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).