Change of Soil Water Content and Light of Several Plantations at Loess Plateau Semi-arid Area and Its Effect on Forest Stand

The soil water content and light under forest of seventeen years old Robinia pseudoacacia plantation of four planting density (1.5 m×2 m, 1.5 m×4 m, 1.5 m×6 m, 1.5 m×8 m), ulmus pumila plantation of two planting density (1.5 m 3 m, 1.5 m 5 m) and Pinus tabulaeformis plantation of two planting density (1.5 m×3 m, 1.5 m×5 m) at Loess Plateau Semi-arid Area of Shanxi province were observed and determined. Studies were conducted on the growth of trees in different forest stand, the species and their number of undergrowth, and the growth and development of undergrowth. The results showed that within the forest with low planting density and big nutrition space the soil water content and light decreased with the increasing of planting density. Within the stand of Robinia pseudoacacia, as the planting density increase, the phenomenon of dry soil emerges as that occurred in mature plantation in arid and semi-arid areas. The abundance ratio and species diversity also decreased. The species gradually change from mesophyte such as Convolvulus arvensis and Themeda minor to typical xerophyte such as Artemisia sacrorum, Lespedeza dahurica and Adiantam capiuarisveneris. The average soil water content of two planting density Pinus tabulaeformis plantation were lower than other stands, which interpreted that the micro-environment, forest tree growth and development of undergrowth varied with the type of stand and planting density. In constructing plantation in semi-arid areas, the species should be selected based on bio-ecological characteristics and growth conditions. The planting should not be too dense so as to increase the productivity and stability of stand, and to prevent the occurrence of soil drought.