Riparian vegetation, Colorado River, and climate: Five decades of spatiotemporal dynamics in the Grand Canyon with river regulation

Abstract

Documentation of the interacting effects of river regulation and climate on riparian vegetation has typically been limited to small segments of rivers or focused on individual plant species. We examine spatiotemporal variability in riparian vegetation for the Colorado River in Grand Canyon relative to river regulation and climate, over the five decades since completion of the upstream Glen Canyon Dam in 1963. Long-term changes along this highly modified, large segment of the river provide insights for management of similar riparian ecosystems around the world. We analyze vegetation extent based on maps and imagery from eight dates between 1965 and 2009, coupled with the instantaneous hydrograph for the entire period. Analysis confirms a net increase in vegetated area since completion of the dam. Magnitude and timing of such vegetation changes are river stage-dependent. Vegetation expansion is coincident with inundation frequency changes and is unlikely to occur for time periods when inundation frequency exceeds approximately 5%. Vegetation expansion at lower zones of the riparian area is greater during the periods with lower peak and higher base flows, while vegetation at higher zones couples with precipitation patterns and decreases during drought. Short pulses of high flow, such as the controlled floods of the Colorado River in 1996, 2004, and 2008, do not keep vegetation from expanding onto bare sand habitat. Management intended to promote resilience of riparian vegetation must contend with communities that are sensitive to the interacting effects of altered flood regimes and water availability from river and precipitation.

Changes in Community-Level Riparian Plant Traits over Inundation Gradients, Colorado River, Grand Canyon

“…Lower runoff expected in the future may reduce flow releases from Glen Canyon Dam into the Colorado River leading to lower average river levels and an altered inundation regime in a direction that started decades ago. Reductions in annual peak flood volume and disturbance since 1963 have increased vegetation cover at lower elevations, particularly woody vegetation (Turner and Karpiscak 1980;Gloss et al 2005;Sankey et al 2015). Changes in some community traits (e.g., SLA) between the wet year of 2013 and the drier year of 2014 suggest quicker response of some traits to changes in environmental conditions.…”

“…Inset in panel a shows a significant interaction between inundation x year for specific leaf area; interactions not significant for other traits. Gray band shows the 95% confidence interval of the linear regression (Sankey et al 2015), drier conditions in upland zones in 2014 should have given C4 species an advantage over C3 species.…”

“…Vegetation encroachment, streamward expansion of xeric communities (terrestrialization), and reduced recruitment of some species commonly occur in riparian ecosystems of southwestern North America following changes to flow regimes (Turner and Karpiscak 1980;Shafroth et al 2002;Merritt and Bateman 2012;Sankey et al 2015). These changes strongly suggest that flooding and water stress influence riparian community-level traits (Kyle and Leishman 2009;Hough-Snee et al 2015).…”

“…Disturbance and abiotic stress are well-known limitations on plant cover (Grime 1977;Stromberg et al 2007). In our study of riparian vegetation in Grand Canyon, the primary disturbances are shear stress, scouring, abrasion, and burial from deposition during floods, and the primary abiotic stresses include seasonal submergence and inundated anaerobic soils near the river, and drought stress at more upland sites within the river corridor (Howard and Dolan 1981;Stevens et al 1995;Sankey et al 2015).…”

“…Our study focuses on riparian vegetation of the Colorado River in Grand Canyon in semi-arid northern Arizona, where completion of Glen Canyon Dam in 1963 greatly reduced annual flood peaks, increased base flow and daily fluctuations, and altered sediment transport processes, which led to the establishment of riparian vegetation at lower elevations, extensive fluvial marsh formation, and expansion of vegetated fluvial surfaces (Turner and Karpiscak 1980;Stevens et al 1995;Gloss et al 2005;Sankey et al 2015). A projected warmer future climate and changes in water availability in this region (Hayhoe et al 2004;Seager et al 2007;Seager and Vecchi 2010;Dominguez et al 2012) may further alter river flow management and consequently affect riparian plant communities, especially those at the upper reaches of the floodplain through direct effects of climate.…”