Numerical simulations of hydraulic redistribution across climates: The role of the root hydraulic conductivities

Abstract

Hydraulic redistribution, a process by which vegetation roots redistribute soil moisture, has been recognized as an important mechanism impacting several processes that regulate plant water uptake, energy and water partitioning, and biogeochemical cycling. We analyze how the magnitude of hydraulic redistribution varies across ecosystems that are exposed to different climates and seasonal patterns of incoming shortwave radiation and precipitation. Numerical simulation studies are performed over 10 Ameriflux sites, which show that hydraulic redistribution predictions are significantly influenced by the specified root hydraulic conductivities. We performed sensitivity analyses by considering expected ranges of root conductivities based on previous experimental studies, and found contrasting patterns in energylimited and water-limited ecosystems. In energy-limited ecosystems, there is a threshold above which high root conductivities enhance hydraulic redistribution with no increase in transpiration, while in water-limited ecosystems increase in root conductivities was always associated with enhancements in both transpiration and hydraulic redistribution. Further we found differences in the magnitude and seasonality of hydraulic redistribution and transpiration across different climates, regulated by interplay between precipitation and transpiration. The annual hydraulic redistribution to transpiration flux ratio (HR/Tr) was significant in Mediterranean climates (HR/Tr 30%), and in the tropical humid climates (HR/Tr 15%). However, in the continental climates hydraulic redistribution occurs only during sporadic precipitation events throughout the summer resulting in lower annual magnitudes (HR/Tr < 5%). These results provide more insights for suitable implementation of numerical models to capture belowground processes in eco-hydrology, and enhance our understanding about the variability of hydraulic redistribution across different climates.

Water potential gradient, root conduit size and root xylem hydraulic conductivity determine the extent of hydraulic redistribution in temperate trees

“…In light of the ‘resistor’ concept, species with smaller xylem conduits showed lower conductance for HR than species with larger conduits and higher k sa . Root conduit anatomy (Hafner et al, ) and xylem hydraulic conductivity (Quijano & Kumar, ) reflected the magnitude of the internal ‘resistor’ of different species for HR, confirming our hypothesis that HR increases with increasing root conduit diameter and root‐xylem hydraulic conductivity. Because the deciduous species of our study had embolism formation in their roots by the end of the experiment (Table ; Figure ), the correlation between the amount of HR water and k sa as surrogate for conductivity was more accurate and resulted in better correlations than the regression with xylem conduit diameter.…”

“…Burgess, Adams, Turner, & Ong, ; Caldwell & Richards, ; Richards & Caldwell, ), HR has also been demonstrated in mesic environments (Dawson, ) and is now considered a general phenomenon occurring across different climates and ecosystems (e.g. Quijano & Kumar, ; Sardans & Peñuelas, ).…”

“…While a number of suggested factors drive HR, including ‘external’ environmental and ‘internal’ plant factors (Neumann & Cardon, ), a mechanistic understanding of the role each factor plays in determining the quantity of HR is still lacking. Reported ‘external’ factors include soil texture, soil Ψ gradients and atmospheric vapour pressure deficit (VPD), while plant‐driven ‘internal’ factors affecting HR include xylem vessel conductivity (Quijano & Kumar, ), rates of (night‐time) transpiration (Dawson et al, ; Howard et al, ) and water refilling of plant storage tissues (Yu et al, ). In addition, root characteristics such as bark thickness (Bauerle et al, ) or the regulation of aquaporins (Li, Santoni, & Maurel, ; Prieto, Armas, & Pugnaire, ) might affect the amount of redistributed water.…”

Influences of Root Hydraulic Redistribution on N2O Emissions at AmeriFlux Sites

“…There are two primary approaches to HR modeling: the scheme proposed by Ryel et al (2002) (e.g., Fu et al, 2016;Li et al, 2012;Wang, 2011) or its variations (e.g., Lee et al, 2005;Yu & D'Odorico, 2015), and the approach proposed by Amenu and Kumar (2008) (e.g., Luo et al, 2013;Quijano & Kumar, 2015;Tang et al, 2015). Ryel et al's scheme is an empirical method that describes HR flux based on the soil water potential (Wang, 2011;Zheng & Wang, 2007) and CLM4.5 (Fu et al, 2016).…”

Plant transpiration and groundwater dynamics in water‐limited climates: Impacts of hydraulic redistribution

“…Both experimental and modeling studies show that HR plays an important role in terrestrial water and energy cycles not only in the arid and semiarid regions [e.g., Caldwell and Richards , ; Dawson , ; Ryel et al ., ] but also in regions experiencing distinct wet and dry seasons (e.g., the regions with Mediterranean‐type climates) [e.g., Lee et al ., ; Wang , ; Luo et al ., ] by promoting dry‐season transpiration and carbon assimilation [e.g., Ryel et al ., ; Lee et al ., ; Amenu and Kumar , ; Wang , ; Luo et al ., ; Quijano and Kumar , ]. The modeling study of Luo et al .…”