Cool News for Tree Lovers

Ryan Jensen is preparing to collect leaf area index and other biophysical measurements in north central Florida.

Jensen chose to compare electricity use in these two cities, which are just 30 miles apart, because of their similarities. The likenesses include similar types of trees, ecosystems, climate, geology, topography, natural vegetation, population, and number of degree-days. "They both have about the same number of lakes nearby, and everything else is just about the same," says Michael Binford, associate professor of geography at the University of Florida, Gainesville. "The only thing that wasn't the same was the tree canopy coverage, which is denser in Gainesville."

Jensen developed an artificial neural network (ANN) to more accurately measure a city's leaf area index (LAI), using remotely sensed imagery collected by satellite. Jensen applied the ANN to area maps of the cities and then computed the average LAI of each city. Gainesville's average LAI was 4.6 m² of leaves per m² of ground, which is more than double Ocala's average of 2.1 m² of leaves per m² of ground.

"I then correlated these results with the average energy consumption per household in each city," says Jensen. The average energy consumption per household in Gainesville was 935 kWh per month, while the average in Ocala was 1,075 kWh hours per month. "When you use the local utility's rate of 7.5 cents per kWh, it results in a yearly savings of $126 per household for the residents of Gainesville," says Jensen. "Of course, there are other factors involved, but I think it's a pretty big factor."

Jensen says that his study concludes that strict tree ordinances, such as the one on the books in Gainesville, can be justified in economic terms. Gainesville's tree ordinance specifies what species and sizes of tree can be removed with a permit. Permits are available free of charge, but an applicant must have a good reason for removing a native tree, such as that the tree will cause structural damage. In addition, the ordinance specifies very strict penalties, such as replacement on an inch-per-inch basis of illegally removed trees, which can be extremely expensive. Ocala also has a tree ordinance, but it is not as strict as Gainesville's.

"But that's not the only issue here," says Jensen. "This study doesn't begin to touch some of the other benefits, like how healthy tree canopies will reduce urban air temperatures by altering surface reflectivity so that heat is not absorbed. Healthy canopies also filter the air to absorb some pollutants, they increase property values, and they even make people's moods better."

Jensen computed the ANN using remotely sensed brightness values from the Landsat Thematic Mapper Satellite. "I cor-related those brightness values--those vegetation reflectance values--with the LAI measurements I made on the ground," says Jensen, who explains that vegetation reflects in a different manner than other ground sources do. "I was able to determine the brightness (or reflectance) values from areas where I did not measure LAI, and I applied this ANN and was then able to arrive at a LAI index surface map for both cities." In essence, both the top leaf coverage and the leaves beneath the canopy were measured. With a recalibration of the neural network to the forests that exist in other cities, this same method can be used elsewhere.

--April Terreri

April Terreri is an independent writer based in Whitefield, New Hampshire, who specializes in business and trade articles for national magazines. She can be reached at wordwman@together.net.

For more information: R.R. Jensen conducted this study as part of his doctoral dissertation, "Measurement, Comparison, and Use of Remotely Derived Leaf Area Index Predictors," for the University of Florida. To get a copy of the full study, contact Dr. Jensen, who is now an assistant professor of geography, at Department of Geography, Geology, and Anthropology, Indiana State University, Terre Haute, IN 47809. Tel:(812)237-2258; E-mail: r-jensen@indstate.edu.

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