Abstract
Abstract. Digital repeat photography has become a widely used tool for assessing the annual course of vegetation phenology of different ecosystems. By using the green chromatic coordinate (GCC) as a greenness measure, we examined the feasibility of digital repeat photography for assessing the vegetation phenology in two contrasting high-latitude ecosystems. Ecosystem–atmosphere CO2 fluxes and various meteorological variables were continuously measured at both sites. While the seasonal changes in GCC were more obvious for the ecosystem that is dominated by annual plants (open wetland), clear seasonal patterns were also observed for the evergreen ecosystem (coniferous forest). Daily and seasonal time periods with sufficient solar radiation were determined based on images of a grey reference plate. The variability in cloudiness had only a minor effect on GCC, and GCC did not depend on the sun angle and direction either. The daily GCC of wetland correlated well with the daily photosynthetic capacity estimated from the CO2 flux measurements. At the forest site, the correlation was high in 2015 but there were discernible deviations during the course of the summer of 2014. The year-to-year differences were most likely generated by meteorological conditions, with higher temperatures coinciding with higher GCCs. In addition to depicting the seasonal course of ecosystem functioning, GCC was shown to respond to environmental changes on a timescale of days. Overall, monitoring of phenological variations with digital images provides a powerful tool for linking gross primary production and phenology.
Highlights
Phenology is an important factor in the ecology of ecosystems
The objectives of this study were to (1) evaluate the digital repeat photography as a method for monitoring the phenology of boreal vegetation at high latitudes, (2) investigate the differences in the phenology between two adjacent but contrasting ecosystems located in northern Finland, and (3) assess whether the data obtained from such cameras can support the interpretation of the micrometeorological measurements of CO2 fluxes conducted at the sites
We demonstrated the feasibility of digital repeat photography for assessing the link between vegetation phenology and CO2 exchange for two contrasting high-latitude ecosystems
Summary
The most distinctive phenomena comprising vegetation phenology are the changes in plant physiology, biomass and leaf area (Migliavacca et al, 2011; Sonnentag et al, 2011, 2012; Bauerle et al, 2012). In part, these changes drive the carbon cycle of ecosystems, and they have various feedbacks to the climate system through effects on surface albedo and aerodynamic roughness, and ecosystem–atmosphere exchanges of various gases (e.g. H2O, CO2 and volatile organic compounds) (Arneth et al, 2010). Gas exchange is modulated by seasonal variations in photosynthesis and respiration (Richardson et al, 2013). Phenological phenomena are largely controlled by abiotic factors such as temperature, water availability and day length
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