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Abstract
Vegetation dynamics on mid-channel bars (MCBs) is essential for supporting ecosystem functions and associated services in river systems, especially in dammed large rivers. Generally, there are two possible changing patterns that vegetation of MCBs downstream a dam would experience. On one hand, the vegetation area may shrink because of a decrease in the MCB area in the post-dam period, which has been observed in many rivers around the world. On the other hand, the MCB vegetation area may expand because flood disturbances would be weakened by dam operation. However, little evidence has been reported to clarify such confusion. Therefore, vegetation dynamics of MCBs in the mid-lower Yangtze River downstream the Three Gorges Dam (TGD; the world’s largest dam) is selected as a case study to address the issue. Using long-term (1987–2017) Landsat archive images, this study reveals the spatiotemporal variations of vegetation area change intensities (VACIs; indicated by dynamic trends) on MCBs in the mid-lower Yangtze River. Results show that an overall VACI in the post-dam period (2003–2017) is about three times faster than that in the pre-dam period (1987–2002). In other words, the rate of vegetation colonization accelerated after the TGD operation began in 2003. Moreover, the VACIs in the post-dam period are size dependent, where large size MCBs are likely to gain higher VACIs: Small-sized MCBs (0.33 km2/yr), medium-sized MCBs (1.23 km2/yr), large-sized MCBs (1.49 km2/yr). In addition, VACIs of individual MCBs in the post-dam period are distance dependent, where the further a MCB was from the TGD, the higher the VACI. It is also suggested that the weakened flood disturbances in the post-dam could explain the rapid vegetation growth and colonization. This work is not only beneficial for managing and protecting MCBs downstream the TGD after its operation, but is also helpful in understanding vegetation dynamics of MCBs in other dammed river systems around the world.
Highlights
The study area consists of Mid-channel bars (MCBs) downstream of the Three Gorges Dam (TGD) located at the junction of the upper and middle reaches of the Yangtze River (Figure 1)
We random this study focused onwere dynamics of the natural vegetation in MCBs, the classified artificial vegetation patches were excluded in the accuracy assessment
This paper is the first time that focus has been paid to the area dynamics of vegetation in MCBs in the downstream TGD of the Yangtze River during 1987–2017
Summary
Mid-channel bars (MCBs) are elevated regions of sediment (such as sand or gravel) that has been deposited by water flow [1]. They are commonly present in various sized rivers around the world [2,3]. In recent decades the ecological functions of MCBs have been gradually recognized thanks to their characteristics of isolation and edge effects [6,7].
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