PROCEEDINGS: Index of Abstracts

MODELING IMPACTS OF CO₂, OZONE, AND CLIMATE CHANGE ON TREE GROWTH

George E. Host-1, Gary W. Theseira-1, and J. G. Isebrands-2

1-Research Associate and Postdoctoral Research Associate, respectively, Natural Resources Research Institute, University of Minnesota, Duluth, MN 55811. 2-Project Leader and Research Plant Physiologist, USDA Forest Service, North Central Forest Experiment Station, Rhinelander, WI 54501.

Understanding the influence of ozone, CO₂, and changing climatic regimes on basic plant physiological processes is essential for predicting the response of forest ecosystems. To understand the relationships among these interacting factors, in the face of genetic and other environmental variability, requires a means of synthesis. Physiological process modeling provides one such tool: it allows the integration of diverse information from research, reflects the interactions among variables, and provides a direction for future research.

To model trace gas effects on aspen, we have adapted an existing growth process model for poplar known as ECOPHYS. ECOPHYS is a mechanistic whole-tree model that simulates growth of poplar in its establishment year (Host et al. 1990a, Isebrands et al. 1989, Rauscher et al. 1990). ECOPHYS uses the individual leaf as the primary biological unit of the model. Hourly solar radiation, temperature, and clonal (genetic) factors acting at the leaf level provide the major driving variables for plant growth. Canopy architecture is modeled by means of a three-dimensional geometric approach. By knowing leaf orientation patterns and tracking solar position over the course of the day, we calculate precise estimates of intercepted radiation, which in turn are supplied to a photosynthate production submodel. Photosynthates are distributed to various growth centers in the plant by means of a radiotracer-based model of carbon allocation (Dickson 1986). The amount of photosynthate arriving at a growth center, after respiratory losses are determined, is used to calculate biomass production and dimensional growth. The model has been subjected to extensive validations both in terms of photosynthesis (Host et al. 1990) and in regional predictions of biomass production (Host and Isebrands 1994).

Our current research has three major facets: the development of three-dimensional soil and root models to complement the existing above-ground portion of the model, the integration of existing trace gas response data into the model framework, and the scaling of the existing model in time and space; specifically to simulate the growth of an interacting population of trees for a number of years. These objectives will allow us to simulate impacts related to global change, and to provide input to models operating at larger temporal and spatial scales.