Abstract
The proliferation of digital cameras co-located with eddy covariance instrumentation provides new opportunities to better understand the relationship between canopy phenology and the seasonality of canopy photosynthesis. In this paper we analyze the abilities and limitations of canopy color metrics measured by digital repeat photography to track seasonal canopy development and photosynthesis, determine phenological transition dates, and estimate intra-annual and interannual variability in canopy photosynthesis. We used 59 site-years of camera imagery and net ecosystem exchange measurements from 17 towers spanning three plant functional types (deciduous broadleaf forest, evergreen needleleaf forest, and grassland/crops) to derive color indices and estimate gross primary productivity (GPP). GPP was strongly correlated with greenness derived from camera imagery in all three plant functional types. Specifically, the beginning of the photosynthetic period in deciduous broadleaf forest and grassland/crops and the end of the photosynthetic period in grassland/crops were both correlated with changes in greenness; changes in redness were correlated with the end of the photosynthetic period in deciduous broadleaf forest. However, it was not possible to accurately identify the beginning or ending of the photosynthetic period using camera greenness in evergreen needleleaf forest. At deciduous broadleaf sites, anomalies in integrated greenness and total GPP were significantly correlated up to 60 days after the mean onset date for the start of spring. More generally, results from this work demonstrate that digital repeat photography can be used to quantify both the duration of the photosynthetically active period as well as total GPP in deciduous broadleaf forest and grassland/crops, but that new and different approaches are required before comparable results can be achieved in evergreen needleleaf forest.
Highlights
Climate change impacts on vegetation phenology have been widely documented across a range of biomes and plant functional types (Richardson et al 2013)
The specific questions guiding this study were: 150 1) Can camera-derived color indices be used to monitor the seasonality of gross primary productivity (GPP) within and across multiple PFTs? 2) How does the relationship between canopy phenology and GPP vary within and across PFTs? 3) What is the relationship between dynamics in greenness measured from digital camera imagery and key phenophase transitions in different PFTs? 4) Can interannual variation in annual GPP be estimated using camera-derived color indices? 157 To address these questions, we used data from the PhenoCam network of co-located cameras and eddy covariance towers to assess the relationship between canopy phenology and the seasonality of photosynthesis
In the section below we elaborate on these themes, discussing four camera-based phenology metrics – start of spring, middle of spring, middle of fall, and end of fall – and their relationship with the seasonality of GPP. 417 At DBF sites, camera-derived spring and fall phenophase transition dates successfully captured 418 spatiotemporal variability in the beginning and end of the photosynthetic period
Summary
Climate change impacts on vegetation phenology have been widely documented across a range of biomes and plant functional types (Richardson et al 2013). 131 Color indices derived from digital repeat photography have been correlated with canopy photosynthesis in deciduous broadleaf forest (Richardson et al 2007, 2009a, Ahrends et al 2009, Mizunuma et al 2012), grasslands (Migliavacca et al 2012), and desert shrublands (Kurc and Benton 2010) Each of these studies was limited to one or two sites and it is unclear how well results from these efforts generalize within and across PFTs at regional to continental scales. Our study, conducted across a range of PFTs, provides the most comprehensive analysis of canopy development and photosynthesis using digital repeat 161 photography to date, and provides useful new understanding regarding the ability of camera[162] derived color indices to track the seasonality of GPP across space and time.
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