Forest Ecology and Management Group, Wageningen University, Post Box 47, 6708 PB, Wageningen, The Netherlands.
Tree Physiol. 2011 Sep;31(9):1024-37. doi: 10.1093/treephys/tpr065. Epub 2011 Sep 5.
Forest trees show large changes in functional traits as they develop from a sapling in the shaded understorey to an adult in the light-exposed canopy. The adaptive function of such changes remains poorly understood. The carbon gain hypothesis suggests that these changes should be adaptive (acclimation) and that they serve to maximize net vegetative or reproductive growth. We explore the carbon gain hypothesis using a mechanistic model that combines an above-ground plant structure, a biochemical photosynthesis model and a biophysical stomatal conductance model. Our simulations show how forest trees that maximize their carbon gain increase their total leaf area, sapwood area and leaf photosynthetic capacity with tree height and light intensity. In turn, they show how forest trees increased crown stomatal conductance and transpiration, and how the carbon budget was affected. These responses in functional traits to tree height (and light availability) largely differed from the responses exhibited by exposed trees. Forest and exposed trees nevertheless shared a number of emergent patterns: they showed a similar decrease in the average leaf water potential and intercellular CO(2) concentration with tree height, and kept almost constant values for the ratio of light absorption to electron transport capacity, the ratio of photosynthetic capacity to water supply capacity, and nitrogen partitioning between electron transport and carboxylation. While most of the predicted qualitative responses in individual traits are consistent with field or lab observations, the empirical support for capacity balances is scarce. We conclude that modelling functional trait optimization and carbon gain maximization from underlying physiological processes and trade-offs generates a set of predictions for functional trait acclimation and maintenance of capacity balances of trees of different height in a forest light gradient, but actual tests of the predicted patterns are still scarce.
森林树木在从林下荫蔽处的幼树发育为林冠光照充足处的成年树的过程中，其功能性状会发生较大变化。这些变化的适应功能仍知之甚少。碳增益假说认为，这些变化应该是适应性的（驯化），并且它们旨在最大限度地提高净营养生长或生殖生长。我们使用一种将地上植物结构、生化光合作用模型和生物物理气孔导度模型结合在一起的机制模型来探索碳增益假说。我们的模拟表明，最大限度地增加碳增益的森林树木如何随着树高和光照强度增加其总叶面积、边材面积和叶片光合作用能力。反过来，它们展示了森林树木如何增加树冠气孔导度和蒸腾作用，以及碳预算如何受到影响。这些功能性状对树高（和光可用性）的响应在很大程度上与暴露树木的响应不同。然而，森林树木和暴露树木共享了许多新兴模式：它们表现出与树高相似的平均叶片水势和胞间 CO2 浓度降低，并且光吸收与电子传递能力的比值、光合作用能力与水分供应能力的比值以及电子传递和羧化之间的氮分配几乎保持恒定。虽然大多数个体性状的预测定性响应与野外或实验室观察一致，但对能力平衡的经验支持却很少。我们的结论是，从潜在的生理过程和权衡中模拟功能性状优化和碳增益最大化，可以为森林光照梯度中不同高度树木的功能性状驯化和能力平衡的维持生成一组预测，但对预测模式的实际检验仍然很少。
