Appropriate amount of plant-resources relationship regulation
Plant growth and resource availability relationship can be divided into different stages. For example, caranaga growth and resource use relationship can be divided into four stages in a growing season in semiarid loess hilly region of water-limited regions (Guyuan, China). The first stage is at the dormant period from January 1st to the second ten of April. At this period, plants are dormant and not sensitive to water stress because plant stop growth. The second stage is the period from the second ten of April (caragana bud germination) to the end of Jun. (full-expansion of leaf) and the plant was not sensitive to water stress at this stage because the leaves of plants are not fully developed, and Caragana only absorbs a small amount of water; The third stage started is at the fast growth period from the end of June when the leaves were totally expansion to the end of August. At this stage,plant grow rapidly and need a lot of water. At this stage, plant suck much water and was very sensitive to soil moisture in this period. If the soil water resources in the maximal infiltration depth are lower than soil water resource use limit by plants, the plant-water relation goes into critical period of plant-water relationship regulation. At this time, the plant water relationship in the key period of plant water relationship regulation is very important, because it determines the maximum yield and benefit of a plant community. The fourth stage is leaf fall period, ranging from September to the end of October (complete leaf drop), the plant was insensitive to water stress. After the leaf fall period from November (the beginning of dormancy) to the first ten of April (the end of dormancy), the plant-water relation goes into the dormant period and the influence of soil moisture on plant growth was very little in this period.
Planting density anddegree of cover relation
The degree of vegetative cover is an important index to evaluate the function of a plant community to conserve soil and water. The effective degree of cover is the degree of vegetative cover when soil loss is equal to allowable soil loss, so effective degree of cover should be as the cultivating standard of adult soil and water conservation vegetation or forest (Guo 2000). The degree of cover increased gradually with age at the same condition of species, initially planting density and site condition. In a growing season of a given species and site condition, the degree of cover in a plant community increased gradually from budding in mid-April to the fully expansion of leaf by the end of June. The degree of cover then remained relatively constant in the period from July to October, the degree of cover can be defined as the stable coverage degree (SCD).
The degree of cover increased with the planting density, and the relationship between the stable cover degree (SCD) and planting density at the third ten of July was given by (Guo and Shao, 2013), see figure 4
SCD = 0.2895 Ln (PD) – 1.8197
Where, SCD is stable cover degree and PD is planting density.