How grazing could restore California’s grasslands
Herbivores, as you well know, eat plants. Some herbivores eat so many plants and so rapidly that they might as well be thought of as biological lawnmowers. But there’s a problem when biological lawnmowers meet invasive grasses. Together, the two can overwhelm native grasses, making them nearly impossible to restore. Indeed, the conversion of California grasslands from ones dominated by native, perennial bunchgrasses to ones dominated by exotic annual species is considered an “ecologically significant biological invasion.”
The invaders suppress the natives both as seedlings and as adults, but that’s not all. They alter the ecosystem’s response to disturbances, cause changes in soil carbon and other nutrients, cause changes in the community of soil microbes, and alter the profile of water in the soil. When combined with endless grazing by cows and other livestock, it’s no wonder that the native grasses can’t compete.
But Carlene Henneman, Nathaniel E. Seavy, and Thomas Gardali, researchers with Point Blue Conservation Science, have a plan. The idea is to sync the timing and intensity of grazing to the exotic annuals cycle, allowing the native perennials as much time to grow and seed as possible in between periods of grazing. While that all makes sense theoretically, the usefulness of the approach hasn’t actually been subject to much testing. So that’s what the researchers did, at a place called TomKat Ranch in Pescadero, California, south of the San Francisco Bay Area.
The ranch is home to some 800 acres of grassland (dominated by exotic annuals), and 100-150 head of cattle. From 2008 to 2011, the cattle were left to graze the land as they wished. But then in 2011, the researchers instituted their experimental grazing schedule.
According to the experimental paradigm, the density of cattle was increased by subdividing the grazing area into sub-units. The cattle were allowed to graze in one area for a specified period of time (between one day and one week), and were then shifted to the next area. Each paddock therefore received between 70 and 120 days of rest in between grazing periods. This went on for two years. In July of 2011, 2012, and 2013, the researchers surveyed for native grasses.
The proportion of “vegetation survey units” that included native grasses at all increased from 8% in 2011 to 80% in 2013. The surface area covered by native grasses remained small throughout the study (less than 5%), but increased significantly over time. In 2011, the researchers spotted only single, dispersed, individual grasses from among the native species. By 2013, they found a number of small but dense patches, each containing multiple individuals.
The gains made by the native grasses were meager, but promising. The results convincingly suggest that switching from season-long open-ended continuous grazing to a more rigorous planned schedule will facilitate the restoration of California’s grassland.
The researchers caution that the generalizability of this sort of plan is somewhat restricted. Grazing effects also depend on other variables, such as weather patterns, soil quality, and so on. “Therefore,” the researchers argue, “grazing management must take an adaptive approach,” with the particulars of the scheduled grazing paradigm altered to fit the unique demands of any given ranch. – Jason G. Goldman | 09 January 2015
Source: Henneman C. & Seavy, N., & Gardali, T. (2014). Restoring Native Perennial Grasses by Changing Grazing Practices in Central Coastal California, Ecological Restoration, 32 (4) 352-354. DOI: http://dx.doi.org/10.3368/er.32.4.352
Header image: shutterstock.com
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