Dam inundation simplifies the plant community composition

Patterns and drivers of plant carbon, nitrogen, and phosphorus stoichiometry in a novel riparian ecosystem

Carbon (C), nitrogen (N), and phosphorus (P) stoichiometry serve as valuable indices for plant nutrient utilization and biogeochemical cycling within ecosystems. However, the allocation of these nutrients among different plant organs and the underlying drivers in dynamic riparian ecosystems remain inadequately understood. In this study, we gathered plant samples from diverse life forms (annuals and perennials) and organs (leaves, stems, and roots) in the riparian zone of the Three Gorges Reservoir Region (TGRR) in China-a novel ecosystem subject to winter flooding. We used random forest analysis and structural equation modeling to find out how flooding, life forms, plant communities, and soil variables affect organs C, N, and P levels. Results showed that the mean concentrations of plant C, N, and P in the riparian zone of the TGRR were 386.65, 19.31, and 5.27 mg/g for leaves respectively, 404.02, 11.23, and 4.81 mg/g for stems respectively, and 388.22, 9.32, and 3.27 mg/g for roots respectively. The C:N, C:P and N:P ratios were 16.15, 191.7 and 5.56 for leaves respectively; 26.98, 273.72 and 4.6 for stems respectively; and 16.63, 223.06 and 4.77 for roots respectively. Riparian plants exhibited nitrogen limitation, with weak carbon sequestration, low nutrient utilization efficiency, and a high capacity for nutrient uptake. Plant C:N:P stoichiometry was significantly different across life forms and organs, with higher N and P concentrations in leaves than stems and roots, and higher in annuals than perennials. While flooding stress triggered distinct responses in the C, N, and P concentrations among annual and perennial plants, they maintained similar stoichiometric ratios along flooding gradients. Furthermore, our investigation identified soil properties and life forms as more influential factors than plant communities in shaping variations in C:N:P stoichiometry in organs. Flooding indirectly impacts plant C:N:P stoichiometry primarily through alterations in plant community composition and soil factors. This study underscores the potential for hydrologic changes to influence plant community composition and soil nutrient dynamics, and further alter plant ecological strategies and biogeochemical cycling in riparian ecosystems.

Water depth-dependent stem elongation of completely submerged Alternanthera philoxeroides is mediated by intra-internodal growth variations

Complete submergence, especially deep submergence, poses a serious threat to the growth and survival of plants. One study previously showed that Alternanthera philoxeroides (a herbaceous perennial plant) submerged at depth of 2 m presented fast stem elongation and reduced stem elongation as water depth increased. In the present study, we aimed to figure out from the morphological and anatomical perspective how the differential growth response of the plant to water depth was achieved. We investigated the elongation of different stem parts and the relationship of stem elongation to cell size and number in A. philoxeroides by conducting experiments using a series of submergence depths (0 m, 2 m, 5 m, and 9 m). The results showed that, in comparison with unsubmerged plants, completely submerged plants exhibited enhanced elongation at depths of 2 m and 5 m but suppressed elongation at depth of 9 m in immature stem internodes, and displayed very little elongation in mature stem internodes at any depths. The stem growth of A. philoxeroides at any submergence depth was chiefly caused by the elongation of the basal parts of immature internodes. The elongation of the basal parts of immature internodes was highly correlated to both cell proliferation and cell enlargement, but the elongation of the middle and upper parts of immature internodes correlated nearly only with cell enlargement. This study provided new information on the growth responses of A. philoxeroides to heterogeneous submergence environments and deepened our understanding of the growth performance of terrestrial plants in habitats prone to deep floods.

