Phytosphere Research

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

Progress report 1:  first year (2004-2005) 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


This progress report presents results from the first year of a three-year study that examines the effects of different grazing regimes on vegetation at the Jepson Prairie Preserve.  The purpose of this study is to test whether the current sheep grazing regimes 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 plants, especially exotic grasses, predominate on the relatively high mound or upland areas of the Preserve, whereas native species typically predominate in low lying areas.

Starting in January 2005, three adjacent field at the Preserve were grazed according to three different grazing regimes.  The grazing regimes differ with respect to when, how long, and how many sheep are present in each field.  In each field, we established eight clusters of study plots.  Each cluster included plots in both high (weed-dominated) and low (generally native dominated) positions.  Adjacent grazed and nongrazed (fenced) plots were set up in both high and low positions in each cluster.  By comparing forage heights in paired grazed and nongrazed plots at approximately monthly intervals, we were able to assess the pattern of forage removal over time at each plot.  This pattern is referred to as the grazing profile for a given plot.  Due to the uneven nature of grazing within fields, we found that grazing profiles commonly varied between plots within a field, i.e., between plots that had the same overall grazing regime.

First-year data showed that adjacent high and low plots within fields were grazed at different intensities by sheep.  Weed-dominated high plots were grazed preferentially in winter months when the low-lying native-dominated areas were periodically 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.

A baseline assessment of native and exotic cover and species diversity within plots was conducted in late April 2004 prior to the start of the experiment.  Plant cover was reassessed in April 2005.  Low plots that were grazed during the peak spring bloom period in late March and April 2005 lost native cover and gained exotic cover compared to the baseline assessment.  This effect was not seen in high plots or in low plots that were not grazed during this period.

After one growing season, plots excluded from grazing had considerably more residual dry matter (RDM) in August than did plots that were grazed.  However, native cover and native species diversity did not differ significantly between grazed and nongrazed plots.

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.