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

氮是自然生态系统和农业生态系统中控制植物生长的重要元素之一。生态生理学理论认为，植物冠层内氮的分布应与截留光的分布相匹配；也就是说，靠近冠层顶部（光照水平最高的地方）的氮含量应该比靠近冠层底部的阴凉区域要多得多。然而，当理论预测与实际的植物冠层进行比较时，结果充其量是喜忧参半。氮在大多数冠层中的分布确实表现出与理论预测相同的一般模式。然而，不同物种间氮含量变化的机制是不同的，从冠层顶部到冠层底部的氮含量差异很少像理论预测的那样大。研究人员提出了一种假说来解释这种机制上的变化，并提出了氮的变化不总是与光分布模式相匹配的一些原因，尤其是在老叶子中。他们提议在三种不同的西部针叶树物种中验证这一假设：道格拉斯冷杉（menziesii），太平洋西北地区最重要的木材物种；太平洋银杉（Abies amabilis）是一种重要的高海拔森林树种；黄松（Pinus ponderosa）是美国西部干旱森林中的主要树种。如果这一假设被证明是正确的，它将更好地理解复杂冠层内嫩枝结构和功能的组织原理，并将有助于努力“扩大”从叶子到冠层的气体通量——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.