Seasonal cattle grazing on the Reserve is conducted annually for the purposes of vegetation management to meet several ecological goals. One of the primary goals for cattle grazing on the Reserve is to prevent pervasive, non-native, European grasses from encroaching into the thousands of rare and protected vernal pools. Without proper grazing, excessive grass growth can lead to thatch (phytomass) build up in pools, which can cause harm to native plants and animals by reducing rare, endemic wetland plants via competition; reduce vernal pool hydroperiods; and degrade of the overall aquatic environment. In additional to control of non-native invasive plant species, such as annual European grasses, properly managed grazing can also provide the following ecological benefits to grasslands; fuel load reduction (which reduces fire intensity and severity), wildlife species habitat enhancement, and improved soil productivity. Each year, seasonal cattle grazing on the Reserve is evaluated via residual dry matter (RDM) monitoring, to provide an estimation of forage utilization and an indication of rangeland health.
Cattle grazing that is properly managed helps maintain vernal pool ecological health by limiting the spread of non-native, European grasses and forbs into and around vernal pools (Barry 1998, Robbins and Vollmar 2002, Marty 2005, Pike and Marty 2005, Swiecki and Bernhardt 2008, Alvarez 2011). The growth of exotic plants into vernal pools leads to high rates of evapotranspiration, which dries out soil and reduces the number of days that pools are inundated (i.e. hydroperiod). Adequate grazing also prevents the living and dead undecomposed plant matter (phytomass or “thatch”) from accumulating in pools. Excessive thatch can lead to degradation and eutrophication of the aquatic environment. Although cattle grazing is considered necessary on the Reserve, prudent environmental stewardship requires monitoring to ensure that grazing levels are appropriate and to identify and mitigate any potential negative impacts from cattle presence on the landscape. Climate and weather are other important factors for arriving at optimal grazing. RDM is expected to vary each year due to climatic conditions alone, regardless of grazing levels. Properly managed grazing that results in optimal RDM levels provides ecological benefits to not just vernal pool ecosystems, but also benefits diurnal raptors (e.g., golden eagles, ferruginous hawks and Swainson’s hawks), nesting birds species that prefer low vegetation (e.g., Western meadows larks and great horned larks), and some small mammal species (e.g., California ground squirrels). Low vegetation also enhances habitat for many amphibian and reptile species, some of which are listed as threatened and endangered (e.g., California tiger salamander).
Residual Dry Matter (RDM) surveys are conducted on the Reserve annually in autumn to assess grazing levels that occurred during the grazing season. RDM refers to the non-living, dried grasses and low forbs that remain standing before the beginning of a new growing season. RDM measurements involve clipping dried grassland vegetation within a small quadrat, weighing the sample, and calculating the pounds per acre of standing dried plants. Rangeland resource managers use this standard field technique to monitor and assess grazing intensity throughout the West (Heady 1956, Heady 1965, Bartolome et al. 2006). Appropriate RDM levels for a grassland help to maximize productivity in subsequent years, reduce erosion, maintain a favorable microclimate for seed germination, and reduce loss of nutrients. RDM is also useful for predicting future forage production and plant species composition (Bartolome pers. comm., in Vollmar 2002). RDM measurements help Reserve staff to assess grassland cattle grazing in order to maintain healthy vernal pools (and their associated listed plants and animals) and to comply with requirements and guidelines set forth in the Reserve’s conservation easements and Management Plan (Airola 2008; Appendix B; 33 pgs). In relation to RDM monitoring, the Management Plan states that “long-term monitoring is required to assess the effectiveness of management actions and to provide feedback information for adaptive grazing management. The primary management assumption is that the removal of non-native annual grass thatch and control of invasive weeds through managed grazing will maintain the populations of native biological resources on the site within a natural range of variability.” Because RDM is an inexact proxy for estimating grassland productivity and rangeland health, Reserve staff are interested in pursuing additional monitoring to further supplement this single indicator. In the near future, additional rangeland health indicators may be considered for informing and further improving adaptive grazing management practices.
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Robins, J.D., and J.E. Vollmar. 2002. Chapter 11: Livestock Grazing and Vernal Pools. pp. 401- 430. In, J.E. Vollmar (Ed.) Wildlife and Rare Plant Ecology of Eastern Merced County’s Vernal Pool Grasslands. Vollmar Consulting. Berkeley, California.
Swiecki, T., and E. Bernhardt. 2008. Effects of grazing on upland vegetation at Jepson Prairie Preserve, Solano County, CA. Report prepared by Photosphere Research for the Solano Land Trust, Fairfield, California.