ASSESSING THE EFFECT OF GENOTYPE IN DETERMINING THE RELATIONSHIP BETWEEN NONSTRUCTURAL CARBOHYDRATE RESERVES AND THE INDUCED CHEMICAL DEFENSE RESPONSE OF POPULUS TREMULOIDES AGAINST THE INVASIVE HERBIVORE, LYMANTRIA DISPAR

Abstract

Trees can mobilize non-structural carbohydrate (NSC) reserves to mitigate suboptimal environmental conditions that occur at different scales in time and space, from daily fluctuations to extreme and longer-term environmental disturbances such as insect outbreaks. Following an extensive insect defoliation event, NSC are required to support vegetative regrowth in addition to other herbivore-induced physiological responses including the production of chemical defenses. While both NSC and chemical defenses are suggested to be largely under genetic control, the role of genetics versus environment in mediating relationships between these variables have rarely been considered in forest disturbance ecology and are notably absent from dynamic global vegetation models. In this thesis, I characterize the dynamics of organ-level NSC, leaf chemical defense production, and their potential relationship in two Populus tremuloides (aspen) chemotypes during and after an invasive insect outbreak of Lymantria dispar (spongy moth). I leveraged historical chemical defense data from a long-term common garden experiment to select chemotypes at either extreme of the defense distribution: 1) high-defense trees that contain high levels of total phenolic glycosides (tPg) (with known resistance against lepidopteran herbivores) and low total condensed tannin concentrations (tCt), and 2) low-defense trees with low concentrations of tPg but high levels of tCt. I found no evidence of selective herbivory between the chemotypes, and that both chemotypes reduced tCt production and increased tPg production during post-defoliation canopy reflush. Notably, however, low-defense trees during early- and late-recovery exhibited relative increases in tPg that were two to three times greater than high-defense trees that constitutively produce phenolic glycosides, which made for nearly identical concentrations of tPg between low-defense leaves and high-defense leaves during recovery. Prior to defoliation, high-defense trees had four times as much starch in their leaves relative to low-defense trees, which in turn had twice as much starch in their twigs compared to their high-defense counterparts. Initial differences in NSC, however, did not predict the induced defense response in new leaves. Rather, tPg across chemotypes and time exhibited threshold-like behavior, only increasing to values greater than 5% dry weight when sucrose levels were extremely low (i.e., during recovery) while sucrose was positively correlated

with [tCt] and explained ~45% of the variation. Together, these results suggest that while constitutive NSC reserve status and chemical defense are under genetic control, severe insect defoliation induces key shifts in the phenylpropanoid pathway that significantly alters the chemical phenotype during recovery from herbivory. Under biotic stress, it appears that this shift in secondary metabolism may be regulated in part by low sucrose concentrations, which potentially serve as both a signal and/or substrate to support tPg production at the cost of tCt production.