Study On Suitable Initial Planting Density Of Caragana Korshinskii Plantation In Semi-arid Loess Hilly Region

The semi-arid Loess Hilly Region is characterized by the scarce native vegetation andserious soil erosion. To cover ground quickly and play the role of conserving soil and wateras soon as possible, artificial forest and grass in this region are often planted with a toohigh initial planting density. Overloaded vegetation accelerates the development of soildesiccation in the deep soil layer, which in turn brings obviously negative effects on itsnormal growth and stable ecological benefits. Therefore, this paper takes Caraganakorshinskii plantations as research objects, after discussing effects of initial plantingdensity 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 onsoil 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 is0-40cm, and the deepest infiltration depth is no more than300cm during the observationperiod. Meanwhile, major depth that resulted from evapotranspiration (ET) is0-200cm,and the deepest depth influenced by ET reaches300cm. The whole soil profile can bedivided into three levels, the active layer (0-40cm), the second active layer (40-200cm)and the relatively stable layer (deeper than200cm).(2) Soil water characteristic curves at different soil depth in abandoned land can bewell described by Gardner power function. Both parameters in the function and soil waterconstants calculated from the function show vertical changing trends of first decrease thenrise. Soil water-holding capacity and water-supplying capacity in80-200cm soil layer areweak compared with0-40cm and240-400cm soil layer. Besides, SWRUL in this regioncan be estimated when soil water content in0-290cm soil layer are all smaller than theircorresponding wilting coefficient, and its value is222.84mm. (3) The relationship between preserving rate, height, basic diameter and initialplanting density of Caragana can each be expressed as a single peak curve with thegreatest value appearing in the1.5plot. At the same time, biomass of Caragana aged from1to4years increases with the increase of initial planting density, but the biomass of1.5plot is more than that of2.0plot at age5.(4) Although Caragana plantation increases the recharge amount of precipitation, theconsumption and depletion depth of soil water are also increased. Thus there is a negativeeffect of afforestation on soil water resources during the observation period (2002-2006),with the exception of2003as it is a rainy year. Soil water resources of each plot havegenerally been approached or reached the SWRUL at the fifth year.(5) Suitable initial planting density can be defined as the planting density when notonly the use of soil water resources is maximized, but also excessive soil desiccation isavoided, then artificial vegetation can grow normally with better ecological benefits. Forartificial Caragana forest, the fitting sowing amount is0.015kg·m-2and the suitableinitial planting density is72plant·m-2. This density should be regulated for the first timeafter five years of afforestation.