**1. Introduction**

The high temperature and low humidity in the arid-hot valley results from a combination of local climatic conditions with strong sunlight and valley wind (particularly winds caused by convection during a forest fire) and evapotranspiration. Poor soil water retention capacity intensifies the difficulty of vegetation growth, leading to serious soil erosion, threatening the human and environment security of Yangtze River basin, especially in the main stream of Jinsha River (the upper Yangtze River) [1]. Since the 1990s, plant introduction and afforestation projects have been carried out in the arid-hot valley in southwest China, but we still do not understand the factors responsible for success or failure of these projects [2].

Exploring the relationship between vegetation and environment is the key to successful vegetation restoration, particularly on mountainous terrain. Many studies have focused on potential impact factors, including climatic factors, topographic conditions, soil properties, and human disturbance [3–5]. A deficiency of soil moisture results in sparse vegetation and decreased restoration effectiveness in the arid-hot valley and, especially at the end of the drought season, the values of soil moisture may be lower than the plant-wilting coefficient [6,7]. In addition, topography plays an important role in vegetation growth and distribution [8], since it affects many aspects of the plant's environment; higher solar radiation and less available water on the sunny slopes of semi-arid valleys would exacerbate

drought stress, resulting in the survival of fewer species, a lower plant density, and lower growth rates of the surviving plants [9]. The literature shows that topographic conditions and soil properties are closely related to plant community composition and distribution [10], vegetation cover [11], and biodiversity [12]. To improve ecological restoration efficiency, it is necessary to deeply understand the relationships between the targeted native vegetation and the local environment (e.g., topography, soil, other habitat conditions), and to identify the key factors that affect the growth, survival, and distribution of vegetation in the arid-hot valley [13,14].

The Jinsha River basin in southwestern China has a typical arid-hot valley, and provides the important source of hydroelectric power, and plays a vital role in ecological security for its special ecosystems and rich biodiversity. Since economic development has been driven by the hydroelectric project, dam and reservoir construction have destroyed much local vegetation, thereby exacerbating the contradictions between civilization and nature. Managers of the project recognize this problem, and have looked for ways to promote greener and more sustainable development, including vegetation restoration projects and soil erosion control [14–16]. Many studies on the change of vegetation cover [17], mechanisms and degree of vegetation resistance, soil characteristics and fertility, and restoration techniques and benefits have been carried out [18]. The dominant native plants, *Dodonaea viscosa* and *Pinus yunnanensis,* with high stress resistance and restoration benefit, are important for soil and water conservation, and both are, therefore, key plants in ecological restoration projects. These native plants have attracted much attention from many researchers, but most of the research selected a single factor, such as soil quality, slope and elevation, to statistically analyze its effect on vegetation; even a few studies have focused on more factors, but just discussed the qualitative influence and were carried out only in the tributaries of the Jinsha River [19]. *D. viscosa* biomass is significantly affected by both slopes and aspects [20], and transpiration of *D. viscosa* is strong during both the rainy and dry seasons [21]. *P. yunnanensis* is widely distributed, but grows poorly on sunny slopes, even though it can tolerate low soil moisture and nutrient levels and, especially, a very low phosphorus content [22,23]. However, there has been no integrated analysis of how environmental factors affect these native plants of the Jinsha River valley.

Using the Longkaikou Reservoir region in the Jinsha River basin as a case study, we obtained field data about changes in the characteristics of *D. viscosa* and *P. yunnanensis* across an elevation gradient, including the vegetation biomass and the response to climatic, topographic, and soil characteristics, by means of multiple field surveys to analyze the differences in soil and vegetation as a function of topography. Our goal was to reveal the relationships among the topographic and soil variables and the biomass of *D. viscosa* and *P. yunnanensis*. Our results will provide scientific support for vegetation restoration projects in the arid-hot valley of the Jinsha River.

#### **2. Materials and Methods**
