4.1. Anti-seasonal flooding substantially reduced species diversity in the WLFZ of the TGR
A unique riparian ecosystem has been created as a result of anti-seasonal and continuous flooding after TGR operations, which notably influences the species diversity distribution patterns of plant communities and their functional characteristics (Li et al., 2022). Before the impoundment of the TGR, there were 405 vascular plants in the riparian area of the TGR (Wang et al., 2002). However, in the early stage of the TGR impoundment in 2009, only a total of 231 species, belonging to 61 families and 169 genera, was found in the WLFZ of the TGR (Liu et al., 2011). After the early 7 years of the TGR impoundment in 2010, Zhang et al., (2013) found that a few shrubs (Boehmeria nivea, Lespedeza davidii, Lespedeza cuneata) and trees (Morus alba, Albizia kalkora and Broussonetia papyrifera) only appeared at the altitude of 170 m. The main vegetation type was herbs in the WLFZ. In the present study, after the early 19 years of the TGR impoundment, a total of 73 vascular plant species were identified in the WLFZ. Annual herbs accounted for the highest percentage of all life forms at each altitude. Annuals, perennials and shrubs accounted for 71.23%, 27.39% and 1.37% of the total number of species, respectively. Thus it could be seen, anti-seational and continuous flooding triggered the dramatic alterations in floristic composition, structure, and distribution pattern of plant communities in the riparian zone.
At the same time, after 19 years of water storage, plant life forms have been altered dramatically in the new riparian forest. This novel anti-seasonal flooding reduced functional diversity, mostly owing to the loss of stress-tolerant woody species and competitive perennial herbs. Essentially new hydromorphological conditions following damming limited recruitment of native shrub and tree species guilds sensitive to floods (to drag forces, inundation, and anoxia). Thus it can be seen that woody plants (trees and shrubs) showed the greatest decrease, and the proportion of perennial herbs also decreased, while the proportion of annual herbs increased significantly, indicating that herbs, especially annual herbs, i.e. Compositae, Gramineae and Leguminosae families, are more suitable for the environment of water level fluctuations in the TGR. Thus it could be seen, the pre-dam vegetation failed to persist under the new riparian ecosystem, and the species richness and diversity of the riparian forests were significantly lower due to the great hydrological shifts by the TGD construction (New & Xie, 2008; Chen et al., 2012; Chen et al., 2022).
A plausible reason was that most annuals, i.e., Compositae, Gramineae and Leguminosae, germinate in spring and fructify in autumn during the low water level of the TGR operation within a growth season, and have the flooding tolerance and the capacity to synchronize germination and growth within a short-exposure period, which underlie the plant species alterations. Therefore, these adaptive annuals survive as dominant species in the WLFZ of the TGR mainly because their phenology do not compound with the submergence occurring time. The recession of the water level left a nearly barren drawdown zone and provided an entire growing season for the forbs and graminoids, especially for the annual and biennial and perennial species. With its short life cycle, annual herbs were able to go from seed to seed life cycle in a relatively short period of time after water receding and before water storage. The next year, a new life cycle began with the emergence of seeds from nearby seed sources or soil seed banks (De Souza et al., 2021). After receding, clonal growth can quickly expand space and gain an advantage in the inter-specific competition by their extensive lateral spread and forming dense, nearly monospecific stands, i.e., C. dactylon, which can quickly re-sprout following continuous inundation and take advantage of the short-exposure period before the reservoir is filled again. Thus, the proportion of annual herbaceous species showed a decreasing trend while the proportion of perennial herbaceous species showed an increasing trend along elevation gradients. The transformed species richness of forbs, graminoids, annuals, biennials and perennials increased significantly, especially the Compositae, Graminaceae and Leguminaceae plants.
