-Leafy Spurge: Targhee National Forest, ID
The Problem: Leafy Spurge (Euphorbia esula) is an aggressive perennial herb that out competes native grasses and vegetation. It propagates through rhizomes and exploding seed pods. It can have up to a 40-foot taproot rendering herbicide management ineffective. As the climate warms, leafy spurge is being found at higher elevations. This is of particular concern near the southwest corner of Yellowstone National Park—where the heavily used Targhee National forest and Agricultural land meet the park border.
-A large leafy spurge site in the Targhee National Forest
Outline: The first task of the project was to find large patches of leafy spurge near the southwest corner of Yellowstone National Park. After a few days of driving around, we found many suitable sites in the Targhee National Forest and the surrounding agricultural land near Ashton, ID. Because leafy spurge thrives in disturbed sites, many “training pixel” sites were found in a new subdivision outside of Ashton. Training pixels are large (30x30 meter) sites of homogenous leafy spurge that can be used to calibrate the software to identify what 100% spurge cover looks like. This will allow a unique spectral signature for leafy spurge to be identified. After enough suitable sites were found, a flight line was devised to encompass the best sites.
-A hawk moth caterpillar gorging on some leafy spurge.
On July 1st an airplane with a hyperspectral sensor flew over the designated flight line. The sensor panned slightly from side to side to allow an imaging strip two kilometers wide and a resolution of less than five meters. The sensor acquired data in 130 spectral bands, allowing for analysis far beyond the visible spectrum.
In order to validate the hyperspectral data, we collected groundtruthing polygons. After obtaining a true color RGB image from the flight, the distinct yellow-green color of the spurge was easily to identify. We recognized previously known sites on the image, but also found many new sites off of roads and trails.
-A Trimble GeoXT GPS unit.
Procedure: After the flight, we downloaded a list of waypoints to a GPS unit. Then we walked the perimeter of all the leafy spurge sites with the GPS unit (A Trimble GeoXT), creating a polygon file for the patch in the process. We set the logging interval to one second, creating a new node on the polygon every second. We also employed Carrier phase correction, and collected a minimum of 45 minutes of carrier time for each rover file. In the image below, one can see that even the uncorrected rover files follow the boundaries of the spurge patches quite closely. The polygons here are plotted over a 1-meter resolution NAIP image. Patch borders are somewhat ambiguous, but areas with consistent densities were used as patch boundaries. Sub-polygons, small areas within larger polygons with very high densities, were also collected. Photos were taken of each polygon, and a GPS photo point and compass bearing were also recorded.
In addition to the spatial polygon file, we collected attribute data for each polygon using a data dictionary on the GeoXT. The following attributes for each polygon were recorded:
- cover type (forest/grass)- percent leafy spurge cover
- mean leafy spurge height
- phenology (flowering/non-flowering)
- yellow toadflax percent
- grass/sedge percent
- conifer percent
- deciduous percent
- bare/rock percent
- native vegetation percent
-The flightline for the Hyperspectral flight.
-Even the uncorrected rover polygons on a 1-meter NAIP image follow the borders of the spurge patches quite closely.
-A zoomed view of a polygon with two high-density sub-polygons inside the main.