Assessing the effectiveness of a central flux tower in representing the spatial variations in gross primary productivity in a semi-arid pine forest

A key challenge in ecosystem studies has been to quantitatively evaluate the extent to which a permanent eddy covariance (EC) flux tower reliably represents a forest that is much larger than its footprint, with potentially significant spatial variations in activities. Here, we addressed this challenge using normalized difference vegetation index (NDVI) and gross primary productivity (GPP), derived from remote sensing measurements of an unmanned aerial vehicle (UAV; 5cm resolution), satellites (Landsat 8 and VENμS; 30 m and 5 m resolution, respectively), and data from the permanent (20 years) EC tower at the center of the 3000 ha semi-arid Yatir forest in Israel. Tree-level NDVI (after partitioning from the soil signal), plot-level GPP, tree size, stand density, and topographic parameters were obtained in 14 plots (0.36 ha each) across the forest over two years. These results were compared with the GPP estimates of the EC flux tower. The results showed significant spatial variations across the forest in plot-level RS-derived GPP but not in the tree-level NDVI. Canopy cover and topographical aspect were the dominant factors influencing the spatial variations in GPP, which increases with canopy cover on north-facing slopes. The plot-level GPP over the tower plot was consistent with the mean and range of values across the forest and agreed with the EC values for the same measurement time. Our results provide confidence in the long-term EC flux measurements at the study site and demonstrate a strategy to optimize the location of flux towers, which have critical contributions to the global efforts to assess biosphere-atmosphere interactions.