Spatiotemporal variations in groundwater and evaporative demand drive ecophysiological functioning of a phreatophyte in drylands

Abstract

Water is the main limiting factor for groundwater-dependent ecosystems (GDEs) in drylands. Predicted climate change (precipitation reductions and temperature increases) and anthropogenic activities such as groundwater drawdown jeopardize the structure and functioning of these ecosystems, presenting new challenges for their management. We developed a trait-based analysis to examine the spatiotemporal variability in the ecophysiology of Ziziphus lotus, a phreatophyte that dominates one of the few terrestrial GDEs of semiarid regions in Europe. We assessed morpho-functional and hydraulic traits along a naturally occurring gradient of depth-to-groundwater (DTGW, 2–25 m) in a coastal aquifer, and throughout the growing season of the species. Increasing DTGW and salinity negatively affected photosynthetic and transpiration rates, increasing plant water stress (lower predawn and midday water potential), and positively affected Huber value (sapwood cross-sectional area per leaf area), reducing leaf area and likely, plant hydraulic demand. However, higher atmospheric evaporative demand fostered higher transpiration rates and water stress. Differences in climatic conditions throughout the growing season drove temporal variability in Z. lotus responses along the DTGW gradient, with warmer and drier conditions promoting carbon assimilation and water loss more intensively at shallow water tables. This multiple-trait analysis allowed us to identify plant ecophysiological thresholds related to the increase in DTGW and evaporative demand during the growing season. These findings highlight the existence of tipping points in the ecophysiological functioning of phreatophytic plants in drylands, which contribute to disentangle the functional responses of the related GDEs under groundwater detriment because of climate change effects.

Interannual controls on riparian plant health in a dryland river

“…Second, the frequent associations with summer-season changes in groundwater elevations likely reflect root access to soil moisture. Such access is often argued to be the greatest limiting factor for plant growth in semi-arid Mediterranean environments (Newman et al, 2006;Torres-García et al, 2021), but as the multi-decadal analysis demonstrates here, there is additionally an over-riding need for winter storms to drive groundwater recharge in areas such as southern California that are subject to high interannual flow variability. This phenomenon has also been documented along other intermittent and dryland rivers around the globe (Scanlon et al, 2006), including systems within Africa (Cuthbert et al, 2019;Lange, 2005), Brazil (Costa et al, 2013), China (Qin et al, 2012) and Australia (McCallum et al, 2014).…”

“…more resistant to atmospheric changes in water availability (i.e., drought) than upland areas (Mayes et al, 2020), recent research indicates that increasing drought severity caused by anthropogenic factors and associated water table declines in groundwater dependent ecosystems (GDEs) results in extensive vegetation stress and mortality (Chiloane et al, 2022;Kibler et al, 2021). Precipitation, streamflow and evaporative demand also exert strong control on vegetation health and survival in dryland riparian areas and other GDEs (Torres-García et al, 2021;Williams et al, 2022;Williams et al, 2023). The relative influence of such abiotic forcing on vegetation health varies based on localized conditions, necessitating an analysis of these relationships at a fine spatial resolution (i.e., site-scale) to provide a holistic understanding of physical controls on plant health and guide management actions that generally occur at the site scale.…”