Groundwater drawdown drives ecophysiological adjustments of woody vegetation in a semi‐arid coastal ecosystem

Abstract

Predicted droughts and anthropogenic water use will increase groundwater lowering rates and intensify groundwater limitation, particularly for Mediterranean semi-arid ecosystems. These hydrological changes may be expected to elicit differential functional responses of vegetation either belowground or aboveground. Yet, our ability to predict the impacts of groundwater changes on these ecosystems is still poor. Thus, we sought to better understand the impact of falling water table on the physiology of woody vegetation. We specifically ask (a) how is woody vegetation ecophysiological performance affected by water table depth during the dry season? and (b) does the vegetation response to increasing depth to groundwater differ among water-use functional types? We examined a suite of physiological parameters and water-uptake depths of the dominant, functionally distinct woody vegetation along a water-table depth gradient in a Mediterranean semi-arid coastal ecosystem that is currently experiencing anthropogenic groundwater extraction pressure. We found that groundwater drawdown did negatively affect the ecophysiological performance of the woody vegetation. Across all studied environmental factors, depth to groundwater was the most important driver of ecophysiological adjustments. Plant functional types, independent of groundwater dependence, showed consistent declines in water content and generally reduced C and N acquisition with increasing depths to groundwater. Functional types showed distinct operating physiological ranges, but common physiological sensitivity to greater water table depth. Thus, although differences in water-source use exist, a physiological convergence appeared to happen among different functional types. These results strongly suggest that hydrological drought has an important impact on fundamental physiological processes, constraining the performance of woody vegetation under semi-arid conditions. By disentangling the functional responses and vulnerability of woody vegetation to groundwater limitation, our study establishes the basis for predicting the physiological responses of woody vegetation in semi-arid coastal ecosystems to groundwater drawdown.

Mapping the suitability of groundwater-dependent vegetation in a semi-arid Mediterranean area

“…Thus, those plants should show almost zero anomaly. In contrast, GDV plants coping well with usual summer drought can be expected to suffer an unusual stress under an extremely dry year, even having access to groundwater , 2014, with a negative impact of groundwater drawdown (Antunes et al, 2018). Therefore, GDV plants should show negative NDWI anomalies.…”

“…Mapping GDEs constitutes a first and fundamental step to their active management. Several approaches have been proposed, from local field surveys measuring plant transpiration of stable isotopes (Antunes et al, 2018) up to larger spatial scales involving remote-sensing techniques (e.g., normalized difference vegetation index -NDVI; Barron et al, 2014;Eamus et al, 2015;Howard and Merrifield, 2010), remote-sensing combined with ground-based observations (Lv et al, 2013), a geographic information system (GIS; Pérez Hoyos et al" 2016a) combining field surveys (Condesso de Melo et al, 2015) or even statistical approaches (Pérez Hoyos et al, 2016b).…”

“…Additionally, stone pine (Pinus pinea L.) has become a commonly co-occurrent species in the last decades (Coelho and Campos, 2009). The use of groundwater has been frequently reported for both the Pinus (Antunes et al, 2018;Filella and Peñuelas, 2004;Grossiord et al, 2016;Peñuelas and Filella, 2003) and Quercus genre (Barbeta and Peñuelas, 2017;Kurz-Besson et al, 2006, 2014. Furthermore, the contribution of groundwater to tree physiology has been shown to be of a greater magnitude for Quercus sp.…”

“…In addition, the species roots are known to reach depths as deep as 13 m in southern Portugal (David et al, 2004). P. pinea has been recently included in the facultative phreatophyte species (Antunes et al, 2018). This species shows a very similar root system (Montero et al, 2004) compared to cork oak , with large sinker roots reaching 5 m depth (Canadell et al, 1996).…”

“…Based on former knowledge gathered from field surveys conducted in the region (Antunes et al, 2018;Condesso de Melo et al, 2015;Kurz-Besson et al, 2006, 2014Pinto et al, 2013) on environmental conditions and the species ecophysiological needs, we hypothesized that (1) groundwater depth together with climatic conditions play one of the most important environmental roles in GDV's distribution and (2) groundwater depth between 1.5 and 15 m associated with xeric conditions should favor a higher density of GDV and thus a larger use of groundwater by the vegetation.…”