Flow and scour constraints on uprooting of pioneer woody seedlings

Abstract

Scour and uprooting during flood events is a major disturbance agent that affects plant mortality rates and subsequent vegetation composition and density, setting the trajectory of physical‐biological interactions in rivers. During flood events, riparian plants may be uprooted if they are subjected to hydraulic drag forces greater than their resisting force. We measured the resisting force of woody seedlings established on river bars with in situ lateral pull tests that simulated flood flows with and without substrate scour. We quantified the influence of seedling size, species (Populus and Tamarix), water‐table depth, and scour depth on resisting force. Seedling size and resisting force were positively related with scour depth and water‐table depth—a proxy for root length—exerting strong and opposing controls on resisting force. Populus required less force to uproot than Tamarix, but displayed a greater increase in uprooting force with seedling size. Further, we found that calculated mean velocities required to uproot seedlings were greater than modeled flood velocities under most conditions. Only when plants were either shallowly rooted or subjected to substrate scour (≥0.3 m) did the calculated velocities required for uprooting decrease to within the range of modeled flood velocities, indicating that drag forces alone are unlikely to uproot seedlings in the absence of extreme events or bar‐scale sediment transport. Seedlings on river bars are most resilient to uprooting when they are large, deeply rooted, and unlikely to experience substrate scour, which has implications for ecogeomorphic evolution and river management.

A Physical Model for the Uprooting of Flexible Vegetation on River Bars

“…With these values, self‐induced scour is either hampered (in the flume experiments with Isolated plants) or limited to a few centimeters (in the field). For similar values of the stem‐to‐grain ratio, Bywater‐Reyes et al () concluded that Type IIa can be important for juvenile seedlings (<1 year), although they did not measure the uprooting resistance of such young plants. In the light of our experimental results, we found out that Type IIa is negligible even for young seedlings in the Isolated configuration.…”

“…We also carried out field measurements of root resistance to Type I uprooting of approximately 1‐ to 3‐year‐old Salix purpurea pioneer plants by following the approach previously adopted by Bywater‐Reyes et al () and Bankhead et al (). A rope was clamped to the base of the plant and slowly pulled in the downstream direction using a hand winch until uprooting occurred.…”

“…For each series, we found that the maximum measured force scales with the plant stem area, that is, the product between the seedling height and diameter. Unlike Bywater‐Reyes et al () and Bankhead et al (), we used the plant stem area due to its measuring simplicity and quickness. Additionally, neither advanced technologies nor computations are required, thus allowing for time and cost savings in extensive field campaigns.…”

Morphodynamics of alternate bars in the presence of riparian vegetation

“…), which occurs when erosion exceeds root depth; see, for example, Bywater‐Reyes et al. (); Caponi and Siviglia (); Edmaier et al. () Perona and Crouzy () Erosion and deposition are recorded over 1 year from September 1st to August 31st, allowing vegetation to recover from partial burial or partial root exposure at the end of each growing season.…”

The influence of a vegetated bar on channel-bend flow dynamics

“…to uprooting during floods and their morphodynamic effects (Bywater-Reyes et al, 2015Diehl et al, 2017a;Kui et al, 2014). Vegetation's impact on altering the velocity (flow steering) is poorly understood despite advances in understanding the reciprocal interactions between riparian vegetation recruitment and river processes (Corenblit et al, 2007;Gurnell, 2014;Osterkamp and Hupp, 2010;Schnauder and Moggridge, 2009).…”