Evaluating the capacity of PlanetScope satellites for fine-scale phenology monitoring across temperate forests in eastern North America

In temperate forests, leaf phenology – the study of the timing of periodic and recurring events in leaves – is a sensitive indicator of climate change and a main regulator of carbon and water cycling. Many studies have evidenced large intra-site leaf phenology variability across individual trees. However, monitoring individual tree-scale leaf phenology with conventional approaches (e.g., field observations and phenoCam observations) is often restricted to a small spatial extent and sample size. The availability of PlanetScope data with high spatial and temporal resolution offer opportunities to overcome this limitation, but comprehensive assessments of its capacity for individual tree-scale phenology monitoring is lacking. To fill this knowledge gap, we proposed a method that integrates 0.1 m resolution airborne imagery and ground phenology records of individual trees with time-series PlanetScope observations to monitor fine-scale phenology. We tested this method at six NEON forest sites in eastern North America. Our results show that PlanetScope-derived land surface phenology is able to 1) characterize significant individual tree-scale leaf phenology variability across different forest sites and years, with r ranging from 0.21 to 0.42 when comparing PlanetScope-derived phenological metrics with their ground correspondences at the individual tree scale. The relationship between PlanetScope-derived phenology and ground phenology observations is stronger at the species level (r=0.57-0.82) when more PlanetScope pixels are included; and 2) capture more variability in fall phenology but also with larger uncertainties (e.g., r=0.82 and RMSE=2.14; species level) compared with spring phenology (r=0.76 and RMSE=0.72). Additionally, when comparing with ground methods, PlanetScope satellites are also advantageous for providing spatially explicit information across large spatial coverages. These findings collectively demonstrate that PlanetScope data displays the capacity for fine-scale leaf phenology monitoring, and it also has the potential to provide rich fine-scale phenology information to advance the field of plant phenology research.