Water, carbon, and nutrient cycles in terrestrial groundwater-dependent ecosystems of drylandsZiziphuslotus’ shrublandsas case study

Resumen

Terrestrial groundwater-dependent ecosystems (GDEs) rely on groundwater to maintain their structure, composition, and functions. In drylands, where water is the most limiting ecological factor, such ecosystems have a unique role for biodiversity and human well-being, given they use an inaccessible source of water for most plants. Climate change, groundwater depletion and pollution, and land-use changes might be reducing water availability for these ecosystems, jeopardizing their ability to provide ecosystem functions and services. Understanding the ecological processes underlying plant water use, carbon and nutrient uptake, and soil-plant interactions in these ecosystems is critical for sustainable groundwater management, biodiversity conservation, and climate change adaptation in drylands. The primary goal of this thesis is to investigate the functioning of a terrestrial GDE from Mediterranean arid regions and its phreatophytic vegetation to: (1) understand its contribution to the water, carbon, and nutrient cycles, and (2) provide scientific evidence to managers and policymakers policymakers who must deal with the sustainable management of groundwater and ensure GDEs’ contributions to people in the face of global change. To do this, I focused on Ziziphus lotus (L.) Lam. (Rhamnaceae), a long-lived, winter deciduous, and deep-rooted phreatophyte that significantly modifies the arid landscape conditions, creating a GDE considered as a priority habitat for conservation in Europe. The general hypothesis is that spatiotemporal variations in groundwater availability and climate variability can alter the functioning of this GDE. I used the coastal plain of the Cabo de Gata-Níjar Natural Park (Spain) as a study area since it provides a spatial gradient in the depth-to-groundwater (DTGW) to test this hypothesis. I developed four studies that address spatiotemporal variations in processes related to water, carbon, and nutrient cycles at different structural levels, from leaves and individuals to vegetation patches. Chapter I focused on plant morpho-functional and physiological traits to identify growth patterns at the individual level and temporal variations throughout the growing season. I described Z. lotus’ growth pattern as the repetition of modular units composed by shoots (short and long) and branches (flowering and plagiotropic) with differentiated functions. Related to the branches, I identified morpho-functionally distinct leaves (i.e., heterophylly) with different water-use patterns. Both modular growth and heterophylly might contribute to prioritizing resource investment in particular functions over time, either for reproduction (in spring when plants might obtain water and nutrients from the topsoil layer) or growth (throughout the growing season). The focus of Chapter II was to assess two main aspects related to the water-use strategy of Z. lotus: the source of water and its transport regulation through the plant. I used stable isotopes (δ2H, δ18O, and Δ13C), stem water potentials, leaf gas-exchange measurements, and leaf nutrient concentrations to uncover the partial groundwater dependency and anisohydric stomatal regulation of Z. lotus across the spatial DTGW gradient. Nevertheless, as DTGW increased, I found that Z. lotus (1) decreased groundwater use and (2) reduced water loss through transpiration while increasing water-use efficiency (i.e., less extreme anisohydric behaviour). I also detected a physiological threshold at 13 14 m, indicating the maximum groundwater depth for better functioning. Chapter III introduced the temporal variability defined by seasonal climatic conditions (e.g., the atmospheric evaporative demand) to identify plant ecophysiological thresholds with a trait-based analysis. I found that some traits were more affected by high DTGW and groundwater salinity (e.g., low photosynthetic rate and stomatal conductance, and high Huber value [ratio between sapwood cross-sectional area to leaf area]). In contrast, other traits were more related to seasonal variations in atmospheric conditions (e.g., high transpiration and more negative predawn and midday water potential in summer). This study confirmed spatial ecophysiological thresholds (Chapter II) that depend on groundwater characteristics (i.e., difficulties to obtain groundwater at high depths [> 14 m] and salinity levels). Additionally, the analysis identified new temporal thresholds related to atmospheric evaporative demand that indicated significantly lower vapor pressure deficit and water stress in spring than in summer. In Chapter IV, I considered the heterogeneous spatial distribution of vegetation in drylands to study soil-plants interactions. I assessed the spatiotemporal coupling between vegetation functioning and soil biological activity and its relationship with soil quality (soil properties and nutrient availability), mineralization rates, and water availability in patches of Z. lotus. I found that soil and vegetation showed a decoupled activity. Whereas soil respiration and mineralization processes promptly responded to rainfall pulses, vegetation activity was overall decoupled from precipitation. The presence of phreatophytes enhanced soil quality and soil biological activity, thereby promoting fertile islands in drylands. In general, the GDE dominated by Z. lotus contributes to enhancing the primary productivity of drylands through its phreatophytic nature, which increases transpiration, carbon assimilation, soil activity, and mineralization processes, fostering nutrient cycling. However, these processes and GDEs are threatened by groundwater overexploitation, climate change effects, and land use changes due to: (1) the dependence of phreatophytes on groundwater (DTGW threshold for better functioning up to 14 m); (2) the necessity of soil water (mostly from precipitation) for nutrient uptake, reproductive investment, and soil microbial activation; and (3) the importance of every single long-lived plant to maintain ecosystem functioning.