Tracking plant physiological properties from multi-angular tower-based remote sensing |
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Authors: | Thomas Hilker Anatoly Gitelson Nicholas C Coops Forrest G Hall T Andrew Black |
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Institution: | (1) Faculty of Forest Resources Management, University of British Columbia, 2424 Main Mall, Vancouver, BC, V6T 1Z4, Canada;(2) School of Natural Resources, University of Nebraska Lincoln, 303 Hardin Hall, 3310 Holdrege, Lincoln, NE 68583-0973, USA;(3) Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, USA;(4) Code 614.4, Goddard Space Flight Center, Greenbelt, MD 20771, USA;(5) Faculty of Land and Food Systems, University of British Columbia, 2357 Main Mall, Vancouver, BC, V6T 1Z4, Canada |
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Abstract: | Imaging spectroscopy is a powerful technique for monitoring the biochemical constituents of vegetation and is critical for
understanding the fluxes of carbon and water between the land surface and the atmosphere. However, spectral observations are
subject to the sun–observer geometry and canopy structure which impose confounding effects on spectral estimates of leaf pigments.
For instance, the sun–observer geometry influences the spectral brightness measured by the sensor. Likewise, when considering
pigment distribution at the stand level scale, the pigment content observed from single view angles may not necessarily be
representative of stand-level conditions as some constituents vary as a function of the degree of leaf illumination and are
therefore not isotropic. As an alternative to mono-angle observations, multi-angular remote sensing can describe the anisotropy
of surface reflectance and yield accurate information on canopy structure. These observations can also be used to describe
the bi-directional reflectance distribution which then allows the modeling of reflectance independently of the observation
geometry. In this paper, we demonstrate a method for estimating pigment contents of chlorophyll and carotenoids continuously
over a year from tower-based, multi-angular spectro-radiometer observations. Estimates of chlorophyll and carotenoid content
were derived at two flux-tower sites in western Canada. Pigment contents derived from inversion of a CR model (PROSAIL) compared
well to those estimated using a semi-analytical approach (r
2 = 0.90 and r
2 = 0.69, P < 0.05 for both sites, respectively). Analysis of the seasonal dynamics indicated that net ecosystem productivity was strongly
related to total canopy chlorophyll content at the deciduous site (r
2 = 0.70, P < 0.001), but not at the coniferous site. Similarly, spectral estimates of photosynthetic light-use efficiency showed strong
seasonal patterns in the deciduous stand, but not in conifers. We conclude that multi-angular, spectral observations can play
a key role in explaining seasonal dynamics of fluxes of carbon and water and provide a valuable addition to flux-tower-based
networks. |
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