Books

Responses of Northern U.S. Forests to Environmental Change
ISBN 0-387-98900-5

Chapter 11: Regional Impacts of Climate Change and Elevated CO₂ on Forest Productivity

Jennifer C. Jenkins, David W. Kicklighter, and John D. Aber

Changes in productivity of ecosystems in the Northeast in response to increasing atmospheric CO₂ and scenarios of climate change were estimated with two ecosystem process models, PnET-II and TEM 4.0. Models were used for this regional analysis because of complex interactions at the ecosystem scale between CO₂ response, biogeochemical cycles, and water and energy fluxes. The two models differ in their representation of above- and below-ground processes, in their mechanisms of response to CO2, and in their approaches to regional parameterization. To the extent possible, input data was consistent so that differences in how the two models represent ecosystem processes could be considered as an explanation of differences in results.

At the regional scale, both models predicted an increase in productivity under climate change scenarios, with PnET-II predicting an average increase of 37.9% and TEM 4.0 an average increase of 30.0%. There was a large difference in model predictions for different climate change scenarios, with both models tending to respond in a similar way. Estimates of NPP with PnET-II showed more sensitivity to different forest types. NPP responses in TEM 4.0 appear to be limited by temperature, while PnET-II appears to be limited by water. Differences in model structures and representations of response to increased CO₂ contributed to variability in predictions of future productivity. Neither model included some factors known or suspected to influence productivity, such as transient (as opposed to stepwise) climate change, nitrogen deposition, and past land use.

Below: Biome-level responses to individual climate variables from Hadley/ suplphate GCM (a, PnET-II; b, TEM 4.0). Within each forest type, each bar represents the mean +/- SD for one run with one Hadley/sulphate climate variable substituted for the equivalent VEMAP contemporary variable. "Altered" variables are from the Hadley/sulphate GCM; all others are from the VEMAP contemporary climate scenario, except for the "all combined" run, in which all of the Hadley/sulphate variables were applied at once.

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