A model study of the combined effect of above and below ground plant traits on the ecomorphodynamics of gravel bars

Abstract

Both above- and below-ground plant traits are known to modulate feedbacks between vegetation and river morphodynamic processes. However, how they collectively influence vegetation establishment on gravel bars remains less clear. Here we develop a numerical model that couples above- and below-ground vegetation dynamics with hydromorphological processes. The model dynamically links plant growth rate to water table fluctuations and includes plant mortality by uprooting and burial. We considered a realistic hydrological regime and used the model to simulate the coevolution of alternate gravel bars and vegetation that displays trade-offs in investment of above- and below-ground biomass. We found that a balanced plant growth above- and below-ground facilitates vegetation to establish on steady, stable bars, because it allows plants to develop traits that maximise growth performance during low flow periods and thus survival during floods. Regardless of the growth strategy, vegetation could not establish on migrating bars because of large plant loss by uprooting during floods. These findings add on previous studies suggesting that morphodynamic processes play a key role on determining plant trait distributions and highlight the importance of including the dynamics of both above- and below-ground plant traits for predicting shifts between bare and vegetated states in river bars.

Hydrologic and geomorphic effects on riparian plant species occurrence and encroachment: Remote sensing of 360 km of the Colorado River in Grand Canyon

“…In our study, distributions of alkali goldenrod and coyote willow were observed to be driven by geomorphic surface as opposed to inundation frequency, both of which occurred with high cover specifically on gravel bars. Gravel bars are high energy, in‐channel flow environments with rocky substrate and host riparian species with specific functional traits like equal amounts of above‐ to below‐ground biomass that allows plants to survive flow fluctuations, as well as roots, rhizomes or stolons that anchor plants during scouring flow (Caponi et al, 2020; Pasquale et al, 2014).…”

“…Consistent with our third hypothesis, modelling showed significant variability between vegetation encroachment and starting vegetation proportion among hydro-geo units. We interpret this as species-specific encroachment behaviour in response to pressures exerted by hydrology, geomorphology and competition for available factor for encroachment compared with the physical pressures of the geomorphic setting (Caponi et al, 2020;Edmaier et al, 2015;Pasquale et al, 2014;Perona & Crouzy, 2018).…”

“…Gravel bars are high energy, in-channel flow environments with rocky substrate and host riparian species with specific functional traits like equal amounts of above-to below-ground biomass that allows plants to survive flow fluctuations, as well as roots, rhizomes or stolons that anchor plants during scouring flow (Caponi et al, 2020;Pasquale et al, 2014).…”

An Ecogeomorphic Framework Coupling Sediment Modeling With Invasive Riparian Vegetation Dynamics

“…Most sediment models that explicitly account for lateral interaction with the floodplain are applied at localized (reach) scales rather than at the larger (catchment) scale pertinent to management problems such as riparian invasions or land use change. Both conceptual (e.g., Corenblit et al., 2007) and numerical (Caponi et al., 2020; Solari et al., 2016) models linking plants and morphodynamics have been developed to highlight different components of these relationships, including the effect of vegetation on river meandering and planform (e.g., Crosato & Saleh, 2011; Perucca et al., 2007), and on flow and sediment dynamics (e.g., Bertoldi et al., 2014; Camporeale et al., 2013; Hooke et al., 2005; Shields et al., 2017).…”

“…Most sediment models that explicitly account for lateral interaction with the floodplain are applied at localized (reach) scales rather than at the larger (catchment) scale pertinent to management problems such as riparian invasions or land use change. Both conceptual (e.g., Corenblit et al, 2007) and numerical (Caponi et al, 2020;Solari et al, 2016) models linking plants and morphodynamics have been developed to highlight different components of these relationships, including the effect of vegetation on river meandering and planform (e.g., Crosato & Saleh, 2011;Perucca et al, 2007), and on flow and sediment dynamics (e.g., Bertoldi et al, 2014;Camporeale et al, 2013;Hooke et al, 2005;Shields et al, 2017 Sediment models focused on bank erosion and channel stability conceptualize the relevant cohesive and erosive forces, including the effects of vegetation, differently at different spatial scales. The cohesive forces are determined by soil characteristics such as clay content (Couper, 2003), pore-water pressure in the soil, and density and configuration of roots (Millar, 2000;Pollen-Bankhead & Simon, 2010).…”