Sendall Kerrie M, Lusk Christopher H, Reich Peter B
Department of Plant Biological Sciences, University of Minnesota, 1445 Gortner Avenue, St. Paul, MN, 55108, USA.
Department of Biological Sciences, Macquarie University, Sydney, NSW, 2109, Australia.
Oecologia. 2015 Dec;179(4):1011-21. doi: 10.1007/s00442-015-3428-x. Epub 2015 Aug 30.
Although shade tolerance is often assumed to be a fixed trait, recent work suggests ontogenetic changes in the light requirements of tree species. We determined the influence of gas exchange, biomass distribution, and self-shading on ontogenetic variation in the instantaneous aboveground carbon balance of Acer saccharum. We quantified the aboveground biomass distributions of 18 juveniles varying in height and growing in low light in a temperate forest understory in Minnesota, USA. Gas exchange rates of leaf and stem tissues were measured, and the crown architecture of each individual was quantified. The YPLANT program was used to estimate the self-shaded fraction of each crown and to model net leaf-level carbon gain. Leaf respiration and photosynthesis per gram of leaf tissue increased with plant size. In contrast, stem respiration rates per gram of stem tissue declined, reflecting a shift in the distribution of stem diameter sizes from smaller (with higher respiration) to larger diameter classes. However, these trends were outweighed by ontogenetic increases in self-shading (which reduces the net photosynthesis realized) and stem mass fraction (which increases the proportion of purely respiratory tissue) in terms of influence on net carbon exchange. As a result, net carbon gain per gram of aboveground plant tissue declined with increasing plant size, and the instantaneous aboveground light compensation point increased. When estimates of root respiration were included to model whole-plant carbon gain and light compensation points, relationships with plant size were even more pronounced. Our findings show how an interplay of gas exchange, self-shading, and biomass distribution shapes ontogenetic changes in shade tolerance.
尽管耐荫性通常被认为是一种固定的特性，但最近的研究表明树种对光照的需求会发生个体发育变化。我们确定了气体交换、生物量分配和自我遮荫对糖枫地上部分瞬时碳平衡个体发育变化的影响。我们对美国明尼苏达州温带森林林下18株高度不同且生长在弱光环境中的幼树的地上生物量分布进行了量化。测量了叶片和茎组织的气体交换率，并对每株个体的树冠结构进行了量化。使用YPLANT程序估计每个树冠的自我遮荫比例，并模拟叶片水平的净碳增益。每克叶片组织的叶片呼吸和光合作用随植株大小增加。相比之下，每克茎组织的茎呼吸速率下降，这反映了茎直径大小分布从较小（呼吸速率较高）向较大直径类别的转变。然而，就对净碳交换的影响而言，这些趋势被自我遮荫（降低实际净光合作用）和茎质量分数（增加纯呼吸组织的比例）的个体发育增加所抵消。结果，每克地上植物组织的净碳增益随植株大小增加而下降，地上瞬时光补偿点增加。当纳入根系呼吸估计值以模拟整株植物的碳增益和光补偿点时，与植株大小的关系更加明显。我们的研究结果表明气体交换、自我遮荫和生物量分配之间的相互作用如何塑造耐荫性的个体发育变化。
