Vegetation productivity of the Lake Poyang Wetland in the post Three Gorges Dam era

Abstract

三峡运行以来,气候变化与人类活动共同驱动了鄱阳湖水文情势的急剧变化,并对其湿地植被产生了显著影响,但当前研究尚未系统量化鄱阳湖湿地植被在此条件下的演变规律与趋势。因此,本研究以增强型植被指数EVI(enhanced vegetation index)的时段最高值作为表征湿地植被生产力的代用指标,定量揭示了后三峡工程时代鄱阳湖湿地植被生产力的气候态特征及其变化,包括变化的量级、显著性及阶段性。结果表明:(1)2000-2020年,鄱阳湖湿地植被EVI最高值在全年及春、秋两季分别为0.37、0.30和0.33,入湖河流三角洲及碟形洼地边滩是其生产力中心;春季EVI最高值在此区域高于秋季,而在湖心区低于秋季。(2)三峡运行后的湿地植被生产力变化在全年及春、秋两季均有极大空间异质性,在湖心深泓线附近显著增加,而在入湖河流三角洲及湿地边缘下降。春季EVI增长区域面积占比极大(84.2%),而秋季增长区面积占比较小(71.2%),即有较大面积区域EVI在秋季下降(38.8%)。(3)三峡运行后,鄱阳湖湿地植被生产力中心经历了原位增长、空间扩张以及湖心向转移3个阶段,最终完成了由支流三角洲向湖心区的转移。从总体来看,鄱阳湖湿地植被生产力在全年及春、秋季均呈增长趋势,增速分别为0.0035、0.0049和0.0028 EVI/a。本研究量化了后三峡工程时代的鄱阳湖湿地植被生产力变化,为该区域进一步的综合管理提供了数据支撑。;Since the operation of the Three Gorges Dam (TGD), the superposition of human activities and climate change has dramatically changed hydrological regimes of the Lake Poyang. These changes significantly affected the structure and function of the surrounding wetland ecosystems. However, vegetation variations in the Lake Poyang Wetland under this changing condition has not yet been systematically quantified. Hence, this study took the maximum EVI (enhanced vegetation index) as a proxy index to characterize vegetation productivity of the Lake Poyang Wetland, aiming to quantitatively reveal its changes in the post TGD era. The results show that: (1) During 2000 to 2020, the regional averaged maximum EVI of the wetland was 0.37, 0.30 and 0.33 in the whole year, spring, and autumn, respectively. River deltas of the lake tributaries and the dished depressions in the wetland were generally with higher EVI values compared to other areas, thus regarding as high productivity center of the wetland. In springs, the maximum EVI in the productivity center was higher than that in autumn, whereas it was lower than that in autumn in other areas with low EVI values such as the center of the lake. (2) Changes of vegetation productivity had great spatial heterogeneity for all the periods including the whole year, springs, and autumns. The maximum EVI values increased significantly in the central of the wetland (surrounding the thalweg), while the maximum EVI values decreased in river deltas of lake tributaries and wetland edge. The areas with increasing EVI values in spring accounted for about 84.2% of the whole wetland, while the area with increasing EVI in autumn accounted merely 71.2% of the wetland. That is, there was still a large proportion of the wetland with decreasing EVI in autumn (38.8%). (3) After TGD, the high productivity center of the wetland experienced three stages: in-situ growth, spatial expansion and towards lake center transfer, thus moved from river deltas to lake center areas. Overall, the maximum EVIs of the wetland showed an increasing trend, with a rate of 0.0035, 0.0049, and 0.0028 EVI/year for the annual, spring and autumn, respectively. The findings systematically quantified vegetation productivity changes of the Lake Poyang Wetland in the post TGD era. It also provided a scientific basis for proposing water resource management strategies to maintain the stability of wetland ecosystem.