Erika Cohen

Robbing Peter to Pay Paul: Tradeoffs between Ecosystem Carbon Sequestration and Water Yield

The United States National Forest System supplies much of the nations drinking water. However, changes in climate, land use and population are stressing the ability of these forests to provide that ecosystem service. Federal land managers are under increasing pressure to increase ecosystem carbon sequestration in an attempt to partially offset greenhouse gases and slow global warming. Unfortunately, the positive relationship between carbon gain and water use in forests, puts the need for water and increased carbon gain at odds with each other. To assess these tradeoffs, a coupled water supply and demand, carbon sequestration, and biodiversity (WaSSI-CB) model was developed. WaSSI-CB was designed to be run with climate, population, and land use change scenarios to examine the interactions between water, carbon gain and biodiversity change across the 2,100 USGS 8 digit USGS Hydrologic Unit Code watersheds that span the lower 48 US. Results from this model using historic climate and landuse data indicated that the greatest increases in water use conservation may be made through improved irrigation practices, that manipulations in forest cover (i.e., massive harvesting) are an impractical way of increasing water supply, and the that the southeastern US has the highest potential for forest carbon sequestration. Biodiversity was calculated under steady state, historic conditions, with the greatest and mammal biodiversity occurring the southern US. The impact of future climate and population change were not included in this paper due to space limitations, but will be presented at the conference.

Comparison among model estimates of critical loads of acidic deposition using different sources and scales of input data

The critical load (CL) of acidic atmospheric deposition represents the load of acidity deposited from the atmosphere to the earths surface at which harmful acidification effects on sensitive biological receptors are thought to occur. In this study, the CL for forest soils was estimated for 27 watersheds throughout the United States using a steady-state mass balance approach based on both national and site-specific data and using different approaches for estimating base cation weathering. Results suggested that the scale and source of input data can have large effects on the calculated CL and that the most important parameter in the steady-state model used to estimate CL is base cation weathering. These results suggest that the data and approach used to estimate weathering must be robust if the calculated CL is to be useful for its intended purpose.

Impact of air pollution induced climate change on water availability and ecosystem productivity in the conterminous United States

Air pollution from greenhouse gases and atmospheric aerosols are the major driving force of climate change that directly alters the terrestrial hydrological cycle and ecosystem functions. However, most current Global Climate Models (GCMs) use prescribed chemical concentrations of limited species; they do not explicitly simulate the time-varying concentrations of trace gases and aerosols and their impacts on climate change. This study investigates the individual and combined impacts of climate change and air pollution on water availability and ecosystem productivity over the conterminous US (CONUS). An ecohydrological model is driven by multiple regional climate scenarios with and without taking into account the impacts of air pollutants on the climate system. The results indicate that regional chemistry-climate feedbacks may largely offset the future warming and wetting trends predicted by GCMs without considering air pollution at the CONUS scale. Consequently, the interactions of air pollution and climate change are expected to significantly reduce water availability by the middle of twenty-first century. On the other hand, the combined impact of climate change and air pollution on ecosystem productivity is less pronounced, but there may still be notable declines in eastern and central regions. The results suggest that air pollution could aggravate regional climate change impacts on water shortage. We conclude that air pollution plays an important role in affecting climate and thus ecohydrological processes. Overlooking the impact of air pollution may cause evident overestimation of future water availability and ecosystem productivity.

Assessment of wildland fire impacts on watershed annual water yield: Analytical framework and case studies in the United States

Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service, Raleigh, North Carolina 27606, USA Oak Ridge Institute for Science and Education, U.S. Department of Energy, Oak Ridge, Tennessee 37830, USA Coweeta Hydrologic Laboratory, Southern Research Station, U.S. Department of Agriculture Forest Service, Otto, North Carolina 28763, USA Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service, Asheville, North Carolina 28804, USA Center for Forest Disturbance Science, Southern Research Station, U.S. Department of Agriculture Forest Service, Athens, Georgia 30602, USA Oak Ridge National Laboratory, U.S. Department of Energy, Grand Rapids, Minnesota 55744, USA Correspondence Dennis W. Hallema, Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service, 920 Main Campus Dr. Suite 300, Raleigh, NC 27606, USA. Email: dwhallem@ncsu.edu Funding information Joint Fire Science Program, U.S. Department of Agriculture Forest Service Southern Research Station.

Divergence of ecosystem services in U.S. National Forests and Grasslands under a changing climate

The 170 National Forests and Grasslands (NFs) in the conterminous United States are public lands that provide important ecosystem services such as clean water and timber supply to the American people. This study investigates the potential impacts of climate change on two key ecosystem functions (i.e., water yield and ecosystem productivity) using the most recent climate projections derived from 20 Global Climate Models (GCMs) of the Coupled Model Intercomparison Project phase 5 (CMIP5). We find that future climate change may result in a significant reduction in water yield but an increase in ecosystem productivity in NFs. On average, gross ecosystem productivity is projected to increase by 76 ~ 229 g C m−2 yr−1 (8% ~ 24%) while water yield is projected to decrease by 18 ~ 31 mm yr−1 (4% ~ 7%) by 2100 as a result of the combination of increased air temperature (+1.8 ~ +5.2 °C) and precipitation (+17 ~ +51 mm yr−1). The notable divergence in ecosystem services of water supply and carbon sequestration is expected to intensify under higher greenhouse gas emission and associated climate change in the future, posing greater challenges to managing NFs for both ecosystem services.

Climate Change and Fire impacts on Ecosystem Critical Nitrogen Load

The federal agencies of the United States (US) are currently developing guidelines for critical nitrogen loads for US forest ecosystems. These guidelines will be used to establish regulations designed to maintain nitrogen inputs below the level shown to damage forests and streams. Traditionally, an ecosystem is considered to be at risk for health impairment when the critical nitrogen load exceeds a level known to impair forest health. The excess over the critical nitrogen load is termed the exceedance, and the larger the exceedance, the greater the risk of ecosystem damage. This definition of critical nitrogen load applies to a single, long-term pollutant exposure. Unfortunately, a single critical nitrogen load level may not accurately reflect ecosystem health risk when an ecosystem is subjected to multiple environmental stresses. In other US regions, fire is a major component of the forest ecosystem. Fire volatilizes organic nitrogen, reduces plant nitrogen uptake, increases nitrogen mineralization and nitrification, and can change the pH level of surface horizons. If multiple stress impacts (i.e., drought and fire) are included in critical nitrogen load assessments, critical nitrogen load may need to be lowered in many areas. This paper explores how fire and climate change and variability influence ecosystem critical nitrogen loads.