Abstract

Neltuma tamarugo is a strict phreatophyte endemic to the Atacama Desert whose persistence is threatened by groundwater decline. To characterize its hydraulic strategy, we assessed stomatal conductance, leaf hydraulic conductance, and whole-plant hydraulic conductance under semi-controlled water stress, and analyzed the relationship between predawn and midday water potential under both semi-controlled and field conditions in the Llamara Salt Flat under contrasting levels of groundwater decline. Under semi-controlled conditions, stomatal closure and near-zero whole-plant hydraulic conductance occurred at very negative midday water potentials (approximately - 4.33 and - 4.49 MPa, respectively), well below the turgor loss point and leaf hydraulic P-50, and close to leaf hydraulic P-88. Water stress also reduced branch growth and leaf area, followed by leaf shedding. In both semi-controlled and field conditions, predawn and midday stem water potentials were strongly coupled, with slopes near unity and relatively large Hydroscape Areas, indicating predominantly anisohydric behavior. In the field, trees under severe groundwater decline showed a lower Psi(tlp), suggesting some acclimation, although midday water potentials still frequently fell below the turgor loss point. These results indicate that N. tamarugo maintains function under severe water deficit through anisohydric regulation and canopy-level adjustment. However, continued groundwater decline likely reduces functional performance and increases long-term mortality risk, highlighting the need to prevent further phreatic decline in this threatened species.