Species diversity distribution along elevation gradients has different patterns. Some studies have demonstrated that species richness patterns from the lowest to highest elevations may show a monotonic decrease, or a monotonic increase; others have revealed hump-shaped patterns with a peak in richness at mid-elevations (Mallen-cooper & Pickering, 2008; Trigas et al., 2013; Arturo & Lauro, 2005). In the present study, the S, H and D increased with the increasing elevation but E showed a contrary trend (Fig. 3). This species distribution pattern might be caused by several synergetic attributes (e.g., the submergence depth, the tolerant capacity to flooding, the life form, the dispersal mode, and the inter-specific competition) (Zhang et al., 2013). The lower elevation area (145–155 m) is generally prone to more severe flooding with a greatest depth of inundation (30 m) and prolonged inundation duration (nearly half 6 months), which resulted in the lower S, and H than the higher elevation area (156–165 m and 165–175 m). Thus, these annual species in the low elevation area may rapidly attain maturity before being submerged with short life cycle (Zhang et al., 2013). At the highest elevations (166–175 m), seed dispersal by wind, water, animals, and humans might be important factors resulting in the higher diversity index (Merritt & Wohl, 2006). However, H showed hump-shaped patterns with a peak at mid-reaches from upstream to downstream (Fig. 4). The middle reaches of the reservoir area were mostly forested on both sides of the Yangtze River, with less agricultural cultivation and less human influence. In the downstream reaches, the Citrus farming industry on both sides of the Xiangxi and Tongzhuang rivers was booming, and the plant communities on both sides of the river was more affected by agricultural farming and human interference. Ecological characteristics of plant guilds as an assemblage of plant population are response to the environment changes and are more pronounced during succession (Yang et al., 2012; Ge et al., 2020). In this study, it was found that the main influence factors affecting the spatial distribution of the plant guilds in the new riparian forest were hydrological factors, such as elevation (different flooding depths) and flooding time. So, the pattern of the plant guilds in the TGR was mainly affected by water level disturbance. This result was largely consistent with other results that hydrological conditions determined the vegetation diversity and aboveground biomass patterns at the elevation gradients of the drawdown zone (Wang et al., 2014). It can be seen that the vegetation spatial distribution of the TGR area was heavily influenced by the hydrological factors, i.e., different flooding depth and flooding time in the reservoir area and the species diversity was significantly reduced.
Soil nutrient concentrations could also strongly affect plant species diversity and evenness (Aerts, et al., 2003). In the present study, the concentrations of TN and TP appeared to be higher at the elevations of 165–175 m than at the elevations of 145–155 m and 155–165 m. The continuous submergence in winter and the high frequency of floods in summer may result in this pattern for soil nutrients may be released when submerged and soil that serves as a nutrient source may be scoured by repeated flooding. Qui & McComb (1996) also reported that the concentration of TN was reduced after continuous submergence. And Roem & Berendse (2000) studied nutrient supply ratio as possible factors determining changes in plant species diversity in grassland and heathland communities, which showed that plant species with high diversity were at balanced N/P ratios between 10 and 14. However, in the present study, N/P ratios of the soil was between 0.98 and 1.56 with an average of 1.26, which showed that N was a limiting factor in the soil in the new riparian forest. The increase of N supply in a N-limited grassland (e.g., N/P ratio < 10) may lead to an increase in biodiversity (Roem & Berendse, 2000). In this study, the distribution patterns of plant guilds were positively associated with TN, TP, and OM, while negatively correlated with pH, NH4+-N, NO3−-N, and AP in the CCA (Fig. 8b.). Of these, the significantly negative correlation between NO3−-N and the distribution patterns of plant species (p < 0.05) was consistent with the results that excess of NO3−-N is known for its negative effect on the diversity of plant guilds (Aerts, et al., 2003). Therefore, N might be a soil nutrient limiting factor in determining the alterations in plant species diversity and plant distribution patterns in addition to elevation gradients and flooding time in the new riparian forest of the TGR.
The results also corroborated the trend of the species diversity index increasing with the elevation gradients. At higher elevations, the number of plant species is increasing and the competition between species becoming more intense. From bottom to top along axis 2, OM and TP rose (Fig. 8b.). This result was basically the same as the pattern shown in Table 2, which showed that the influence of elevation and flooding on the distribution pattern of the plant guilds in the TGR was more obvious than that of OM and TP, and the first order axis could explain the interrelationship between the plant guild and the habitat in the declining zone (Fig. 8c.). That is, although the spatial distribution of the plant guilds in the subduction zone of the TGR area was the result of a combination of multiple factors, the influence of elevation gradients and flooding time played a dominant role in the formation of the spatial pattern of the plant guilds in the new riparian forest of the TGR and the second factors were TN, TP, and OM.
