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

• Lidar penetration metrics are used to map urban effective LAI at 10 m resolution.
• We develop a new method for correcting variable pulse path length through canopy.
• We map LAI at the tree crown scale using lidar and an allometric approach.
• Dimensions of automatically segmented crowns are accurately measured with lidar.
• Citywide LAI maps generated using allometry and penetration metrics are compared.

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 (LAIe) 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 (r2 = 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., r2 = 0.87 for crown length). LAI estimates based on the lidar crown measurements corresponded well with estimates from field measurements (r2 = 0.84, y = 0.97x + 0.10). Consistency of the LPM and allometric lidar methods was also strong at 71 validation plots (r2 = 0.88) and at 450 additional sample locations across the entire study area (r2 = 0.72). This level of correspondence exceeded that of the canopy hemispherical photography and allometric, ground-based estimates (r2 = 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.