Collaborative Research: Distribution of Photosynthetic Capacity in Conifer Canopies: The Role of Shoot Geometry, Leaf Morphology and Nitrogen Concentration

9613350在自然和农业生态系统中，氮素是控制植物生长的最重要的元素之一。生态生理学理论认为，植物冠层内氮素的分布应该与截获的光的分布相匹配；也就是说，冠层顶部(光照水平最高的地方)附近的氮素含量应该比冠层底部附近较暗的区域多得多。然而，当理论预测与实际植物冠层进行比较时，结果充其量是喜忧参半。氮素在大多数冠层中的分布与理论预测的大体一致。然而，不同物种氮含量变化的机制不同，从树冠顶部到树冠底部的氮素水平差异很少像理论预测的那样大。研究人员提出了一个假说，解释了这种机制上的变化，并提出了一些原因，为什么氮的变化并不总是与光的分布模式相匹配，特别是在老叶中。他们建议在三种不同的西部针叶树种上验证这一假设：太平洋西北部最重要的木材树种--道格拉斯-冷杉；高海拔森林的重要树种--太平洋银杉；以及美国西部干燥森林的优势树种--黄皮松。如果这一假设被证明是正确的，它将提供对复杂树冠内树枝结构和功能的组织原则的更好理解，并将有助于“放大”从树叶到树冠的气体通量--这是试图模拟和预测西部针叶林气候变化后果的关键组成部分。

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.