Impacts of the Three Gorges Dam on riparian vegetation in the Yangtze River Basin under climate change

As the largest hydroelectric project in the world, the Three Gorges Dam (TGD) is expected to have significant environmental and ecological impacts on riparian vegetation in the Yangtze River Basin (YRB). However, existing studies have mainly focused on small segments of the YRB. In addition, few studies have quantified the responses of riparian vegetation to both climatic factors and dam construction. In this study, we investigated riparian vegetation dynamics over the entire YRB before, during, and after the construction of TGD from 1982 to 2015 using the normalized difference vegetation index (NDVI). Furthermore, the effects of climatic factors and dam construction on riparian vegetation were quantitatively analyzed using path analysis. The results demonstrate that the YRB has experienced a generally greening trend after TGD construction. The impacts of climate change on riparian vegetation have exhibited notable spatial heterogeneity and temperature is the main climatic factor that affects riparian vegetation growth. Moreover, TGD becomes the major contributor to riparian vegetation dynamics in the YRB after TGD construction. TGD has not only directly enhanced riparian vegetation but also indirectly affected riparian vegetation by regulating the microclimate. This study highlights the significance of anthropogenic interference when evaluating the relationships between riparian vegetation and climatic factors, providing useful insights for the effective management and conservation of large-scale riparian ecosystems.

The differences in plant invasion in two types of shorelines under flow regulation of the Three Gorges Reservoir

Riparian zones, crucial for linking fluvial and terrestrial habitats, are among the most diverse ecosystems. However, they are intensively invaded by alien plants, particularly in dam-regulated rivers. Therefore, understanding the mechanisms underlying plant invasion in dam-regulated river systems has become increasingly important, given that over two-thirds of global rivers are artificially regulated. Regulated rivers may flood upland areas or pristine riparian zones, resulting in shorelines developed from pre-upland and pre-riparian areas. However, differences in invasion intensities, adaptive strategies of invasive plants, and native species' resistance (namely the diversity-invasibility relationship) across these shorelines are unclear. To address these uncertainties, we performed field investigations in the Three Gorges Reservoir (TGR) on the upper Yangtze River, where both pre-upland and pre-riparian shorelines are present. Our findings indicate that pre-upland shorelines are more intensively invaded, showing higher relative richness and cover of invasive species. Invasive plants in this area displayed more conservative resource strategies and greater drought tolerance, exhibiting lower community-weighted mean (CWM) specific leaf area, higher CWM leaf dry mass content, and larger CWM seed mass. Pre-upland shorelines' invasibility decreased as the richness and cover of native species increased, a trend not observed in pre-riparian shorelines. The observed variations in plant invasion between the two shoreline types are primarily driven by differences in resident plant presence, soil moisture levels, and hydrological disturbances. This study provides valuable insights for policymakers and practitioners involved in managing invasive plants in regulated river ecosystems.

Dam inundation duration as a dominant constraint on riparian vegetation recovery

The identification of limiting factors is essential for the ecological restoration of riparian ecosystems degraded by the damming of rivers, but remains unclear. Here, we quantitatively assessed the relative importance of environmental factors and revealed the main limiting factors for riparian vegetation restoration and their influencing mechanisms, using riparian plant and environmental data of seven large reservoirs in southwest China. We found that inundation duration had a significantly greater effect on riparian vegetation distribution, cover and diversity than environmental factors such as inundation depth, rainfall, humidity, temperature, sunshine hours, aspect, slope, surface relief, soil pH, available nitrogen (AN), available phosphorus (AP), and available potassium (AK); vegetation cover, species richness, complexity and dominance were highly significantly negatively correlated with inundation duration (p < 0.01); inundation for 5 months is close to the tolerance limit of most plants and poses a significant limiting effect on the vegetation restoration in the reservoir riparian. Therefore, the inundation duration should be highlighted in riparian vegetation restoration. Meanwhile, incorporating the riparian inundation into the river ecological scheduling objectives to shorten the inundation duration and thus radically alleviate the limitation is a new opportunity for vegetation restoration in the reservoir riparian.

