Mortality of riparian box elder from sediment mobilization and extended inundation

Friedman, Jonathon; Auble, Gregor

"In order to explore how high flows limit the streamward extent of riparian vegetation the effects were quantified of sediment mobilization and extended inundation on box elder (Acer negundo) saplings along the cobble-bed Gunnison River in Black Canyon of the Gunnison National Monument, Colorado (USA). Box elders were counted and aged in 144 plots of 37.2 m2, and a hydraulic model combined with the hydrologic record to determine the maximum shear stress and number of growing-season days inundated for each plot in each year of the record. The effects of the 2 mortality factors were quantified by calculating the extreme values which were survived during the lifetime of trees sampled in 1994 and by recounting box elders in the plots following a high flow in 1995. Both mortality factors can be modelled as threshold functions: box elders are killed either by inundation for more than 85 days during the growing season or by shear stress that exceeds the critical value for mobilization of the underlying sediment particles. Construction of upstream reservoirs in the 1960s and 1970s reduced the proportion of the canyon bottom annually cleared of box elders by high flows. Furthermore, because the dams decreased the magnitude of high flows more than their duration, flow regulation has decreased the importance of sediment mobilization relative to extended inundation. The threshold functions and cross-section data were used to develop a response surface predicting the proportion of the canyon bottom cleared at any combination of flow magnitude and duration. This response surface allows vegetation removal to be incorporated into quantitative multi-objective water management decisions." [from Tree CD] 2 pages of references

Mortality of riparian box elder from sediment mobilization and extended inundation

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