Mortality of riparian box elder from sediment mobilization and extended inundation

Abstract

To explore how high flows limit the streamward extent of riparian vegetation we quantified the effects 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. We counted and aged box elders in 144 plots of 37.2 m2, and combined a hydraulic model 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. We quantified the effects of the two mortality factors by calculating the extreme values 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 modeled 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. We use the threshold functions and cross‐section data 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. Copyright © 1999 John Wiley & Sons, Ltd.

Forecasting Environmental Responses to Restoration of Rivers Used as Log Floatways: An Interdisciplinary Challenge

“…Local-scale flooding and sediment deposition also contribute nutrients to riparian soils and facilitate the dispersion of riparian propagules (Nilsson and others 1991;Andersson and others 2000). In contrast, infrequent floods of high intensity or duration may denude large areas of riparian vegetation by dislodging or burying plants (Bendix 1999), or by creating anaerobic soil conditions (Blom and Voesenek 1996;Friedman and Auble 1999). Because floatway structures shield riparian plant communities from all but the most infrequent, catastrophic floods, their removal is expected to result in more frequent but less intense disturbance and correspondingly enhanced species richness.…”

Restoration of Rivers Used for Timber Floating: Effects on Riparian Plant Diversity

“…Frequent low-intensity floods limit competitive exclusion by dominant species and create open patches for colonization by opportunistic species Grelsson 1990, Auble andScott 1998). In contrast, infrequent floods of high intensity or duration may denude large areas of riparian vegetation by dislodging or burying plants (Bendix 1999) or by creating anaerobic soil conditions (Blom andVoesenek 1996, Friedman andAuble 1999). Local-scale flooding and sediment deposition also affect soil composition and nutrient dynamics (Richards 1982, Naiman and De´-camps 1997) and facilitate the dispersion of riparian propagules (Nilsson et al 1991.…”

Riparian trees and aridland streams of the southwestern United States: An assessment of the past, present, and future

“…There were considerable trends for earlier peak discharges at the Gunnison River site and at the Gila River site, but weak trends in this variable at the other sites. Earlier peak discharge at the Gunnison River site could be attributed to accelerated runoff due to warming or to hydrological changes that have occurred following construction of several upstream dams (Friedman and Auble, 1999). Earlier peaks at the Gila River site were likely attributable to the recent heavy precipitation events that occurred during winter months.…”

“…These often co-occurring taxa usually grow in alluvial reaches below 2000 m. Their presence along each of the streams indicates that they have adapted to a variety of flow regimes in regulated and unregulated reaches. Boxelders are present in the upper reaches of Southern Rockies and Central Highland streams and are often the dominant trees in canyon-bound segments (Friedman and Auble, 1999). Boxelder can also be a component of mixed deciduous montane riparian communities in alluvial reaches (Stoleson and Finch, 2003).…”

“…This channel narrowing has, however, reduced the area suitable for boxelder establishment along regulated streams including the Green River (Dewine and Cooper, 2007;Coble and Kolb, 2013). Reduction in peak discharge following construction of dams reduced mortality of boxelder from shear stress along the Gunnison River (Friedman and Auble, 1999). We are unaware of studies showing response of Arizona sycamores to changes in discharge variables, but we expect that they would be similar to those of cottonwood and Goodding's willow because of their similar requirements for growth, survival, and reproduction.…”