4.2. Comparisons between P. fruticosa shrub and alpine
grassland
A series of water line equations between δ18O and
δ2H were established to compare the differences in
water interaction from shrub and grassland sites (Figure 5). Soil and
plant water samples were well described by linear regressions, resulting
of laying at an angle to the LMWL, this is consistent with previous
studies (Goldsmith et al., 2012; Evaristo et al., 2015; Che, et al.,
2019). The slopes and intercepts of these water lines were determined by
the relative evaporation rates of the different water isotopes (Crawford
et al., 2014; Benettin et al., 2018; Bowen et al., 2018; Chi et al.,
2019), indicating the different magnitudes of evaporative enrichment of
isotopes in soil water and plant water. The slope and intercept of soil
water at the grassland site (5.46 and -8.66, respectively) were slightly
lower than those at the shrub site (5.83 and -4.83, respectively),
suggesting soil evaporation was slightly greater at the grassland site
than at the shrub site. This was probably because the shrub site had a
denser coverage shading the soil surface compared with the grassland
site, just as daily evaporation rates were slightly lower for understory
vegetation than for grassland during the growing season (Crawford, et
al., 2014; Schwärzel et al., 2020). Nevertheless, the slopes and
intercepts of δ18O and δ2H in plant
water at the grassland site (2.46 and -24.57, respectively) were higher
than the slope and intercept at the shrub site (1.54 and -33.75,
respectively). The differences likely resulted from substantial
variabilities for leaf water at shrub and grassland sites, evaporative
distinctions of 2H/1H and18O/17O on the primary of leaf
water(Farquhar et al., 2007), and the transpiration from an area of
grass is greater than the transpiration from a similar area of shrubs.
Because the two sampling sites neighbored each other, they had similar
topographical and geological conditions, and precipitation
δ18O and δ2H were also similar. The
δ18O in soil water and plant water showed significant
differences between shrub and grassland sites (p < 0.05) in
contrast to the non-significant differences in δ2H
between the two sites (p = 0.068 and 0.06 for soil water and plant
water, respectively), resulting in different slopes and intercepts for
the water line equations from soil water and plant water between the two
sites. Isotope-fractionated differences between δ18O
and δ2H were probably associated with local
microenvironment and heterogeneity differences of surroundings between
the grassland and shrub sites despite the same general conditions
(Ellsworth&Williams, 2007).