Detecting drought stress from the leaf to the landscape: is canopy structure or leaf desiccation more influential in tree canopy spectral responses to declining water?

Remotely sensed measurements of equivalent water thickness (EWT) derived from visible-shortwave infrared imaging spectroscopy data have the potential to revolutionize how we monitor drought in natural ecosystems. However, the mechanistic underpinnings of EWT measurements are still relatively unknown. We collected coincident measurements of leaf spectra, leaf water content, and leaf water potential over the course of a tabletop leaf drydown for 13 tree species. We then used a vegetation radiative transfer model to scale these measurements from the leaf to canopy scale, simulating how a leaf drying event would appear from an airborne or satellite instrument. We found that EWT responses to changes in leaf water content are strongly species-specific at both the leaf and canopy level. Furthermore, changes in EWT associated with declining leaf water are relatively small compared to those associated with potential canopy structural changes, such as leaf shedding or shifting leaf angle distribution, and with retrieval errors of current instruments. Our results suggest that single point-based EWT measurements are unlikely to reliably detect drought stress across species due to several confounding factors, but that time series data (newly available through operational and upcoming spaceborne imaging spectrometers) may enable approaches that circumvent many of these challenges.