Oliveira Rafael S, Dawson Todd E, Burgess Stephen S O, Nepstad Daniel C
Department of Integrative Biology, University of California, Berkeley, CA 94720, USA.
Oecologia. 2005 Sep;145(3):354-63. doi: 10.1007/s00442-005-0108-2. Epub 2005 Sep 29.
About half of the Amazon rainforest is subject to seasonal droughts of 3 months or more. Despite this drought, several studies have shown that these forests, under a strongly seasonal climate, do not exhibit significant water stress during the dry season. In addition to deep soil water uptake, another contributing explanation for the absence of plant water stress during drought is the process of hydraulic redistribution; the nocturnal transfer of water by roots from moist to dry regions of the soil profile. Here, we present data on patterns of soil moisture and sap flow in roots of three dimorphic-rooted species in the Tapajós Forest, Amazônia, which demonstrate both upward (hydraulic lift) and downward hydraulic redistribution. We measured sap flow in lateral and tap roots of our three study species over a 2-year period using the heat ratio method, a sap-flow technique that allows bi-directional measurement of water flow. On certain nights during the dry season, reverse or acropetal flow (i.e.,in the direction of the soil) in the lateral roots and positive or basipetal sap flow (toward the plant) in the tap roots of Coussarea racemosa (caferana), Manilkara huberi (maçaranduba) and Protium robustum (breu) were observed, a pattern consistent with upward hydraulic redistribution (hydraulic lift). With the onset of heavy rains, this pattern reversed, with continuous night-time acropetal sap flow in the tap root and basipetal sap flow in lateral roots, indicating water movement from wet top soil to dry deeper soils (downward hydraulic redistribution). Both patterns were present in trees within a rainfall exclusion plot (Seca Floresta) and to a more limited extent in the control plot. Although hydraulic redistribution has traditionally been associated with arid or strongly seasonal environments, our findings now suggest that it is important in ameliorating water stress and improving rain infiltration in Amazonian rainforests. This has broad implications for understanding and modeling ecosystem process and forest function in this important biome.
大约一半的亚马逊雨林会经历长达三个月或更长时间的季节性干旱。尽管存在这种干旱情况，但多项研究表明，这些处于强烈季节性气候下的森林在旱季并未表现出明显的水分胁迫。除了深层土壤水分吸收外，干旱期间植物不存在水分胁迫的另一个原因是水力再分配过程；即根系在夜间将水分从土壤剖面的湿润区域转移到干燥区域。在此，我们展示了亚马逊州塔帕若斯森林中三种具有二态根系的物种的土壤湿度和根系液流模式数据，这些数据表明了向上（水力提升）和向下的水力再分配。我们使用热比法在两年时间内测量了这三个研究物种侧根和主根的液流，热比法是一种能够双向测量水流的液流技术。在旱季的某些夜晚，我们观察到总序苦树（caferana）、马氏铁线子（maçaranduba）和粗壮原檀（breu）的侧根出现反向或向顶流（即流向土壤的方向），主根出现正向或向基液流（流向植物），这种模式与向上的水力再分配（水力提升）一致。随着大雨的到来，这种模式发生逆转，主根夜间持续出现向顶液流，侧根出现向基液流，表明水分从湿润的表层土壤向干燥的深层土壤移动（向下水力再分配）。这两种模式在降雨排除区（Seca Floresta）的树木中都存在，在对照区的程度则较为有限。尽管传统上水力再分配与干旱或强烈季节性环境相关，但我们的研究结果表明，它对于缓解亚马逊雨林的水分胁迫和改善雨水渗透非常重要。这对于理解和模拟这个重要生物群落中的生态系统过程和森林功能具有广泛的意义。
