4.1 Effects of gravel content on vegetation restoration

The differences in soil physical structure and nutrients with different gravel contents directly affect the development of vegetation communities (Bhattacharya., 2021; Briat et al., 2020). The results of our study in the southeast Tibetan cold wet regions are different from that in other alpine arid areas in Tibet (Wang et al., 2011).The physical and chemical properties of soil with low gravel content are better than medium and high gravel content (Table 1). An increase in soil gravel content does not necessarily promote soil total porosity and plant biomass, indicating that the interaction between gravel and fine soil changed the soil skeleton structure, so that soil with high gravel content had more large voids than soil with medium gravel content, which reduced root mechanical resistance. Increased soil water content in the soil available space promoted the development of plant traits (Table.1, 2, Fig. 1) (Clark et al., 2003; Shi et al.,2012; Zhang et al.,2016). Appropriate gravel content would maintain low bulk density and high porosity (Gargiulo et al., 2016), which is conducive to water infiltration and solute migration (Zhou et al., 2009), and further to the absorption and utilization of nutrients by plants. This lays a foundation for the vigorous metabolism of plants and adequate formation of photosynthates (Ye et al., 2021). However, excellent water infiltration and solute migration in soil with 50% gravel content will inevitably lead to poor soil water retention and nutrient loss, resulting in water and nutrient stress on plants (Wang et al., 2011; Ceacero et al., 2020). In our study, there was a significant positive correlation between plant traits and total porosity (p<0.01), continuous rainfall during the growing season alleviated soil water stress and provided abundant water resources and concentrated transport capacity of soil nutrients for plants in the eastern Tibetan Plateau (Table 3 and 4).
Table4 RDA sequencing of soil properties affecting plant traits.
Note: Fe, Festuca elata ; Ed, Elymus dahuricus ; Ms,Medicago sativa ; Fe+Ed, Festuca elata plus Elymus dahuricus ; Fe+Ms, Festuca elata plus Medicago sativa ; Ms+Ed,Medicago sativa plus Elymus dahuricus ;Fe+Ms+Ed, Festuca elata plus Medicago sativa plus Elymus dahuricus ; TN, soil total nitrogen; TP, soil total phosphorus; SOC, soil organic carbon; BD, bulk density; e , total porosity.
The abundance of soil nutrients will promote plant growth (Vesic et al., 2020; Wu et al., 2022). For instance, soil with 10% gravel content had richer nutrients, better water retention, and lower bulk density, resulting in significantly better plant growth than soil with 50% and 30% gravel content (Table 2 and Fig. 1) (Yang et al., 2022). Further study showed that the contents of TN, TP and SOC in rhizosphere soil, roots and plant aboveground parts of low gravel soil were significantly higher than those in soil containing 50% gravel and soil containing 30% gravel. Plant traits were closely related to the measurement characteristics of C, N and P. With the increase in growth rate, the N:P and C:P of plant organs tended to decrease. P content showed an increasing trend (Fig. 2) (Yan, 2022). At the same time, TN content in the above-ground parts of plants with 50% and 30% gravel content is lower than that in other parts of China, while TP content is higher than that in other parts of China, and there is a positive and significant correlation between TN and above-ground parts of plants, indicating that plant growth in the soil with 50% and 30% gravel content is also limited by nitrogen (Table.4) (Tang et al., 2018).

4.2 Effects of plant species on vegetation restoration

Plant functional trait expression is closely related to plant environmental adaptability and ecosystem structure and function. Due to the harsh, unique and sensitive natural environment and short growing season, the average height and total biomass of Festuca elata andMedicago sativa were significantly lower than those of Inner Mongolia and the Loess Plateau (Fang et al. 2021; Ren et al. 2022; Liu et al. 2022). At the same time, the average height, total coverage and total biomass of Festuca elata and Medicago sativa plants in our study were significantly higher than those of Elymus dahuricus (p<0.05) (Fig. 1), indicating that in the ecological restoration of the Qinghai-Tibet Plateau, Both Festuca elata andMedicago sativa can achieve good ecological restoration effect (McNickle and Brown 2014, Weidlich et al. 2018), but monotone landscape effect is easy to be formed by unicast plants. Moreover, the single life history strategy of Festuca elata and Medicago sativaunicast communities is difficult to cope with the long-term changes of soil with high gravel content and complex ecological environment in the Tibetan Plateau (Bennett et al. 2016) (Fig. 1 and 2). At the same time, it was also confirmed that the mixed planting effect of different niche plants such as Festuca elata and Medicago sativa was better than that of single planting of Festuca elata andMedicago sativa in the ecological restoration of the Tibetan Plateau, because the mixed sowing of Gramineae and legumes avoided the interspecific competition to a certain extent. The intraspecific competition was alleviated, the harmonious growth relationship was presented, and the community biomass was promoted (Sturludottir, xie et al., 2020; Diaz et al. 1998), however, Festuca elata plusElymus dahuricus with heterogeneous and niche plants had adverse effects on community productivity due to the same life-history strategies and similar soil resource requirements (Huangfu et al., 2021, Miao et al. 2022) .
In general, soil bulk density and total porosity were the main factors affecting plant traits in all planting patterns (Fig. 1, 2, 5). However, the responses of plant traits to soil physical structure varied greatly among different plant species and planting patterns and the community may not be affected by nutrients, For example Medicago sativa andMedicago sativa plus Elymus dahuricus were affected by soil compactness (p<0.05). Total porosity had the strongest explanatory power for shallow-rooted, fast-growing tall fescue (p<0.01) (Persi et al. 2022) (Fig. 1, 4, 5). Elymus dahuricus was less affected by nutrients in terms of nitrogen fixation of root rhizobium and higher TP content in the soil, However, soil with 30% and 50% gravel content could not provide enough nutrients forFestuca elata and Festuca elata plus Elymus dahuricus . Plant growth is limited by TN (Gusewell et al., 2004; Narangerel, 2022; Leinauer, 2021) (Fig. 4, 5). Most previous studies on the Qinghai-Tibet Plateau focused on the restriction of soil nitrogen and phosphorus on plant growth (Shaver et al., 1980; Hong et al., 2014). In our study, SOC all had a significant positive correlation withFestuca elata , Lymus dahuricus , Festuca elata plus Elymus dahuricus , and Festuca elata plus Medicago sativa(p<0.01), which may be due to the low organic matter in the gravel soils, which prevented plants from absorbing water-soluble organic matter and inhibited the development of plant roots and shoot parts (Jones et al., 2012; Christ et al., 1996). The study also found that the total biomass of Festuca elata plus Medicago sativa plus Elymus dahuricus was significantly higher than that of other seeding modes (P<0.05), but did not receive nutrient restriction. This may be due to the relatively reasonable proportion of ecological factors of each component in mixed sowing, which can enhance spatial complementary, increase water and nutrient availability, reduce climate pressure or improve soil stability, and thus promote community yield in the short term (Chen et al. 2022; Huangfu et al. 2022; Craine et al. 2006) (Fig 1, 4, 5).