition, further increasing carbon loss. The forecasts also show that urbanization will increasingly affect ecosystem nitrogen storage. Climate change and carbon dioxide fertilization along with declining nitrogen deposition from decreases in fossil fuel use are projected to drive nitrogen oligotrophication-slowing forest growth and becoming more severe as nitrogen inputs decrease. In addition, urbanization and climate change are expected to substantially reduce snowpack and shorten snow cover duration in southern New England, with potential consequences for regional water dynamics. These trends highlight the need to integrate future climate, air quality, and land-use projections into forest management strategies for both urban and rural ecosystems.
Introduction
Introduction
The northeastern United States forest is an important biome in North America as it plays a critical role in the biogeochemical cycles of water, carbon, and nutrients for the region. However, the structure and function of northeastern forests have been substantially impacted by human activities for centuries. Land-use changes such as deforestation, agriculture, and urban expansion have directly reduced the forest extent. Satellite images reveal that New England lost approximately one hundred twenty-nine thousand hectares, two point eight percent, of forest cover between nineteen ninety and two thousand five.
Urbanization also aggravates air pollution and atmospheric deposition. Industrial activities, electricity generation, and vehicle emissions increase the emissions of air pollutants, which coupled with urban heat island effects have exacerbated air quality problems in urban areas. Higher carbon dioxide concentrations in urban landscapes compared with those in surrounding rural areas, a phenomenon known as the "urban carbon dioxide dome", has been reported globally. Nitrogen deposition is likewise elevated in urban regions due to concentrated anthropogenic emissions. In contrast, ozone exhibits a more complex spatial pattern. Higher emissions of volatile organic compounds associated with human activities and higher air temperature are likely to increase ozone formation, while high emissions of nitric oxide can consume ozone and decrease concentrations, resulting in a shorter ozone lifetime in urban areas. Consequently, the spatial distribution of ozone along an urbanization gradient is largely determined by the nitric oxide to nitrogen oxides emission rate. In areas with a high nitric oxide to nitrogen oxides ratio, ozone concentrations tend to decrease with increasing impervious surface area.
Urban environments, characterized by higher air temperatures and elevated concentrations of air pollutants, have complex effects on carbon, nitrogen, and water dynamics in ecosystems. Increasing air temperatures, elevated atmospheric carbon dioxide concentrations, and enhanced nitrogen deposition can stimulate plant growth. Elevated ozone concentrations damage pigments and photosynthetic enzymes, resulting in reduced forest productivity. Enhanced plant growth increases plant nitrogen demand, while higher temperatures accelerate soil decomposition, jointly decreasing soil nitrogen availability. Elevated nitrogen deposition in urban areas can partially offset those soil nitrogen losses.
Furthermore, water also regulates nitrogen cycling by serving as a medium for plant uptake and influencing soil decomposition rates. Higher temperatures increase the vapor pressure deficit and reduce winter snowpack, which can intensify soil water stress and potentially cause root damage during freeze-thaw events, ultimately altering ecosystem carbon and nitrogen dynamics. In contrast, elevated carbon dioxide concentrations reduce stomatal conductance, leading to an increase in water-use efficiency and a decrease in transpiration, which mitigates drought stress.
The influence of human activities on forest ecosystems in the northeastern United States is expected to intensify with continued urban expansion and climate change. According to United States Census Bureau data, the population in New England grew by one point two million, eight point six percent, between two thousand and twenty twenty, with most of this increase concentrated in Massachusetts, fifty-seven point one percent, followed by Connecticut, sixteen point eight percent, and New Hampshire, eleven point nine percent. The population is projected to continue increasing in the coming decades. Model projections suggest that the percentage of urban land in New England will increase from ten point five percent to eighteen percent by twenty fifty.
Long Term Ecological Research in the eastern United States has documented an average annual temperature increase by zero point one five to zero point two degrees Celsius per decade, along with an annual precipitation increase by eighteen to twenty-nine millimeters from nineteen fifty to twenty nineteen, with a more rapid change observed in recent years. Various modeling studies project a warmer and wetter future climate in New England, although the magnitude of projected increases in temperature and precipitation varies among studies.
In this study, we present a modeling framework to characterize and quantify how forests in the northeastern United States may respond to ongoing environmental changes by the mid-twenty-first century, with a particular focus on the complex interactions that occur in urbanized areas. We considered a series of changes in land cover, including an aggressive urbanization scenario driven by substantial population influx but limited urban planning, Growing Global, a scenario with a similar high population influx but improved urban planning to minimize impacts on surrounding natural areas, Yankee Cosmopolitan, a scenario in which population and urban expansion continue at the current rate, Recent Trends, and a reference scenario in which urbanization remains at current levels, Constant scenario. Details of scenario construction are provided in the methods section. Building on our previous work, which quantified changes in air pollutant concentrations and air temperature along an urbanization gradient and the future land-cover scenarios described above for New England, we also developed projections of future urban climate and air quality scenarios.
To project ecosystem responses to these future scenarios, we applied the new PnET-CN-daily model, an updated version of the well-established PnET-CN model that has been widely used for ecosystem simulations in the region for decades. The PnET-CN model requires relatively simple input data while simulating complex ecosystem processes, making it well suited for regional-scale applications. In PnET-CN-daily, the simulation time step was refined from monthly to daily to improve process resolution and model accuracy. Using this enhanced model, we projected changes in carbon, nitrogen, and water across New England under a range of future scenarios and additionally quantified the distinct contributions of climate, land cover, and atmospheric chemistry to these ecosystem dynamics.