Abstract
There is currently a lack of evidence surrounding changes in the health of riparian zones under different land-use patterns within mega-reservoirs and around dams. Scientific evidence for the quantitative effects of stress indicators is vague and varies significantly among reservoirs and dams worldwide. In this study, we used a field-based approach to evaluate riparian health changes—influenced by pressure indicators—across 274 transects from three land-use areas (rural, rural–urban transitional, and urban) in the Three Gorges Dam Reservoir (TGDR) in China during 2019. Multivariate statistical techniques were applied to test for riparian zone changes under these variant land-use patterns. Our results showed that 13 pressure indicators significantly influenced 27 health indicators (including parameters for habitat, plant cover, regeneration, erosion, and exotics) of the riparian zones from the three land-use areas differently. Our results also showed that parameters for plant cover, erosion, and exotics were major contributors within the selected riparian health indicators, whereas land-use designs, farming systems, and pollutant activity variables were the pressure indicators with the strongest impact. Pearson correlation (with r ranging from -0.731 to 0.989) showed that urban transects exhibited the strongest comparative interaction, whereas rural–urban transitional transects formed the weakest association. Furthermore, the agglomerative hierarchical cluster analysis revealed similarities between rural and rural–urban transitional sites while confirming substantial dissimilarity in urban locations. These comprehensive and relevant results provide essential information for reservoir administrators to implement functional changes suited to TGDR land-use scenarios.
Keywords: land-use patterns, rapid appraisal, stress indicators, quantitative assessment, riparian health condition, Three Gorges Dam Reservoir
1. Introduction
Human activities have caused substantial disorder in reservoir riparian zones across several countries (Arif et al., 2021; He et al., 2020), such as deterioration caused by urbanization and other industrial development (Belete et al., 2020; Bombino et al., 2019). Riparian zones ensure preliminary reservoir safety and, ultimately, naturally improve water quality on a large scale (Mello et al., 2018; Shahariar et al. 2021). Reservoirs and lakes are essential water sources that promote human welfare, production, and life-cycle conservation, and they provide 90% of the biosphere’s freshwater (Dai et al., 2017). Reservoir riparian zones are biodiversity hotspots that form some of the world’s most important natural ecological landscapes (Tape et al., 2016; Yu et al., 2016). However, these buffer zones are also among the most sensitive and non-recoverable natural ecosystems. Despite the increasing positive global effect of reservoirs, their riparian zones are changing and deteriorating owing to variations in land-use patterns (Dempsey et al., 2017; Weldegebriel et al. 2021). The impacts of land-use patterns on riparian health are complex (Castillo et al., 2012), and there is an urgent need to understand the relationship between land-use variation and the condition of riparian zones in reservoirs (Mello et al., 2018; Yu et al., 2016). Research on this topic is a prerequisite for managing large dams; it would produce vital information to help determine the potential impacts of land-use changes on the riparian zones of large reservoirs such as the Three Gorges Dam Reservoir (TGDR).
Due to the demand for urban and industrial land use, natural land surfaces are repeatedly converted to artificial land surfaces, consequently weakening marginal areas and causing increased heavy-metal loads and low water quality (Woźniak et al., 2022; Yu et al., 2016). These changes directly modify the land’s physical features and alter riparian ecosystems along with their associated biodiversity, instream habitats, and water characteristics (Bombino et al., 2019; Puntenney-Desmond et al., 2020; Tape et al., 2016). Land use with a higher level of human activity, including industrial, commercial, residential, and transport activities, tends to permanently impact the riparian zone (Bombino et al., 2008; Catford et al., 2013; Galia et al., 2016). In earlier quantitative studies, negative correlations were found between artificial land use and riparian ecosystems (Rivaes et al., 2015; Shieh et al., 2007). However, the relationships between riparian health conditions and pressure activities, such as agricultural cropping and the implementation of farming systems, are more complicated. Previous studies have indicated that land-use changes and extensive environmentally unfriendly events might influence the vegetation in buffer zones, thereby significantly affecting riparian health and rapidly altering the area’s physical features (Castillo et al., 2012; Sowińska-Świerkosz et al., 2014; Tao et al., 2020). Multipurpose studies have been conducted on riparian zones in various parts of China, as well as worldwide, and their impacts were assessed under different land-use scenarios (Yu et al., 2016; Zheng et al., 2021a). However, the literature is limited in terms of studies investigating the riparian zone response against pressure indicators from variant land-use patterns in extra-large reservoirs and around mega-dams such as the TGDR. Therefore, there is an urgent need to examine how riparian zones react under different land uses to ensure the conservation and sustainable management of these ecological landscapes.
