Abstract
The fraction of absorbed photosynthetic active radiation (FAPAR) is an essential climate variable for assessing the productivity of ecosystems. Satellite remote sensing provides spatially distributed FAPAR products, but their accurate and efficient validation is challenging in forest environments. As the FAPAR is linked to the canopy structure, it may be approximated by the fractional vegetation cover (FCOVER) under the assumption that incoming radiation is either absorbed or passed through gaps in the canopy. With FCOVER being easier to retrieve, FAPAR validation activities could benefit from a priori information on FCOVER. Spatially distributed FCOVER is available from satellite remote sensing or can be retrieved from imagery of Unmanned Aerial Vehicles (UAVs) at a centimetric resolution. We investigated remote sensing-derived FCOVER as a proxy for in situ FAPAR in a dense mixed-coniferous forest, considering both absolute values and spatiotemporal variability. Therefore, direct FAPAR measurements, acquired with a Wireless Sensor Network, were related to FCOVER derived from UAV and Sentinel-2 (S2) imagery at different seasons. The results indicated that spatially aggregated UAV-derived FCOVER was close (RMSE = 0.02) to in situ FAPAR during the peak vegetation period when the canopy was almost closed. The S2 FCOVER product underestimated both the in situ FAPAR and UAV-derived FCOVER (RMSE > 0.3), which we attributed to the generic nature of the retrieval algorithm and the coarser resolution of the product. We concluded that UAV-derived FCOVER may be used as a proxy for direct FAPAR measurements in dense canopies. As another key finding, the spatial variability of the FCOVER consistently surpassed that of the in situ FAPAR, which was also well-reflected in the S2 FAPAR and FCOVER products. We recommend integrating this experimental finding as consistency criteria in the context of ECV quality assessments. To facilitate the FAPAR sampling activities, we further suggest assessing the spatial variability of UAV-derived FCOVER to benchmark sampling sizes for in situ FAPAR measurements. Finally, our study contributes to refining the FAPAR sampling protocols needed for the validation and improvement of FAPAR estimates in forest environments.
Highlights
Introduction distributed under the terms andThe fraction of absorbed photosynthetic active radiation (FAPAR) links PAR to the absorption of plants and characterizes the vegetation structure, as well as the energy and carbon cycle [1,2]
We assessed the potential of remote sensing-derived fractional vegetation cover (FCOVER) as a proxy for in situ FAPAR
We evaluated in situ FAPAR based on direct PAR measurements, UAVderived FCOVER and the S2 FAPAR and FCOVER products in a dense mixed-coniferous forest, ensuring almost identical illumination conditions at their acquisition times
Summary
Introduction distributed under the terms andThe fraction of absorbed photosynthetic active radiation (FAPAR) links PAR to the absorption of plants and characterizes the vegetation structure, as well as the energy and carbon cycle [1,2]. Satellite remote sensing provides spatially distributed information on vegetation spectral properties, and numerous retrieval algorithms have been developed to retrieve FAPAR from space [7,8,9,10,11,12]. There is an increasing availability of FAPAR datasets from hectometric (Terra & Aqua/MODIS [13], SPOT/VEGETATION [14], PROBA-V [15] and Sentinel-3/OLCI [16]) to decametric (Sentinel-2/MSI [17]) spatial resolutions. Most of these products are available at relatively coarse spatial resolutions, thereby suitable only for regional-to-global FAPAR studies. With the availability of a harmonized Sentinel-2 and Landsat-8 surface reflectance product [20], new approaches for determining vegetation parameters are expected [21]
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