Resource use limit by plants
Generally, as plant grow, plant suck and use resource, and then resource amount reduces because the resource amount in a given space in which plant crown or plant root develop is limit. Therefore, there is a resource use limit by plants in soil and water conservation vegetation, expressed by the indicator species in a plant or population community, and resource use limit by plants can be defined as control amount of plant utilization resources such as soil water resources use limit by plant. For example, soil water is the most important factor influencing and limiting plant growth in water-limited regions. After soil water resources appeared (Budagovski 1986), soil water resources can be used to express soil water condition because root system vertical distribute in the soil and take soil water in a given soil body, see figure 1. Two-curve method can be used to determine maximum infiltration depth (MID) (Guo 2020), see figure 2. The utilization limit of soil water resources by plants refers to the soil water resources with the maximum infiltration depth when the soil water content is equal to the wilting coefficient. Soil water resources use limit by plant changes with species and site condition (Guo 2014). The utilization limit of plants to soil water resources is the control limit plant use soil water in water-limited regions, which change with plant species and location. For example, soil water resources use limit by plant in the 16-years-old Caragana (Caragana Korshinskii ) shrub land in the fish scale pit on the top of Heici mountain is 252.8mm, see table 1. And soil water resource use limit by plants in caragana shrub land in the middle of the Heici mountain is 222.8mm, see table 2. The soil water resource use limit by plants in red plum apricot plantation is 212.7 mm in the semiarid loess hilly region (Guyuan county, China).
The soil water contents at different soil suctions using a centrifuge or pressure chamber, were measured and established the characteristic soil moisture curve using least square method. And then estimate the wilting coefficient at different soil depths. The wilting coefficient is the soil water content when soil suctions equal 15bar based on the characteristic soil moisture curve at different soil depth, see figure 3. As plant grow, soil water resource reduces (Guo 2010,2014,2020,2021b).
Indicator species is the main cultivation or goal species for artificial vegetation such as caragana in caragana shrubland. If soil and water conservation vegetation is made up 2 or three plant species, the indicator plant is the dominant tree species or the main tree species with large coverage in mixed forest. The resource use limit by plants includes space resource use limit by plants in areas rich in water and fertilizer, soil water resource use limit by plants in the water-limited regions (Guo 2010; Guo 2014,2021a and b) and soil fertilizer resource use limit by plants in the fertilizer -limited regions (Guo 2021b).