Faculty of Geo-information and Earth Observation (ITC), University of Twente, Enschede, Hengelosestraat 99, AE Enschede, The Netherlands.
Department of Geography, King's College London, London, UK.
Tree Physiol. 2018 Jul 1;38(7):1053-1070. doi: 10.1093/treephys/tpy004.
It has been suggested that vigorous secondary tropical forests can have very high transpiration rates, but sap flow and stomatal conductance dynamics of trees and shrubs in these forests are understudied. In an effort to address this knowledge gap, sap flow (thermal dissipation method, 12 trees) and stomatal conductance (porometry, six trees) were measured for young (5-7 years) Psiadia altissima (DC.) Drake trees, a widely occurring species dominating young regrowth following abandonment of swidden agriculture in upland eastern Madagascar. In addition, stomatal conductance (gs) was determined for three individuals of two locally common invasive shrubs (Lantana camara L. and Rubus moluccanus L.) during three periods with contrasting soil moisture conditions. Values of gs for the three investigated species were significantly higher and more sensitive to climatic conditions during the wet period compared with the dry period. Further, gs of the understorey shrubs was much more sensitive to soil moisture content than that of the trees. Tree transpiration rates (Ec) were relatively stable during the dry season and were only affected somewhat by soil water content at the end of the dry season, suggesting the trees had continued access to soil water despite drying out of the topsoil. The Ec exhibited a plateau-shaped relation with vapour pressure deficit (VPD), which was attributed to stomatal closure at high VPD. Vapour pressure deficit was the major driver of variation in Ec, during both the wet and the dry season. Overall water use of the trees was modest, possibly reflecting low site fertility after three swidden cultivation cycles. The observed contrast in gs response to soil water and climatic conditions for the trees and shrubs underscores the need to take root distributions into account when modelling transpiration from regenerating tropical forests.
有人认为，热带次生林的蒸腾速率可能非常高，但这些森林中树木和灌木的液流和气孔导度动态仍研究不足。为了填补这一知识空白，我们测量了年轻（5-7 年）Psidia altissima（DC.） Drake 树的液流（热消散法，12 棵树）和气孔导度（porometry，六棵树），这种树在高地马达加斯加开垦农业废弃后，广泛存在于年轻的再生林中。此外，在三个具有不同土壤水分条件的时期，我们还测定了三种当地常见入侵灌木（Lantana camara L. 和 Rubus moluccanus L.）的气孔导度（gs）。在湿润期，三种研究物种的 gs 值显著更高，对气候条件也更敏感，而在干燥期则相反。此外，林下灌木的 gs 对土壤水分含量的敏感性远高于树木。在旱季，树木蒸腾速率（Ec）相对稳定，仅在旱季末期受土壤含水量的影响略有变化，这表明尽管表土变干，树木仍能继续获得土壤水分。Ec 与水汽压亏缺（VPD）呈平台状关系，这归因于高 VPD 下的气孔关闭。在湿润和干燥季节，VPD 都是影响 Ec 变化的主要因素。总的来说，树木的用水量适中，这可能反映了经过三个刀耕火种周期后，立地肥力较低。树木和灌木对土壤水分和气候条件的 gs 响应的观察结果表明，在对再生热带森林的蒸腾作用进行建模时，需要考虑根的分布。
