| Through studying the soil water dynamic in the sands and characteristic of Nitraria tangutorum community in Wulanbuhe Desert is studied. The results are showed below.1 The rainfall is the main factor to affect the soil water dynamic in the sands. The changes of the sand water content are closely related with rainy season and rainfall. The variety of the sand water content is in 0 cm— 60 cm layer, the active soil moisture layer, is greater than in 60 cm depth below. There exist three distinctive layers of soil moisture from the top to the bottom: the surface dry sand layer, the variety layer and stable layer of the soil moisture. The sand water content in April and May is the lowest during the whole year while it is the highest in the rainy season from June to September. The sand water storage is linear with the rainfall (y=16.962+0.147x(y is the sand water storage of the layers from 0 cm to 100 cm(mm), x is the sum of rainfall before every sampling(mm))and it is showed that 14.7% of the rainfall from April to September can be absorbed by soil, in the moving dune, the rest part all evaporates.2 The sand water content in the different slope direction is different and the order is the windward slope> the atypical slope direction >the lee slope in the moving dune. The sand water content in the different slope position is different and the order is the bottom of slope>the middle slope >the top of slope on the windward slope in Wulanbuhe Desert.3 The amount of the soil water storage in the layer from 0 cm to 100 cm is 16mm~ 32mm in the sand soil while it is 70mm~220mm in the clay.4 The relationship between the water content of sands and water potential can be expressed by equation y= -14.577 +14.539X(l-EXP(-1.960x)) (x is the water content of sand, y is water potential). The relationship between the water content of clay and water potential can be expressed by equation y= -8.772 + 8.612×(l-EXP(-0.1999x)) (x is the water content of clay, y is water potential).5 In Wulanbuhe Desert, the aboveground part of Nitraria tangutorum can be divided into four types, the new branch, the old branch I, the old branch II and the withered twig. It can be expressed by equation y=a (Dm~2Hm) ^b in the relationship among the total biomass (y), the maximum branch height (Hm) and the maximum ground diameter (Dm) of new branch. The relationship between the total biomass (y) and Hm of old branch I and old branch II can be expressed by y=aHm^b. The relationship between the total biomass and the branch biomass can be expressed by y=aEXP (-bx^k), (y is the branchbiomass, x is the total biomass). The relationship between the total biomass and the leaf biomass or the leaf area can be expressed by y=a+b (1-EXP (-kx)), (y is the leaf biomass or the leaf area, x is the total biomass).6 On the development of the Nitraria tangutorum shrubs, in Wulanbuhe Desert, the Nitraria tangutorum shrubs is in a developing state during the proportion of the biomass of new branch in the total biomass is more 52%, or it is in a degenerating state.7 Nitraria tangutorum shrubs can be divided into four types based on the biomass of aboveground part of Nitraria tangutorum, leaves area index and other characteristics. The proportion of the biomass of new branch in the total biomass decreases from type I to IV, and type III has the greatest biomass and leaves area index, and it is the best for Nitraria tangutorum shrub.
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