Insights from long-term ungrazed and grazed watersheds in western drylands

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This article is part of the Spring 2019 issue of the Earth Science Matters Newsletter.

Domestic livestock grazing is the most widespread land-use type in moisture-limited ecosystems (drylands) globally, including the Colorado Plateau. Livestock grazing, especially overgrazing, in drylands can negatively affect plant communities, biological soil crusts (lichens, mosses, and cyanobacteria growing on the soil surface), and soils. This can have multiple impacts including increased occurrence of undesirable nonnative plant species, dramatic changes in plant community, increased erosion, and increased runoff. Understanding the consequences of grazing can provide useful information on how to better manage livestock in the future, as well as restore damaged dryland systems. Studies of long-term grazing experiments are particularly useful in understanding grazing and its ecological impacts.

In 1953, a Federal research group established four paired watersheds in western Colorado, USA to study how grazing by domestic livestock affected runoff and sediment yield. Exclusion of livestock from half of the watersheds dramatically reduced runoff and sediment yield after the first 10 years, primarily due to changes in ground cover (biological soil crusts, plant litter); there was no observable change in vegetation.

collection of figures characterizing grazed verses ungrazed lands in the southwest

A) Map depicting location of the study area within the four-corner’s region of the southwestern US (black star), Colorado Plateau (shaded region), and distribution of the saline Mancos Shale deposits (blue); B) soil cover examples with low groundcover in grazed area; C) high groundcover due to high biological soil crust cover in ungrazed watersheds; D) county-level cattle and sheep totals from the study area; E) comparisons of ground cover in 1953 (before grazing exclosures erected), 1963, 1972, and 2004 (error bars represent standard error of the means; watersheds as the experimental units, n = 4 per treatment). (Adapted from Duniway et al., 2018).

(Credit: Michael Duniway, USGS. Public domain.)

In a recent study, USGS scientists and researchers from Colorado Mesa University, reported the results of repeated soil and vegetation assessments after more than 50 years of grazing exclusion. Within each watershed, six to thirteen 30-m long transects were established in 2004 for sampling. Results show that many of the differences in soil conditions between grazed and ungrazed watersheds observed in the 1950s and 1960s were still present in 2004, despite a reduction in the numbers of livestock over the years.

Overall, vegetation remained similar between grazed and ungrazed areas; where plants did respond to grazing, those responses depended on the soil type. For example, on one of the three soil types in the watersheds, grazing increased the frequency of cheatgrass (an aggressive nonnative grass), but not on the other two soil types.

Soil conditions in the grazed watersheds differed from ungrazed areas, with decreased soil lichen abundance and soil stability, increased soil compaction, and the occurrence of physical crusts at the soil surface. Comparisons of ground cover measured in 2004 to those measured in 1953, 1966, and 1972 suggest that many of the differences between grazed and ungrazed watersheds were likely driven by high sheep numbers grazing during 1950s drought years.

These persistent differences in watershed soil conditions, despite large reductions in livestock use, suggest the combination of overgrazing and drought may have pushed these salt desert ecosystems into a persistent, degraded ecological state. The results of this study support many current assessment and monitoring approaches that include a focus on plants and soil quality indicators to evaluate rangeland condition.

The paper “Insights from Long-Term Ungrazed and Grazed Watersheds in a Salt Desert Colorado Plateau Ecosystem” was published in Rangeland Ecology & Management and is available here: https://www.sciencedirect.com/science/article/abs/pii/S1550742418300320

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