Elm (Ulmaceae) widely distributes in the temperate zone of northern Hemisphere, especially in Russia, England, America, Denmark etc. Depending on its drought tolerance, salinity adaptation and adaptation to various climates, elm (Ulmus pumila) contributes to the maintenance of savanna鄄like ligneous plant community in typical grassland region of the north China. This sparse elm community, generally provides with tree canopy covers less than 25% and average height of mature trees lower than 5 m, and appears a complex mosaic landscape as a whole. Elm woodlands are primarily located in the sandlands in Hunshandak, Kerqin and Ordos Plateau of Inner Mongolia, and in Songnen Plains of northeastern China. Elm woodlands offer wide expanses of grazing land for large herbivores. On the other hand, it can be utilized by agriculture cultivation. Some studies demonstrate that elm woodlands play important roles in decreasing soil erosion and stabilizing sand dune. At larger spatial scale, elm woodland is considered as a natural barrier to sandstorm in the eastern Asia. It greatly contributes to alleviating environmental deterioration and maintaining ecosystem stability. In the past two decades, most elm woodlands of the northern China have been over鄄utilized by grazing or cultivation. Such excessive utilization of elm woodland leads to the decrease in basal area and productivity, the scarcity of U. pumila juveniles, and the increase in species composition of community. This greatly weakens ecological protective functions of elm woodland. Some researchers assumed that the degradation of elm woodland is associated with drought climate, overgrazing and tree cutting. In order to anchor the essential reason that caused degradation of elm woodland, we carried out following

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