The semi-arid Loess Hilly Region is characterized by the scarce native vegetation and
serious soil erosion. To cover ground quickly and play the role of conserving soil and water
as soon as possible, artificial forest and grass in this region are often planted with a too
high initial planting density. Overloaded vegetation accelerates the development of soil
desiccation in the deep soil layer, which in turn brings obviously negative effects on its
normal growth and stable ecological benefits. Therefore, this paper takes Caragana
korshinskii plantations as research objects, after discussing effects of initial planting
density on soil water conditions of forestland as well as growth dynamics of Caragana,
and applying the theory of initiation stage to regulate plant growth and soil water based on
soil water resources use limit (SWRUL), the suitable initial planting density is calculated.
Main conclusions draw from this study are as follows:
(1) The common depth of soil affected by rainfall infiltration in abandoned land is
0-40 cm, and the deepest infiltration depth is no more than 300 cm during the observation
period. Meanwhile, major depth that resulted from evapotranspiration (ET) is 0-200 cm,
and the deepest depth influenced by ET reaches 300 cm. The whole soil profile can be
divided into three levels, the active layer (0-40 cm), the second active layer (40-200 cm)
and the relatively stable layer (deeper than 200 cm).
(2) Soil water characteristic curves at different soil depth in abandoned land can be
well described by Gardner power function. Both parameters in the function and soil water
constants calculated from the function show vertical changing trends of first decrease then
rise. Soil water-holding capacity and water-supplying capacity in 80-200 cm soil layer are
weak compared with 0-40 cm and 240-400 cm soil layer. Besides, SWRUL in this region
can be estimated when soil water content in 0-290 cm soil layer are all smaller than their
corresponding wilting coefficient, and its value is 222.84 mm. (3) The relationship between preserving rate, height, basic diameter and initial
planting density of Caragana can each be expressed as a single peak curve with the
greatest value appearing in the 1.5 plot. At the same time, biomass of Caragana aged from
1 to 4 years increases with the increase of initial planting density, but the biomass of 1.5
plot is more than that of 2.0 plot at age 5.
(4) Although Caragana plantation increases the recharge amount of precipitation, the
consumption and depletion depth of soil water are also increased. Thus there is a negative
effect of afforestation on soil water resources during the observation period (2002-2006),
with the exception of 2003 as it is a rainy year. Soil water resources of each plot have
generally been approached or reached the SWRUL at the fifth year.
(5) Suitable initial planting density can be defined as the planting density when not
only the use of soil water resources is maximized, but also excessive soil desiccation is
avoided, then artificial vegetation can grow normally with better ecological benefits. For
artificial Caragana forest, the fitting sowing amount is 0.015 kg· m -2 and the suitable
initial planting density is 72 plant·m -2 . This density should be regulated for the first time
after five years of afforestation.
Key words: Caragana korshinskii, initial planting density, growth, soil water, soil water
resources use limit