Greenness indices from digital cameras predict the timing and seasonal dynamics of canopy‐scale photosynthesis

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.

Using Near-Infrared-Enabled Digital Repeat Photography to Track Structural and Physiological Phenology in Mediterranean Tree–Grass Ecosystems

“…Our results about the differences between GPP and GCC are contrasting to previous studies [20,32,75], which mostly focused on temperate deciduous forests, evergreen needleleaf forests or grasslands, while the relationship between GCC and GPP is comparable to a study focused on a grassland understory of a "open forest savanna" in Australia [30]. One possible explanation for the discrepancy between previous studies [20,32,75] and this study, can be related to the patterns of re-greening of the vegetation in autumn after the onset of the rainy season. At the beginning of the Green-up period, the canopy of the grass is characterized by a relevant amount of dry biomass (~38% senescent grasses as measured between October to November 2014-2015).…”

“…However, the POS1 in our study is not comparable to the timing of the maximum value in other studies, as the POS1 in our study is in the winter time, while others are in the late spring or summer [32,75]. By contrast, the POS2 extracted from GPP is delayed compared to the POS2 extracted from VIs during the spring period (Figure 7), which agrees with previous studies [32,75].…”

“…For this reason, the GPP increases systematically earlier than GCC. However, there is evidence in the literature that this systematic delay is dependent on the greening patterns and mechanisms of vegetation phenology, therefore, it is vegetation type-dependent [20,32,75].…”

“…Past studies related to derived PTDs from PhenoCam mainly focus on temperate/boreal forests and grassland (e.g., [32]), and only a few recent studies have focused on seasonally dry tree-grass ecosystems [43,44]. Considering that tree-grass ecosystems are a widely distributed land cover type, which occupies 16-35% of the Earth's land surface [45][46][47], it is necessary to further investigate the methods to extract PTDs for these ecosystems.…”

“…However, to our knowledge, only a few studies pay special attention to the differences between phenology of ecosystem structure and of ecosystem functioning and carbon fluxes [28,32].…”