Responses of elm (Ulmus pumila) woodland to different disturbances in northeastern China

Abstract

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-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-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 experiments: in the Songnen grassland of China (40°30'-48°05'N, 122°12'-126°20°'E), we sampled an undisturbed Ulmus woodland in 1983, and we maintanenced that the Ulmus woodlands were seldom disturbed in past two decades. In 2004, we sampled five sites: the undisturbed site; two farming sites with 40% and 90% crop coverage; and two sites with light and heavy grazing as determined by distance to the nearest village (600 and 200 m, respectively). In each site, we sampled ten 4 m2 plots, and species' names, number and aboveground biomass (measured as community productivity) were measured. Species diversity was calculated. We found that: 1) there were not significant species and productivity changes of elm woodland in 1983 and 2004, therefore we concluded that climate changes was not the main reason caused the degradation of elm woodland; 2) in the contrast with the undisturbed elm woodland, the plowing and grazing elm woodlands showed great differences in species composition and productivity. These leaded us to such conclusion that human activities, such as plowing and grazing, directly decreased the productivity and species diversity of elm woodland, and also simplified the structure of elm woodland. 3) Furthermore, high intensity of grazing caused the constructive specie (U. pumila) was thinner and shorter; therefore, elm represented much more shrub-like in grazing elm woodland. Finally, we concluded that human activities were the main and direct reason caused the degradation of elm woodland.