With the policies of conversion of cropland to forest and grassland and exclosure under no grazing carried on, the desert stppe ecosystems is under restoration and regeneration to some extent. However, long鄄term exclosure did not assure the restoration of plant communities in desert steppe. As was concerned, Glyeyrrhiza uralensis community tended to degrade. Taking the Glyeyrrhiza uralensis community under exclosrue of different ages after excavation as a subject, the influence of enclosure on Glyeyrrhiza uralensis population community and distribution pattern was discussed, by the method of the mean variance ratio and indices of aggregation intensity. The result showed:1) Under enclosure along with the increasing ages, species richness index and diversity index of Glyeyrrhiza uralensis community tended to increase. Evenness index firstly increased and then decreased. 2) The spatial pattern of Glyeyrrhiza uralensis, Agropyron cristatum, Artemisia sieversiana and Artemisia scoparia population were all changed. That of Glyeyrrhiza uralensis population and Agropyron cristatum population changed from random into aggregation, but the intercompetition was opposite. That of Artemisia sieversiana and Artemisia scoparia population changed from aggregation into random. 3) In arid and semi鄄arid region, the recovery of Glyeyrrhiza uralensis species needed 5 years, and the grass yield and the ground biological indexes are better. At that time, Glyeyrrhiza uralensis should be reasonably excavated, thus beneficial for the improvement of population development and related productivity.
Litter is the important components of grassland ecosystem, and plays an important role in grassland, influencing ecosystem structure, process and functioning. Litter accumulation can change the nutrient availability and modify local physical environmental conditions, and litter of grassland has gradually become one of the important issues in grassland ecology, but the factors related to the decomposition litter and litter decomposition rates in desert steppe is still unknown. In this study, four typical plant litters of desert steppe, Glycyrrhiza uralensis, Leymus secalinus, Agropyron crisatum and Artemisia desertorum communities were selected in Yanchi Country of Ningxia, and the loss rate of litter mass, the chemical composition, the water content of four typical plant communities were measured, and climatic factors and their partial http: / / www.ecologica.cn correlations were analyzed, and statistics and numerical taxonomy methods were used to identify the effects of litter accumulation. The results showed that the loss rate of litter mass was increased with decomposition time. However, the relationship between the loss rate of litter mass and decomposition time was not linear correlation. The loss rate of litter mass of four typical plant communities was Glycyrrhiza uralensis community > Leymus secalinus community > Agropyron crisatum community>Artemisia desertorum community. The litter decomposition rates of desert steppe was best represented by the he Olson model of single exponential decay, and the litter decomposition rate of Glycyrrhiza uralensis was the fastest, while the Artemisia desertorum was the slowest. The contents of N, P, K of litters of Agropyron crisatum, Leymus secalinus and Glycyrrhiza uralensis communities were significantly higher than Artemisia desertorum community, but contents of C, lignin, cellulose, and rates of C 颐 N and lignin 颐 N and cellulose 颐 N were significantly lower than litter of Artemisia desertorum community. The decomposition rates of four plant communities was positively correlated with the initial contents of N,P,K in litters,. Partial correlation analysis results showed the decomposition rates of four plant communities was significantly correlated with rainfall and the water content of litter. It was concluded that litter decomposition was complex physical and chemical and biological processes, which Influenced by many factors, and plant species was an major factor decisioned the rate of decomposition. The contents of 50% of litters decomposed in desert steppe was about 2-5 years, while 95% was about 8-24 years, which could provide scientific basis for the response and adaptation mechanism of grassland on the environmental factors and adaptation strategy.
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