Findings
Phase 2 Transient Dynamics

VEMAP Findings

Background:

The Vegetation-Ecosystem Modeling and Analysis Project (VEMAP) was a large, collaborative, multi-agency program to simulate and understand ecosystem dynamics for the continental U.S. The project involved the development of common data sets for model input including a high-resolution topographically-adjusted climate history of the U.S. from 1895-1993 on a 0.5 grid, with soils and vegetation cover. The vegetation cover data set includes a detailed agricultural data base based on USDA statistics and remote sensing, as well as natural vegetation (also derived from satellite imagery) (selected animations of input climate).

Experimental Design:

Two principal model experiments were run. First, a series of ecosystem models were run from 1895 to 1993 to simulate current ecosystem biogeochemistry. Second, these same models were integrated forward using the output from two climate system models (CCC (Canadian Climate Centre) and Hadley Centre models) using climate results translated into the VEMAP grid and re-adjusted for high-resolution topography for the simulated period 1994-2100.

Findings:

The models agree fairly well for the mean fluxes in the late 20th century, suggesting a sink due to CO2 fertilization, climate, and agriculture of less than 0.1 gigatons of carbon (Gt C) per year. The models agree in the mean within about 25%, although they differ somewhat in their simulation of interannual variability. The models agree in simulating a high sensitivity to drought, which they predict should release carbon to the atmosphere (Schimel et al., 2000 [Abstract]). The estimated value for the CO2 fertilization sink, 0.08 Gt C per year, is a small fraction of the sink actually estimated for the U.S. using direct observations (0.3-0.7) and suggests that changes in land use practices, including agricultural abandonment and fire suppression, dominate the sink in the U.S. The models provide an interesting view of the future. Three of the models used do not include any disturbance (e.g., fire or harvest) practices. They agree reasonably well in suggesting a steady sink from CO2 fertilization interacting with climate change (This figure shows results from the VEMAP models, showing the historical (1895-1993) and future climate scenario results.).Three of the models include disturbance. In two models, LPJ (Lund-Potsdam-Jena) and MC1 (MAPSS-Century 1), disturbance is included via a prognostic fire model. In the third, Century, the model is forced by assumed fire and harvest return frequencies. All three models suggest a much lower accumulation of carbon because of chronic losses due to disturbance. One model (MC1) suggests that climate changes in the mid-century could trigger large scale fires with substantial losses of carbon. The other is less sensitive to the mid-century climate but shows losses accelerating in the late 21st century. Clearly, the role of disturbance and land management must be the priority for the next round of model development, testing, and applications.

Key Publications


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