Peterson, Eric B. .
Estimating Cover of Bromus tectorum (cheatgrass) using Tobit Regression and Phenology derived from Two Dates of Satellite Images.
Bromus tectorum (cheatgrass) is an annual Eurasian grass that has invaded rangelands of the western USA. Being both a fire follower and a fire promoter, it can rapidly exclude native vegetation and is among the greatest threats to conservation in the intermountain west. Key to land management is a strong understanding of B. tectorum distrubution and density. Percent ground cover of B. tectorum was estimated and mapped as a continuous variable over 13.3 million hectares in Nevada, USA. Estimation involved a statistical model derived from 262 training plots, two dates of 6 Landsat 7 ETM+ scenes (from 2001), and elevation. Absence of B. tectorum in many plots lead to a zero-truncated data set. Tobit Regression, a method for modeling truncated data, was found to produce better models from these data than multiple linear regression. The two dates of the imagery were used to derive a variable representing phenology of the landscape. The derived phenology (in quadratic form), elevation, and the late-season green band were statistically significant in the model development. Additionally, the average brightness across imagery bands was used to limit estimates. Final map accuracy determined from assessment plots showed good correspondance between sampled and estimated B. tectorum ground cover (R2=0.51) and the Root-Mean-Square Error for estimated ground cover is 9.14 percent.
1 - Nevada Natural Heritage Program, 1550 East College Parkway, Carson City, Nevada, 89706, USA
Presentation Type: Poster
Location: Special Event Center (Cliff Lodge)
Date: Tuesday, August 3rd, 2004
Time: 12:30 PM