Unnatural flooding alters the functional diversity of riparian vegetation of the Three Gorges Reservoir

Abstract

The response of riparian vegetation to flow regulation has been a research focus for decades. Several studies have shed light on the effects of flow stabilisation on riparian woody species, but other life forms exposed to intensified inundation have been overlooked. Furthermore, studies from a functional perspective are scarce. We evaluated the functional response of riparian vegetation along the shores of the Three Gorges Reservoir on the Yangtze River in China to unnaturally long annual flooding (>7 months) after the first year of filling. We aimed to answer the following: (1) can we derive well‐defined flow‐response guilds from the riparian zones of the Yangtze River? and (2) which plant traits and guilds are favoured or disfavoured by the unnaturally long flooding environment? Woody and herbaceous species were inventoried in 12 reaches along the shorelines of the Three Gorges Reservoir and another 12 reaches along the free‐flowing Yangtze River. We performed a cluster analysis to derive riparian guilds using abundance data (projective coverage) from 40 riparian plant species and 13 responsive traits. Structural composition and functional diversity of the unnaturally and naturally flooded riparian vegetation were compared. Unnaturally long flooding substantially reduced species richness, but it did not change the riparian vegetation cover. This novel flooding reduced functional diversity, mostly owing to the loss of stress‐tolerant woody species and competitive perennial herbs. However, competitive annual herbs and flood‐tolerant riparian herbs, as the most abundant functional guilds, were favoured even under such long‐term hypoxic conditions. These guilds under regulation revealed a high functional resilience to prolonged flooding along the upstream reaches of the Yangtze River. Flooding tolerance and the capacity to synchronise germination and growth with short‐exposure periods underlie the plant species changes. Our findings are useful for anticipating the effects of long‐lasting inundation on riparian areas triggered by flow regulation or warmer climates. The functional perspective lends confidence that our conclusions can be generalised to other geographical regions despite not sharing the same species pool. Finally, the plant species that showed a high flooding tolerance should be considered for the restoration of riparian areas affected by the Three Gorges Reservoir.

The taxonomic structure but not the functioning of riparian herbaceous communities varies with hydrological conditions on a large, highly regulated river: Evidence from a 2‐year replicated study

“…However, over the 2 years studied, the magnitude of flooding varied within a narrow range, and the peak flows recorded cannot be characterized as intermediate disturbances (Figure S2). At the same time, it has been shown that frequently flooded sites with low spatial variation in flood frequencies are species‐poor (Pollock et al, 1998) but also that in dam‐regulated rivers, long unnatural inundations have a negatively effect on species richness (e.g., Su et al, 2020; Zheng et al, 2021). On the Rhone, the spatial structuring of inundations, longer in 2019 at low levels on pebble‐dominated bars, and of species richness, lower in 2019 in low‐lying quadrats composed of coarse‐grained sediments, supports these previous finding.…”

“…In contrast to species richness, functional dispersion values remained stable over time at the corridor and gravel bar scales, with the exception of GB2. This suggests that the herbaceous species that disappeared between 2017 and 2019 on the Rhone gravel bars had redundant trait values in the community, showing some functional resilience of this regulated system to environmental changes (Su et al, 2020). Also, trait dispersion for SLA was higher on GB1, which is located in the only bypassed reach where bedload transport is still active (Vázquez‐Tarrío et al, 2019).…”

“…However, over the 2 years studied, the magnitude of flooding varied within a narrow range, and the peak flows recorded cannot be characterized as intermediate disturbances (Figure S2). At the same time, it has been shown that frequently flooded sites with low spatial variation in flood frequencies are species-poor (Pollock et al, 1998) but also that in dam-regulated rivers, long unnatural inundations have a negatively effect on species richness (e.g., Su et al, 2020;Zheng et al, 2021) Since along the gravel bars of the Rhone, as in other rivers (Kalníková et al, 2018;Škornik et al, 2017), species richness is highest in these riparian habitats, we believe that their conservation should be a priority (Gilvear et al, 2008;Rottenborn et al, 2018;Zeng et al, 2015).…”

“…In riparian ecology, several recent studies have mobilized a functional trait‐based approach to understand the effects of hydraulic alterations on vegetation (Aguiar et al, 2018; Bejarano et al, 2018; Janssen, Stella, et al, 2020; Su et al, 2020; Tabacchi et al, 2019). Combined with a taxonomic approach, this allows us to focus both on conservation issues, by studying variations in species richness and composition, but also to overcome the limits of taxonomic studies, under the control of the local species pool, by studying the underlying ecological mechanisms captured by functional traits (Lavorel et al, 2008).…”

“…Although many studies have documented the impacts of anthropogenic environmental changes on riparian vegetation, many have focused on woody species and forested habitats established on riverbanks (Aguiar et al, 2018; Bejarano et al, 2018; Janssen, Stella, et al, 2020; Johnson et al, 2012). In regulated and channelized rivers, knowledge of the factors that structure herbaceous communities at fine spatial and temporal scales is thus still scarce (Su et al, 2020), particularly for riparian habitats of high conservation value, such as gravel bars (Gilvear et al, 2008). Composed of accumulations of alluvial sediments, these dynamic and morphologically complex habitats are nevertheless a key element of the river landscape and shelter numerous threatened plant and animal species (Kalníková et al, 2018; Rottenborn et al, 2018; Zeng et al, 2015).…”