Distinguishing the mechanisms driving multifaceted plant diversity in subtropical reservoir riparian zones

Understanding the multifaceted plant diversity and its maintenance mechanisms is crucial for biodiversity conservation. Dam-induced water level fluctuations dramatically alter various aspects of riparian diversity, such as taxonomic (TD), phylogenetic (PD), or functional (FD) diversity. However, few studies simultaneously evaluated plant TD, FD, and PD, especially in the subtropical reservoir riparian zone. Here we sampled plant diversity and environmental drivers along inundation gradients of the Three Gorges Reservoir Region in China. We integrated multifaceted plant diversity to assess how distinct ecological processes affect the plant community assembly and how they respond to inundation gradients, spatial variability, climate, and soils in dam-regulated riparian zones. We found that alpha TD, PD, and FD diversity exhibited decreasing trends with increasing inundation gradients and significant positive correlations with soil organic matter. The number of clustering plant communities increases along the inundation gradients. Beta TD and PD diversity were mainly dominated by species turnover with fewer contributions from nestedness, while beta FD diversity was mainly dominated by nestedness with fewer contributions from species turnover. The explainable rates of different dimensions of beta diversity, turnover, and nestedness ranged from 11% to 61%, with spatial factors explaining the highest beta diversity in different dimensions, followed by inundation gradients, soil properties, and climate variables. Our results suggest dispersal limitations are more important for species turnover in dam-regulated riparian zones at regional scales, while inundation gradients and soil fertility are more critical in shaping plant community assemblages at the local scale. This study emphasizes that environmental and spatial gradients are critical for understanding the assembly mechanisms driving multifaceted plant communities at local and regional scales and reinforces the importance of protecting seed sources and dispersal pathways and maintaining river connectivity when implementing restoration projects.

Effect of topographical features on hydrologically connected riparian landscapes across different land-use patterns in colossal dams and reservoirs

Topographic features impact the riparian landscape, which shapes reservoir ecosystems. We know little about ecological network parameter (ENP) responses to topographical features (riparian width, stream-channel width, slope, and elevation) from three land-use areas (rural, urban, and rural-urban transitional) in larger dams and reservoirs globally. This study used a field-based approach with 305 transects on an inundated area of 58,000 km² inside the Three Gorges Dam Reservoir (TGDR) in China. We discovered that topographical features influenced ENPs differently, involving parameters of plant cover, regeneration, exotics, erosion, habitat, and stressors. As per the Pearson correlation (p < 0.05), riparian width had the most significant effect on transitional ENPs and the least impact on urban ENPs. Riparian width showed the most important influence on the parameters of exotics (with r ≤ -0.44) and erosion (r ≤ 0.56). In contrast, stream-channel widths had the greatest effect on rural ENPs and the least on urban and transitional ENPs. The erosion parameters were the most affected (r ≤ -0.26) by stream width. The slope showed relationships with the fewest ENPs in all three areas and influenced the stress (with a range of -0.51 <r < 0.85) and erosion (r ≤ -0.39) parameters. The impact of elevation was higher in urban areas and was positively correlated with the parameters of plant cover (r ≤ 0.70), erosion (r ≤ 0.58), and habitat (r ≤ 0.69). These results justify the policy emphasis on riparian areas that are managed using the same techniques, which generally ignores their topographical features.

The impact of ecotourism on ecosystem functioning along main rivers and tributaries: Implications for management and policy changes

