Following the Footprint of Invasive Trees

Scientists at the U.S. Department of Agriculture (USDA) have been investigating ways of using aerial photography and computer programs to quickly identify and measure areas affected by invasive trees. As a result, Agricultural Research Service (ARS) rangeland scientists Kirk Davies and Matt Madsen have developed tools that could help land managers protect sagebrush ecosystems and control invasive vegetation more economically and efficiently.

ARS is USDA's chief intramural scientific research agency, and this work supports the USDA priority of responding to climate change.

The expansion of western juniper trees on some arid U.S. rangelands is pushing out other plant species, reducing sagebrush habitat and livestock forage, and at times fueling catastrophic wildfires. Davies and Madsen, who work at the Eastern Oregon Agricultural Research Center in Burns, Ore., led studies on using National Agricultural Imagery Program (NAIP) high-resolution aerial images to estimate western juniper cover.

Working at a 30,500-acre site in Idaho, the scientists collected information about juniper distribution and then used this data for "training" image software to identify juniper on the NAIP images. Using this calibrated approach, the team attained a tree cover classification overall accuracy rate of 92 percent.

For instance, the NAIP/software combination calculated that 26.8 percent of one site was covered with juniper, when initial ground scouting had determined that 24.7 percent of the site was covered with juniper. Imagery analysis indicated that the maximum juniper cover in the study area was 82 percent, a figure the scientists validated with ground-truthing surveys that indicated the maximum juniper cover was 78.7 percent.

Madsen also led a project that examined the use of high-resolution aerial imagery to study the expanse of pinyon-juniper forests in Utah. He "trained" his feature extraction software to identify pinyon and juniper trees within aerial photographs, and then used this data to estimate pinyon-juniper tree cover and density. This method yielded a 93 percent accuracy rate for tree cover. Madsen also obtained a 95 percent accuracy rate in estimating the density of mature trees that were at least 4.5 feet wide.

Findings from the two studies were published in Rangeland Ecology and Management and Environmental Management.

Read more about this research in the July 2013 issue of Agricultural Research magazine.