Steven G. McNulty

Interactive effects of air pollution and climate change on forest ecosystems in the United States: current understanding and future scenarios

A review of the current status of air pollution and climate change (CC) in the United States from a perspective of their impacts on forest ecosystems is provided. Ambient ozone (O3) and nitrogen (N) deposition have important and widespread ecological impacts in U.S. forests. Effects of sulphurous (S) air pollutants and other trace pollutants have significant ecological importance only at much smaller geographic scales. Complex interactive effects of air pollution and CC for selected future CC scenarios are reviewed. In addition, simulations of past, present, and future hydrologic, nutrient, and growth changes caused by interactive effects of air pollution and CC are described for two U.S. forest ecosystems. Impacts of O3, N deposition, and CC on growth and hydrology of mixed conifer forests in the San Bernardino Mountains in southern California were projected with the DayCent model. Effects of N deposition, CO2 fertilization, N deposition, and CC on northern hardwood forests at the Hubbard Brook Experimental Forest in New Hampshire were simulated with the PnET-BGC model. Projected changes in these forests can influence the provision of ecosystem services such as C sequestration and water supply. The extent of these effects will vary depending on the future intensity and extent of CC, air pollutant emission levels, the distribution of air pollution, and other factors such as drought, pest outbreaks, fire, etc. Our chapter ends with research and management recommendations intended to increase our ability to cope with uncertainties related to the future interactive effects of multiple air pollutants, atmospheric deposition, CC, and other biotic and abiotic stressors.

Surface storm flow prediction on hillslopes based on topography and hydrologic connectivity

BackgroundHillslopes provide critical watershed ecosystem services such as soil erosion control and storm flow regulation through collecting, storing, and releasing rain water. During intense rainstorms, rainfall intensity and infiltration capacity on the hillslope control Hortonian runoff while the topographic attributes of the hillslope (e.g., slope, aspect, curvature) and the channel network define the structural hydraulic connectivity that determines how rapidly excess water is transferred. This paper discusses literature on the link between topographic attributes and hydrologic connectivity and demonstrates how this link can be used to define a parsimonious model for predicting surface runoff during high intensity rainfall.Main textFirst, we provide a topographic characterization of the hillslope necessary to determine the structural hydrologic connectivity of surface flow based on existing literature. Subsequently, we demonstrate a hydrologic surface response model that routes the geomorphologic unit hydrograph (GIUH) through a spatial domain of representative elementary hillslopes reflecting the structural hydrologic connectivity. Topographic attributes impact flow and travel time distributions by affecting gravitational acceleration of overland flow and channel, solar irradiance, flow deceleration by vegetation, and flow divergence/convergence.ConclusionsWe show with an example where we apply the GIUH-based model to hypothetical hillslopes that the spatial organization of the channel network is critical in the flow and travel time distribution, and that topographic attributes are key in obtaining simple yet accurate representations of hydrologic connectivity. Parsimonious GIUH models of surface runoff that use this hydrologic connectivity have the advantage of low data requirements, being scalable and applicable regardless of the spatial complexity of the hillslope, and have the potential to fundamentally improve flood forecasting tools used in the assessment of ecosystem services.

Burned forests impact water supplies

Wildland fire impacts on surface freshwater resources have not previously been measured, nor factored into regional water management strategies. But, large wildland fires are increasing and raise concerns about fire impacts on potable water. Here we synthesize long-term records of wildland fire, climate, and river flow for 168 locations across the United States. We show that annual river flow changed in 32 locations, where more than 19% of the basin area was burned. Wildland fires enhanced annual river flow in the western regions with a warm temperate or humid continental climate. Wildland fires increased annual river flow most in the semi-arid Lower Colorado region, in spite of frequent droughts in this region. In contrast, prescribed burns in the subtropical Southeast did not significantly alter river flow. These extremely variable outcomes offer new insights into the potential role of wildfire and prescribed fire in regional water resource management, under a changing climate.Wildland fire seasons in the Unitedxa0States are getting longer, yet the impacts of fire on water availability at the regional scale are unclear. Here the authors show that fire increased annual river flow throughout the West, while prescribed burns in the subtropical Southeast had limited impact on river flow.