基于MODIS的鄱阳湖湿地植被变化及其对水位的响应研究

ANALYSIS OF POYANG LAKE WETLAND VEGETATION GROWTH DYNAMICS AND ITS RESPONSE TO LAKE WATER LEVEL BASED ON MODIS

针对鄱阳湖水情变化如何影响湖区湿地植被生长过程的问题,在鄱阳湖国家级自然保护区内的蚌湖—吴城生态断面上,选取3个梯度分布MODIS像元样点,基于2001～2010年MODIS增强型植被指数(EVI)时间序列,分析了近10a来退水期植被生长动态变化过程;结合星子站水位资料,探讨了鄱阳湖退水期水位变化对湿地植被生长的影响。结果表明:各样点EVI年内均值表现为先上升后下降的单峰特征,多年平均值在10月份最大,高程越高的样点其EVI达到年最大值的时间越早;不同水位持续长短及其起讫时期影响植被的生长变化,EVI均值与生长期的拟合优度(0.75)高于最大值与生长期的拟合优度(0.49);样点每提前出露10d,EVI均值约升高0.02,二者相关性R2达到0.55。研究结果对湖区植被资源管理以及湖泊水位调控效应研究等具有积极的参考意义。

The Poyang Lake is the largest freshwater lake in China and it is one of the most important wetlands in the world. Wetland vegetation plays an important role in material circulation and energy flow as well as other ecological process. Plant community in wetland exhibits significant responses to water content and water level fluctuation in wetland. As for the Poyang Lake, water level varies significantly within an order of 10m in a single year,with the bottom of lake emerging at low water level. Only in these dry seasons hygrophilous vegetation, such as Carex can grow. Hence, water level-induced vegetation variations have been substantially discussed in recent literatures. The relationship between vegetation growth and water level dynamics has been currently carried out. The launch of MODIS （moderate resolution imaging sepctroradiometer）provided an opportunity for us to study the vegetation growth dynamic process. In this study,time series of Enhanced Vegetation Index （EVI） calculated from multi-year MODIS images were used to analyze vegetation variations in the course of low water level for the year 2001-2010. For this purpose, three hierarchically distributed MODIS pixels were selected along the Banghu-Wucheng ecological transect in the Poyang Lake National Nature Reserve （PLNNR）. The average Wusong elevation of the three sample points was 13.8m, 15.2m, 15.8m, respectively. More over,we also explored the influence on vegetation growth imposed by fluctuation of water level assisted with data from Xingzi hydrometric station. Results indicated that intra-annual EVI of each sampling site exhibited a trend of increasing firstly and then decreasing. The annual mean EVI culminated in October and pixel with higher terrain corresponded to the early occurrence of peak EVI. No significant trends were found in the inter-annual EVI. Water level determines time span of outcrop emergence, thereby affects the growing season of vegetation. Mean EVI had a better goodness of fit（0.75） with the growing phase than that of the maximum of EVI （0.49）. Mean EVI would increase by 0.02 when the bottom of sample site emergenced 10 days in advance compared with the year 2005 as reference year, and the determination coefficient reached 0.55. In the typical dry year （2006）,the three sample points with high,middle and low elevation were exposed 72,60,52 days in advance respectively,and the mean EVI of each sample significantly increased by 0.12,0.16 and 0.11. In the typical wet year （2002）,only the maximum EVI significantly increased due to more precipitation in the study period. We can conclude that the long-lasting water level extends the length of the vegetation growth. It is propitious to the growth of wetland plant community. And the precipitation is an important climatic factor for the vegetation. Results can be used to guide the vegetation resource management in the Poyang Lake and the lake level regulation.