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).