The contribution of hydraulic redistribution to transpiration in the humid tropics

Abstract:

Hydraulic redistribution (HR) is defined as the root transfer of water from moist to dry layers within the soil profile following a water potential gradient. This ecohydrological phenomenon is thought to occur across ecosystem types, including the Amazon rainforest. In particular, HR is more likely for species that depend on water and nutrient acquisition at shallow soil layers but have to maintain roots at depth for uptake during dry periods. Still, the occurrence and extent of HR and its contribution to transpiration are not well quantified. The objective of this study was to better understand the importance of HR in helping trees cope with drought conditions. This study was conducted near the K-34 tower site at the ZF-2 Tropical Forestry Experimental Station (02°36′33″S; 60°12′33″W) in Central Amazon. A suite of ecohydrological and meteorological data were continuously monitored from 2017 to 2019. First, we examined diurnal patterns of soil moisture and soil matric potential for soil profiles of 1 m depth and co-located with six trees. Second, the sap flux velocity of these six trees was measured at 2, 4, and 6 cm sapwood depths, and used to estimate transpiration; we also monitored vapor pressure deficit and rainfall. Third, we identified the conditions under which HR may occur as: (1) nighttime hours and when vapor pressure deficit approaches zero; and (2) when soil is distinctly stratified between dry shallow soil and wet deep soil (i.e. hydraulic lift). Lastly, we calculated the percent contribution of HR to both tree-level transpiration and total soil water extracted within the top 1 m. Initial results indicate that HR may reach 0.1 mm per day at 10 cm depth, which may replace up to 30% of the daily water extracted from that layer during dry periods. We expect that the occurrence and timing of HR might be related to the initiation and cessation of sap flow through tropical trees. Moreover, we expect spatial patterns of HR to be dependent on root density and surrounding tree basal area. These results would provide insight into how transpiration fluxes in the tropics may be affected by HR during drought conditions, thereby improving the representation of transpiration in modeling efforts.