Planting density and soil loss relationship
The main type of soil erosion in Chinese Loess Plateau is water erosion. The influence of planting density on soil losses can be expressed by the sediment concentrations in the runoff plots. The sediment in the runoff gradually increased with the reduction of planting density. The relationship between the sediment concentration SC (gram per liter) in the surface runoff and planting density D can be written as follow:
SC =
Here, SC sediment concentration, g per liter, d is plant density, shrubs per 100 m2
Under the same species, age and site condition, the biomass, the canopy coverage and the canopy interception per unit area increased with planting density, leading to a smaller surface runoff, suggesting that the benefit of vegetation to conserve soil and water is remarkable (Guo and Shao 2013).
Appropriate amount of plant- resources relationship regulation
The duration of resource scarcity refers to the period when the resource storage is less than resource use limit by plants. If the resource storage is smaller than resource use limit by plants, the plant- resource relation goes into critical period of plant- resource relationship regulation, the plant- resource relation enters the critical period of plant-water relationship regulation; the plant- resource relation should be considered because it decides the maximal yield and benefits of soil and water conservation vegetation.
After the plant-resources relation enters the critical period of plant- resources relationship regulation, if the duration of resource scarcity is more than the critical period of plant resources relationship regulation or the existing plant density is more than the vegetation carrying capacity in the critical period of plant resources relationship regulation, vegetation carrying capacity should be evaluated and plant growth should be controlled by cutting some tree to reduce plant density and regulating plant-resources relationship and get the maximal yield or service.
The vegetation carrying capacity is the ability of land resources to bear vegetation, which changes with vegetation type caused by tree species, time period and location (Guo 2014,2021a and b) and is a suitable measure of assessing the plant-resources relationship. vegetation carrying capacity includes space carrying capacity for vegetation in areas rich in water and fertilizer, soil water carrying capacity for vegetation in water-limited regions and soil fertilizer carrying capacity for vegetation in fertilizer limited regions (Guo 2019). Total coverage of a plant community when plant density of indicator species in a plant community is equal to vegetation carrying capacity is the suitable limit of vegetation restoration.
If the plant density is more than the soil water carrying capacity for vegetation in the critical period of plant-resources relationship regulation, plant-resources relationship has to be regulated to get the maximal yield and benefits and carry out sustainable use of soil water resources and high-quality and sustainable development (Guo 2014,2021b). Even if the goods and benefits produced by non-native ecosystem meet the need of increase in the population, but non-native species change plant-resources relationship of origin vegetation and its self-regulation and resilience is limit and cannot recover to the balance condition once soil water resources with maximum infiltration depth in Caragana forest land is smaller than soil water resources use limit by plants. For example, If the soil water resources in the maximal infiltration depth is more than soil water resources use limit by plant, the plant water relation enters the critical period of plant-water relationship regulation. The soil water sever deficit is the soil water resources in the maximal infiltration depth is more than soil water resources use limit by plant. If the duration of soil water sever deficit is more than critical period of plant-water relationship regulation, 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 should be estimated. The relationship between planting density and soil water supply SWS, soil water consumption SWC, the amount of water taken up by the plants in the critical period of plant-water relationship regulation in the 16-year-old caragana shrubland of semiarid loess hilly region (Guo and Shao 2004; Guo 2014,2021) was determined by:
SWS = 92.494- 0. 29D
SWC = 0. 0118D2- 0. 7575D+ 64.759
Where SWS is soil water supply in mm, SWC is soil water consumption in mm.
By solving the equation set, the soil water carrying capacity for vegetation was 72 shrubs per hm2, and the corresponding cover degree was 0.8 according to the formula:CD= 0.289 5ln72- 0.486 4. The degree of cover is the limit of canaraga restoration. When exceeding this limit, the canaraga shrubland would deteriorate soil water environment, and soil desiccation will be emerged or aggravated, soil degradation and vegetation decline will happen, which in turn influence benefits of water and soil conservation of forest vegetation.