"Modeling the Hydrological and Ecological Impact of Hydraulic Redistribution in the Amazon"
Hydraulic Redistribution (HR), the nocturnal transport of moisture by plant roots from wetter to drier portions of the root zone, has been documented for over 60 species, mostly from relatively dry regions. Recently tree species in the middle of the Amazonia have been added to the list, indicating that HR might be more common than originally thought. Despite the general notion of HR increasing dry season transpiration, little is known regarding whether and how the hydrological impact of HR may depend on climate characteristics and what the long-term ecological impact might be. For example, where do we expect to see the most substantial impact of HR on hydrological processes? How does the impact of HR differ between dry and wet years? Does HR always increase plant water availability? Do plants benefit from HR? What is the long-term ecological impact? Such questions are explored in this study using the NCAR CLM3-DGVM and focusing on the Amazon region. It is found that HR influences transpiration and total evapotranspiration the strongest in places and during seasons of inter-mediate soil wetness. While HR increases the long-term mean of dry season transpiration, it reduces dry season transpiration in extremely dry years when the HR-induced acceleration of moisture depletion reduced water availability later in the dry season. Such negative hydrological impact of HR can lead to extremely low or negative NPP later in the dry season that limits the growth of trees that are not in dry-season senescence, i.e., evergreen trees. As a result, HR leads to a shift in vegetation competition in the model, favoring tropical broadleaf drought deciduous trees at the expense of tropical broadleaf evergreen trees. Climate extremes play a dominant role in this ecological impact of HR.
Results from this study can be used to help locate future field experiments sites that optimize the detection of HR signal in the Amazonia. For example, the site of the Oliveira et al. study was in east-central Amazonia, at 2.8968ºS, 54.9519ºW. However, according to our experiments based on the present vegetation distribution, hydrometeorological conditions at that location are conducive to relatively weak HR signal. The signal might have been much stronger if the experiment were carried out in the southern part of Amazonia for example. The HR-induced shift of vegetation competition needs to be validated based on remote sensing data during extremely dry years such as the 1998 El Nino drought.