Combined LiDAR and Hyperspectral Imagery for Landscape Forest Reproduction across the United States

Individual seed production (ISP) determines forest regeneration, and it is increasingly important following the large forest transformations with contemporary climate change. Seed production determines the reproductive potential of each species, but a comprehensive understanding requires methods that can be implemented remotely across broad areas. Foliar carbon (C) and nutrients, including nitrogen (N), phosphorous (P), potassium (K), magnesium (Mg), calcium (Ca), zinc (Zn), are important for tree photosynthesis and vegetative growth. Foliar nutrients could affect ISP through their relationships with allocation between reproduction and other physiological functions. Studies on ISP and its relationship with foliar nutrients are limited to a few locations and few species, due to the large investment needed for field measurements on both variables. The challenge of limited data is compounded by the low signal-to-noise ratio in ISP data, which thus requires massive sample sizes to estimates effects. Inference from ISP data has to account for attributes of the individual tree (e.g., species, size, competition with neighbors), its local habitat (e.g., soil), and regional climate (e.g., temperature and moisture). We synthesized ISP estimates from the Masting Inference and Forecasting (MASTIF) network with a 1-meter foliar nutrients map derived from hyperspectral remote sensing at the National Ecolology Observation Network (NEON). Individual tree crowns were delineated using LiDAR data at 93 NEON plots across the United States. The linkages between ISP from MASTIF and foliar nutrients, calibrated from field observations, were evaluated for 33,461 tree-years from 72 tree species. ISP responses to foliar nutrients are species-specific. Foliar P, K, and C promote ISP in species of several coniferous genera such as Picea, Pinus, Tsuga, and Pseudotsuga while reducing ISP in deciduous genus Acer. By contrast, increased seed production is related to high foliar Mg and Ca in Acer, but not in the conifers. Effects of foliar nutrients on ISP in Quercus, Betula, and Fraxinus are not significant. ISP is not clearly related to N and Zn. By establishing the link between canopy condition and ISP, this first study on combined LiDAR and hyperspectral imagery indicates promise for future monitoring of reproductive potential.