Can environmental flows moderate riparian invasions? The influence of seedling morphology and density on scour losses in experimental floods

Abstract

Environmental flow releases are an effective tool to meet multiple management objectives, including maintaining river conveyance, restoring naturally functioning riparian plant communities, and controlling invasive species. In this context, predicting plant mortality during floods remains a key area of uncertainty for both river managers and ecologists, particularly with respect to how flood hydraulics and sediment dynamics interact with the plants’ own traits to influence their vulnerability to scour and burial. To understand these processes better, we conducted flume experiments to quantify different plant species’ vulnerability to flooding across a range of plant sizes, patch densities, and sediment condition (equilibrium transport versus sediment deficit), using sand‐bed rivers in the U.S. southwest as our reference system. We ran 10 experimental floods in a 0.6 m wide flume using live seedlings of cottonwood and tamarisk, which have contrasting morphologies. Sediment supply, plant morphology, and patch composition all had significant impacts on plant vulnerability during floods. Floods under sediment deficit conditions, which typically occur downstream of dams, resulted in bed degradation and a 35% greater risk of plant loss compared to equilibrium sediment conditions. Plants in sparse patches dislodged five times more frequently than in dense patches. Tamarisk plants and patches had greater frontal area, larger basal diameter, longer roots, and lower crown position compared to cottonwood across all seedling heights. These traits were associated with a 75% reduction in tamarisk seedlings’ vulnerability to scour compared to cottonwood. Synthesis and applications. Tamarisk's greater survivability helps to explain its vigorous establishment and persistence on regulated rivers where flood magnitudes have been reduced. Furthermore, its documented influence on hydraulics, sediment deposition, and scour patterns in flumes is amplified at larger scales in strongly altered river channels where it has broadly invaded. Efforts to remove riparian vegetation using flow releases to maintain open floodways and/or control the spread of non‐native species will need to consider the target plants’ size, density, and species‐specific traits, in addition to the balance of sediment transport capacity and supply in the river system.

Riparian Vegetation and Sediment Supply Regulate the Morphodynamic Response of an Experimental Stream to Floods

“…These differences likely reflect contrasts in these species' morphological-effect traits (Diehl et al, 2017a), and in turn the distinct turbulent structures expected to occur around these plants. Tamarisk seedlings have thicker, more rigid, and more multiple stems than cottonwood, as well as less foliage high on the plant ( Table 2), such that tamarisk pronate less than cottonwood and induce greater aggradation rates, as observed not only here but also in a set of experiments we completed in a smaller, straight flume (Kui et al, 2014(Kui et al, , 2019Diehl et al, 2017b). The high turbulence (u rms ) values associated with cottonwood (Run 1) would be expected to promote sediment suspension and thus limit deposition, analogous to the behavior of sparse cylinders in flume trials .…”

“…In the experiments documented here, transport capacity was limited because the maximum flood we tested was analogous to a bankfull flood, such that plant dislodgment did not occur (Kui et al, 2014). However, Kui et al (2019) documented a 35% greater risk of plant loss under sediment-deficit and associated bed degradation, compared to equilibrium sediment conditions. Vulnerability to scour also depended on density (plants in sparse patches dislodged five times more frequently than in dense patches) and species (tamarisk seedlings were less vulnerable to scour than cottonwood; Kui et al, 2019).…”

“…However, Kui et al (2019) documented a 35% greater risk of plant loss under sediment-deficit and associated bed degradation, compared to equilibrium sediment conditions. Vulnerability to scour also depended on density (plants in sparse patches dislodged five times more frequently than in dense patches) and species (tamarisk seedlings were less vulnerable to scour than cottonwood; Kui et al, 2019). Similarly, field observations have shown that seedling uprooting is facilitated by scour around the base of plants, which exposes root systems and thus reduces plant resistance to dislodgement (Bywater-Reyes et al, 2015).…”

A Green New Balance: Interactions among riparian vegetation plant traits and morphodynamics in alluvial rivers

“…Because the degree to which a plant bends over in the flow is important for understanding how plants mediate ecogeomorphic feedbacks in riparian systems, we evaluate differences in the force necessary to bend over Populus and Tamarix by combining data from Diehl et al (2017a), Kui et al (2018), and unpublished records from Bywater‐Reyes et al (2015) (Supporting Information and Appendix ; Figure 5). Bending forces for riparian seedlings and young trees were measured using force gauges attached to stems.…”

Flow‐ecology modelling to inform reservoir releases for riparian restoration and management

“…Incorporating quantitative estimates of mortality thresholds associated with scour or burial (e.g., Bywater‐Reyes et al, 2015; Edmaier et al, 2011; Friedman & Auble, 1999; Kui et al, 2019) could also allow for better estimates of realized establishment areas. Although our results are limited to one growing season, many seedlings in this region commonly grow 1–3 m tall in the first 2 years and would likely be less vulnerable to burial and scour events or drought in future years than smaller seedlings (Wilcox & Shafroth, 2013).…”