Models of climate change predict a global increase in atmospheric temperatures by the end of the century; hence future droughts will be superimposed upon warmer conditions. Persistence of terrestrial vegetation under climate change largely depends on adaptation of plants capacity to exploit soil water resources. However, little is known on the water uptake dynamics and their consequences on tree functioning in response to atmospheric warming combined to reduced precipitation, although it may be an important adaptive response of trees to climate change. In a semi-arid woodland, seasonal dynamics of water extraction depth were analyzed through stable water isotopes in soil and xylem samples of piñon pine (Pinus edulis) and juniper (Juniperus monosperma), subjected to three years of precipitation reduction, atmospheric warming and their simultaneous effects. Predawn leaf water potential, maximum CO2assimilation and stomatal conductance were measured simultaneously on all trees.
During the summer, juniper trees subjected to precipitation reduction acquired water from deeper sources relative to ambient trees while water was taken up more superficially under warming and when both stressors acted at once. During the summer and autumn, piñon trees subjected to precipitation reduction and both stressors simultaneously took up water more superficially or deeper, respectively, relative to ambient trees. For both species, changes in water uptake depth in response to the treatments were associated with higher water stress and lower gas exchange rates. Our findings highlight that climate change could modify water sources of trees. Temperature rise may lead to maladaptive acclimation of juniper, decreasing its strong plasticity in water uptake depth. Inversely, drier soil conditions may alter the activity of superficial roots and decrease the ability to exploit shallow water resources for piñon. Overall, this study demonstrates that adaptive acclimation in the use of water sources may not necessarily occur to increase the resistance of forests to climate change.