Study On Dynamic Changes And Vegetation Characteristics Of Wetland In Yanchi County Of Ningxia Over The Past 20 Years

Taking the wetland in Yanchi County of Ningxia as the research objectives, the dynamic changes and vegetation characteristics of study site for more than about recent 20 years were studied, based on the TM remote sensing data in 1988,1994,2003,2010 and 2013 and the field investigation data of Seitan wetland from 2007 to 2014. The dynamic changes of wetland and its effects on vegetation characteristics such as main plant populations’fluctuations, diversity, niche and stability were analyzed and the main reason which caused the above changes was revealed. Finally, the priority conservation grade of 6 wetland in Haba Lake Nature Reserve was evaluated, aimed to put forward the reasonable suggestions for the protection of different grade wetlands. The results showed as follows:(1) The wetland area in Yanchi County decreased as whole during 1988-2013. The amount and density of patches decreased and the mean patch area increased, which showed that landscape heterogeneity became lower than the past. The area of transfer between wetlands and other land use systems were given priority on the unused land and grassland. On the county, the transfer area between wetland and forest land or farmland was very small, and there was no transfer between wetland and construction land. The maximum dynamic degree (i.e. the rate of change of wetland) of wetland of study site, depletion degree (i.e. the rate of decrease of wetland) and the development degree (i.e. the rate of increase of wetland) appeared from 2010 to 2013, the minimum happened from 2003 to 2010.(2) The 4 fluctuation forms of plant populations can be divided in Siertan wetland from 2007 to 2014, i.e. stable types, fluctuating upward types, fluctuating downward and climate fluctuation types. The highest vegetation coverage, height, density and biomass in Siertan wetland appeared in 2012, while the lowest was found in 2009. The study results of vegetation fluctuation ratio showed the largest positive fluctuations happened in 2012 and the maximum negative fluctuations appeared in 2009. The main reasons were that 2011 was the rainy year and 2008 was the drought year during the year 2007-2014, which reflected the largest hysteresis effects of precipitation on vegetation.(3) In 2007 and 2013, the vegetation in Siertan wetland scored the higher a-diversity and β-diversity, and the lowest in 2012. The maximum a-diversity value was found in the north line and the least in the east line. The results of β diversity showed that the vegetation similarity between the east line and north line was more than that of the others. "Three strips" (i.e. wetland zone, ecotone zone and arid zone) in Siertan wetland were decided by using the moving-split window technique. The results of a-diversity of different vegetation strips from 2007 to 2014 indicated that the ecotone zone almost had the maximum a-diversity except for evenness indices which was largest found in arid zone. The results of β-diversity showed that the vegetation similarity between wetland zone and ecotone zone or the ecotone and arid zone were more than those of the others.(4) The results of the niche breath in Siertan wetland from 2007～2014 showed that the Suaeda salsa (L) Pall, Nitraria tangutorum Bobr. and Kalidium foliatum (Pall.) Moq. was the highest in wetland zone, Bassia dasyphylla O.Kuntze, Salsola ruthenica and Psammochlou villosa in ecotone zone, and Heteropappus altaicus (Wild.) Hovopokr., Sophora alopecuroides Linn, and Thermopsis shischkinii Czefr in dryland zone. The linear relationship bewtween the niche breadth and niche overlap was not obvious.(5) The M. Godron stability measurement results showed that the vegetation communities in Siertan wetland were in long-term unstable state. The stability of vegetation fluctuated largely in wetland zone influenced by the rainfall and the water area of wetland. The main factors which determined the stability of wetland was the size of wetland area.(6) The priority protection grade of six wetlands in Haba Lake Reserve were evaluated and divided into 4 levels based on the expert weighting method, and the measures for the purposes of conservation and restoration were put forward according to the 4 protection levels. The factors which mainly caused the dynamic changes of wetland in Yanchi County included the natural and human factors. Human disturbance played a two-way role on the development of wetland.