Abstract
This study aimed to understand which vegetation indices (VIs) are an ideal proxy for describing phenology and interannual variability of Gross Primary Productivity (GPP) in short-rotation coppice (SRC) plantations. Canopy structure- and chlorophyll-sensitive VIs derived from Sentinel-2 images were used to estimate the start and end of the growing season (SOS and EOS, respectively) during the period 2016–2018, for an SRC poplar (Populus spp.) plantation in Lochristi (Belgium). Three different filtering methods (Savitzky–Golay (SavGol), polynomial (Polyfit) and Harmonic Analysis of Time Series (HANTS)) and five SOS- and EOS threshold methods (first derivative function, 10% and 20% percentages and 10% and 20% percentiles) were applied to identify the optimal methods for the determination of phenophases. Our results showed that the MEdium Resolution Imaging Spectrometer (MERIS) Terrestrial Chlorophyll Index (MTCI) had the best fit with GPP phenology, as derived from eddy covariance measurements, in identifying SOS- and EOS-dates. For SOS, the performance was only slightly better than for several other indices, whereas for EOS, MTCI performed markedly better. The relationship between SOS/EOS derived from GPP and VIs varied interannually. MTCI described best the seasonal pattern of the SRC plantation’s GPP (R2 = 0.52 when combining all three years). However, during the extreme dry year 2018, the Chlorophyll Red Edge Index performed slightly better in reproducing growing season GPP variability than MTCI (R2 = 0.59; R2 = 0.49, respectively). Regarding smoothing functions, Polyfit and HANTS methods showed the best (and very similar) performances. We further found that defining SOS as the date at which the 10% or 20% percentile occurred, yielded the best agreement between the VIs and the GPP; while for EOS the dates of the 10% percentile threshold came out as the best.
Highlights
Short-rotation coppice (SRC) plantations provide multiple economic and ecological benefits [1]
In this study we explored for the first time the use of eight canopy structure- and chlorophyll-sensitive vegetation indices (VIs), derived from Sentinel-2, for describing the phenology and seasonal variability of Gross Primary Productivity (GPP), derived from eddy covariance (EC), of an SRC poplar plantation in Belgium
Our results revealed that MERIS Terrestrial Chlorophyll Index (MTCI) was the most suitable proxy for describing the GPP phenology (i.e., it exhibited the lower distance in identifying the start of the season (SOS)- and end of the season (EOS)-dates (Tables 4 and 5; Figures 4 and 5), and the highest R2 and lowest root mean square error (RMSE) in reconstructing the seasonal pattern of GPP of the SRC plantation (Figure 7)
Summary
Short-rotation coppice (SRC) plantations provide multiple economic and ecological benefits [1]. They are ideal for producing biomass for energy [2] and can, substitute for fossil fuels to reduce anthropogenic CO2 emissions [3]. They enrich farm-scale biodiversity [4], increase soil organic carbon stock [5,6], improve groundwater quality [7] and help in preventing pests and diseases. In the last 20 years, SRC plantations increased globally to cover an area of 31.8 million ha in 2016 [9]. Poplars (Populus spp.) are widely used as bioenergy plantations and for wood production in the Northern Hemisphere representing 99% (31.4 million ha) of SRC area worldwide
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