**4. Discussion**

#### *4.1. Effects of Habitat Conditions on Vegetation*

The arid-hot valley has fragile habitats because of the highly variable spatial and temporal distribution of precipitation, and barren nutrient conditions. Our results showed large differences in soil moisture between the rainy and dry seasons, between the shade and sunny slopes, and between elevations, which is consistent with previous research results [6,29]. The soil moisture at an elevation (≤1520 m) averaged 0.055, and was below the wilting point of 0.059 for *D. viscosa* [30], possibly because the continuously intense evaporation during the dry season leads to low soil moisture [31]. Soil moisture was higher at 1380 m than 1440 m, and soil fertility was highest at the lowest elevation (1380 m), probably due to the increased relative humidity near the river. In fact, increased relative humidity and livestock grazing at lower elevations would accelerate the decomposition and accumulation of nutrients in the soil [32]. Our results also show differences in vegetation biomass and morphology along the elevation gradient, though the two species showed different patterns: *D. viscosa* generally either increased in size and then decreased with increasing elevation, or decreased steadily, whereas *P. yunnanensis* tended to increase in size with increasing elevation.

It will important to build on the present results to improve our understanding of the key factors that affect the growth, development, and distribution of organisms along the elevation gradient in our study area. The complex and diverse relationships between soil properties and topographic conditions affect the composition and biodiversity of the regional vegetation community [3,33], and provide insights into the characteristics that will make a plant suitable for local vegetation restoration.

Our RDA results sugges<sup>t</sup> that topographic conditions explain more of the vegetation variation than soil properties, especially for elevation. This results from the high mountains and deep valleys

in the Jinsha River basin. Rossi et al. [34] showed that topographic conditions, and especially the elevation and slope, could explain local vegetation patterns well in high mountain areas. The significant correlations among the elevation, distance from quadrats to the river, and soil moisture, and their significant effects on the biomass and height of *D. viscosa* and *P. yunnanensis*, confirmed that elevation was the dominant factor that affected vegetation growth and distribution in our study area. Elevation is often regarded as a comprehensive physical index that integrates the effects of changes in precipitation, temperature, relative humidity, and other environmental conditions, all of which affect soil properties and vegetation growth. Close relationships among the elevation, soil moisture, and vegetation were also found in China's Loess Plateau [11]. The topographic wetness index has also been strongly correlated with vegetation cover, indicating that it can reflect the overall growth of the vegetation community through its ability to account for microtopographic water distribution and perhaps salinity redistribution [35]. *D. viscosa* and *P. yunnanensis* are widely distributed in our study area, due to their strong tolerance for drought barren soil, so the correlation was weak between vegetation parameters and soil nutrients in our study region.

The decreased precipitation and intense evaporation in the dry season exacerbate the effects of drought stress at low elevations, and led to little or no growth of *P. yunnanensis*, but sparse growth of *D. viscosa* on the sunny slopes. The slightly higher soil moisture on the shade slopes could be crucial to help plants survive the drought period, and this may be the main reason for differences in the plant community and ecological landscape in the arid-hot valley [29]. At an elevation of 1640 m, the growth of *D. viscosa* decreased, whereas *P. yunnanensis* responded strongly to the obviously increased soil moisture at that elevation, suggesting that the increase of soil moisture, combined with the lower temperature at that elevation, was more beneficial for the growth of *P. yunnanensis*. Xiong et al. [36] thought that the geotechnical properties of the site, such as infiltration and lithology, determined the soil moisture and vegetation types in arid-hot valley, but thought that elevation was not important. However, the results of our RDA analyses (Figures 5 and 6) showed that elevation and distance from the quadrats to the river provide a comprehensive index that reflects the change of community characteristics, especially for biomass. Thus, understanding the soil moisture conditions at each site will be a key to successful vegetation restoration.

