Hikosaka Kouki, Sudoh Sinya, Hirose Tadaki
Biological Institute, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan e-mail:
Oecologia. 1999 Mar;118(3):388-396. doi: 10.1007/s004420050740.
The importance of light acquisition and utilization by individuals in intraspecific competition was evaluated by determining growth and photosynthesis of individual plants in a dense monospecific stand of an annual, Xanthium canadense. Photosynthesis of individual plants in the stand was calculated using a canopy photosynthesis model in which leaf photosynthesis was assumed to be function of leaf nitrogen content and light availability. The estimated photosynthetic rates of individuals were strongly correlated with the measured growth rates. Photosynthetic rates per unit aboveground mass (RPR, relative photosynthetic rate) increased with increasing aboveground mass, suggesting asymmetric (one-sided) competition in the stand. However, larger individuals had similar RPRs, suggesting symmetric (two-sided) competition. These results were consistent with the observation that size inequality over the whole stand increased with growth, but it remained stable among the larger individuals. The RPR of an individual was calculated as the product of absorbed photon flux per unit aboveground mass (Φ) and light use efficiency (LUE, photosynthesis per unit absorbed photon flux). Φ indicates the efficiency of light acquisition, and was higher in larger individuals in the stand, while LUE was highest in individuals with intermediate aboveground mass. LUE depends on leaf nitrogen content. At an early stage, leaf nitrogen contents of smaller individuals were similar to those that maximize LUE. Light availability to smaller individuals decreased as they grew, while their nitrogen contents did not change markedly, which decreased their LUE. We concluded that asymmetric competition among individuals in the stand resulted mainly from lower efficiencies in both light acquisition and light use by smaller individuals.
通过测定一年生植物加拿大苍耳单一种植密集群落中个体植株的生长和光合作用，评估了种内竞争中个体对光的获取和利用的重要性。利用冠层光合作用模型计算群落中个体植株的光合作用，该模型假定叶片光合作用是叶片氮含量和光照可利用性的函数。个体的估计光合速率与实测生长速率密切相关。单位地上部质量的光合速率（RPR，相对光合速率）随地上部质量的增加而增加，表明群落中存在不对称（单侧）竞争。然而，较大个体的RPR相似，表明存在对称（双侧）竞争。这些结果与以下观察结果一致：整个群落中大小不平等随生长而增加，但在较大个体中保持稳定。个体的RPR计算为单位地上部质量吸收的光子通量（Φ）与光利用效率（LUE，单位吸收光子通量的光合作用）的乘积。Φ表示光获取效率，在群落中较大个体中更高，而LUE在地上部质量中等的个体中最高。LUE取决于叶片氮含量。在早期，较小个体的叶片氮含量与使LUE最大化的含量相似。随着较小个体的生长，它们可获得的光照减少，而其氮含量没有明显变化，这降低了它们的LUE。我们得出结论，群落中个体间的不对称竞争主要源于较小个体在光获取和光利用方面效率较低。
