22 Jensen and Hardin: Estimating Urban Leaf Area sensor. A single image resolution element (pixel) may be measuring the spectral response of a land cover mixture rather than a single land cover type. For example, a subur- ban pixel may represent a mixture of grass, asphalt, con- crete, and roof shingles. This kind of spectral mixing makes urban remote sensing less amenable to statistical methods that assume normal distributions and no measurement error. Newer spaceborne instruments, having finer spatial resolutions, reduce the constraint and provide better data for urban remote sensing (Jensen et al. 2003). The improve- ment in resolution is fortunate, because governments (e.g., state, county, city) and private companies annually invest hundreds of millions of dollars acquiring remotely sensed data that detail the urban landscape more effectively than through traditional “windshield surveys” (Jensen 2000). DATA AND METHODS Study Area The city of Terre Haute is located in Vigo County along the banks of the Wabash River in west central Indiana, U.S. (39° 25′ N, 87° 25′ W). Terre Haute government officials have made a conscious effort to maintain the urban tree canopy through a comprehensive tree ordinance that governs both tree removal and planting. The ordinance is administered by a tree advisory board consisting of city residents appointed by the city officials to make suggestions and recommenda- tions to the mayor, city forester, city engineer, and city council. LAI Field Measurements Traditional field measurement of LAI has taken two ap- proaches. The first approach requires the destructive harvesting of leaves within a vertical column passing upward through the entire tree canopy. The second involves collection of leaf litterfall. These direct methods are similar: They are time intensive and require many replicates to account for spatial variability in the canopy (Green et al. 1997). However, these direct LAI measurements are accurate for a very specific geographic location, are relatively easy to perform by untrained personnel, and are well understood by ecologists. Gap-fraction analysis is a nondestructive field method that has been developed to estimate LAI. Gap-fraction analysis is predicated on the theory that the decrease in light intensity (light attenuation) with increasing depth in vegeta- tive canopies can be described by the relationship: IL IO = e −kLAI ( L ) , (1) where IL/IO is the fraction of incident light at the top of the canopy (IO) reaching depth L in the canopy, LAI(L) is the cumulative LAI from the top of the canopy to point L, k is a stand or species specific constant, and e is the natural ©2005 International Society of Arboriculture logarithm base (Larcher 1975; Aber and Melillo 1991). Different types of vegetation have different k values, causing different rates of light attenuation for the same leaf area. The principal factor causing this is “twig angles and the angles that the foliage subtends with the twig” (Barclay 1998; see also Larcher 1975). Field-measured LAI using gap-fraction analysis assumes that leaf area can be calcu- lated from the fraction of direct solar energy that penetrates the canopy (canopy transmittance). Gap-fraction techniques have been used to study LAI in many different forest settings (Pierce and Running 1988; Chason et al. 1991; Ellsworth and Reich 1993; Nel and Wessman 1993; Green et al. 1997). In this study, LAI was measured using the gap-fraction approach in 143 random locations (sampling sites) through- out the study area during July and August 2001. Like most urban areas, land cover in Terre Haute consists of a wide variety of vegetated and nonvegetated patches. Vegetated areas sampled included trees, shrubs, grasses, and agricul- tural fields growing different varieties of corn and soybeans. Unvegetated areas included buildings, streets, parking lots, ponds, lakes, and the Wabash River. The randomly selected sampling sites represented all major land cover types in Terre Haute. Each of the 143 sampling sites was defined as a 20 × 20 m (65.6 × 65.6 ft) quadrat identified by the global position- ing system coordinates of its center. At each sampling point, 16 below-canopy, photosynthetically active radiation (PAR) measurements were collected, one in each cardinal direc- tion at each corner of the 20 m quadrat. The PAR measure- ments were collected using a Decagon AccuPar Ceptometer™ held approximately 1 m (3.3 ft) above the ground beneath the tree cover. The AccuPar Ceptometer consists of a linear array of 80 adjacent, 1 cm2 (0.16 in2 ) PAR sensors mounted rigidly along a bar and oriented so that when the operator holds the ceptometer horizontally, the PAR passing downward through the canopy can be mea- sured. The ceptometer stored the 16 PAR samples taken at each sampling site and calculated the LAI average automati- cally. This sitewide LAI average was then recorded along with general operator notes regarding the sampling site character. Satellite Sensor Data Data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor were used for comparison to the field LAI measurements. ASTER data are collected in several wavelengths, often referred to as bands. This study employed ASTER bands 1, 2, and 3 measuring the green, red, and near-infrared segments of the electromag- netic spectrum (520–600 nm, 630–690 nm, and 790–860 nm), respectively. These wavelengths are used in vegetation studies because of their correlation to the quantity and
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