Tourism along river basins benefits both tourists and the economy, but its management necessitates trade-offs between nature-based recreation and ecological functioning. Despite ecosystem services being helpful in managing environmental challenges, there are limited data on the impact of tourism activities on ecosystem functioning across different river types globally. This study investigates how people's recreational activities and values affect ecosystem functioning in high-order rivers. The original field data were collected from 308 transects along the main river and tributaries of the Three Gorges Dam Reservoir in China during 2019. Kruskal-Wallis tests (p < 0.01) revealed that the ecosystem functioning indices were significantly higher than the recreational activity and value indices around the rivers and that ecosystem functioning was highest around tributaries. The critical variables of ecotourism activities and ecosystem functioning identified by principal component analysis accounted for 66.49% of the total variance. The Pearson correlation coefficient strengths among tourism and ecosystem functioning parameters were correlated mildly to moderately, but they exhibited positive and negative connections with a range of r = -0.27 to 0.37 (p < 0.05). Furthermore, the distribution patterns of these parameters that were determined by hierarchical cluster analysis were diverse for both the main river and its tributaries. The findings suggest that the development and enforcement of zoning may be necessary for the long-term use of natural resources by all sectors of society. Therefore, it is imperative to raise public awareness and urge governments to adopt more progressive ecotourism policies.

Nature-based tourism influences ecosystem functioning along waterways: Implications for conservation and management

Nature-based tourism has an influence on ecosystem functioning around watercourses, but this influence lacks scientific evidence. Additionally, strategic and operational management of streams necessitates trade-offs between the recreational activities and values of tourists and riparian zone hospitality services. This paper aims to assist environmentalists and planners by exploring the effects of tourism-based recreational activities on ecosystem functioning along the drawdown zone. The study uses multivariate statistical techniques to delineate the relevant global tourism issues for planners. Kruskal-Wallis tests (p < 0.01) were conducted using quantitative data from 284 transects within the Three Gorges Dam Reservoir in China. The results revealed higher ecosystem function indices than tourism indices. Indicators of tourism contributed both positively and negatively to ecological indicators, with the Pearson correlation coefficients ranging from minor to moderate (r = ̶ 0.24 to 0.38, p < 0.05). Principal component analysis revealed that the critical variables of ecosystem functioning and tourism activities explained 72.26 % of the overall variance. Nevertheless, hierarchical cluster analysis revealed that these indicators responded differently in the upstream, midstream, and downstream sections. Our findings suggest that policymakers should consider the different characteristics of riparian zones in future planning, as doing so will improve both national and global strategic and operational management.

Environmental literacy affects riparian clean production near major waterways and tributaries

Although environmental illiteracy threatens the functioning of landscapes throughout the world, it is frequently ignored. The traditional wisdom assumes that suspicions will evaporate when the public and government authorities are provided with new information. Despite significant efforts to enhance riparian corridor output, limited data are available on the effect of environmental literacy metrics (ELMs) on clean production elements (CPEs) across various streams (e.g., main rivers and tributaries) within impoundments. This study examined such effects within the China Three Gorges Dam Reservoir area (TGDRA) by collecting 336 transects that assessed the breadth of effects on 58,000 km² in 2019. The network visualization revealed 7234 papers published over the last 121 years, each of which focused on themes such as plant cover, regeneration, exotics, erosion, habitat, and stressors. The bar graph showed that the general public lacked understanding of environmental literacy (e.g., knowledge, attitudes, and behavior), which influenced plant cover elements most in tributary zones but had little direct effect on regeneration. Locals' environmental literacy had the greatest impact on CPEs, with Pearson correlation coefficients ranging from -0.69 <r < 0.96 in the main river zones. Moreover, public employees' environmental literacy had a stronger correlation with CPEs (-0.58 <r < 0.83) within the main river regions. Farming systems, exposed soil, dominant grass regeneration, and instream structures, including pollution, were among the most notable CPEs within the TGDRA. According to hierarchical approaches, CPEs and ELMs change substantially across stream types. CPEs and ELMs vary significantly around main rivers and tributaries, requiring efforts to raise the public understanding of the worldwide impacts of stream health on humans.

