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 language | English |
|---|---|
| Pages (from-to) | 9190-9206 |
| Number of pages | 17 |
| Journal | Water Resources Research |
| Volume | 51 |
| Issue number | 11 |
| DOIs | |
| State | Published - Nov 2015 |
Funding
Supporting information contains seedling results (Data Set S1), mean seedling characteristics (supporting information Table S1), BR grain-size data (supporting information Figure S1), additional details on HEC-RAS modeling and results (supporting information Table S2), modeled reaches and velocities (supporting information Figures S2 and S3), and statistical analysis results (supporting information Table S3 through S8). This research was funded by the National Science Foundation (EAR 1024652 and EPS-1101342) and EPA STAR Graduate Fellowship. We thank Austin Maphis, Ben Gardner, Franklin Dekker, Abraham Schmidt, Jewel Case, Robert Livesay, Brian Reyes, and Katie Monaco for assistance in the field. We thank Paolo Perona, Nicola Pasquale, Patrick Shafroth, Rebecca Manners, Li Kui, and Krysia Skorko for insights, discussion and review of this research. We thank Mark Reiling, Philip Ramsey, and MPG Ranch for access to the BR site. We thank Missoula County and a NCALM Seed Grant for providing LiDAR, and Steve Niday for survey assistance. SMR LiDAR data acquisition and processing completed by the National Center for Airborne Laser Mapping (NCALM; http://www.ncalm.org). NCALM funding provided by NSF''s Division of Earth Sciences, Instrumentation and Facilities Program (EAR-1043051). We also thank the Associate Editor, two anonymous reviewers, and Simon Dixon for insightful review comments.
| Funder number |
|---|
| 1443108, 1024820, EAR 1024652, 1339015, EPS-1101342 |
| EAR-1043051 |
Keywords
- Populus
- Tamarix
- ecogeomorphology
- fluvial erosion
- pull tests
- vegetation drag