Invasive insect effects on nitrogen cycling and host physiology are not tightly linked

Invasive insects may dramatically alter resource cycling and productivity in forest ecosystems. Yet, although responses of individual trees should both reflect and affect ecosystem-scale responses, relationships between physiological- and ecosystem-scale responses to invasive insects have not been extensively studied. To address this issue, we examined changes in soil nitrogen (N) cycling, N uptake and allocation, and needle biochemistry and physiology in eastern hemlock (Tsuga canadensis (L) Carr) saplings, associated with infestation by the hemlock woolly adelgid (HWA) (Adelges tsugae Annand), an invasive insect causing widespread decline of eastern hemlock in the eastern USA. Compared with uninfested saplings, infested saplings had soils that exhibited faster nitrification rates, and more needle (15)N uptake, N and total protein concentrations. However, these variables did not clearly covary. Further, within infested saplings, needle N concentration did not vary with HWA density. Light-saturated net photosynthetic rates (Asat) declined by 42% as HWA density increased from 0 to 3 adelgids per needle, but did not vary with needle N concentration. Rather, Asat varied with stomatal conductance, which was highest at the lowest HWA density and accounted for 79% of the variation in Asat. Photosynthetic light response did not differ among HWA densities. Our results suggest that the effects of HWA infestation on soil N pools and fluxes, (15)N uptake, needle N and protein concentrations, and needle physiology may not be tightly coupled under at least some conditions. This pattern may reflect direct effects of the HWA on N uptake by host trees, as well as effects of other scale-dependent factors, such as tree hydrology, affected by HWA activity.

Terpene chemistry of eastern hemlocks resistant to hemlock woolly adelgid

Recent studies have identified a small number of individual eastern hemlock trees that demonstrate relative resistance to the introduced sap-feeding insect, the hemlock woolly adelgid. Using gas chromatography, we compared concentrations of twenty-two terpenoids in susceptible and relatively resistant trees, both in the forest and in propagated cuttings in a common-garden setting. Terpenoid concentrations were higher in twig tissue of resistant versus susceptible trees, across six sampling dates and at both sites. These changes may be particularly important because the hemlock woolly adelgid feeds on twig tissue. Because the common-garden cuttings were free of herbivores, the higher terpenoid concentrations are interpreted as a constitutive defense. Increased levels of monoterpenes and sesquiterpenes imply an overall increase in the input of carbon precursors to both terpenoid synthesis pathways.

Hemlock woolly adelgid (Hemiptera: Adelgidae) induces twig volatiles of eastern hemlock in a forest setting

The hemlock woolly adelgid (Adelges tsugae Annand) is an invasive species causing high mortality of eastern hemlock (Tsuga canadensis (L) Carriere) in the forests of eastern North America. Recent findings revealed that sapling eastern hemlocks artificially infested with hemlock woolly adelgid in a plantation setting responded to the insect with an array of induced resin volatile changes. Here we determine if eastern hemlocks growing beneath a forest canopy respond to hemlock woolly adelgid infestation with the same patterns of constitutive and inducible volatile resin production as those plantation specimens. We inoculated previously uninfested branches of mature and immature hemlocks in a central New England forest with hemlock woolly adelgid. We then sampled twig tissue of infested and uninfested trees in late spring, early summer, and mid-autumn, after known intervals of adelgid activity when an induced response might be expected. We identified and quantified resin volatiles by gas chromatography-mass spectrometry. Constitutive levels of total monoterpenoids, sesquiterpenoids, and combined resin volatiles were all several-fold more abundant in forest trees than those previously measured in a plantation setting, pointing to further study of the influence of site factors on hemlock volatile production. Hemlock woolly adelgid infestation induced an array of changes in eastern hemlock's volatile profile, including many-fold increases in benzyl alcohol and methyl salicylate accumulation. Despite differences in constitutive concentrations of volatiles between the two sites, our findings verify that hemlock woolly adelgid elicits patterns of resin volatile induction in forest-grown eastern hemlocks quite similar to those previously observed in plantation grown trees.