Differential Growth Responses of Alternanthera philoxeroides as Affected by Submergence Depths

Global climate change has resulted in an increase in intensity and frequency of flooding, plants living in lowlands, and shore areas have to confront submergence caused by flooding, submergence-tolerant plants usually respond by adopting either escape or quiescence strategies. While certain plants exhibit a changeover from escape strategy upon partial submergence to quiescence strategy under complete shallow submergence, it remains unknown whether plants completely submerged at different water depths would adjust their strategies to cope with the change in submergence depth. Alternanthera philoxeroides is an ideal species to explore this adjustment as it is widely distributed in flood-disturbed habitats and exhibits an escape strategy when completely submerged in shallow waters. We investigated the responses of A. philoxeroides in terms of morphology, anatomy, and non-structural carbohydrate metabolism by conducting experiments using a series of submergence depths (0, 2, 5, and 9 m). During the submergence treatment, environmental factors such as light, dissolved oxygen, and temperature for submerged plants were kept constant. The results showed that A. philoxeroides plants submerged at depth of 2 m presented an escape strategy via fast stem elongation, extensive pith cavity development, and small biomass loss. However, the retarded stem elongation, reduced pith cavity transverse area, and increased biomass loss along the water depth gradient indicated that A. philoxeroides altered its growth response as water depth increased from 2 to 9 m. It is found that the changeover of response strategies occurred at higher submergence depths (5-9 m). Based on the results of our experiments, we demonstrated that water depth played an important role in driving the change in strategy. The water-depth-dependent growth performance of A. philoxeroides would benefit the species in habit exploration and exploitation. Further studies should focus on the performances of plants when submerged at varied water depths with different light climates and dissolved oxygen content, and how water depths drive the response behaviors of the submerged plants.

An Insight into Abiotic Stress and Influx Tolerance Mechanisms in Plants to Cope in Saline Environments

Simple Summary

This review focuses on plant growth and development harmed by abiotic stress, primarily salt stress. Salt stress raises the intracellular osmotic pressure, leading to hazardous sodium buildup. Plants react to salt stress signals by regulating ion homeostasis, activating the osmotic stress pathway, modulating plant hormone signaling, and altering cytoskeleton dynamics and cell wall composition. Understanding the processes underlying these physiological and biochemical responses to salt stress could lead to more effective agricultural crop yield measures. In this review, researchers outline recent advances in plant salt stress control. The study of plant salt tolerance processes is essential, both theoretically and practically, to improve agricultural output, produce novel salt-tolerant cultivars, and make full use of saline soil. Based on past research, this paper discusses the adverse effects of salt stress on plants, including photosynthesis suppression, ion homeostasis disturbance, and membrane peroxidation. The authors have also covered the physiological mechanisms of salt tolerance, such as the scavenging of reactive oxygen species and osmotic adjustment. This study further identifies specific salt stress-responsive mechanisms linked to physiological systems. Based on previous studies, this article reviews the current methodologies and techniques for improving plant salt tolerance. Overall, it is hoped that the above-mentioned points will impart helpful background information for future agricultural and crop plant production.

Abstract

Salinity is significant abiotic stress that affects the majority of agricultural, irrigated, and cultivated land. It is an issue of global importance, causing many socio-economic problems. Salt stress mainly occurs due to two factors: (1) soil type and (2) irrigation water. It is a major environmental constraint, limiting crop growth, plant productivity, and agricultural yield. Soil salinity is a major problem that considerably distorts ecological habitats in arid and semi-arid regions. Excess salts in the soil affect plant nutrient uptake and osmotic balance, leading to osmotic and ionic stress. Plant adaptation or tolerance to salinity stress involves complex physiological traits, metabolic pathways, the production of enzymes, compatible solutes, metabolites, and molecular or genetic networks. Different plant species have different salt overly sensitive pathways and high-affinity K⁺ channel transporters that maintain ion homeostasis. However, little progress has been made in developing salt-tolerant crop varieties using different breeding approaches. This review highlights the interlinking of plant morpho-physiological, molecular, biochemical, and genetic approaches to produce salt-tolerant plant species. Most of the research emphasizes the significance of plant growth-promoting rhizobacteria in protecting plants from biotic and abiotic stressors. Plant growth, survival, and yield can be stabilized by utilizing this knowledge using different breeding and agronomical techniques. This information marks existing research areas and future gaps that require more attention to reveal new salt tolerance determinants in plants—in the future, creating genetically modified plants could help increase crop growth and the toleration of saline environments.

