Discussions
Before restoring soil and water conservation vegetation, Excellent tree species are preferred for soil and water conservation vegetation (Guo et al, 1990) and then appropriate sowing rate and planting density shall be adopted (Guo 2014). After sowing or planting, with plant grow, resources reduce, and then degree of cover increases with the increase of planting densities, basal diameter, surface runoff and the sediment concentration declined, but the canopy coverage and interception increased. The planting density was linear related to basal diameter, logarithm relevant to degree of cover and surface runoff, and exponentially related to canopy interception. The relationship between sediment concentrations in runoff and planting density can be described by an “inverse-S shape” curve. Though the dense planting density can increase degree of cover and thus increase benefits of water and soil conservation forestor vegetation, the restoration of artificial forest vegetation should have a maximum limit due to the limited water resource. For example, the limit is the cover degree of 2002 in caragana shrubland in the semiarid loess hilly region of the water-limited region (Guo and Shao 2013; Guo 2021a and b).
With increase in planting density, the degree of canopies’ cover and the precipitation intercepted by the canopies increase, but the runoff and sediment concentration decline, the benefit of forest vegetation for soil and water conservation is more obvious (Liu et al. 2008).Under the same rainfall, rainfall intensity and rain even duration condition, with increase in planting density, the rainwater intercept by the forest canopies and water loss increase, the evaporation and transpiration increases at the same time, and then soil consumption increase, soil water supply reduces, which lead to serious soil degradation in the form of soil desiccation in the woodland and vegetation degradation. It is hostile to the heath and stability of forest vegetation ecosystem, but the heath and stability of the ecosystem is the foundation to maintain sustainable and stable ecological benefit. So, in the long-term, there should be a balance between water supply and consumption in the process of forest vegetation restoration. The planting density of indicator plant, such as Caragana in this study at the balance point was called soil water carrying capacity for vegetation (Guo 2014,2021a and b). The cover degree of forest vegetation at the point was the maximum degree of restoration. It should be drafted as the basis for determining the target of forest vegetation for soil and water conservation. The maximum yield and benefits of soil and water conservation is the rational maximum yield and benefits of soil and water conservation when plant density is equal to soil water carry capacity for vegetation in the critical period of plant-water relationship regulation. If the plant-water relation goes into critical period of plant-water relationship regulation and planting density is more than the vegetation carrying capacity, then the degree of restoration exceeded the limit, more production and more soil and water conservation may be obtained temporarily at the expense of environment, which caused soil degradation in form of excessive soil drying under both perennial grasses and forests and finally lead to desertification in dry years or soil water resources waste in wet years, which did not correspond with strategy of high-quality and sustainable development, so we should ensure the cover degree of soil and water conservation meets the requirement of 60% of the construction standard of Soil and water conservation in Spring (Guo 1996), and then investigate the soil water condition and plant growth. Once soil water resources within maximum infiltration depth of the forest land is lower than soil water resources use limit by plants. Soil water influence plant growth and soil and water conservation. The relationship between soil water supply or soil water consumption and plant density and soil water carrying capacity for vegetation in the critical period of plant-water relationship regulation was estimated. The condition of caragana growth in the critical period of plant-water relationship regulation decide the benefits of soil and water conservation in the whole growing season. The soil water carry capacity for vegetation change with vegetation type, period and location (Guo 2014,2021a and b). If the plant density is more than soil water carry capacity for vegetation in the critical period of plant-water relationship regulation, we must regulate plant-water relationship be reducing the density and give full play to the maximum benefit of soil and water conservation forest to ensure sustainable use of soil water resources and the high-quality and sustainable development of soil and water conservation vegetation in water-limited regions.