Comparing Phenology of a Temperate Deciduous Forest Captured by Solar-Induced Fluorescence and Vegetation Indices  
[PERSON]  
1 Us Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Ave., SW, Washington, DC 20375, USA 11 US Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Ave.  
Satellite measurements of GPP are advancing carbon budget calculations, identifying feedback under climate change, and have potential as a tool to examine vegetation health, including seasonal timing and productivity, in regions where the data are lacking. In temperate regions, deciduous broadleaf forests (DBFs) have a distinct growing season, with leaf phenology and GPP variability related to the timing and magnitude of biosphere-atmosphere exchanges of atmospheric CO\({}_{2}\), with temperature and humidity as the key controls [1; 2; 3; 4; 5; 6; 7]. Therefore, a rapidly changing climate is of particular concern to DBF health, the resilience of DBF forests, and, more generally, the carbon cycle. In the eastern United States, and particularly in the region of eastern Kentucky, protected, healthy DBFs have the potential to fill socioeconomic roles that were previously filled by coal mining. As of 2017, Kentucky's forestry industry provides approximately four times more jobs than the coal industry, and estimates of forest products and recreation contribute approximately USD 26 billion to the state's economy ([[https://forestry.ca.uky.edu/economic-report](https://forestry.ca.uky.edu/economic-report)]([https://forestry.ca.uky.edu/economic-report](https://forestry.ca.uky.edu/economic-report)), accessed 20 August 2023). However, in order to reap the benefits of this resource, we need to overcome the dearth of forest health data. In particular, the metrics of forest structure, function, and seasonal shifts that can facilitate regional monitoring of health are needed. An increasing number of satellite-based remote sensing measurements promise to fill the current gaps in our understanding of DBF function and structure because these measurements are found to track co-incident changes in GPP over time, such as traditional vegetation indices and, more recently, solar-induced chlorophyll fluorescence. Yet, no comparison among satellite-based proxies of DBF GPP has been carried out on a regional scale. An investigation of how these proxies reflect regional temperate DBF phenology will aid in the monitoring of forest health and decision-making for the essential data needed for future ground monitoring stations. Reflectance-based vegetation indices (VIs) from remote sensing platforms, such as the normalized difference vegetation index (NDVI), are widely used to examine, model, and monitor GPP, large-scale vegetation health, and phenology, primarily. These VIs capture changes in canopy structure, such as biomass, leaf shape and orientation, and canopy architecture, and leaf chemistry, such as chlorophyll absorption, which coincide with changes in GPP [8]. The enhanced vegetation index (EVI), which exploits blue wavebands, has shown improved performance over NDVI in dense canopies as it is less prone to saturation and less sensitive to atmospheric contamination [9]. However, because these VIs rely on structural and chemical changes and suffer from contamination by the atmosphere and canopy, they may miss physiological responses, i.e., photosynthetic rate changes in plants that are better indicators of GPP [10; 11; 12; 13; 14; 15; 16; 17; 18]. Solar-induced fluorescence from space-based observations provides high spectral and spatial resolution measurements and promises to better constrain uncertainties in measuring GPP than VIs. For example, strong empirical links have been made between solar-induced fluorescence (SIF) and GPP in diverse ecosystems and at multiple spatial and temporal scales (e.g., [13; 17; 19; 20; 21; 22]). [PERSON] et al. [23] showed strong agreement in yearly and monthly satellite SIF and GPP derived from eddy co-variance towers at the biome level for temperate forests in North America.