Hydraulic redistribution buffers climate variability and regulates grass‐tree interactions in a semiarid riparian savanna

Abstract

Anticipating the ability of ecosystems to maintain functional integrity across predicted altered precipitation regimes remains a grand ecohydrological challenge. Overstory trees and understory grasses within semiarid savannas vary in their structure and sensitivity to environmental pressures, underscoring the need to examine the ecohydrological implications of this climatic variability. Whereas precipitation has long been recognized as a key driver of landscape ecohydrology, understanding a site's hydraulic redistribution regime (the balance in downward and upward movement of water and the seasonality of these bidirectional flows) may be equally important to understanding moisture availability to vegetation in these dryland ecosystems. As a result, we linked measures of ecosystem‐scale carbon exchange, overstory tree sap flux and leaf‐level gas exchange to understory whole‐plot and leaf‐level carbon and water exchange within intact and trenched plots (isolating trees from grasses) in a riparian savanna ecosystem. We maintained measurements across 2 years with distinct precipitation regimes. We found that interannual precipitation variability yielded a categorical shift in the directionality and magnitude of the hydraulic redistribution regime—even within this single site. Additionally, we found that connectivity between overstory trees and understory grasses through hydraulic redistribution created a short period of competition within an average rain year but that facilitation of understory function by overstory trees was much greater and lasted longer during drier years. Together, these findings suggest that hydraulic redistribution can serve as a hydrologic buffer against interannual precipitation variability. Given current climate projections of more variable precipitation within and across years, understanding how hydraulic redistribution regimes vary through time will greatly enhance our capacity to anticipate future ecohydrological function.

Plant functional diversity influences water and carbon fluxes and their use efficiencies in native and disturbed dryland ecosystems

“…Those ecohydrological changes allowed deep‐rooted facultative evergreens to extend their CUP in the savanna (Table 1). Buffelgrass could also resprout and regain growth, either after light rains in winter (Castellanos‐Villegas et al, 2010) or possibly from water redistribution activity from evergreen trees (Barron‐Gafford et al, 2021; Hultine et al, 2004). Extending the photosynthetic activity increased the CUP and contributed up to 40% of the total annual NEP of the savanna (Table 2 and Figure 5).…”

Convergent Hydraulic Redistribution and Groundwater Access Supported Facilitative Dependency Between Trees and Grasses in a Semi‐Arid Environment

“…Amenu and Kumar (2008) provides detailed model setup and equations used for hydraulic redistribution and below‐ground ecohydrologic processes and Quijano Kumar (2015) provides a sensitivity analysis of root conductivities for the model. Woody plants are known to redistribute moisture through their roots and large perennial grasses with a considerably deep rooting depth of ∼3 m might perform HR (Barron‐Gafford et al., 2020; Quijano et al., 2012). However, whether the sacaton bunchgrasses perform HR at this site or not remains unclear.…”

“…Site Specific Parameters for Riparian Charleston Mesquite (CMW) and Upland Santa Rita Mesquite (SRM) Sites moisture through their roots and large perennial grasses with a considerably deep rooting depth of ∼3 m might perform HR (Barron-Gafford et al, 2020;Quijano et al, 2012). However, whether the sacaton bunchgrasses perform HR at this site or not remains unclear.…”

“…This downward water movement by deep‐rooted overstory vegetation may create a competitive effect since the amount of near‐surface soil‐water available for shallow‐rooted plants decreases (Barron‐Gafford et al., 2017; Lee et al., 2018). On the other hand, hydraulic lift, where deep‐rooted trees transport water from deep to shallow soil layers, can induce a facilitative relationship between coexisting vegetation in terms of enhanced plant nutrients and available water (Cardon et al., 2013; Matimati et al., 2014; Quijano et al., 2013; Sun et al., 2014) and by providing a buffering mechanism during the dry season (Barron‐Gafford et al., 2020). Either the deep‐rooted trees and/or the shallower rooted plants can use the hydraulically lifted water to support transpiration.…”

“…In upland ecosystems when plants do not have access to groundwater, field measurements and numerical modeling show that grasses use moisture that is available in shallow depths whereas deep‐rooted trees utilize deeper soil moisture (Barron‐Gafford et al., 2017; Lee et al., 2018; Quijano & Kumar, 2015; Quijano et al., 2012). However, in the riparian ecosystems, the presence of accessible groundwater adds extra complexity to the above‐ and below‐ground ecohydrological dynamics and furthermore alters ecophysiological characteristics and photosynthetic function of coexisting vegetation species and the exchange of water, energy, and nutrients (Barron‐Gafford et al., 2020). Despite multiple studies on ecohydrologic significances of HR, the dynamics of HR when roots have access to stable deep soil‐water sources (e.g., groundwater) is unknown.…”