Target-oriented habitat and wildlife management: estimating forage quantity and quality of semi-natural grasslands with Sentinel-1 and Sentinel-2 data

Semi-natural grasslands represent ecosystems with high biodiversity. Their conservation depends on the removal of biomass, for example, through grazing by livestock or wildlife. For this, spatially explicit information about grassland forage quantity and quality is a prerequisite for efficient management. The recent advancements of the Sentinel satellite mission offer new possibilities to support the conservation of semi-natural grasslands. In this study, the combined use of radar (Sentinel-1) and multispectral (Sentinel-2) data to predict forage quantity and quality indicators of semi-natural grassland in Germany was investigated. Field data for organic acid detergent fibre concentration (oADF), crude protein concentration (CP), compressed sward height (CSH) and standing biomass dry weight (DM) collected between 2015 and 2017 were related to remote sensing data using the random forest regression algorithm. In total, 102 optical- and radar-based predictor variables were used to derive an optimized dataset, maximizing the predictive power of the respective model. High R-2 values were obtained for the grassland quality indicators oADF (R-2 = 0.79, RMSE = 2.29%) and CP (R-2 = 0.72, RMSE = 1.70%) using 15 and 8 predictor variables respectively. Lower R-2 values were achieved for the quantity indicators CSH (R-2 = 0.60, RMSE = 2.77 cm) and DM (R-2 = 0.45, RMSE = 90.84 g/m(2)). A permutation-based variable importance measure indicated a strong contribution of simple ratio-based optical indices to the model performance. In particular, the ratios between the narrow near-infrared and red-edge region were among the most important variables. The model performance for oADF, CP and CSH was only marginally increased by adding Sentinel-1 data. For DM, no positive effect on the model performance was observed by combining Sentinel-1 and Sentinel-2 data. Thus, optical Sentinel-2 data might be sufficient to accurately predict forage quality, and to some extent also quantity indicators of semi-natural grassland.