#### *4.2. Restoration Potential of P. yunnanensis and D. viscosa*

Different plant types have different adaptation mechanisms that determine how they respond to environmental stress. Under drought stress, *D. viscosa* decreases its net photosynthetic rate and improves its water-use efficiency [37], whereas *P. yunnanensis* distributes more photosynthate to root organs and reduces water consumption by stems and leaves [38]. Thus, *D. viscosa* can still survive when soil moisture is extremely low. However, when the two species coexist under suitable soil moisture levels, *P. yunnanensis* grows more vigorously, and its shade may limit the growth of *D. viscosa*, in which the overhead shading lead to the decreasing of *D. viscosa* biomass and increasing of soil moisture. Based on these different responses, it should be possible to select the most appropriate species according to soil moisture conditions at each site where vegetation restoration will occur. Without performing this analysis, the restoration may fail or achieve poor results, and if the species uses more water than the site can sustainably provide, this may exacerbate soil drought [39,40]. Elevation was negatively correlated with *D. viscosa* growth, but positively correlated with *P. yunnanensis* growth, indicating that drought-tolerant *D. viscosa* should be restored at the lower elevations and on sunny slopes, where drought stress is most likely to be severe. The positive correlation between plant parameters for slopes suggests that a suitable slope is beneficial for the growth of *D. viscosa*. *P. yunnanensis* was not sensitive to slope, but its growth was negatively correlated with total nitrogen and positively correlated with available phosphorus. Controlled experiments found that *P. yunnanensis* seedlings could adapt well to nitrogen stress, but were obviously affected by a phosphorus deficiency [41], but a field investigation showed that nitrogen and phosphorus were positively correlated with the growth characteristics of *P. yunnanensis*, with the greatest constraint created by low nitrogen [42]. Although response mechanisms may be different, we concluded that soil nutrients is important for *P. yunnanensis* restoration.

For successful vegetation restoration in the arid-hot valley, we should focus on which factors will most strongly constrain survival and growth; our results and previous research sugges<sup>t</sup> that soil moisture will be critical. *D. viscosa* is highly resistant to drought stress, and also develops considerable biomass, so it is a promising species for vegetation restoration in arid and rocky sites, such as those in our study area. In addition, it has ornamental and medicinal value [43,44]. Some experiments in arid-hot valley have shown that *D. viscosa* grows fast, sprouts many tillers, and can restore a degraded site rapidly [45]. However, *D. viscosa* may be unable to survive drought stress combined with competition from other species and damage caused by grazing, especially at the seedling stage; thus, it requires suitable protection, such as clear weeds, moderate grazing, and increased soil moisture using a water-retaining agen<sup>t</sup> [46], to ensure its survival and growth. *P. yunnanensis* is also widely distributed in our study area, and has produced an obvious improvement of water yields, thereby reducing soil erosion and improving water storage, which together can improve soil carbon storage [47]. However, it is less able than *D. viscosa* to withstand drought and poor soil fertility, so the survival and growth of *P. yunnanensis* plantations could be improved by soil managemen<sup>t</sup> supplying sufficient nitrogen and phosphorus to mitigate any soil limitations, combined with scientific planting at an appropriate density, which agrees with previous recommendations [19]. In actual vegetation restoration, we must also consider the economic suitability and maintenance needs of the selected resistant plants, which could be considered in our future researches.

Through sampling and analysis, we have improved our understanding of the relationships among vegetation, topographic, and soil properties in the arid-hot valley. In the future, additional work should be carried out to build on our findings. First, we performed our field study only during two typical seasons. Continuous monitoring of surface soil moisture throughout the year, along with changes in plant physiology, such as the degree of water stress, would provide a clearer understanding of its variation and its effects on plants. In addition, we based the relationship between vegetation and environmental conditions mainly on correlations in field data, rather than using controlled experiments that would allow a detailed exploration of the underlying mechanisms. Future research should focus on such experiments to clarify the underlying processes that define the relationships between the vegetation and environmental factors. Finally, it will be necessary to understand the water balance in the study ecosystem based on the relationships among plants, topography, and soil, so that we can choose suitable restoration species for each combination of these conditions, and improve the likelihood of successful restoration.
