Satellites track trees - measuring tree cover

"In my mind, the Savannah Tree Foundation's study was the top-rated project in Georgia last year and in the whole history of the Georgia Forestry Commission," says Sharon Dolliver, the Commission's urban forestry coordinator. The research was funded by grant money provided by the U.S. Forest Service.

The principal investigator, Charles C. Watson Jr., is a remote-sensing specialist formerly with NASA. His study documents changes in tree cover in Savannah, the Isle of Hope, and the surrounding county during the 20-year period from 1973 to 1992.

Watson used images from the LANDSAT satellite for the years 1973, 1978, 1986, and 1992. The satellite takes photographs of any given location on earth approximately once every 16 days. For Watson's study, images were photographed in the spring during April and May to ensure that the trees were fully foliated and to limit the effects of haze and thunderstorms common in the summer months.

The satellite images are in the form of digital data and thus are capable of being analyzed in a computer-based geographical information system (GIS). Watson processed the data into "pixels," or representations of 50-meter squares of ground. Trees, grass, and buildings all reflect light in unique ways, so he was able to determine the percentage of a particular terrain type in each pixel. A pixel might be 75 percent building and 25 percent tree canopy, for instance. Watson could then determine the percentage of tree canopy that covered the subject area during the four study years.

Additional manipulation was required to correct for spacecraft motion and the earth's rotation. "Any remote-sensing method, no matter how sound theoretically, must be subjected to extensive confidence checks," Watson wrote in his study.

The scientist used field verification and aerial photography to cross-check the data, and the resulting confidence level in the accuracy of his results in 95 percent or better. In plain language, the percentages of tree canopy that he estimated from studying the satellite images are within 5 percent of what is out there in the real world.

Watson's estimates of the net change to the tree canopy during the 20-year study period are even more accurate. Since he used the same kind of satellite imagery for 1973 and 1992, any errors will be consistent. For example, if the 1973 tree cover was overestimated by 3 percent, the 1992 cover would be overestimated by 3 percent as well. The study's results--which do not quantify the absolute number of trees lost during the period but rather the net change during the 20 years--are therefore amazingly accurate.

What his study shows is that more than one-fourth of the county lost significant amounts of tree canopy (defined as 20 percent or more) during the past two decades. The study also indicates that 60 percent of the loss occurred in residential neighborhoods rather than in commercial areas. Most alarming, it shows that the rate of loss has increased during the past eight years compared with the previous 11.

Satellite imagery and aerial photography provide a comprehensive picture of the entire urban forests canopy, whereas inventories taken on the ground produce information only on public trees--those along the street. The 90 percent of the urban tree canopy that is on private property--in backyards, for example--is off-limits to on-the-ground inventories.

"Looking at the entire canopy allows us to see urban areas from an ecological perspective," notes Gary Moll, AMERICAN FORESTS' vice president for urban forestry. Inventorying the canopy provides the basis for understanding the interconnections between urban tree loss and degradation of the urban ecosystem--water and air quality, the urban-heat-island effect, noise pollution, and wildlife habitat. Studying the impacts of tree loss will help city planners assess the value of the urban forest and use that information in planning capital improvements such as tree plantings.

The Watson study puts Savannah on the cutting edge in the use of satellite imagery for urban planning. The irony in Savannah, however, is that Gwen McKee's neighbor has been unable to sell any lots. Prospective buyers drive by--and keep going when they see fill dirt packed so hard even grass is having trouble growing.

AMERICAN FORESTS' Global ReLeaf for New Communities program is helping prevent needless tragedies like this by spreading the word to developers that houses on wooded lots command premium prices and sell faster than those on treeless lots.

COPYRIGHT 1994 American Forests
COPYRIGHT 2004 Gale Group