EP51C-0929
Interactions between fluvial forces and vegetation size, density and morphology influence plant mortality during experimental floods

Friday, 18 December 2015
Poster Hall (Moscone South)
John C Stella1,2, LI Kui3, Rebecca Manners4, Andrew C Wilcox4, Anne Lightbody5 and Leonard S Sklar6, (1)SUNY College of Environmental Science and Forestry, Forest and Natural Resources Management, Syracuse, NY, United States, (2)Nature Conservancy, Freshwater Science Program, San Francisco, CA, United States, (3)SUNY College of Environmental Science and Forestry, Environmental Science, Syracuse, NY, United States, (4)University of Montana, Geosciences, Missoula, MT, United States, (5)University of New Hampshire, Earth Sciences, Durham, NH, United States, (6)San Francisco State University, San Francisco, CA, United States
Abstract:
Introduction and methods

Fluvial disturbance is a key driver of riparian vegetation dynamics in river corridors. Despite an increasing understanding of ecohydraulic interactions between plants and fluvial forces, the interactive influences of plant morphology and sediment supply on plant mortality, a key demographic factor, are largely unknown. To better understand these processes, we designed and conducted a series of flume experiments to: (1) quantify effects of plant traits that interact with flow and sediment transport on plant loss to scour during floods; and (2) predict plant dislodgement for different species across a range of plant sizes, patch densities, and sediment condition (equilibrium transport versus sediment deficit). We ran ten experimental floods in a 28 m long × 0.6 m wide × 0.71 m tall flume, using live, 1–3 year-old tamarisk and cottonwood seedlings with contrasting morphologies with varied combinations of size and density.

Results and discussion

Both sediment supply and plant traits (morphology and composition) have significant impacts on plant vulnerability during floods. Sediment deficit resulted in bed degradation and a 35% greater risk of plant loss compared to equilibrium sediment conditions. The probability of plant dislodgement in sparse patches was 4.5 times greater than in dense patches. Tamarisk plants and patches had greater frontal area, basal diameter and longer roots compared to cottonwood across all seedling heights. These traits, as well as its lower crown position reduced tamarisk’s vulnerability to scour by 75%. Compared with cottonwood, tamarisk exhibits better resistance to floods, due to its greater root biomass and longer roots that stabilize soil, and its greater frontal area and lower crown that effectively trap sediment. These traits likely contribute to riverscape-scale changes in channel morphology that are evident where tamarisk has invaded native riparian communities, and explain the persistence of tamarisk species on regulated rivers where catastrophic floods have been reduced.

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