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Yellow Starthistle Mapping Project: Caltrans Survey 1999

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Intensive State Highway Survey and Mapping

The area between State Highway 80 and north of, and including, State Highway
140, east of State Highway 99 and as far east as the highest highway elevation.
This is of special concern to CALTRANS. CALTRANS funded the work of two
CDFA seasonal employees who mapped roadside occurrence of YST with a Global Positioning System (GPS) receiver in 1999.

I. Methods

Between July and October, 1999, seasonal employees of the California Department of Food and Agriculture surveyed highways in the Sierra Nevada for yellow starthistle (YST). The surveyors were using Trimble GeoExplorer equipment that
was mounted to their vehicle, and would mark points when they saw YST on the roadside. An overview of their survey can be seen on Map 1.

Approximate gradations of the level of YST infestation were made by gathering points of three types: low, high, trace (trace being one or two plants seen at one time).

The surveying was generally broken into three legs. The first was a general
highway roadside survey. The second was a survey that looked beyond the right
of way to see if YST extended past the roadside. The last leg was a re-survey of upper elevations to catch plants that had germinated later in the season.

Once the data had been collected, it was differentially corrected for better accuracy and incorporated into a GIS using MapInfo software. Analysis was performed visually and with the use of GIS software. A distinction was made between lower and higher density locations using a buffering technique. If three points or fewer were separated by more than a kilometer from other points, they were considered low density. These lower density locations (regardless of their infestation level) are considered outlier populations when they are found on the eastern extreme of a road. These infestations are the leading edge of yellow starthistle in the Sierra Nevada, and are crucial to control to stop the eastward spread of YST.

II. General Findings

Right of Way Survey: Yellow starthistle infests a majority of the roadside area surveyed. It was detected on every road surveyed except Highway 89 between South Lake Tahoe and Truckee . Of the points taken within the right-of-way, 15414 were "high" and 5920 were "low" or "trace" points. Generally speaking, YST's presence drops considerably between 3000 and 4000 feet in elevation on the roadside, so that east of this "edge" YST is less prevalent.

Outside Right of Way: The following roads were surveyed between August 17th and August 25th for YST presence outside the right of way:

  • Highway 49
  • Highway 50 (east of 49)
  • Highway 120 (east of 49)
  • Highway 26 (east of 49)
  • Highway 88 (east of 49)
  • Highway 4 (east of 49)
  • Highway 80

In terms of how far YST extends up into the Sierra Nevada outside the right-of-way, YST appears to diminish in presence outside the right-of way lower than it drops out along the roadside (see Map 2). However, steep terrain sometimes made it difficult to see past the immediate roadside, which makes this data less reliable than the roadside data.

III. Project maps: