Document: JER-3-99-159

A phenological model of global C₃ and C₄ grass distribution with application for climatic change under a VEMAP elevated CO₂ scenario.

WINSLOW, J.C.* ¹, E.R.HUNT, JR. ² and S.C.PIPER ³

University of Montana, Missoula MT 59812 USA ¹
USDA ARS RSML, Beltsville, MD 20705 USA ²
University of California-San Diego, La Jolla, CA 92093 USA ³

Abstract:
Atmosphere/biosphere interactions differ between C₃ and C₄ ecosystems. Therefore, the relative abundance of C₃ and C₄ vegetation must be known to characterize ecosystem exchange processes. Since climatic change and increased atmospheric CO₂ are hypothesized to change this relative abundance, inquiries into climatic-change effects on biospheric processes must also address changes in the vegetation distribution. Our model, based upon the timing of the active growing periods for each type of grass and the water available during those growing periods, can extrapolate, based upon climate, the grass distribution determined by available surveys. When applied to 14 years of global weather data our model successfully estimated the surveyed grass distribution at over 140 points in North America, Argentina, Australia, South Africa, Kenya, and Egypt. The model correctly classified the surveyed grass as all C₃, mixed C₃-₄ and all C₄ grasses at 76% of the compared points. In the mixed region, the model produced an R² = 0.54 with a Mean Absolute Error of 0.19 when the predicted C₃ / C₄ grass biomass distribution was compared to the surveys. To examine the effects of a changing climate, we applied the model to the VEMAP historical climate data and a plausible climate change scenario for the U.S. Our model predicted northward movement of C₄ grasses in the great plains consistent with the effects of a warmer climate. However, when we introduced the hypothesized increase in photosynthesis and a decrease in stomatal conductance which are expected to occur under elevated CO₂, the effects of a warmer climate were partially, but not completely, ameliorated and our model predicted a decrease in the relative amounts of C₃ biomass.

Keywords: atmosphere,biosphere,interactions

This abstract is being presented at: 10:30 AM in session:
Poster Session #18: Elevated CO₂.