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Additional key words: weed management, vernal pools, GIS, GPS
This report describes the development of a monitoring system to assess populations of selected exotic and native plants at the Jepson Prairie Preserve in Solano County, California. The monitoring system is based on the use of permanent belt transects (20 m wide) that traverse all of the pastures at the preserve. Transects are divided into segments 50 m in length for purposes of data collection and analysis. Percent cover of each monitored species within each transect segment is estimated visually and assigned to one of four cover ranks. In addition to transect-based monitoring, populations of uncommon weeds located outside of transects are mapped with the aid of a GPS receiver. Initial monitoring along the transects was conducted during a one-week period in mid-April 2001 and transects were resurveyed in April 2002. The results of these surveys are reported in this report and a previous report (Swiecki and Bernhardt 2001).
We used paired comparisons of data collected along the same transect by different sets of evaluators to assess the amount of variation in cover ranks that could be attributed to differences between evaluators and to GPS receiver position error. Measurement error associated with these factors was acceptably low. Quality control on data collected in successive years can be maintained by ensuring that evaluators are adequately trained, plant phenology is optimal for observations, and differentially-corrected GPS readings are used.
For most monitored species, cover ratings in 2001 and 2002 did not differ significantly. However, some species, including the native grass Pleuropogon californicus and the exotics Carduus pycnocephalus (Italian thistle) and Lactuca serriola (prickly lettuce) showed general increases in cover that may be related to differences in rainfall patterns in the two years. Monitoring data also documented that controlled burns in 2000 and 2001 provided virtually complete suppression of the exotic grass Taeniatherum caput-medusae (medusahead) for at least 2 years in some pastures. However, burns conducted in some pastures in the same years were ineffective at controlling T. caput-medusae. Analysis of paired pre- and post-fire data also indicates that the cover of Triphysaria eriantha, Viola pedunculata, and Erodium spp. was significantly elevated in burned transect segments relative to matched nonburned segments.
Among the target weeds surveyed, Lepidium latifolium (perennial pepperweed) and C. pycnocephalus appear to have spread significantly since 1995. Centaurea calcitrapa (purple star-thistle) may also show some expansion in its distribution since 1995. However, 1995 baseline data are inadequate to determine whether most target weed species show long-term gains in cover. A number of target weed species that have limited distribution at the preserve were rarely detected in transects, but mapped points and polygons document that they occur at various locations throughout the preserve.
The combination of annual transect-based monitoring and point/polygon mapping of uncommon target weeds provides a practical means for assessing the spread of invasive species at the preserve. It can also provide information on the efficacy of management activities such as burning and the impact of management actions on various native species. Adaptive management of Jepson Prairie grassland vegetation will be facilitated by consistently implementing the monitoring protocols described in this report.