However, land-use effects may vary in buffer zones depending on their particular use (Ferreira et al., 2005). Agricultural activities significantly affect rural sections, while concrete structures impact urban areas (Zheng et al., 2021b; Petts & Gurnell, 2005). Results from quantitative studies that compared correlations between near- and far-field developments showed a higher correlation in the near-field events, confirming that pressure impact was progressive within these regions, owing to their limited width (Johansen et al., 2007). Therefore, there is a need to understand the relationship between land use and riparian health variables on different scales. Such an understanding would be critical to the logical establishment of the whole riparian catchment zone and support the monitoring process.
Similarly, watershed protection strategies are mainly subject to the land-use patterns and management practices of individual regions (Nilsson & Svedmark, 2002). The demand for effective policies in China is at present higher than in the 1950s. The increasing population incentivizes planners to introduce substantial developments in the agricultural sector to meet the public’s needs, which is the principal factor that resulted in the status change of water bodies. The area reduction ratio is particularly high for these water bodies, increasing from 135 to 8,700 km2 from the 1970s to 2016 (Dai et al., 2017). The Chinese government is continuously implementing strategies to conserve water resources at the national level and is building new dams to ensure the optimal use of water sources. Due to the long history of human settlement in the area, the rivers in southwest China are among the most threatened ecosystems (Yuan et al., 2021), and their riparian zones are susceptible to prolonged submergence (Sarneel et al., 2019). Since big reservoirs can hold water for an extended period, riparian ecosystems and biodiversity can quickly deteriorate in reservoir regions (Zheng et al., 2021a; Wang et al., 2016; Yang et al., 2014). Compared with other rivers, the Yangtze River—the primary source of the TGDR—is extremely fragile. A previous report highlighted major pressures the river faced and described it as the world’s most polluted waterway (Arif et al., 2021). These environmental and physical variations are the consequences of human disturbance that have altered the Yangtze River’s land-use pattern.
This study focuses on various water sources flowing into the TGDR, the largest deep-water reservoir in China and the primary freshwater source of southwest China (Chen et al., 2021; He et al., 2021). The riparian zones of the TGDR are very vulnerable owing to their geomorphology and specific geographic locations, which are particularly important with regards to their ecological and environmental role (Sang et al., 2019). Once the river ecosystem is damaged, it is challenging to restore it to its original condition (Ding et al., 2021). Riparian health conditions in the TGDR are declining, and the problem has intensified because of recent land-use variations. However, the pressure effects cannot be fundamentally determined in the reservoir basin merely via the point source (Dempsey et al., 2017). Statistical models and theoretical studies are needed, as these generate technical information to combat the pressure effects of different land uses in various riparian zones of mega-dams. This study’s primary objective was to investigate changes within the riparian zones of reservoirs and dams under the influence of stress indicators from different land uses (i.e., rural, rural–urban transitional, and urban). More precisely, we examined the variation between riparian health and pressure situations observed in rural, rural–urban transitional, and urban areas and identified the key indicators responsible for the total variation in these three regions’ riparian zones. Furthermore, we explored the correlations between sets of pressure indicators and riparian health indicators (RHIs) and measured the ability of variables to interact with one another. Finally, we classified the statistical similarities between land-use sites along the TGDR. This research will help establish strategies to manage riparian zones under variant land uses on a large scale.