PARENT SESSION
Oral Session #4: Plant Ecology: Gas Exchange. Presiding: J. Pereira.
Monday, August 6, 2001. 8:00 AM to 12:00 PM. Hall of Ideas P&Q.

Relationships between ecosystem CO₂ flux and vegetation spectral reflectance in Southern California chaparral.

Sims, Daniel ¹, Kwon, Hyojung², Oechel, Walter², Gamon, John¹, ^{1 2}

ABSTRACT- Comparisons between eddy covariance measurements of CO₂ flux and remote sensing of spectral reflectance are often limited by differences in their temporal and spatial scales. Remote sensing images are typically made at only one point in time but have a large amount of spatial detail. Eddy flux measurements are made continuously through time but measure only one point in space. In order to better match the temporal and spatial scales of the remote sensing measurements to those of the eddy flux, we installed automated systems (optical sampling instruments on a tram system) within eddy covariance tower footprints at Sky Oaks field station near San Diego, CA. This system measured hyperspectral (narrow-band) reflectance over a 100 m transect throughout diurnal and seasonal cycles. These tram measurements had very high spatial precision, allowing us to measure optical changes in a spatial context. We parameterized and applied a simple light use efficiency model for CO₂ flux. Light absorption was modeled as a function of NDVI and incident PPFD, and light use efficiency as a function of the photochemical reflectance index (PRI). The model captured the large diurnal and seasonal changes in light use efficiency associated with periodic photosynthetic downregulation in this evergreen dominated chaparral ecosystem. The potential for applying this model at the regional scale will also be discussed.

KEY WORDS: carbon flux, remote sensing, modeling, photosynthesis