In our study, PFGs responded differently to environmental changes and community competition. In this study area, the climate is dry and cold (Fig. 1), so the dominant functional groups of the community are grasses and forbs (Fig. 2). This is mainly because the forbs are the main dominant PFGs in the desert area and has strong adaptability to the arid environments (Ning et al., 2017). On the other hand, grasses have a conservative resource-use strategies and are strongly cold-tolerant, so they can adapt well to high altitude environment (Suding et al., 2015). This is also consistent with the finding from other study area of the central Tibetan Plateau at the same altitude, where the grassland vegetation types are dominated by grasses (Niu et al., 2019). At the same time, previous studies have shown that the growth of legumes will consume more water, and legumes could not survive in nutrient-poor desert areas, especially mobile dunes (Guo et al., 2017; Cui et al., 2018). Therefore, in this study area, the dominant functional group of each sample site was forbs or grasses, and the importance value of both reached more than 50% of the community. The mass ratio hypothesis predicts that the species with the greatest proportion in the community will have the greatest impact on the ecological function of the community (Grime, 1998). In this study, forbs fit this hypothesis and have significant correlation with community biomass; however, the grasses, legume and sedge did not fit the hypothesis, and the increase of grass functional groups only affected the root biomass of the community (Table 1). The main reason is that grass function group allocates less photosynthate to roots than to aboveground organs in alpine regions (Wu et al., 2013). The results indicated that the forbs had a dominant influence on the community in this area. This was different from the previous functional group removal experiments in arid northern grassland of Canada, in which the effect of grasses functional group on community biomass was dominant (Mclaren and Turkington, 2010).
In addition to community biomass, PFGs are also related to community species diversity. Within the study area, species diversity, richness and evenness of the community were significantly affected by legumes (Fig. 3). The results indicated that the legumes functional groups reflected the community succession and development in the alpine grassland communities with forbs and grasses as dominant functional groups (Hu et al., 2016). In addition, the dominant functional groups determined the community structure, but could not reflect the development trend of the community. In other words, if a plant community is dominated by a few functional groups, then differences in species composition will have a greater impact on the whole system (Hooper and Vitousek, 1997). Therefore, the discovery of legumes functional groups has a large effect on the prediction and discovery of ecosystem functioning in degraded grassland and extreme climate zones (Spehn et al., 2002; Scherer-Lorenzen et al., 2003). However, in previous studies in arid areas, legumes could not survive in nutrient-poor areas (Guo et al., 2017), so the changes of different PFGs in the community were related to nutrients.
Soil nutrients are the basis of plant growth, and the allocation of plant nutrients reflects the ecological strategies of plants to cope with environmental changes (Elser, et al., 2010; Craine and Dybzinski, 2013). In this study area, forbs and grasses were the main components of plant communities, so the P concentration of plant communities and the C, N, P concentration of underground roots were mainly related to the functional groups of forbs and grasses (Fig. 4). This is mainly because plants spend more resources to roots in less diverse and resource-poor environments, which is consistent with the alpine grassland ecological strategy of invest preferentially in structures for persistence (K-strategy) in previous studies (Patty et al., 2010; Ning et al., 2021). The legume functional group, as the key functional group affecting community diversity, was only significantly correlated with P in the community (Fig. 4). Studies have shown that legume are important species affecting plant community diversity. These results indicated that P content was an important factor limiting grassland species diversity (Kattge et al., 2011). Nutrient limitation affects the composition of community functional groups and response strategies to environmental change. The study area is located at a high altitude of more than 3700m, where the climate is dry and the soil nutrient content is low. In all the communities we studied, grass and forb were significantly correlated with soil P concentration (Table 3), and as dominant functional groups, grass and forb had important effects on community composition. These results indicated that soil P content was the main limiting factor affecting the plant community construction and the changes of main functional groups in this study area. This is consistent with previous studies that plant growth on the Qinghai-Tibet Plateau is mainly restricted by soil N and P, and the availability of N and P nutrients has an important effect on species composition and community structure (Janssens et al., 1998; Nie et al., 2010). The functional groups of legumes affected the contents of C and N in soil, the changes of nitrogen fixation and community composition affected the absorption and transformation of C and N in soil. (Hooper and Vitousek, 1998; Spehn et al., 2002). In the alpine desert grassland, the increase of legumes function group not only affected the diversity of community species, but also reduced the loss of soil C and N (Wen et al., 2013; Wu et al., 2018). The results indicated that different plant functional groups would adapt to the changes of external environment according to their own traits under the same resource environment. In addition to soil nutrient, soil moisture is also one of the important factors affecting plant growth (Angers and Caron, 1998; Xu et al., 2018). However, in our study, only sedge function group and soil moisture had significant correlation, while the major functional groups (forbs and grass) in the community showed no response to soil moisture (Table 3). The results indicated that functional groups such as grasses and forbs were less dependent on soil moisture than legumes and sedges (Cui et al., 2018). This is consistent with the previous researches that the increasing functional groups of forbs and grasses in arid areas can reduce the consumption of soil water and have a strong adaptability to arid habitats (Wu et al., 2019). These studies suggest that the desert PFGs can maintain a high utilization efficiency for soil nutrients under resource-limited conditions as a result of the function group’s plasticity and ability to adapt to extreme environment (Aerts and Chapin, 1999; Berendse and Aerts, 1987).
As an indicator of changes from species to community level, PFGs are effective tools for biomass allocation, diversity change and community composition (Wu et al., 2013; Bora et al., 2020). Changes in PFG tend not to be influenced by a single factor, but by multiple factors, such as rainfall patterns, elevation gradients, nutrient availability, and atmospheric CO2 (Cramer et al., 2001; Weltzin et al., 2003). In this study, grasses and forbs, as dominant functional groups, were strongly responsive to a number of abiotic factors (Table 4; Fig. 5). There was a strong direct correlation between grasses and forbs with community nutrients (N and P). In the analysis of the functional of forbs, the aboveground community N concentration had a negative effect, while the underground root N concentration had a positive effect, but the results for grasses were the opposite of those for forbs (Table 4; Fig. 5). This is mainly because most grasses have highly branched fibrous root systems than forbs plants and can absorb nutrients in the surface soil more efficiently (Chapin et al., 1995; You et al., 2017). Therefore, the forbs allocate more nutrients to the root system to sustain growth, while the grasses allocate more nutrients to the growth of above-ground organs. This indicates that the plant community construction in this study area is mainly affected by soil N content, which is consistent with the current studies that the Qinghai-Tibet Plateau is mainly affected by N and P in previous studies (Nie et al., 2010). In addition, grasses had negative effects on soil moisture and soil TP while being affected by N content (Table 4 b; Fig. 5 b). These results indicate that grasses have more competitive advantages over the fords due to their conservative resource-use strategies in extreme environments (Suding et al., 2005). This is inconsistent with our third hypothesis.