Dam-induced difference of invasive plant species distribution along the riparian habitats

Riparian ecosystem is structurally unstable due to the frequent disturbances from water fluctuation. Moreover, dams on large rivers tend to trigger fundamental changes of the composition and structure of riparian plant communities, which provides high odds for invasive species to colonize. Yet, how the invasive species distribute along a dam-induced riparian habitat, and how the native species resist to plant invasion are still puzzles. In this study, we investigated spatial distribution of invasive floral species and its correlation with the distance from dam and the dam-triggered flooding stresses, as well as the resistance of native species to plant invasion in the water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) along the Yangtze River. By our investigation, a total of 43 alien plant species belonging to 14 families and 34 genera were found, including 20 existed and 23 newly discovered alien species recorded. Most of the new invasive species are annual herbs of the Asteraceae family. At the current successional stage, the new invasive species had not yet fully occupied the habitats of the existed invasive species. Longitudinally, number and coverages of the new invasive species showed an opposite distribution pattern to the existed invasive species, but vertically they demonstrated similar pattern. Currently, the new dominant invasive species are mainly concentrated at the intermediate elevation of WLFZ in the middle section of the reservoir, whereas the existed dominant invasive species have proliferated across the whole WLFZ. Additionally, native species showed a weak resistance to plant invasion, and water fluctuation along the elevation exerted the most significant influence on plant invasion. The results indicated that, after a decade of riparian community succession, the invasiveness of alien species remain persisted. The potential penetration site of the invasion may locate at the intermediate section along the vertical and longitudinal dimension.

Invasive Plants and Species Richness Impact Litter Decomposition in Riparian Zones

Natural ecosystems generally include litter decomposition as part of the natural cycle since the material properties and the environment greatly influence the decomposition rate. The invasion of exotic plants alters the species diversity and growth characteristics of plant communities, but its impact on litter decomposition is unknown in the riparian zone. This study examines how invasive plants affect the early stages of litter decomposition and how species richness impacts them. This experiment involved a random litter mixture of exotic (Alternanthera philoxeroides and Bidens pilosa) and native species in the riparian zone of the Three Gorges Dam Reservoir in China. There were 43 species mixture types, with various species richness ranging from 1 to 6. Litterbags were placed in the hydro-fluctuation zone and terrestrial zone, where they decomposed over the course of 55 days. Invasive plants decompose rapidly compared to native plants (35.71% of the remaining mass of the invasive plant). The invasive plant A. philoxeroides has the potential to accelerate native plant decomposition (0.29 of non-added synergetic effect), but Bidens pilosa cannot. Nonetheless, species richness had little effect on the decomposition rate. These effects are dependent upon differences in chemical functional characteristics among the species. The initial traits of the plants, specifically C, N, and C/N, were significantly and linearly correlated with the loss of mixed litter mass and mixing effect strength (P < 0.01). In addition, submergence decomposition conditions reduce the disturbance of invasive plants and predict decomposition rates based on litter characteristics. Invasive plants can therefore impact the material cycle of an ecosystem. There is a need to examine decomposition time, which may also involve considering other factors.

Plant-soil interactions and C:N:P stoichiometric homeostasis of plant organs in riparian plantation

