4.3. Implications for meadow eco-hydrological processes
In this study, δ18O and δ2H at the grassland site were greater than at the shrub site during the growing season, but almost the same during the withering season, indicating that grassland plant water had a stronger dynamic fractionation effect in the process of transportation and was more sensitive to environmental conditions. Furthermore, plants at the shrub site displayed more flexible water use patterns that shifted shrub water sources between shallow water and deep soil water, and the grassland site was more susceptible to drought stress. From this perspective, alpine shrub sites on the QPT were formed from the long-term encroachment of P. squamosa . This result is consistent with the results published previously by many other researchers who found that shrub populations are a result of the proliferation and range expansion of woody plant species in arid and semiarid grassland ecosystems (Van Auken, 2000; Maestre et al., 2009; Archer et al., 2017) , and that areal expansion of shrubs is one of the most threatening forms of grassland degradation in arid and semiarid areas (Eldridge et al., 2011). Shrubs from alpine grassland leaded to changes in grassland water use, thereby changing soil water storage(Li et al., 2022).
Many previous studies have shown that vegetation has profound effects on maintaining local hydrological processes (Feng, et al., 2016; Jia et al., 2017). Plant water use strategies can be used to exploring all available water sources by isotopic compositions of xylem water (Wu, et al., 2019). The contrasting plant water use patterns identified in our study contribute to the differential sensitivity to interannual variations of available moisture input. Hence, appropriate management measures, such as recovering natural environmental characteristics and moisture status, should be implemented to maintain grassland ecosystem sustainability.
5. Conclusions and future directions
The δ18O and δ2H values of precipitation, soil water, and plant water varied significantly over months and water sources at the alpine grassland and P. fruticosashrub sites on the QTP. The relationships of δ18O and δ2H indicated that both soil evaporation and plant transpiration at the P. fruticosa shrub site were relatively lower than they were at the alpine grassland site. The grassland plant water had a stronger dynamic fractionation effect in the process of transportation and was more sensitive to environmental conditions, but the plants at the P. fruticosa shrub site displayed more flexible water use patterns that shifted shrub water sources between shallow soil water and deep soil water. Moreover, the relationship in δ18O and δ2H between precipitation and plant water, and the factors influencing precipitation maybe resulted from various possibilities. These results promote better understanding of the interface between plant and surrounding soils between P. fruticosa shrub and alpine grassland sites, provide guidance for meadow management from the perspective of eco-hydrological processes on the QTP.
Although our study used advanced technology exploring the interrelation of soil water and plant water at the alpine grassland and P. fruticosa shrub sites and drawn important conclusions in-depth understanding of eco-hydrological processes, the factors influencing δ18O and δ2H values maybe resulted from various possibilities. These possibilities need to be further examined through greater variety of species, more potential water sources, multi-site continuous observation, and longer time scales in the future.