If one looks at hillslopes in mountainous landscapes, one will often see that patterns of vegetation seem to coincide with the orientation of the slopes. Sunny hillslopes facing south may have no trees, while shaded hillslopes facing north may be densely covered with trees. Similarly, in swales or small valleys, trees may appear taller and healthier. These patterns exist because of the effects of microclimate on plant growth. Microclimate is the variation in temperature, moisture, and sunlight which occurs in very local areas. While microclimate affects plant growth, the distribution and activity of plants in turn affects the larger climate system, by their release of water through transpiration. If we will ever be able to predict the effects of human disturbance to our climate, we must understand how these patterns of plant activity interact with climate.
We believe we understand the principles of forest growth well enough to be able to describe them using a computer model. This computer model can be used to predict the patterns of forest photosynthesis and transpiration over the landscape, and eventually how these patterns will effect and be affected by climate. For our study area in the mountainous terrain of western Montana (Figure 1) , the model calculates the responses of conifers such as pine and fir to the microclimate which develops on hillslopes. The model requires certain starting conditions, which we derive from satellite and aircraft data.
One of those starting conditions is the amount of leaf surface area within the forest canopies, or leaf area index (LAI) as shown in Figure 2 . Results of the model include daily evapotranspiration (the amount of water going into the atmosphere through evaporation and transpiration) and photosynthesis (the amount of carbon being taken in by the forests). Using the model is the only means available to acquire information for an entire region. This information is organized by hillslope, which can be defined at any scale. This is one of the first attempts to understand regional patterns of evapotranspiration and photosynthesis from forests.
USA Research Site: Montana
COLLABORATORS: School of Forestry, University of Montana
For more information on the RHESSys modeling project, click here.
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