Phytosphere Research

Effects of grazing on upland vegetation at Jepson Prairie Preserve, Solano County, CA

Second year (2006) results

T. J. Swiecki and E. A. Bernhardt
Phytosphere Research, Vacaville CA

Download the full report (Adobe Acrobat PDF - 2.7 MB)

Additional key words: weed management, vegetation management, vernal pools, grazing impacts, microplot


The purpose of this study is to test whether the current sheep grazing regimes at Jepson Prairie Preserve can be altered to increase cover of native plants in areas that are currently weed-dominated without adversely affecting areas that are currently dominated by native species.  Weedy plant cover, especially exotic grass cover, predominates on the relatively high mound or upland areas of the Preserve, whereas native cover predominates in low lying areas. This progress report presents results from the second year of this three year study.
Eight plot locations were established in each of three adjacent fields.  Each field was grazed with a different prescribed grazing regime.  The amount of forage removed over the season for each plot location was determined by measuring forage height in adjacent grazed and nongrazed plots.  First-year data showed that adjacent high and low plots within fields were grazed at different intensities by sheep.  This effect was also observed during the second year of the study.  Weed-dominated high plots were grazed preferentially when the low-lying native-dominated areas were flooded.  As the season progressed and the exotic grasses began to dry out and set seed in the high plots, sheep preferentially grazed the native-dominated low plots. 
The amount of forage removed from each grazed plot over the season was incorporated into a grazing profile variable.  Grazing profiles varied among plots within fields that had the same topographic position (high or low).  Some plots in different fields were more similar with respect to their grazing profiles than were plots within the same field. 
A baseline assessment of native and exotic cover and species diversity was conducted in late April 2004 prior to the start of the experiment.  Cover was reassessed again in April 2005 and 2006.  Cover of native and exotic species in high plots was not significantly affected by different grazing profiles and did not differ between grazed and nongrazed plots overall.  However, compared to the 2004 baseline assessment, native cover was significantly reduced and exotic cover increased in low plots that had not been grazed for two years.  As was seen in 2005, low plots that were grazed heavily during the peak spring bloom period (late March and April) in 2006 had reduced native cover and increased exotic cover.  However, plots that were grazed heavily during bloom in 2005 but only lightly during bloom in April 2006 rebounded to near 2004 cover levels.  Within-season grazing profiles were better predictors of vegetation outcomes in low plots than were grazing profiles that included grazing that occurred in the previous year.
After one growing season, high and low plots excluded from grazing had substantially more residual dry matter (RDM) in August than did plots that were grazed.  This difference continued in 2006, but the difference between the grazed and nongrazed plots was not statistically greater in 2006 than in 2005.  Mulch levels were also greater in grazed plots overall than in nongrazed. 
Overall, the study to date has demonstrated substantial effects associated with cessation of grazing.  All the grazing prescriptions tested, including very low intensity regimes, showed substantial differences in weedy cover in low plots, and thatch accumulation in high and low plots, compared to nongrazed plots.  However, none of the grazing treatments tested to date appear to constitute an improvement over the pre-experiment grazing regimes. 

This study is funded by a grant from the California Bay Delta Authority to the Solano Land Trust (Grant Agreement #ERP-02-P21) with additional funding from the Solano County Water Agency.