Carbon (C), nitrogen (N), and phosphorus (P) stoichiometric ratios give valuable insight into ecosystem function. The purpose of the present study is to probe into the C, N, and P stoichiometric characteristics in various organs and their relationships with soil factors of the dominant deciduous conifer plant species (Taxodium ascendens and Taxodium distichum) during afforestation in the riparian zone of Three Gorges Reservoir. The results showed only a small change in the concentration of C in different plant organs and soils. T. ascendens contained mean N and P concentrations of 7.63 and 1.54 g/kg in fine roots, 5.10 and 0.56 g/kg in stems, and 15.48 and 2.30 g/kg in leaves, respectively. Whereas T. distichum had a mean N and P concentration of 7.08 and 1.37 g/kg in fine roots, 4.84 and 0.59 g/kg in stems, and 16.89 and 2.23 g/kg in leaves. The N:P ratios in all organs were below 14, indicating that N may have inhibited tree growth. The fine roots P and N:P of T. distichum were weak plasticity and weak homeostasis, and those of T. ascendens were plasticity and weak plasticity. Their stems and leaves adhere to strict homeostasis. N concentrations were significantly positively related to P concentrations in every tissue (except the stems of T. ascendens), and C concentrations were significantly positively associated with P concentrations in the stems and leaves of T. ascendens and T. distichum (p < 0.05). Likewise, soil P and fine root P were positively associated (p < 0.01). This study contributes to the understanding of deciduous conifer plant stoichiometry. It demonstrates N, P, and N:P stoichiometric homeostasis in T. ascendens and T. distichum, which can withstand flooding and are suitable for vegetation restoration in the hydro-fluctuation zone.

Effects of South-to-North Water Diversion Project Cascade Dams on Riparian Vegetation Along the Middle and Lower Reaches of the Hanjiang River, China

The influence of the construction of dams for water diversion on the ecological environment has attracted recent widespread attention. Over time, dams have emerged as one of the most important factors affecting the vegetation along the riparian zones of rivers. To elucidate the effects of cascade dams on riparian vegetation along the middle and lower reaches of the Hanjiang River, we examined riparian vegetation types upstream and downstream from dams. A total of 14 sample sites and 131 quadrats perpendicular to the river were investigated in June 2019, and 14 sample sites and 134 quadrats were investigated in October 2019. The riparian vegetation was divided into 15 (in June) and 11 (in October) vegetation types by two-way indicator species analysis (TWINSPAN). Significant differences were found between the vegetation types upstream and downstream of dams. Redundancy analysis (RDA) showed that soil moisture content, distance from the water, altitude and soil total nitrogen (TN) were the main environmental factors affecting plants distributions, and soil moisture content was the main factor affecting the zonal distribution of vegetation. By analyzing the impact of cascade dams on the hydrological regime, we found that the construction of cascade dams led to the differentiation of vegetation types upstream and downstream of the dam, and the riparian habitats were fragmented by these dams. This study provides both an important reference for the protection of riparian vegetation and riparian ecosystems and a basis for the management and restoration of river ecosystems after the construction of cascade dams.

Response of Annual Herbaceous Plant Leaching and Decomposition to Periodic Submergence in Mega-Reservoirs: Changes in Litter Nutrients and Soil Properties for Restoration

Simple Summary

This research focuses on the leaching and decomposition of riparian zone plants, which lose mass and release nutrients due to changing water levels during their vigorous growth period. While different factors greatly influence litter decomposition, the change in soil characteristics over various depths and their relationship to litter are largely unknown in mega-reservoir settings. Current research explores how annual plants decompose and release nutrients while they are submerged in soggy circumstances. Flooding circumstances can hasten plant mass loss and nutrient release, as well as change soil and water characteristics. This research found that sediment hindered the loss of mass and C, N, and P elements while stimulating the release of the K element. The litter decomposition of annual herbaceous plants has minimal impact on the overall amount of carbon and nutrients in the soil when the soil is saturated with water. This is linked to water leaching and soil element transformation. However, this does not imply that the significance of litter for soil nutrition is minor. It is essential to investigate the continuing production of residual soil litter nutrients after the water level has receded.

