Water use by broadleaved tree species in response to changes in precipitation in a mountainous area of Beijing

Extreme weather events are expected to occur more frequently in the future, which may influence water distribution and uptake patterns in rocky mountainous areas of North China. To examine how plants respond and adapt to the extreme environment, we investigated water sources of the broadleaved tree species Quercus variabilis under a precipitation gradient (A: no natural precipitation, B: half of natural precipitation, C: natural precipitation, and D: twice the natural precipitation) using the stable isotope approach in the Beijing mountainous area. The results indicated that Q. variabilis exhibited considerable plasticity in the depth of water uptake and showed a strong dependence on deeper soil layers under the precipitation gradient. Q. variabilis mainly absorbed water from the 0-20 cm (20.02-28.61%) and 60-80 cm (22.81-29.44%) soil layers, and also used groundwater (26.32-30.54%) under the low soil water conditions of the A treatment. In the B treatment, Q. variabilis absorbed water from the groundwater (29.60-33.53%) and 60-80 cm (26.05-29.44%) soil layers and a small amount of water from 0 to 20 cm (10.67-12.25%). In the C and D treatments, the soil water content was greater than that of the B treatment; Q. variabilis still predominantly absorbed water from the 60-80 cm (26.88% and 27.35%, respectively) soil layer and groundwater (33.52% and 46.17%, respectively), and took minimal water from the 0-20 cm and 20-40 cm soil layers. We found that Q. variabilis showed no obvious responses to precipitation under the precipitation gradient, and its dimorphic root system allowed it to uptake different water sources under drought and humid environmental conditions and thus maintain its transpiration. Therefore, we can infer that Q. variabilis is well adapted to extreme precipitation events and has a strong adaptability to both extreme drought and flood conditions.