三峡工程运用后长江中游典型江心洲植被动态变化趋势及机制探析

受三峡工程运用的影响,长江中游水沙情势剧变,江心洲生境结构发生变化,洲上植被密度、活力和分布情况也随之改变。因此亟需开展针对江心洲植被的长期观测以厘清其对三峡调控的响应机制,从而制定有效的洲滩植被保护和修复策略。卫星遥感技术是开展长时间、长河段地貌观测的常用手段。目前应用卫星遥感技术监测江心洲植被动态的研究,主要聚焦于三峡工程运用对江心洲植被面积和覆盖度的影响,而较少深入探讨植物长势和分布模式的变化趋势。因此,本文以长江中游4个典型江心洲为研究对象,提出了一系列可用于反演江心洲淹没范围和滩面上植被动态的方法,并量化分析三峡工程运用对植被动态的影响。结果表明:(1)三峡工程运用后,江心洲整体淹没频率降低,部分原有边滩逐步满足耐水植物的生长条件,促使江心洲植被面积呈现增长的趋势;原分布在高滩上的不耐水植物逐步蔓延至低滩,导致江心洲植被茂密程度整体上升;(2)三峡工程运行前,2002年含沙量较大的漫滩洪水有促进植物第二年生长的趋势;而三峡工程运行后,2016年具有同样规模但含沙量减少近80%的漫滩洪水则有抑制植物生长的作用;(3)部分形态稳定的江心洲,其高、低滩植被分布模式之间的异质性受三峡...

Investigation on vegetation dynamics of mid-channel bars in the middle Yangtze River in response to the operation of the Three Gorges Project

Owing to the changing hydrological and sediment regimes following the operation of the Three Gorges Project (TGP), vegetation dynamics of mid-channel bars in the Middle Yangtze River (MYR) have undergone significant alterations. Continuous monitoring of vegetation dynamics is therefore critical to understand the response of vegetation communities to the upstream damming while to foster proactive bar restoration projects. Satellite remote sensing techniques have been widely used for long-term river-scale investigations. Of the many studies that have applied satellite remote sensing technique to assess the vegetation dynamics of mid-channel bars, the vast majority have focused on obtaining the changes in the vegetation cover area or rate but paid less attraction on the trends of vegetation growth and distribution patterns. Therefore, this paper devised a set of methods to retrieve the inundation and vegetation dynamics of mid-channel bars. By taking four representative mid-channel bars in the MYR as examples, the impact of TGP on the vegetation dynamics was quantitively assessed. The main findings included: (i) due to the operation of the TGP, the inundation frequency of the mid-channel bars drastically decreased. Hence, there were more areas meeting the germination conditions of flood-tolerant species, leading to an increase in the vegetation area. Meanwhile, flood-intolerant species on high-elevation regions gradually colonised low-elevation areas, leading to an increment in the vegetation dynamics; (ii) the post-TGD flood in 2016, carrying a mean sediment concentration of 0.08 kg/m³, exerted a more detrimental impact on the vegetation than the pre-TGD flooding of a similar scale in 2002 (mean sediment concentration equaling 0.39 kg/m³); and (iii) some morphologically stable mid-channel bars were transitioned into a degraded state in approximately 10 years after the operation of the TGP. Concerning morphologically unstable mid-channel bars, their vegetation pattern were changed along with the geomorphology.