Xiong Wei, Oren Ram, Wang Yanhui, Yu Pengtao, Liu Hailong, Cao Gongxiang, Xu Lihong, Wang Yunni, Zuo Haijun
Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, P.R. China
Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, USA Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83 Umeå, Sweden.
Tree Physiol. 2015 May;35(5):470-84. doi: 10.1093/treephys/tpv022. Epub 2015 Apr 1.
Small differences in the sensitivity of stomatal conductance to light intensity on leaf surfaces may lead to large differences in total canopy transpiration (EC) with increasing canopy leaf area (L). Typically, the increase of L would more than compensate for the decrease of transpiration per unit of leaf area (EL), resulting in concurrent increase of EC. However, highly shade-intolerant species, such as Larix principis-rupprechtii Mayr., may be so sensitive to increased shading that such compensation is not complete. We hypothesized that in such a stand, windfall-induced spatial variation at a decameter scale would result in greatly reduced EL in patches of high L leading to lower EC than low competition patches of sparse canopy. We further hypothesized that quicker extraction of soil moisture in patches of lower competition will result in earlier onset of drought symptoms in these patches. Thus, patches of low L will transition from light to soil moisture as the factor dominating EL. This process should progressively homogenize EC in the stand even as the variation of soil moisture is increasing. We tested the hypotheses utilizing sap flux of nine trees, and associated environmental and stand variables. The results were consistent with only some of the expectations. Under non-limiting soil moisture, EL was very sensitive to the spatial variation of L, decreasing sharply with increasing L and associated decrease of mean light intensity on leaf surfaces. Thus, under the conditions of ample soil moisture maximum EC decreased with increasing patch-scale L. Annual EC and biomass production also decreased with L, albeit more weakly. Furthermore, variation of EC among patches decreased as average stand soil moisture declined between rain events. However, contrary to expectation, high L plots which transpired less showed a greater EL sensitivity to decreasing stand-scale soil moisture, suggesting a different mechanism than simple control by decreasing soil moisture. We offer potential explanations to the observed phenomenon. Our results demonstrate that spatial variation of L at decameter scale, even within relatively homogeneous, single-species, even-aged stands, can produce large variation of transpiration, soil moisture and biomass production and should be considered in 1-D soil-plant-atmosphere models.
叶片表面气孔导度对光强的敏感性存在细微差异，随着冠层叶面积（L）的增加，可能导致总冠层蒸腾量（EC）出现较大差异。通常情况下，L的增加将超过单位叶面积蒸腾量（EL）的减少，从而导致EC同时增加。然而，像华北落叶松（Larix principis-rupprechtii Mayr.）这样对遮荫高度敏感的物种，可能对遮荫增加极为敏感，以至于这种补偿并不完全。我们推测，在这样的林分中，由风倒引起的十米尺度的空间变化会导致高L斑块中的EL大幅降低，从而使EC低于稀疏冠层的低竞争斑块。我们进一步推测，低竞争斑块中土壤水分的更快提取将导致这些斑块更早出现干旱症状。因此，低L斑块将从光照主导EL的因素转变为土壤水分主导。即使土壤水分变化在增加，这个过程也应使林分中的EC逐渐趋于均匀。我们利用九棵树的液流通量以及相关的环境和林分变量对这些推测进行了检验。结果仅部分符合预期。在非限制性土壤水分条件下，EL对L的空间变化非常敏感，随着L的增加以及叶片表面平均光强的相应降低而急剧下降。因此，在土壤水分充足的条件下，最大EC随着斑块尺度L的增加而降低。年EC和生物量生产也随L降低，尽管降幅较小。此外，降雨事件之间，随着林分平均土壤水分下降，斑块间EC的变化也减小。然而，与预期相反，蒸腾较少的高L样地对林分尺度土壤水分下降表现出更大的EL敏感性，这表明其机制不同于单纯的土壤水分减少控制。我们对观察到的现象提供了可能的解释。我们的结果表明，即使在相对均匀、单一物种、同龄的林分中，十米尺度的L空间变化也会导致蒸腾、土壤水分和生物量生产产生很大变化，在一维土壤-植物-大气模型中应予以考虑。
