Jennifer Moore Myers

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.

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.