Abstract

Litter decomposition is an important soil nutrient source that promotes vegetation in deteriorated riparian zones worldwide. The periodic submergence and sediment burial effects on two prominent annual herbaceous plants (Echinochloa crusgali and Bidens tripartite) are little known in mega-reservoir settings. Our study focuses on the mass and carbon loss and nutrient release from E. crusgali and B. tripartitle litter and changes in soil properties, which are important for riparian zone rehabilitation in the Three Gorges Dam Reservoir, China. This study adopted the litter bag method to explore the nutrient change characteristics and changes in soil properties at different sediment burial depths under flooding scenarios. Three burial depths (0 cm, 5 cm, and 10 cm) were used for these two plants, and the experiment lasted for 180 days. The results revealed that the litter decay rate was high at first in the incubation experiment, and the nutrient loss rate followed the pattern of K > P > N > C. The relationship between % C remaining and % mass remaining was nearly 1:1, and the total amount of P exhibited a leaching–enrichment–release state in the decomposition process. Nutrients were changed significantly in the soil and overlying water at the first decomposition stage. Still, the total soil nutrient change was insignificant at the end, except for the 10 cm burial of B. tripartitle. Moreover, oxidation–reduction potential was the main factor in the litter decomposition process at different burial depths. This study indicated that sediment deposition reduced litter mass loss, slowed down the release of N and P, and retained more C, but promoted the release of K. Conclusively, in litter decomposition under waterlogging, the total soil nutrient content changed little. However, litter does more to the soil than that. Therefore, it is necessary to study the residual soil litter’s continuous output after the water level declines for restoration purposes.

Responses of Ecological Stoichiometric Characteristics of Carbon, Nitrogen, and Phosphorus to Periodic Submergence in Mega-Reservoir: Growth of Taxodium distichum and Taxodium ascendens

Ecological stoichiometric studies can be useful for managing the deteriorated riparian zones of mega-reservoirs in which nutrients significantly impact the balanced vegetation cover. The present study aims to explore the effects of periodic submergence on the stoichiometric ecological characteristics of carbon (C), nitrogen (N), and phosphorus (P), as well as the growth conditions of two leading conifer species (Taxodium distichum and Taxodium ascendens) in the hydro-fluctuation zone of the Three Gorges Reservoir (TGR) region, China. The stoichiometrical contents of C, N, and P in fine roots, leaves, and branches, and the growth conditions of T. distichum and T. ascendens were measured in July 2019. The results showed that periodic submergence affected the stoichiometric characteristics and growth conditions of these two woody species, and the impact was restrained, but both grew well. The effects of inundation on the C, N, and P ecological stoichiometric characteristics differed in different parts of trees. In general, the C contents showed the following pattern: leaves > branches > fine roots. The N and P content showed the following pattern: leaves > fine roots > branches, while the C/N and C/P ratios showed an opposite trend to that of N and P. The N and P content in all parts of T. distichum (with means of 17.18 and 1.70 g/kg for leaves, 4.80 and 0.57 g/kg for branches, and 6.88 and 1.10 g/kg for fine roots, respectively) and T. ascendens (with means of 14.56 and 1.87 g/kg for leaves, 5.03 and 0.63 g/kg for branches, and 8.17 and 1.66 g/kg for fine roots, respectively) were higher than the national average level (with means of 14.14 and 1.11 g/kg for leaves, 3.04 and 0.31 g/kg for branches, and 4.85 and 0.47 g/kg for fine roots, respectively). Except for N and P contents in the leaves of T. distichum, there was a significant correlation between N and P elements in other parts (p < 0.05). Nevertheless, the N/P ratio (10.15, 8.52, 6.44, and 7.93, 8.12, 5.20 in leaves, branches, and fine roots of T. distichum and T. ascendens, respectively) was lower than the critical ratio of 14. The growth conditions of T. distichum and T. ascendens were significantly negatively correlated with their leaf C contents and significantly positively correlated with their fine root N and P contents. This study showed that T. distichum and T. ascendens could maintain their normal growth needs by properly allocating nutrients between different organs to adapt to the long periodic submergence in the hydro-fluctuation zone of the TGR region.