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

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.

The relationship between drainage density, erosion rate, and hilltop curvature: Implications for sediment transport processes

“…For example, we assume a linear relationship between A and x t . Alternative model formulations are possible, such as that of a power law relationship, where A = bx y t [Pelletier et al, 2016]. However, using this alternative power law relationship predicts the same relationship between D d and E as that of our simpler scenario (see supporting information).…”

“…Furthermore, our model scenario neglects colluvial infilling of the valley head (equation (4)). Pelletier et al [2016] present a transport-limited model for predicting drainage density, calibrated for the Walnut Gulch Experimental Watershed, Arizona, where they assume that the valley head occurs where the fluvial erosion rate is greater than the colluvial deposition rate by an amount equal to the net erosion rate E. In the model outlined above, we follow a similar approach to Tucker and Bras [1998] by assuming that the valley head occurs where the fluvial erosion rate is greater than colluvial erosion. Including colluvial deposition at the valley head in our analytical model may lead to decreased absolute values of drainage density.…”

Nonlocal Sediment Transport on Steep Lateral Moraines, Eastern Sierra Nevada, California, USA

“…This provides an opportunity to hypothesize about the impact of changing ecological or climatological conditions. For example, imagine a substantial shift in climate which drives an ecologic change from a grassland to a shrubland (Pelletier et al, ). Under these new conditions, long‐distance transport events may become more common as there are fewer vegetative anchors for the soil which reflects an increase in E .…”

“…This type of analysis is similar to efforts that explore what values of the diffusivity‐like rate constant D might take on in different settings. It is largely suggested that it varies as a function of climate and ecology (Hughes et al, ; Istanbulluoglu & Bras, ; Pelletier et al, ; Tucker & Bras, ). However, this problem is a complex one that involves the specifics of ecology, geology, and climatology.…”

Steady 10Be‐derived paleoerosion rates across the Plio‐Pleistocene climate transition, Fish Creek‐Vallecito basin, California

“…This is part of a larger pattern of increased upwelling and reduction of the extent of the tropical warm-water pool toward the end of the Pliocene [Brierley et al, 2009] associated with intensification of Hadley circulation [Fedorov et al, 2013]. Amplified climate variation after 3 Ma [Zachos et al, 2001] may especially enhance erosion and sediment export from semiarid shrubland regions, including the upper elevations of catchments draining into the FCVB, due to cyclic changes of vegetation cover [Bull, 1991;Pelletier, 2014;Pelletier et al, 2016;Dosseto and Schaller, 2016]. Alternatively, tectonic controls may have driven the progradation.…”