Assessing Genetically-Controlled Chemical Defense in Trembling Aspen in Response to Insect Herbivore Impact with Imaging Spectroscopy and LiDAR Data

Intraspecific genetic variation in trembling aspen (Populus tremuloides) determines functional trait composition among individuals, especially in defensive traits (e.g., phenolic glycosides-PG, condensed tannins-CT), and its chemical responses to insect herbivory. Field measurements limit our ability to characterize genetic-trait relationships across space, which can be overcome using imaging spectroscopy. To understand the potential genetic underpinnings of differential chemical responses among aspen genotypes, we investigate the impact of spongy moth (Lymantria dispar) on variation in foliar defensive and functional traits (nitrogen, non-structural carbohydrate-NSC, leaf mass per area-LMA, leaf area density-LAD) across 519 genotypes in Wisconsin, USA. We integrate time-series airborne hyperspectral imagery and LiDAR data with foliar chemical measurements, genetic data from genome-wide association (GWA) analyses, and BioSIM biological modeling to characterize the chemical response. The results show that while LiDAR can accurately characterize canopy defoliation, imaging spectroscopy and leaf-level reflectance offer great potential to quantify defensive and physiological traits. While CT increased during defoliation and decreased when leaves reflushed, PG only increased during reflushing, indicating a strong defense ability in this species. The degree, onset, and duration of the defensive chemical variation were distinct among genotypes, regardless of their pre-defoliation defensive chemical concentrations. As well, the diverse genetic variation in foliar chemistry corresponded to substantial spectral diversity. The increase in defensive traits during and following defoliation accompanied a decrease in foliar nitrogen, showing that resources were reallocated to defense as a consequence of herbivory. The results provide insights into genetic variation in plant chemical response for predicting the performance and spread of insects in host species. Once global spaceborne imaging spectrometers are launched later this decade, they will provide unprecedented capacity not only to understand the impacts and response of disturbances such as herbivory on vegetation, but also to better understand how multiple factors influence those responses across a species' range.