In the TGR area, the plant guilds were distributed from left to right along the elevation and flooding time, as follows Ass. C. dactylon + E. crusgalli + C. rotundus; Ass. C. dactylon + A. theophrasti + S. plebeia; Ass. E. crusgalli + D. sanguinalis + S. viridis; Ass. C. dactylon; Ass. B. pilosa; Ass. B. tripartita; Ass. D. sanguinalis; Ass. H. scandens; Ass. S. viridis; Ass. C. canadensis + B. pilosa; Ass. E. prostrata + C. dactylon; Ass. C. dactylon + M. officinalis (Fig. 8a.). There were some overlaps between the types. Ass. C. dactylon + E. crusgalli + C. rotundus and Ass. C. dactylon + A. theophrasti + S. plebeia located at the altitude gradient of 145–155 m, with long flooding durations and short growth durations. The medium elevation gradient (155–165 m) was for the Ass. E. crusgalli + D. sanguinalis + S. viridis and Ass. C. dactylon. Ass.B. pilosa, Ass. B. tripartita, Ass. D. sanguinalis, Ass. H. scandens, Ass. S. viridis, Ass. C. canadensis + B. pilosa, Ass. E. prostrata + C. dactylon and Ass. C. dactylon + M. officinalis were distributed at high altitudes (165–175 m) in the WLFZs, with less or almost unaffected by flooding. In the early stages of flooding in 2010, there were 18 main plant guilds in the WLFZ of the TGR, including 5 xerophyte, 6 hygrophyte and 7 mesophyte guilds (Chen et al., 2012). In the present study, after 19 times of water level fluctuations in the TGR, the 12 main plant guild types were discovered, belonging to hygrophyte and mesophyte communities. Xerophyte guilds almost disappeared. Thus, the taxonomic and functional characteristics of communities were differently as they may respond to important drivers differently. The vegetation composition of the WLFZs in the TGR showed a significant change with a transition from xerophytes to hygrophytes and mesophytes with the increasing flooding time. The observed riparian plant guild response patterns to prolonged submergence in the WLFZs of Yangtze River might hopefully be transferred to similiar rivers with little or no existing information in other regions regardless of whether or not they share species. The guild approach helps develop general frameworks to predict vegetation responses to changing environmental conditions.
4.2. Niche structure and utilization of limited resources
Niche breadth describes a suite of environments or resources, in the broadest sense, which a species can inhabit or use, which measures the range of resource characteristics across which a species exists, and indicates the extent that a species utilizes different types of resources (Slatyer et al., 2013), while niche overlap refers to the partial or complete sharing of resources or other ecological factors (predators, foraging space, soil type, and so on) by two or more species (Colwell & Futuyma, 1971). The measures of niche breadth and overlap are all based on the distribution of individual organisms, by species, within a set of resource states (Colwell & Futuyma, 1971). In the present study, the dominant species were C. dactylon, X. sibiricum, C. rotundus, E. crusgalli, S. viridis, B. pilosa, D. sanguinalis and P. hydropiper in the riparian forest of the TGR according to the importance value and niche breadth. The correlations between niche breadths and important values of each elevation were positively correlated in the surveyed sample plots under three types of elevations (Fig. 3.). Under different altitude sections according to niche breadth, C. dactylon (7.0836) >E. crusgalli (3.3973) >B. pilosa (3.1698) at the lower elevations (145–155 m); C. dactylon (8.176) >X. sibiricum (4.436) >E. crusgalli (3.721) at the middle elevations (155–165 m); C. dactylon (6.659) >X. sibiricum (6.0956) >E. prostrata (4.889) at the highest elevations (165–175 m). C. dactylon was the most dominant species in the novel riparian forest with highest importance value and niche breadth at each altitude. C. dactylon might have evolved morphological, physiological, and biochemical adaptations to oxygen deficiency, such as dormant tubers or rhizomes (Zhang et al., 2013). So, it could germinate quickly after submerged period to against the coming dry period to achieve the greatest competitive advantages in the WLFZ. There were the most of species pairs with the niche overlap index less than 0.2 or 0 at the middle altitude (155–165 m). The vegetation in the middle altitude (155–165 m) were the least affected by the Yangtze River flooding during the dry period. C. dactylon almost formed a single-species community at the middle altitude (155–165 m), because of its strong acclimation and fast growth as well as facile vegetative propagation compared with other species.
