Comparing Sap Flow and Groundwater Uptake by Trees Across a Range of Antecedent Soil Moisture Conditions in a Humid Riparian Zone

Abstract:

Trees in different ecosystems often utilize vadose-zone moisture to meet water needs but can also use water from the capillary fringe or water table, if accessible. Riparian zones generally have shallow depths to the water table and riparian trees may have direct access to groundwater most of the year. However, the reliance on groundwater in riparian zones likely varies depending on vadose-zone moisture availability, rooting depth, water-table depth, and other environmental and plant physiological factors. When groundwater is accessible it can act to buffer trees from drought induced stress.

In this study, we investigated the role of antecedent moisture on the sap-flow rate and groundwater uptake (T_G) of trees across a riparian zone at the Panola Mountain Research Watershed. We focused on three separate periods that were free of precipitation and represented different levels of soil-moisture storage in the top one meter of the soil profile. These three periods are classified as wet, dry, and severe drought and correspond to soil-moisture storage exceedance probabilities of 0.65, 0.87, and 0.975, respectively. The periods occurred from June to September of 2019; therefore, we also examined normalized rates to account for differences in solar radiation and day length, to allow for direct comparisons over time.

Sap-flow rates changed little over the periods evaluated; however, sap flow was generally highest when soils were moist and lowest during the drought. T_G was lowest when soils were wet and, on average, increased by 45% in the dry period, followed by a 13% decrease during the drought. When considering normalized T_G fluxes, the pattern changed little but often led to T_G rates during the drought period being equal to, or slightly greater than T_G fluxes during the dry period. This suggests that in addition to vadose-zone moisture availability, plant physiological factors may be influencing water uptake. While we controlled for day length, we couldn’t control for changes in leaf area, which is directly related to water-use. Also, many riparian tree species at the site may be considered isohydric, indicating more rapid stomatal control in response to decreasing soil matric potential and therefore, reduced water uptake. Work is ongoing to further characterize the timing and magnitude of groundwater uptake by riparian trees.