LB 33-277 - Does the reserve status of a tree affect induced chemical defenses against an invasive herbivore?

Quaking aspen (Populus tremuloides) populations are experiencing widespread mortality across temperate forests of North America. One contributing factor to this population decline is the rise of invasive insect outbreaks, including the spongy moth (Lymantria dispar). Aspen mortality associated with extensive and repeated defoliation by L. dispar is hypothesized to occur in part due to a reduction in tree non-structural carbohydrate pools (NSC; starch and soluble sugars), which are critical in supporting key physiological functions. Thus, the relative investment of stress-limited carbon stores towards growth, maintenance, and defense will have important consequences for survival. Phenolic glycosides (PG) are highly effective chemical deterrents against L. dispar herbivory. Some genotypes of aspen are known to constitutively produce high levels of PG, which may increase survival during an outbreak, but could also have longer-term costs if the NSC reserves are initially low and then become depleted. Due to the high carbon costs associated with PG synthesis, we thus hypothesize that larger constitutive NSC reserve pools (in the form of starch) can bolster induced PG defense under conditions of extreme herbivore stress while also buffering trees from future stresses.To test this hypothesis, we leveraged a long-term quaking aspen common garden experiment located in Arlington, WI. Based on over ten years of defensive chemistry data, we selected ten genotypes from two chemotypic extremes: trees exhibiting either high (+PG) or low/negligible (-PG) levels of constitutive PG defense. An outbreak of L.dispar in 2021 caused extensive defoliation across the garden, allowing us to quantify relationships between NSC pools and PG concentrations throughout the defoliation (May-September). To assess whole-tree NSC pools, leaf, twig, main stem phloem, and root tissues samples were collected monthly and analyzed for starch and soluble sugars. Leaf samples were also analyzed for PG. Results show that prior to defoliation, +PG trees stored ~3 times more starch in their leaves while -PG trees stored ~2 times more starch in the twigs. During defoliation, +PG trees induced defenses immediately and continued to synthesize them in the reflushed foliage. Conversely, -PG trees only induced during reflush in July. Both chemotypes exhibited ~70% decrease in leaf starch content from peak herbivory to leaf regrowth, associated with a significant increase in PG investment. These results demonstrate strong chemotypic differences in the timing and magnitude of PG induction, which is likely driven by tissue-specific starch storage early in the season and temporal dynamics of mobilization required for secondary metabolism.