Climate consequences of temperate forest conversion to open pasture or silvopasture

Land use change, particularly the conversion of forest to agriculture, is an important driver of climate change but is rarely considered a major factor in northern temperate regions. This study explored the climate impacts of temperate forest clearing for agriculture in the northeastern United States, hypothesizing that compared to managed cool-season pasture, forest conversion to silvopasture would moderate changes in surface temperature, soil greenhouse fluxes, and soil carbon and nutrient losses. To test this hypothesis, we conducted two land use change experiments, one in New York and the other in New Hampshire, USA. In both locations, forests have regenerated following a period of forest clearing, intensive agricultural management, and farm abandonment, such that secondary forests currently comprise ~60–80% of the land base. In both experiments, we measured microclimatic variables of air and soil temperature and soil moisture, soil greenhouse gas emissions of CO2 and N2O, and soil carbon and nitrogen content. Using a mixed-effects modeling framework, we found that near-surface air and soil temperatures were highest in converted pasture plots, lowest in reference secondary forest areas, and intermediate in the converted silvopasture treatment. Soil respiration followed a similar pattern, with elevated soil CO2 fluxes in open pastures as compared to converted silvopasture and reference secondary forest sites. Although we detected few changes in soil C among treatments, we observed higher soil N stocks and soil N2O fluxes in converted pastures but not in the converted silvopasture treatment. Our results suggest that silvopasture may offer a biogeochemical “middle ground” between intact secondary forests and managed open fields, retaining the climate benefits of forests while enabling expansion of the agricultural land base. Understanding the climate impacts of forest conversion to open pasture or silvopasture is critical to better anticipate the climate consequences of potentially re-emerging agricultural land uses, both across the northeastern US and other temperate forest regions globally.