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Title: The influence of Holocene vegetation changes on topography and erosion rates: a case study at Walnut Gulch Experimental Watershed

item PELLETIER, J.D. - University Of Arizona
item Nichols, Mary
item Nearing, Mark

Submitted to: Earth Surface Dynamics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2016
Publication Date: 6/22/2016
Citation: Pelletier, J., Nichols, M.H., Nearing, M.A. 2016. The influence of Holocene vegetation changes on topography and erosion rates: a case study at Walnut Gulch Experimental Watershed. Earth Surface Dynamics. 4:417-488.

Interpretive Summary: The Walnut Gulch Experimental Watershed is operated by the USDA Agricultural Research Service in southeastern Arizona. Walnut Gulch is representative of large areas of land in the southwestern United States in terms of it hydrologic, erosion, ecological, and landscape processes. One process that has major implications for land use across the entire western United States is that of woody encroachment, or the displacement of grassland ecosystems with those composed of largely woody plants such as mesquite or acacia. Walnut Gulch is both an excellent example of this process, as well as a good place to study and understand the effects associated with it. In Walnut Gulch approximately half of the area is grass covered, and half has been encroached by various woody shrub species. One important effect of this change has been the impact it has had on the surface morphology (shape) of the landscape. As woody species replace the grasses the land erodes and becomes more incised, water drains much more quickly during rain events, and the landscape tends to hold less water and become generally less productive for grazing and wildlife. In this study we developed a conceptual and mathematical model that, for the first time, explains to a large degree how and why this process takes place. This study puts into context the difference between the shrublands and grasslands. We know from our previous work using radionuclides and watershed monitoring that the hillslope erosion rates are not the direct, primary determining factor for explaining the vastly different sediment loads between the two ecosystems, but rather it is the surface morphology. Now with this study, with this new model, we show how the vegetation difference explains the morphological differences.

Technical Abstract: The Walnut Gulch Experimental Watershed (WGEW) is dominated by grasslands at high elevations and shrublands at low elevations. Paleovegetation data suggest that portions of WGEW higher than approximately 1430 m a.s.l. have been grasslands and/or woodlands throughout the Quaternary, while lower elevations shifted from a grassland/woodland to a shrubland c. 2-4 ka. Areas below 1430 m a.s.l. have decadal-scale erosion rates approximately ten times higher, drainage densities three times higher, and hillslope scale relief approximately three times lower than elevations above 1430 m. We leverage the abundant geomorphic data that have been collected at WGEW over the past several decades to calibrate a mathematical model for landscape evolution over geologic time scales and test the hypothesis that the differences among erosion rates, drainage densities, and relief between the shrubland and grassland sites are partly the result of a late Holocene shift from grasslands/woodlands to shrublands in the lower elevations of WGEW, using the upper elevations as a control. Model predictions for the increase in drainage density associated with the shift from grasslands/woodlands to shrublands are broadly consistent with measured values. Using modern erosion rates and the magnitude of relief reduction associated with the transition from grasslands/woodlands to shrublands, we estimate the timing of the grassland-to-shrubland transition in the lower elevations of WGEW to be approximately 2 ka, i.e., broadly consistent with paleovegetation data. Our results provide support for the hypothesis that changing vegetation cover can exert significant influence on drainage basin evolution.