4 Discussions
In this study, we found that increasing water availability increased the
total biomass of three of the four target species. However, the total
biomass of S. grandis followed a unimodal pattern with increasing
water availability. Interestingly, the four target plant species adopted
different strategies to deal with water availability, where with
increasing water availability, R: S of species L. chinensis andA. frigida followed an unimodal pattern, where the lowest points
were 83 mm and 475 mm, respectively. L. chinensis has strong
forage ability as a rhizomatous species (Wang et al. 2004), which
enables this species to allocate more biomass to its roots when growing
on wet conditions (Yang and Yang 1998). R: S of species P.
acaulis decreased, while R: S of species S. grandis was not
significantly affected. Our findings are in line with previous studies
that plants modified their biomass allocation strategies to adapt to
their growing conditions (Schmid 1987; Enquist et al., 2002). We
expected that species dominant in lightly and heavily degraded
grasslands responded differently to the water availability. In line with
our expectation, R: S of A. frigida and P. acaulidecreased, to some extent, with the increasing water availability (Fig.
5c-d). Such findings are also consistent with the results of species
such as Salix psammophila , Hedysarum leave ,Artemisia ordosica and Caragana korshinskii in the
previous studies (Dong et al., 1999; Xiao et al., 2001). However, the
responses of R: S of L. chinensis and S. grandis to
increasing water availability in this study indicate that some other
strategies merit further research.
A unimodal relationship between water availability and total biomass was
found in species S. grandis and A. frigida , where the
highest points were 519 mm and 850 mm, respectively. In other words,
increasing water availability firstly increased and then decreased their
total biomass when the rainfall became detrimental, which might be
caused by a lack of oxygen since it is fatal to the root respiration
(Lambers and Steingrover 1978). Such result indicates that speciesS. grandis and A. frigida could tolerate dry conditions,
while the latter species has a much wider range of water availability.
Under extreme wet conditions, both S. grandis and A.
frigida decreased their total biomass, while the former reduced both
its aboveground biomass (Fig. 3b) and belowground biomass (Fig. 4b),
while the latter decreased only its aboveground biomass (Fig. 4c). Such
difference could be a key driver of species coexistence (Sanaei et al.,
2018). Species adopted different biomass allocation strategies to
respond to water availability, suggesting that fitness of some species
such as S. grandis is not significantly affected by the water
availability, and such dominant species could further impact the
community structure and ecosystem functioning. However, increasing water
availability inhibited the growth and productivity of species such asA. frigida and P. acaulis , but improved the plant fitness
through reducing the availability of the grasslands (Neilson 1995).