HR: 16:30h
AN: B54A-03 INVITED    [Abstracts]
TI: Monitoring regional patterns of canopy-scale phenology with a network of digitial webcams
AU: * Richardson, A D
EM: andrew.richardson@unh.edu
AF: University of New Hampshire, CSRC-EOS, Morse Hall, 39 College Road, Durham, NH 03824, United States
AU: Braswell, B H
EM: rob.braswell@unh.edu
AF: University of New Hampshire, CSRC-EOS, Morse Hall, 39 College Road, Durham, NH 03824, United States
AU: Hollinger, D Y
EM: davidh@hypatia.unh.edu
AF: USDA Forest Service, Northern Research Station, 271 Mast Road, Durham, NH 03824, United States
AU: Jenkins, J P
EM: julian.jenkins@unh.edu
AF: University of New Hampshire, CSRC-EOS, Morse Hall, 39 College Road, Durham, NH 03824, United States
AB: Understanding relationships between canopy structure and the seasonal dynamics of photosynthetic uptake of CO2 by forest canopies requires improved knowledge of canopy phenology at eddy covariance flux tower sites. We investigated whether digital webcam images could be used to monitor the trajectories of spring green-up and autumn senescence in a deciduous northern hardwood forest. A standard, commercially available webcam was mounted at the top of the eddy covariance tower at the Bartlett AmeriFlux site. Images were collected each day around mid-day. Red, green and blue color channel brightness data for a 640 x 100 pixel region-of-interest were extracted from each image. We evaluated the green-up signal extracted from webcam images against changes in fAPAR (the fraction of incident photosynthetically active radiation that is absorbed by the canopy), broadband NDVI, and Amax (the light-saturated rate of canopy photosynthesis, inferred from eddy flux measurements). The relative brightness of the green channel (green %) was relatively stable through the winter months. A steady rising trend in green % began around day 120 and continued through day 160, at which point a stable plateau was reached. The relative brightness of the blue channel (blue %) also responded to spring green-up, although there was more day-to-day variation in the signal because blue % was more sensitive to changes in the quality (spectral distribution) of incident radiation. Seasonal changes in blue % were most similar to those in fAPAR and broadband NDVI, whereas changes in green % proceeded more slowly, and were drawn out over a longer period of time. Changes in Amax lagged green-up by at least a week. The onset of autumn senescence was marked by a decrease in green %, which preceded a spike in red % when autumn coloration peaked at day 270. A decrease in red % was observed over the next 30 days (through day 300) as senescence progressed and the deciduous canopy was shed. We conclude that webcams offer an inexpensive means by which phenological changes in canopy state could be quantified using instrument-based "near" remote sensing. We describe a recently-established camera network (13 sites in the northeastern US and Canada, of which 5 sites are instrumented for eddy flux measurements) which will provide a regional perspective on both spatial and temporal variation in patterns of canopy phenology.
DE: 0428 Carbon cycling (4806)
DE: 0439 Ecosystems, structure and dynamics (4815)
DE: 0452 Instruments and techniques
SC: Biogeosciences [B]
MN: 2007 Fall Meeting