Urbanization and Fragmentation Mediate Temperate Forest Carbon Cycle Response to Climate

Abstract:

Forest fragmentation is ubiquitous across urban and rural areas. While there is mounting evidence that forest fragmentation alters the terrestrial carbon cycle, the extent to which differences in ambient growing conditions between urban and rural landscapes mediate forest response to fragmentation and climate remains unexamined. This study integrates field measurements of forest structure, growth rates, and soil respiration with climate data and high-resolution land-cover maps to quantify forest carbon storage and sequestration patterns along edge-to-interior transects. These data were used to contrast the response of temperate broadleaf forests to non-forest edges within rural and urban landscapes. We find that forest growth rates in both rural and urban landscapes nearly double from the forest interior to edge. Additionally, these edge-induced enhancements in forest growth are not offset by concurrent increases in soil respiration observed across our sites. We also find that forest productivity generally increases near edges because of increases in leaf area, but elevated air temperature at the edge tempers this response and imparts greater sensitivity of forest growth to heat. In particular, the adverse impacts of heat on forest growth are two to three times larger in urban than rural landscapes. We demonstrate that the highly fragmented nature of urban forests compared to rural forests makes them a stronger carbon sink per unit area, highlighting the outsized role that remnant forests play in developed landscapes. However, in light of our findings that both urbanization and fragmentation increase forest sensitivity to heat, warming climate conditions will likely have a disproportionately negative impact on rates of forest carbon sequestration across urban compared to rural landscapes. The important effects of fragmentation and urbanization on forest structure and carbon cycling shown here highlight the need to include the influences of both of these facets of land cover change when quantifying regional carbon balance and its response to a changing climate.