Squandering water in drylands: the water‐use strategy of the phreatophyte Ziziphus lotus in a groundwater‐dependent ecosystem

Abstract

Premise Water is the most limiting factor in dryland ecosystems, and plants are adapted to cope with this constraint. Particularly vulnerable are phreatophytic plants from groundwater‐dependent ecosystems (GDEs) in regions that have to face water regime alterations due to the impacts of climate and land‐use changes. Methods We investigated two aspects related to the water‐use strategy of a keystone species that dominates one of the few terrestrial GDEs in European drylands (Ziziphus lotus): where it obtains water and how it regulates its use. We (1) evaluated plants’ water sources and use patterns using a multiple‐isotope approach (δ2H, δ18O, and Δ13C); (2) assessed the regulation of plant water potential by characterizing the species on an isohydric–anisohydric continuum; and (3) evaluated plants’ response to increasing water stress along a depth‐to‐groundwater (DTGW) gradient by measuring foliar gas exchange and nutrient concentrations. Results Ziziphus lotus behaves as a facultative or partial phreatophyte with extreme anisohydric stomatal regulation. However, as DTGW increased, Z. lotus (1) reduced the use of groundwater, (2) reduced total water uptake, and (3) limited transpiration water loss while increasing water‐use efficiency. We also found a physiological threshold at 14 m depth to groundwater, which could indicate maximum rooting length beyond which optimal plant function could not be sustained. Conclusions Species such as Z. lotus survive by squandering water in drylands because of a substantial groundwater uptake. However, the identification of DTGW thresholds indicates that drawdowns in groundwater level would jeopardize the functioning of the GDE.

A multiple-trait analysis of ecohydrological acclimatisation in a dryland phreatophytic shrub

“…Nevertheless, we observed that neither carbon assimilation nor water loss was completely compromised at any point of the growing season and at any DTGW, since the lowest mean rates of A and g s were observed in May, reaching 6.84 µmol CO 2 m −2 s −1 and 0.14 mol H 2 O m −2 s −1 , respectively, at bore 5 (Table 1 ). This result can be explained by Z. lotus accessing and using groundwater continuously during its growing season to avoid stomatal closure, even in summer (Torres-García et al 2021 ). In this sense, the low values of WUEi we observed in summer and the lack of relationship with DTGW agree with having access to a water source, likely groundwater, since large WUEi is widely associated with groundwater usage where precipitation in scarce (Eamus et al 2013 ; Cleverly et al 2016 ; Rumman et al 2018 ).…”

“…In addition, Z. lotus transpiration rate did not decline in summer; in fact, it increased with VPD, more significantly at shallow water tables, suggesting that summer conditions could induce higher rates when sufficient groundwater is available (Nolan et al 2018 ; Eamus and Prior, 2001 ), and that groundwater availability to the plant depends on climatic conditions. Despite the risk of hydraulic failure due to this anisohydric behaviour (Torres-García et al 2021 ) and the physiological limitations of tapping water from deep sources, Z. lotus plants can maintain high gas exchange under current conditions.…”

“…Instead, we found that the combination of deep groundwater and high salinity away from the coast might promote water stress in the root zone as well as a drought-like condition in the plant (Kath et al 2015 ). Although Z. lotus showed little evidence of water deficit (slightly negative water potentials even in summer) and have continuous access to groundwater during its growing season (Torres-García et al 2021 ), Ψ pd and Ψ md correlated with DTGW and salinity. Particularly, salinity might have induced lower water potential at the root surface, reducing water uptake and photosynthetic rate in Z. lotus , as in other species from GDEs (Kath et al 2015 ).…”

“…The winter-deciduous phreatophyte Z. lotus is the dominant species of this coastal plain ecosystem, which is comprised of Z. lotus and other shallow-rooted Mediterranean shrubs such as Lycium intricatum , Salsola oppositifolia, and Withania frutescens (Tirado 2009 ). Z. lotus distributes along the Mediterranean basin, being native from North Africa, the Middle East, and southern Europe, mainly Spain, where it constitutes one of the few terrestrial GDEs in European drylands (Guirado et al 2018 ; Torres-García et al 2021 ). It is a slow-growing, long-lived shrub that has not substantially changed in size or shape in the past 70 years in the study area.…”

“…Z. lotus is responsible for most of the photosynthetic activity during summer, whereas the rest of the vegetation constituting the island grows in winter, entailing a replacement in the drivers of the primary productivity of the ecosystem (Guirado et al 2018 ). Z. lotus partially depends on groundwater to survive (Torres-García et al 2021 ) by developing a dual root system that can reach up to 60 m deep (Le Houérou 2006 ) whilst also maintaining active roots near the surface. Vegetation sampling was made on a total of 16 adult individuals of Z. lotus (1–3 m tall and 50–200 m 2 area) selected next to each bore (two per bore at a maximum distance of 130 m) (Fig.…”