Assessing the genetic basis of remotely evaluated drought response in a beech (Fagus sylvatica) population

European common beech (Fagus sylvatica) is a dominant broadleaf tree forming European temperate forests. Currently, European tree populations face an increasing frequency of extreme drought events. Understanding how today’s forest trees respond to successive stress events is essential for predicting their future under current climate conditions.

We sampled leaves of 276 trees within a single stand in Switzerland along with their geolocations. Using whole-genome sequencing, we identified genetic intraspecific variation and using airborne imaging spectroscopy data collected over multiple years during peak greenness, we quantified the response of the studied temperate forest stand to documented drought events in Switzerland by measuring changes in a spectral index of canopy water content (NDWI). To derive the spectral index at the individual tree level, we combined airborne, geolocation, and drone lidar data to extract median values for each defined tree canopy. We then examined the link between intraspecific genetic variation within a single forest stand and remotely sensed intraspecific trait variation. To do this, a genome-wide association study was performed, integrating genetic variation from the 276 individuals with remotely sensed reflectance data of the tree canopy.

This study addressed various challenges, including defining tree crowns using airborne and drone lidar data and applying different models for genome-wide association studies that account for genetic structure, terrain, and topographical covariates. Our objectives were to 1) establish the overall genetic contribution to variations in remotely assessed drought responses and 2) identify genetic differences associated with specific drought responses within beech tree stands. We present our findings and discuss the challenges of estimating the genetic component of remotely sensed canopy trait variation using genome-wide association studies in natural beech populations. By linking remote sensing with forest genomics, this study aims to highlight challenges and potential outputs for long-term monitoring of the genetic basis of drought response across the temporal and spatial scales of forest species.