Abstract
In support of NASA’s next-generation spectrometer—the Hyperspectral Infrared Imager (HyspIRI)—we are working towards assessing sub-pixel vegetation structure from imaging spectroscopy data. Of particular interest is Leaf Area Index (LAI), which is an informative, yet notoriously challenging parameter to efficiently measure in situ. While photosynthetically-active radiation (PAR) sensors have been validated for measuring crop LAI, there is limited literature on the efficacy of PAR-based LAI measurement in the forest environment. This study (i) validates PAR-based LAI measurement in forest environments, and (ii) proposes a suitable collection protocol, which balances efficiency with measurement variation, e.g., due to sun flecks and various-sized canopy gaps. A synthetic PAR sensor model was developed in the Digital Imaging and Remote Sensing Image Generation (DIRSIG) model and used to validate LAI measurement based on first-principles and explicitly-known leaf geometry. Simulated collection parameters were adjusted to empirically identify optimal collection protocols. These collection protocols were then validated in the field by correlating PAR-based LAI measurement to the normalized difference vegetation index (NDVI) extracted from the “classic” Airborne Visible Infrared Imaging Spectrometer (AVIRIS-C) data ( was 0.61). The results indicate that our proposed collecting protocol is suitable for measuring the LAI of sparse forest (LAI < 3–5 ()).
Highlights
The Hyperspectral Infrared Imager (HyspIRI) mission has been proposed by the Jet PropulsionLaboratory (JPL) in response to NASA’s Earth Science Decadal Survey [1]; this mission is geared to provide global imaging spectroscopy data to benefit research in domains ranging from coastal bathymetry to oceanography, wildfire science, volcanology, and terrestrial ecosystems
We investigated the relationship between Leaf Area Index (LAI), calculated from photosynthetically-active radiation (PAR), and the normalized difference vegetation index (NDVI), extracted from imagery
The simulated PAR is slightly larger than the measured PAR during the late afternoon
Summary
Laboratory (JPL) in response to NASA’s Earth Science Decadal Survey [1]; this mission is geared to provide global imaging spectroscopy data to benefit research in domains ranging from coastal bathymetry to oceanography, wildfire science, volcanology, and terrestrial ecosystems. Its 185 km swath width provides global coverage and 15-day revisit times, its large ground sample distance (GSD) introduces uncertainty in subpixel spectral variation. This is especially relevant in the open woodland forest environment, where trees are mixed with shrub, grass, bare soil, and rock. Our team participated in this mission by investigating the impact of sub-pixel structural variation on the assessment of vegetation structure via imaging spectroscopy data [2]. Of particular interest is Leaf Area Index (LAI), which is defined as the ratio of one-sided leaf area per unit ground area for flat broadleaf species [3]
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