Fluvial Geomorphology, Root Distribution, and Tensile Strength of the Invasive Giant Reed, Arundo Donax and Its Role on Stream Bank Stability in the Santa Clara River, Southern California

Abstract

Arundo donax (giant reed) is a large, perennial grass that invades semi-arid riparian systems where it competes with native vegetation and modifies channel geomorphology. For the Santa Clara River, CA, changes in channel width and intensity of braiding over several decades are linked in part to high flow events that remove A. donax. Nevertheless, the area of A. donax at the two study sites increased fivefold over a period of 28 years at one site and fourfold over 15 years at the second site. Effects of A. donax on bank stability are compared to those of a common native riparian tree-Salix laevigata (red willow)-at two sites on the banks and floodplain of the Santa Clara River. There is a significant difference of root density of A. donax compared to S. laevigata and the latter has a higher number of roots per unit area at nearly all depths of the soil profile. Tensile root strength for S. laevigata (for roots of 1-6 mm in diameter) is about five times stronger than for A. donax and adds twice the apparent cohesion to weakly cohesive bank materials than does A. donax (8.6 kPa compared to 3.3 kPa, respectively). Modeling of bank stability for banks of variable height suggests that S. laevigata, as compared to A. donax, increases the factor of safety (FS) by~60% for banks 1 m high,~55% for banks 2 m high and~40% for banks 3 m high. For 3 m high banks, the FS for banks with A. donax is <1. This has geomorphic significance because, in the case of A. donax growing near the water line of alluvial banks, the upper 10-20 cm has a hard, resistant near-surface layer overlying more erodible banks just below the near-surface rhizomal layer. Such banks may be easily undercut during high flow events, resulting in overhanging blocks of soil and A. donax that slump and collapse into the active channel, facilitating lateral bank erosion. Therefore, there is a decrease in the lateral stability of channels if the mixed riparian forest is converted to dominance by A. donax.

Invasive Japanese knotweed (Reynoutria japonica Houtt.) and related knotweeds as catalysts for streambank erosion

“…impacts are rare (Lavoie, 2017), and we hope our arguments spur future work in this direction. Such studies could build on robust bases of complementary research, including: Existing models that use a vegetation‐root component, such as RipRoot (Pollen‐Bankhead & Simon, 2009), as part of a process‐based bank‐stability model, such as BSTEM (Klavon et al, 2017) which have been used to investigate the impact of Arundo donax on erosion (Stover, Keller, Dudley, & Langendoen, 2018). Pairing such models with field studies or experiments would provide especially valuable insights into the relationship between these species and erosive processes. Photogrammetric analyses can provide useful insights into general fluvial biogeomorphic dynamics (Hortobágyi et al, 2017), and satellite and unmanned aerial vehicles (UAVs) can accurately map knotweed s.l .…”

Japanese knotweed increases soil erosion on riverbanks

“…Other plant invaders facilitate riverbank erosion. For example, the creation of a shallow network of rhizomes by an invasive species (e.g., Arundo donax ) may result in undercutting, causing the riverbank to retreat with each flood episode (Stover, Keller, Dudley, & Langendoen, 2018). The soil may also be left bare when the stems of the invader die (e.g., Impatiens glandulifera ), leaving the riverbanks vulnerable to erosion during subsequent floods (Greenwood, Baumann, Pulley, & Kuhn, 2018; Greenwood, Gange, & Kuhn, 2020; Greenwood & Kuhn, 2014).…”

An Ecogeomorphic Framework Coupling Sediment Modeling With Invasive Riparian Vegetation Dynamics

“…Arundo establishes well in depositional environments with high water availability, such as riparian zones. Arundo has a lower tensile strength than most riparian species native to Mediterranean climates, and has shallow rooting depths compared with many of these plants (Stover et al., 2018). These factors result in reduced bank stability where arundo establishes compared to banks colonized by native vegetation (Stover et al., 2018), potentially facilitating channel change and additional uprooting and transport of arundo plants.…”

“…Arundo has a lower tensile strength than most riparian species native to Mediterranean climates, and has shallow rooting depths compared with many of these plants (Stover et al., 2018). These factors result in reduced bank stability where arundo establishes compared to banks colonized by native vegetation (Stover et al., 2018), potentially facilitating channel change and additional uprooting and transport of arundo plants. Consequently, bank erosion and sediment transport are important components in understanding and modeling interactions between fluvial processes and this riparian species.…”