Diurnal observations of forest spectral traits from UAS platforms

Abstract:

Humans rely on healthy forests for many important ecosystem services, such as carbon storage, biodiversity, climate regulation, and the supply of clean water, energy, building materials, and food. However, forests are subject to stress from changing environmental conditions and disturbances. Observations of diurnal and seasonal dynamics of canopy light use efficiency and productivity are needed to better understand how forests are responding to their dynamic conditions. Our goal is to combine diurnal spectral reflectance and 3D structure measurements acquired through UAS flights with eddy covariance flux tower measurements for the same area. Flux towers provide high temporal frequency estimates of ecosystem productivity and micrometeorology within the footprint of the tower. We use flexible UAS-mounted instruments to acquire high-resolution data in the morning, noon, and afternoon to assess the ability of these systems to capture light use efficiency and gross ecosystem productivity (GEP) as it varies throughout the day, validated by the flux tower measurements. We also examine methods to compute shadow fraction, which is required when observing forest canopies at different times during the day with remote sensing observations. Our work will contribute to the reduction of error caused by shadow fraction in optical imagery and relatively low-cost methods (compared to traditional aircraft) to extend the measurement of light use efficiency and GEP throughout the day in areas beyond the footprint of flux towers.