Determinants of the reproductive yield of competing plants in a stand

研究了CO_2浓度升高对加拿大苍耳繁殖产量的影响。虽然随着CO_2浓度的升高，植株生长和总生殖产量（种子加蒴果）增加，但种子产量没有增加。通过对植物生长过程中氮素利用的分析发现，高CO_2浓度下，由于叶片寿命的延长和低温的影响，氮素从叶片向种子的再转运受到抑制。生长季后期气温较低，降低了转运量。因此，种子生产受到限制的N的可用性。我们开发了一种新的方法来分离的影响，从邻居的阴影，并将其应用于建立不同密度的加拿大苍耳林分的自遮荫。我们估计了（1）林分中植物的遮荫强度和（2）孤立于林分的植物的自遮荫。邻居的遮荫估计为（1）减去（2）。结果表明，林分中个体间的相互作用较强，且随着密度的增加而增强，通过对种子生产中个体大小与数量的权衡分析，提出了一个库限制生长模型，该模型假设了维持呼吸作用导致的资源损失，限制了对母株资源的吸收速率。该模型预测，种子大小不会随着种子数量的减少而增加。这是因为使用储存物质的效率随着种子数量的减少而降低，因此有利于以种子尺寸为代价增加种子数量。最佳种子大小小于Smith和Fretwell预测的。

We studied the effect of CO_2 elevation on the reproductive yield of Xanthium canadense. Although the plant growth and total reproductive yield (seeds plus capsules) increased with CO_2 elevation, seed production did not increase. Analysing N use in plant growth, we found retranslocation of N from leaves to seeds was retarded at high CO_2 due to increased longevity of leaves and low temperature. Translocation was lowered by lower air temperature at later growing season. Thus seed production was constrained by the availability of N.We developed a new method to separate the effect of self-shading from that of shading from neighbors and applied it to stands of Xanthium canadense established with different densities. We estimated (1) the intensity of shading for plants in the stand and (2) self-shading for plants isolated from the stand. Shading from neighbors was estimated as a subtraction of (2) from (1). We found that the interaction among individuals was strong in a stand and that the intensity increased with increasing plant densities.From an analysis of trade-off between size and number in seed production, we proposed a sink-limiting growth model, where contraints in the absorption rate of resources from mother plants were assumed with a loss of resources due to maintenance respiration. This model predicts that seed size does not increase reciprocally to the reduction in seed number. This is because an efficiency in using storage substance decreases with a smaller number of seeds and thus benefits increasing seed number at the expense of seed size. The optimal seed size is smaller than that predicted by Smith and Fretwell.