Flow and scour constraints on uprooting of pioneer woody seedlings

Sharon Bywater-Reyes, Andrew C. Wilcox, John C. Stella, Anne F. Lightbody

Research output: Contribution to journalArticlepeer-review

57 Scopus citations

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.

Original languageEnglish
Pages (from-to)9190-9206
Number of pages17
JournalWater Resources Research
Volume51
Issue number11
DOIs
StatePublished - Nov 2015

Keywords

  • Populus
  • Tamarix
  • ecogeomorphology
  • fluvial erosion
  • pull tests
  • vegetation drag

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