Niche breadth in the study area had high niche overlap between species, but in some habitat conditions, the species with narrower niche breadth appeared larger niche overlap. There were some plants, such as H. scandens, L. crustacea and C. ambrosioides, niche overlap value was 1.00, almost perfect overlap. Most of these species had lower niche overlap with those dominant species. In fact, the niche occupation of resource space between two species was only infinitely close, so the overlap was only infinitely close to 1.00. This indicated that there was no direct linear relationship between niche breadth and niche overlap, which was caused by the heterogeneity of spatial distribution of environmental resources available to species (Chen et al., 2019). The TAOih in the different altitude area was highest at the altitude of 145–155 m (0.3642), lower at 165–175 m (0. 2619) and 155–165 m (0.2524) in descending order. The vegetation in the lower elevations (145–155 m) suffered the longest periods of winter flooding and summer flood. So they had the shortest time to complete their life cycle under the influence of both winter storage and summer flood. The results showed that anti-seasonal and continuous flooding would lead to the gradual disappearance of the original diverse niches, resulting in more uniform habitats, and more obvious competition among species with similar resource requirements. The comprehensive ecological level analysis concluded that, after 19-year inundation of the TGR, the vegetation of the new riparian forest was still in the high niche overlap, intense competition, and species specialization, which showed that the vegetation was still in the early stage of primary succession, ecosystem stability was poor, and habitat fragmentation was severe in the TGR area.
4.3. Practical implications for vegetation restoration and reconstruction
The anti-seasonal and continuous flooding precipitated loss of the original vegetation, especially trees and shrubs after the filling of the TGR. We found a significant decrease in the number of vascular plants compared to the pre-flooding period. The proportion of annual herbs, especially Compositae, Gramineae and Leguminosae plants have significantly increased in the riparian forest as a result of their adaptation strategies. And we found some invasive plants such as E. annuus began to show dominance in the vegetation composition of the new riparian forest in the TGR. The TGR area is not only one of the most biodiverse areas in China, but also one of the most endemic species areas in the world (Jin et al., 1984). The dominance of invasive plants can cause great harm to the gene pool and genetic diversity in the TGR area (Yang et al., 2012; Ge et al., 2020). The present results showed the high heterogeneity of species diversity and environmental factors in the TGR areas with significant habitat changes and poor ecosystem stability. Therefore, there may be some differences in the governance strategies adopted in different areas of the novel riparian ecosystem for vegetation restoration of the riparian forests.
According to the comparative analysis of the vegetation status in the TGR area, the following four implications are proposed:
(1) more attentions should be given to the indigenous species in the selection of species for the restoration and reconstruction of the novel riparian forests. The exploration and study of indigenous species in the reservoir may be a more effective and safe means of artificial vegetation restoration. Therefore, the above- described indigenous plant guilds should be prioritized in vegetation restoration efforts;
(2) differences between regions should be taken into account in the implement of vegetation restoration measures in the reservoir area. The construction of artificial guilds during vegetation restoration should be tailored to the local context and plant adaptations to local conditions;
(3) in general, when the artificial guilds are restored in the reservoir area, it may have a better effect with herbaceous plants as the main part, supplemented by shrubs or small trees in the middle-high elevation areas such as Distylium chinense (Sun et al., 2020) and Taxodium distichum (Li et al., 2010);
(4) studies have shown that stabilization of vegetation in the depression zone may take 70 years or more (Nilsson & Aradóttir, 2013; Nilsson et al., 2013; Nilsson et al., 1997; Nilsson et al., 2015). Although it has been 19 years since the formation of the WFLZ of the TGR, the niche differentiation between different dominant plants was lower, the inter-specific competition was more intense and the stability of plant guilds was still worse. Therefore, long term investigations and observations should be continued in this area to identify and monitor alterations in the characteristics of the plant guilds and soil properties due to anti-seasonal and continuous inundation on riparian areas triggered by flow regulation or global warmer climates.