4.3. Implications for grassland shrubbization of alpine meadow
The plants at alpine P. fruticosa shrub extracted water from the
middle soil layer to deep soil water preferentially from May to June,
and extracted water from shallow, middle and deep soil in a balanced
manner from July to October. The results were different from the results
carried out on north of Qilian Mountains, where has a drier climate.
Their results had shown that P. fruticosa used the shallow soil
water except for August, when used the middle soil water (Zhang et al.,
2022). This difference may be due to the different ages of plant (Wang,
et al., 2021a), the different geographical locations (Li et al., 2022),
or the different symbiotic species (Wu, et al., 2019). Additionally, the
different sources of the soil water from the eco-hydrological processes
maybe another reason.
Both δ18O and δ2H at the grassland
site were enricher than at the shrub site during the growing season, but
almost the same during the withering season. Grassland was more
susceptible to drought stress, plants at grassland site had a stronger
partitioning effect in dynamic transport, comparing with plants at the
shrub site displayed more flexible water utilisations. From the
perspective of plant water use patterns and evolution, alpine shrub on
the northern QPT were formed from the long-term encroachment of P.
fruticosa . This result is consistent with the results published
previously by Klein et al. (2007) , who found that shrub populations are
a result of proliferation and expansion of woody plant, and that areal
expansion of shrubs is one of the most threatening forms of grassland
degradation (Eldridge et al., 2011) . The contrasting plant water use
patterns identified in our study provided theoretical basis for the
alpine grassland shrubbization, contributed to predict vegetation
functional dynamics and the ecohydrological process under current and
future climatic conditions should take species-specific plasticity and
responses to short-term water supply fluctuations into account.
5. Conclusions and future
directions
The δ18O and δ2H values of
precipitation, soil water, and plant water varied significantly over
months at the alpine grassland and P. fruticosa shrub sites on
the northern QTP, those of groundwater remained relatively stable during
the sampling period. Considering groundwater, precipitation, soil water,
and plant water, a progressive enrichment in δ18O or
δ2H existed from groundwater and precipitation to soil
water to plant water for each month. Alpine grassland was more
susceptible to drought stress, had a stronger partitioning effect in
dynamic transport than shrub. The P. fruticosa shrub displayed
more flexible water utilisations, and was more competitive for water
than grasslands. Furthermore, the plants in alpine shrub and grassland
reached water use balance in August to October. Shrubs degraded from
alpine grassland changed water use pattern of grassland, thereby
changing soil water storage. These results contribute to understand the
alpine grassland shrubbization from water use patterns of grassland and
shrub plants on the northern Qinghai-Tibet Plateau, promote better
understanding of the interface between plant and surrounding soils from
the perspective of eco-hydrological processes on the QTP.
Our study explored the interaction of soil water and plant water at the
alpine grassland and P. fruticosa shrub sites using stable
isotopes, drawn important conclusions in the process of alpine grassland
shrubbization. However, greater variety of species, more potential water
sources (dew, creek and deeper groundwater), multi-site continuous
observation, and longer time scales need to be took into consideration
to further examine in the future.