Why Does Forest Growth Decline with Stand Age? An Experimental Test of Carbon Acquisition and Allocation Over Stand Development

9306356 Fownes为什么森林生长随着林龄而下降？林分发育过程中碳获取与分配的实验研究。这项研究的总体目标是了解平均年龄森林的生产生物学。平均年龄森林的生物量积累速率通常先增加，达到峰值，然后下降。传统的解释是，叶面积指数，因此总木本生物量，因此呼吸，继续增加，直到相对较少的光合作用为新生长留下。然而，这一命题尚未得到直接检验。最近的证据，无论是林分呼吸的直接估计还是生长效率的间接指数，都表明这种解释是不正确的。我们提出了三个假设的综合实验测试：(1)呼吸作用确实导致生长速度下降；(2)养分可用性随着时间的推移而减少，从而降低叶面积、光合作用或两者兼而有之；(3)这种模式是由净产量分配转移到不经常测量的成分（特别是根）引起的。这项研究的意义在于回答了森林生态系统生物学的一个基本问题。如果呼吸假说被驳倒，传统的林分生长动态解释将被推翻。了解林分生长和碳分配的控制对预测热带森林对气候变化的响应至关重要。该研究的实际意义在于制定一个综合的人工林管理方法，并回答诸如如何优化植物密度/轮作长度/收获大小的关系，以及轮作后期施肥是否可能有益等问题。正在认真考虑在试验点周围地区开展商业桉树种植工业，其结果将很容易用于规划和管理。

9306356 Fownes Why does forest growth decline with stand age? An experimental test of carbon acquisition and allocation over stand development. the overall goal of this research is to understand the production biology of even-aged forests. Biomass accumulation rates of even-aged forests typically increase at first, peak, and then decline. The conventional explanation has been that leaf area index, and thus gross while woody biomass, and thus respiration, continues to increase until relatively little photosynthate remains for new growth. However, this proposition has not been tested directly. Recent evidence, both direct estimates of stand respiration and indirect indices of growth efficiency, suggest that this explanation cannot be correct. We propose an integrated experimental test of three hypotheses: (1) that respiration does account for declining growth rate, (2) that nutrient availability decreases over time, thereby lowering leaf area, photosynthesis, or both, or (3) that the pattern is caused by shifting allocation of net production to infrequently-measured components, particularly roots. %%% The significance of the research lies in answering one of the fundamental questions in forest ecosystem biology. If the respiration hypothesis is refuted, conventional explanations of stand growth dynamics will be overthrown. Understanding the controls of stand growth and carbon allocation will be crucial for predicting the response of tropical forest to changing climate. The practical significance of the research will be in formulating a comprehensive approach to plantation management, and in answering such questions as how to optimize the plant-density/ rotation-length/ harvest-size relationship, and whether late-rotation fertilization is likely to be beneficial. The region around the experimental site is receiving serious consideration for a commercial Eucalyptus plantation industry, and results will be readily adopted into planning and management.