9613350 Yoder Nitrogen is one of the most important elements controlling plant growth in both natural and agricultural ecosystems. Ecophysiology theory suggests that the distribution of nitrogen within a plant canopy should match the distribution of intercepted light; that is, there should be much more nitrogen near the top of the canopy (where light levels are highest) than in the more shaded areas near the bottom of the canopy. However, when theoretical predictions have been compared with actual plant canopies, the results have been mixed at best. Nitrogen distribution in most canopies does show the same general pattern as that predicted by theory. However, the mechanism by which nitrogen content changes differs between species, and the difference in nitrogen levels from the top to the bottom of the canopy is rarely as large as the theory predicts that it should be. The investigators have developed a hypothesis that explains this variation in mechanismsand also suggests some reasons why the variation in nitrogen does notalways match the pattern of light distribution, especially in older leaves. They propose to test this hypothesis in three different western conifer species: Douglas-fir (Pseudotsuga menziesii), the most important timber species in the Pacific Northwest; Pacific silver fir (Abies amabilis) an important tree of high altitude forests; and ponderosa pine (Pinus ponderosa), the dominant tree in dry forests throughout the western United States. If the hypothesis proves correct, it will provide a better understanding of the organizing principles for shoot structure and function within complex canopies, and will assist in efforts to "scale up" gas fluxes from leaves to canopies--a critical component of attempts to model and predict the consequences of climate change in western conifer forests.
