Abstract
Plantations of fast-growing forest species such as black locust (Robinia Pseudoacacia) can contribute to energy transformation, mitigate industrial pollution, and restore degraded, marginal land. In this study, the synergistic use of Sentinel-2 and Sentinel-1 time series data is explored for modeling aboveground biomass (AGB) in black locust short-rotation plantations in northeastern Greece. Optimal modeling dates and EO sensor data are also identified through the analysis. Random forest (RF) models were originally developed using monthly Sentinel-2 spectral indices, while, progressively, monthly Sentinel-1 bands were incorporated in the statistical analysis. The highest accuracy was observed for the models generated using Sentinel-2 August composites (R2 = 0.52). The inclusion of Sentinel-1 bands in the spectral indices’ models had a negligible effect on modeling accuracy during the leaf-on period. The correlation and comparative performance of the spectral indices in terms of pairwise correlation with AGB varied among the phenophases of the forest plantations. Overall, the field-measured AGB in the forest plantations plots presented a higher correlation with the optical Sentinel-2 images. The synergy of Sentinel-1 and Sentinel-2 data proved to be a non-efficient approach for improving forest biomass RF models throughout the year within the geographical and environmental context of our study.
Highlights
Europe’s Green Deal strategy sets out the pathway for transforming the European Union (EU) towards climate neutrality by 2050
The symmetrical seasonal change demonstrated from the Normalized Difference Vegetation Index (NDVI) values records the opening of the buds and mid-spring leaf development, with peak foliage development occurring in late May–early June
The leaf yellowing in summer is related to drought stress and high temperatures and is noted in the NDVI decrease over this period. These findings, related to black locust phenology, are confirmed by NDWI seasonal variation (Figure 4) since positive values have been linked to healthy vegetation [33]
Summary
Europe’s Green Deal strategy sets out the pathway for transforming the European Union (EU) towards climate neutrality by 2050. Long-term ecological, economic, and social sustainability of biomass production, optimization of cost-efficient management, minimization of susceptibility and exposure of short-rotation forest plantations to hazards, and regulation of biomass supply in the biomass market [4] require accurate spatial and temporal information related to the extent, status, and silvicultural parameters of plantations Such information can nowadays be provided by remote sensing sensors [8]. The synergetic use of passive and active data for AGB estimation in short-rotation plantations has not been previously explored in such an environmental (i.e., afforested agricultural land) research framework (i.e., optimal modeling dates). Forests 2021, 12, 902 the synergetic use of passive and active data for AGB estimation in short-rotation plantations has not been previously explored in such an environmental (i.e., afforested agr3icouf 1l-6 tural land) research framework (i.e., optimal modeling dates).
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