Mapping urban forest leaf area index with airborne lidar using penetration metrics and allometry

a r t i c l e i n f o Keywords: Airborne lidar Leaf area index Urban ecosystem analysis Hemispherical photography Allometry Vegetation structure In urban areas, leaf area index (LAI) is a key ecosystem structural attribute with implications for energy and water balance, gas exchange, and anthropogenic energy use. In this study, we estimated LAI spatially using airborne lidar in downtown Santa Barbara, California, USA. We implemented two different modeling approaches. First, we directly estimated effective LAI (LAI e) using scan angle-and clump-corrected lidar laser penetration metrics (LPM). Second, we adapted existing allometric equations to estimate crown structural metrics including tree height and crown base height using lidar. The latter approach allowed for LAI estimates at the individual tree-crown scale. The LPM method, at both high and decimated point densities, resulted in good linear agreement with estimates from ground-based hemispherical photography (r 2 = 0.82, y = 0.99x) using a model that assumed a spherical leaf angle distribution. Within individual tree crown segments, the lidar estimates of crown structure closely paralleled field measurements (e.g., r 2 = 0.87 for crown length). LAI estimates based on the lidar crown measurements corresponded well with estimates from field measurements (r 2 = 0.84, y = 0.97x + 0.10). Consistency of the LPM and allometric lidar methods was also strong at 71 validation plots (r 2 = 0.88) and at 450 additional sample locations across the entire study area (r 2 = 0.72). This level of correspondence exceeded that of the canopy hemispherical photography and allometric, ground-based estimates (r 2 = 0.53). The first-order alignment of these two disparate methods may indicate that the error bounds for mapping LAI in cities are small enough to pursue large scale, spatially explicit estimation. Urban trees provide a broad array of ecosystem services that are governed by tree species, canopy structure, and locational context Leaf Area Index (LAI), commonly defined as one half of the total green leaf area per unit ground area (Chen & Black, 1992), is a critical structural attribute that has implications for urban energy balance, gas exchange, hydrological throughput, and anthropogenic energy use. It is an eco-physiological measure of leaf surface available for photosynthesis and transpiration (Chen, Rich, Gower, Norman, & Plummer, 1997). In addition , dry depositional uptake and intercellular suspension of air pollutants such as O 3 , NO 2 , SO 2 , CO, and PM x is …