Abstract
Phenological shifts in events such as flowering and bud break are important indicators of ecosystem processes, and are therefore of particular significance for carbon (C) cycle research. Using long-term flux data from three contrasting plant functional type (evergreen and deciduous) boreal forest sites, we evaluated and compared the responses of annual C fluxes to multiple spring phenological indicators, including the C-uptake period onset (CUP onset), spring temperature (average value from March to May), and satellite-derived enhanced vegetation index (EVI) (average value from March to May). We found that the CUP onset was negatively correlated with annual gross primary production (GPP) for all three sites, but that its predictive strength for annual net ecosystem production (NEP) differed substantially among plant functional types. Spring temperature demonstrated particularly good potential for predicting both annual GPP and NEP for the evergreen sites, but not for the deciduous site. Spring EVI was demonstrated to have potential for predicting annual NEP for all sites. However, both plant functional types confounded the correlation of annual NEP with annual GPP. Although none of these phenological indicators provided consistent insight into annual C fluxes, using various currently available datasets our results remain potentially useful for the assessment of forest C cycling with future climate change. Previous analyses using only a single phenological metric should be considered with caution.
Highlights
The timing of plant canopy phenological events is an important indicator of the temporal and spatial variability of ecosystem-atmosphere fluxes [1,2]
The gross primary production (GPP) of SK-OBS was independent of spring enhanced vegetation index (EVI), our results suggest that spring variation in canopy greenness may have potential as an indicator of the annual net ecosystem production (NEP) of both evergreen and deciduous sites in boreal regions
The final uncertainty is that previous analyses have determined that carbon-uptake period (CUP) onset is a good indicator of annual NEP, which was not supported at the SK-OA and SK-OJP sites in our evaluation
Summary
The timing of plant canopy phenological events is an important indicator of the temporal and spatial variability of ecosystem-atmosphere fluxes [1,2]. Higher spring temperatures tend to increase annual NEP in various ecosystems, likely due to the earlier onset of canopy greenness [13,14]. Autumn warming would have a negative influence on annual NEP if ecosystem respiration rates were more sensitive to temperature changes [15,16]. These results imply that seasonal changes in climate are important indicators of the annual NEP, as these changes can shift phenological transitions and affect plant growth rhythm
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