Abstract
While satellite data has proved to be a powerful tool in estimating C3 and C4 grass species Aboveground Biomass (AGB), finding an appropriate sensor that can accurately characterize the inherent variations remains a challenge. This limitation has hampered the remote sensing community from continuously and precisely monitoring their productivity. This study assessed the potential of a Sentinel 2 MultiSpectral Instrument, Landsat 8 Operational Land Imager, and WorldView-2 sensors, with improved earth imaging characteristics, in estimating C3 and C4 grasses AGB in the Cathedral Peak, South Africa. Overall, all sensors have shown considerable potential in estimating species AGB; with the use of different combinations of the derived spectral bands and vegetation indices producing better accuracies. However, WorldView-2 derived variables yielded better predictive accuracies (R2 ranging between 0.71 and 0.83; RMSEs between 6.92% and 9.84%), followed by Sentinel 2, with R2 between 0.60 and 0.79; and an RMSE 7.66% and 14.66%. Comparatively, Landsat 8 yielded weaker estimates, with R2 ranging between 0.52 and 0.71 and high RMSEs ranging between 9.07% and 19.88%. In addition, spectral bands located within the red edge (e.g., centered at 0.705 and 0.745 µm for Sentinel 2), SWIR, and NIR, as well as the derived indices, were found to be very important in predicting C3 and C4 AGB from the three sensors. The competence of these bands, especially of the free-available Landsat 8 and Sentinel 2 dataset, was also confirmed from the fusion of the datasets. Most importantly, the three sensors managed to capture and show the spatial variations in AGB for the target C3 and C4 grassland area. This work therefore provides a new horizon and a fundamental step towards C3 and C4 grass productivity monitoring for carbon accounting, forage mapping, and modelling the influence of environmental changes on their productivity.
Highlights
C3 and C4 grass species Aboveground Biomass (AGB) indicates the productivity of grasses with common phenological, physiological, and morphological characteristics [1,2]
The measured AGB of T. triandra varied from 0.6 kg/m2 to as high as 1.276 kg/m2, whereas for F. costata, it varied between 0.52 kg/m2 and 1.16 kg/m2
The present study has shown the feasibility of using the new generation sensors to estimate and determine the subtle spatial variations of C3 and C4 grasses AGB in the temperate regions of South Africa
Summary
C3 and C4 grass species Aboveground Biomass (AGB) indicates the productivity of grasses with common phenological, physiological, and morphological characteristics [1,2]. C3 and C4 grass species are, facing considerable threats from environmental changes and these are anticipated to vary significantly, according to species functional types [6,7] Most importantly, as they have different environmental tolerances and requirements, C3 and C4 AGB will respond differently to environmental changes, anthropogenic pressure, management practices, and invasion. There is a need to identify robust methods, which have the ability to spatially and temporarily characterize the AGB of these grasses, with better and reliable accuracies. This is critical to improve the monitoring of C3 and C4 grasses’ productivity, and the associated response to environmental and anthropogenic pressure
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