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.