Hydraulic redistribution of soil water by roots affects whole‐stand evapotranspiration and net ecosystem carbon exchange

Abstract

Summary• Hydraulic redistribution (HR) of water via roots from moist to drier portions of the soil occurs in many ecosystems, potentially influencing both water use and carbon assimilation.• By measuring soil water content, sap flow and eddy covariance, we investigated the temporal variability of HR in a loblolly pine (Pinus taeda) plantation during months of normal and below-normal precipitation, and examined its effects on tree transpiration, ecosystem water use and carbon exchange.• The occurrence of HR was explained by courses of reverse flow through roots. As the drought progressed, HR maintained soil moisture above 0.15 cm 3 cm )3 and increased transpiration by 30-50%. HR accounted for 15-25% of measured total site water depletion seasonally, peaking at 1.05 mm d )1. The understory species depended on water redistributed by the deep-rooted overstory pine trees for their early summer water supply. Modeling carbon flux showed that in the absence of HR, gross ecosystem productivity and net ecosystem exchange could be reduced by 750 and 400 g C m )2 yr )1 , respectively.• Hydraulic redistribution mitigated the effects of soil drying on understory and stand evapotranspiration and had important implications for net primary productivity by maintaining this whole ecosystem as a carbon sink.

Using StorAge Selection functions to quantify ecohydrological controls on the time-variant age of evapotranspiration, soil water, and recharge

“…3e, 3i) showed isotopic compositions different from either the simulated fast or slow domain isotopic compositions. This could be indicative of additional undifferentiated water sources in the rooting zone or redistribution of soil 10 water due to root water potential (Domec et al, 2010;Volpe et al, 2013) (e.g. upward flux of water from deeper soils).…”

Hydro-ecological controls on dissolved carbon dynamics in groundwater and export to streams in a temperate pine forest

“…2c), revealing that soil water uptake by the pine trees directly lowers the water table. Soil water retention properties usually vary with depth and thus soil water uptake by plant roots generally occurs from areas in the soil with the highest water potential (Domec et al, 2010;Warren et al, 2005). Previous studies suggest that the ordinary soil depth at which most water is taken up in pines is usually 30-40 cm (Klein et al, 2014;Querejeta et al, 2001) where nutrient concentrations are also the highest (Achat et al, 2008).…”

“…However, estimations of terrestrial carbon leaching from direct simultaneous measurements in groundwater and streams are scarce. These studies are generally restricted to submarine and coastal environments (Atkins et al, 2013;Sadat-Noori et al, 2016;Santos et al, 2012) and boreal lakes (Einarsdottir et al, 2017), but rarely streams. The few studies that estimate exports of carbon from forested landscapes to streams are generally (i) based on carbon observations in soil water (i.e., in the unsaturated zone of the soil) combined with a soil water model that simulates the volume of soil water leached to streams (Öquist et al, 2009;Kindler et al, 2011;Leith et al, 2015), (ii) based on carbon observations in stream combined with stream discharge (Billett et al, 2004;Dawson et al, 2002;Olefeldt et al, 2013;Shibata et al, 2001Shibata et al, , 2005, or (iii) as described by the active pipe concept (Cole et al, 2007), as the sum of the three major riverine carbon fluxes: CO 2 degassing, organic carbon burial in sediments and carbon export downstream (Jonsson et al, 2007).…”

Estimating groundwater evapotranspiration by a subtropical pine plantation using diurnal water table fluctuations: Implications from night-time water use

“…capillary fringe or groundwater) to drier parts through hydraulic lift also has to be taken into account (Warren et al 2007;Domec et al 2010). This could support the higher T flux during conditions with deeper groundwater tables, as redistribution of groundwater to the unsaturated zone will affect the calculation of the night-time slope similar to T n .…”