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Title: Greenness indices from digital cameras predict the timing and seasonal dynamics of canopy-scale photosynthesis

Author
item TOOMEY, MICHAEL - Harvard University
item FRIEDL, MARK - Desiderio Finamore Veterinary Research Institute (FEPAGRO)
item FROLKING, STEVEN - University Of New Hampshire
item HUFKENS, KOEN - Ghent University
item KLOSTERMAN, STEPHEN - Harvard University
item SONNENTAG, OLIVER - University Of Montreal
item BALDOCCHI, DENNIS - University Of California
item Bernacchi, Carl
item BRZOSTEK, EDWARD - Indiana University
item BURNS, SEAN - University Of Colorado

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2014
Publication Date: 1/1/2015
Citation: Toomey, M., Friedl, M., Frolking, S., Hufkens, K., Klosterman, S., Sonnentag, O., Baldocchi, D.D., Bernacchi, C.J., Brzostek, E., Burns, S.P. et al. 2015. Greenness indices from digital cameras predict the timing and seasonal dynamics of canopy-scale photosynthesis. Ecological Applications. 25(1):99-115.

Interpretive Summary: Recently, a large number of cameras have been setup to monitor the growth of ecosystems (referred to as PhenoCams). Many of these are located in fields instrumented with a wide range of sensors to measure the exchange of carbon dioxide between the ecosystem and the atmosphere. Where these are co-located provide a novel opportunity to better understand the relationship between growth stages and canopy photosynthesis throughout the seasons. In this paper, we describe the abilities and limitations of webcams to track seasonal canopy development – as measured using image “greenness” – and photosynthesis, determine phenological transition dates, and estimate intra-annual and interannual variability in canopy photosynthesis for three plant functional types: deciduous broadleaf forest, evergreen needleleaf forest and grassland/crops. We used 59 site-years of camera imagery from the PhenoCam archive, and net ecosystem exchange measurements from 17 towers, to derive color indices and estimate gross primary productivity (GPP), respectively. GPP was strongly correlated with relative greenness in all functional types. The timing of the beginning of the photosynthetic period in both deciduous broadleaf forest and grassland/crops, and the end of the photosynthetic period in grassland/crops was related to changes in greenness. In evergreen forests it was not possible to accurately identify the beginning or ending of the photosynthetic period from camera greenness because these ecosystems are always green. Interannual variability in integrated greenness was related to total spring GPP: at one long-term deciduous broadleaf forest site, integrated greenness was strongly correlated with total GPP during the first 15 days following the mean onset date of start of spring, and was significantly correlated for up to 30 days. Across all deciduous broadleaf site-years, there was a significant correlation between anomalies in integrated greenness and total GPP for up to 60 days after the mean onset date of start of spring. Hence, at least in deciduous broadleaf forest and grassland/crops, webcams may be used to estimate the impacts of shifts in phenology on both the duration of the photosynthetically active period as well as total GPP. This research can help to extrapolate understanding of how ecosystems response to their environment using webcams set up around the world for other purposes.

Technical Abstract: The proliferation of tower-mounted cameras co-located with eddy covariance instrumentation provides a novel opportunity to better understand the relationship between canopy phenology and the seasonality of canopy photosynthesis. In this paper, we describe the abilities and limitations of webcams to track seasonal canopy development – as measured using image “greenness” – and photosynthesis, determine phenological transition dates, and estimate intra-annual and interannual variability in canopy photosynthesis for three plant functional types: deciduous broadleaf forest, evergreen needleleaf forest and grassland/crops. We used 59 site-years of camera imagery from the PhenoCam archive, and net ecosystem exchange measurements from 17 towers, to derive color indices and estimate gross primary productivity (GPP), respectively. GPP was strongly correlated with relative greenness in all functional types. The timing of the beginning of the photosynthetic period in both deciduous broadleaf forest and grassland/crops, and the end of the photosynthetic period in grassland/crops was related to changes in greenness. In evergreen forests it was not possible to accurately identify the beginning or ending of the photosynthetic period from camera greenness. Interannual variability in integrated greenness was related to total spring GPP: at one long-term deciduous broadleaf forest site, integrated greenness was strongly correlated with total GPP during the first 15 days following the mean onset date of start of spring, and was significantly correlated for up to 30 days. Across all deciduous broadleaf site-years, there was a significant correlation between anomalies in integrated greenness and total GPP for up to 60 days after the mean onset date of start of spring. Hence, at least in deciduous broadleaf forest and grassland/crops, webcams may be used to estimate the impacts of shifts in phenology on both the duration of the photosynthetically active period as well as total GPP.