Effects Of Disturbance On Above-ground Vegetation And Offspring Recruitment In A Semiarid Steppe On The Loess Plateau

Disturbance is an important reason for grassland degradation,desertification, salinization, and is the dominant factor of biodiversity changes in the grassland ecosystems. In order to provide a theoretic basis for vegetation restoration and biodiversity maintaining on the Loess Plateau, we study how disturbance affected community structure and offspring. In this study, we chose semi-arid perennial grassland on the Loess Plateau as a research site, and observed differences in species diversity, plant cover and adundance, litter thickness, litter biomass, regeneration strategy in the burnt and unburnt grasslands. The results were as follows:1 Grazing and grazing exclusion significantly affected above-ground vegetation cover,density,biomass,litter thickness and litter biomass. Long enclosure significantly reduced above-ground vegetation density, but improved vegetation cover, biomass, litter thickness and litter biomass compared with grazing. Grazing and grazing exclusion significantly affected species richness, Margalef index and Shannon-Wiener index, but had no significant effect on Simpson index and Pielou index. Long enclosure significantly reduced species richness, Margalef index and Shannon-Wiener index compared with grazing. DCA analysis showed that grazing and grazing exclusion alter the community structure and species composition.2 Fire significantly reduced total cover, species diversity, litter thickness and litter biomass, and significantly increased species density, however it did not alter species richness. Fire had an important effects on plant community structure and species composition in the short time.3 The asexual offspring recruitment contributed more to vegetation regeneration than the seedling recruitment, and perennial species reproduced asexually mainly via tillers, rhizomes and branches. Management practices significantly affect offspring recruitment diversity and numbers. Grazed sites had significantly higher species richness assessed by offspring recruitment than grasslands with 23 years grazing exclusion. Management practices had no significant effect on the sexual recruitment density, but significantly affect asexual recruitment numbers. Grazing numerically increased the number of sexual recruits compared with 23 years grazing exclusion sites. Grazed grassland had a higher density in tiller, rhizome, and branch offspring recruitment than grasslands with 23 years grazing exclusion, however, recruitment from root suckers and stolons was not significantly different among these two treatments.Asexual recruitment play more important role in the semi-arid perennial steppe, and grazing and fencing measures had greater impact on asexual recruitment than seedling recruitment. The effects of management on grassland species diversity and density could be adjusted by plant recruitment and regeneration.4 Fire significantly increased offspring recruitment numbers,but not species richness. Fire significantly increased asexual recruitment numbers, but had no effects on sexual recruitment numbers.The increase of asexual recruitment after fire made a major contribution to the increase of total offspring number. Burnt grassland had a significanly higher density in tiller and rhizome offspring recruitment number than the unburnt grasslands, however, recruitment from root suckers, branch and stolons was not significantly different among these two treatments.Our results indicated that asexual recruitment contributed more to vegetation regeneration of the perennialsteppe community than seedling recruitment. Lack of sexual recruitment is not related to fire management but to species composition.