Interactions of Climate Change with Acidic Deposition in a Forest Watershed over the 21st Century Using a Dynamic Biogeochemical Model (PnET-BGC)

Afshin Pourmokhtarian1*, Charles T. Driscoll1; John L. Campbell2 and Katharine Hayhoe3

Climate is an important regulator of the hydrology and biogeochemistry of forest watersheds. The ecological responses to climate change have been assessed by observational, gradient, laboratory and field studies; however, models are the only practical approach to investigate how future changes in climate are likely to interact with other drivers of global change such as atmospheric deposition. Biogeochemical watershed models are an important tool to help to understand the long-term effects of climate change on ecosystems. In this study, we are using a biogeochemical model (PnET-BGC) coupled with long-term measurements to evaluate the effects of potential future changes in temperature, precipitation, solar radiation, atmospheric deposition and atmospheric CO2 on pools and fluxes of major elements at the Huntington Forest (HF) in New York. Future emissions scenarios were developed from monthly output from three atmosphere-ocean general circulation models (AOGCMs; HadCM3, PCM, GFDL) in conjunction with potential lower and upper bounds of projected atmospheric CO2 (550 and 970 ppm by 2099, respectively). We also evaluated the interaction of climate change with changes in acidic deposition. Estimates of atmospheric deposition are based on a “business-as-usual” deposition scenario. We compared the results of the “business as usual” scenario with two additional scenarios, which consider additional moderate and aggressive emission controls on sulfate and nitrate.

AOGCM results over the 21st century indicate an average increase in temperature ranging from 1.9 to 7.0°C with simultaneous increases in precipitation ranging from 5.0 to 22.3% above the long term mean (1970-1999). Long-term measurements and watershed modeling results show a significant shift in hydrology with earlier spring discharge (snowmelt), greater evapotranspiration and longer growing season (due to CO2 fertilization), and later snowpack development. Model results also show an increase in NO3- leaching over the second half of the century due to increases in net mineralization and nitrification. The extent of this response is dependent on the fertilization effect that increasing atmospheric CO2 has on forest vegetation. The watershed responses of other major elements such as SO42- and Ca2+, and chemical characteristics such as pH and ANC varied based on future climate and deposition scenarios. Model predictions showed that equivalent decreases in sulfate deposition were twice as effective as decreases in nitrate on recovery of soil and streamwater chemistry. Model projections also suggest marked decreases in soil exchangeable calcium, magnesium and potassium with simultaneous decline in soil base saturation and Ca/Al ratio over the next century.

1*Email: , Cell number: 315-877-3442, Office number: 315-443-3303, 151 Link Hall, Syracuse University, Syracuse, New York, 13244,
1 Dept. of Civil & Environmental Engineering, Syracuse University, Syracuse, NY 13244, U.S.A.,
2 US Forest Service, Northern Research Station, Durham, NH 03824, U.S.A.,
3 Dept. of Geosciences, Texas Tech University, Lubbock, TX 79409, U.S.A.