近40年来南四湖湿地NDVI变化特征及其控制因子分析

NDVI changes and control factors of the wetland in the Lake Nansi,Shandong Province, in the past 40 years

利用Landsat系列卫星的MSS、TM和ETM+遥感数据,计算了研究区的归一化植被指数(NDVI),并以此为湿地植被活动的指标,研究1973 2011年间该湿地植被变化特征及年内季节变化特征,揭示植被活动在年内和年际变化的控制因子以及湿地植被对于气候变化、人类活动和极端干旱事件的响应特征.结果表明:(1)近40年来南四湖湿地植被各个季节的变化特征不尽相同.春季NDVI呈现先降低后增加的特征,主要先后受到研究区围垦、渔业养殖等人为活动和气候变化(增温)的影响;夏季和冬季的NDVI呈现显著降低趋势,主要受到围垦、渔业养殖等人类活动的影响;秋季NDVI的变化不显著.(2)年内季节变化方面,湿地植被面积和NDVI都呈现单峰的变化特征,从春季开始增加,在夏季末(全年的第202和205 d)达到最大值,然后开始下降,到冬季降至最低.植被的年内季节变化特征主要受到月均温度的控制.(3)干旱在一定程度上不是湖泊湿地NDVI增加的限制因子.干旱导致湖泊水位下降,滨湖滩地及湖底露出,可能会促进湿地植被生长和植被面积的扩大,使得湿地NDVI增加.

The normalized difference vegetation indexes （NDVI） of the wetland in Lake Nansi are calculated on the basis of MSS, TM and ETM ＋ remote sensing data collected by the Landsat satellites. The activities of the wetland vegetations are deduced by these indexes. By studying the characteristics of seasonal vegetation activities of the wetland during 1973 -2011, we revealed the control factors of intra-annual and inter-annual vegetation activities. Moreover, our study showed the response characteristics of wetland vegetation to climate changes, human activities and extreme drought events. The results can be summarized as follows： （ 1 ） The characteristics of the seasonal variation of wetland vegetation in Lake Nansi are not identical in the past 40 years. The NDVI in spring decreased first and then increased, which may be caused by the climate variation （temperature increase） and human activities such as reclamation and aquaculture in the study area; The NDV! in summer and winter present a significant decreasing trend due to the effects of human activities. However, the variation of the NDV1 in autumn is comparatively small. （2） The NDVI and area of wetland vegetation all present single-peaked variation characteristics in term of seasonal variations in one year. They in- crease in spring and reach the maximum in late summer （the 202nd day and 205th day in the year）, then decrease and reach thelowest values in winter. （3） To certain degree, drought is not the limiting factor for NDVI increase. In fact, since drought results in the drawdown of water level of the lake, the beach and lake bottom are exposed. All these factors probably increase the NDVI by prompting the growth of vegetation and enlarging vegetation area in the wetland.