Abstract
Abstract. Quantifying how landscapes have responded and will respond to vegetation changes is an essential goal of geomorphology. The Walnut Gulch Experimental Watershed (WGEW) offers a unique opportunity to quantify the impact of vegetation changes on landscape evolution over geologic timescales. The 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 late Quaternary, while elevations lower than 1430 m a.s.l. changed from a grassland/woodland to a shrubland ca. 2–4 ka. Elevations below 1430 m a.s.l. have decadal timescale erosion rates approximately 10 times higher, drainage densities approximately 3 times higher, and hillslope-scale relief approximately 3 times lower than elevations above 1430 m. We leverage the abundant geomorphic data collected at WGEW over the past several decades to calibrate a mathematical model that predicts the equilibrium drainage density in shrublands and grasslands/woodlands at WGEW. We use this model to test the hypothesis that the difference in drainage density between the shrublands and grassland/woodlands at WGEW is partly the result of a late Holocene vegetation change 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 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 3 ka, i.e., broadly consistent with paleovegetation studies. Our results provide support for the hypothesis that common vegetation changes in semi-arid environments (e.g., from grassland to shrubland) can change erosion rates by more than an order of magnitude, with important consequences for landscape morphology.
Highlights
Understanding how climate change controls landscape evolution is a central problem in geomorphology
Due to the complex nature of the rock types exposed in the southern portion of the watershed, we focus this study on the northern portion of the watershed, which is dominated by the Gleeson Road Conglomerate (GRC)
In all of the topographic analyses described we used a 1 m pixel−1 digital elevation model (DEM) for Walnut Gulch Experimental Watershed (WGEW) derived from airborne laser swath mapping (Heilman et al, 2008)
Summary
Understanding how climate change controls landscape evolution is a central problem in geomorphology. With changes in temperature (mean and variability), precipitation (mean and variability), and vegetation cover (type and density) often occurring simultaneously. The multifaceted nature of climatic changes can make it difficult to identify which aspects of climate change are most important in driving landscape modification in specific cases. Given the accelerated climatic changes expected to occur in the coming decades, understanding how landforms are likely to respond to specific climatic drivers, acting alone or in concert, is critically important to society (e.g., Pelletier et al, 2015). Pelletier et al.: The influence of Holocene vegetation changes on topography and erosion rates
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