Fireweed (Chamerion angustifolium) is attacked by a wide range of herbivores, from leaf-feeding beetles to stem gallers to ungulate browsers. Understanding the effects of herbivores on seed production and clonal spread of this plant requires investigating the interactions among herbivores as well as direct effects of each herbivore on plant success. We used observational and experimental methods to test whether the effects of each herbivore were independent of other herbivores, in fireweed populations around the Rocky Mountain Biological Lab in Colorado. For example, deer browsing of the apical meristem is typically followed by a flush of lateral growth, so we tested whether leaves on those lateral branches were more or less susceptible to insect damage. On the other hand, leaf damage and stem galling slow the rate of inflorescence development, so we tested whether deer preferentially browsed larger or smaller inflorescences.
Results/Conclusions

Deer browsing had the largest direct effect on fireweed growth and inflorescence development, but appeared to have little effect on subsequent herbivory. New lateral leaves following simulated browsing had lower levels of putatively defensive raphides compared to similarly-aged apical leaves from unbrowsed plants, suggesting that compensatory growth may be less well defended. However, specialist beetles (Adoxus obscurus, Chrysomelidae) showed no preference in choice trials between lateral and apical leaves. Those beetles and other leaf-feeders can slow the growth of stems and inflorescences, which may yield some protection from browsing, as stems browsed by deer were taller and had larger inflorescences than unbrowsed stems. In only a few places beetle damage was severe enough to suppress inflorescence development; more moderate amounts of damage may yield a net benefit to the plant if they reduce the probability of browsing without strongly impacting inflorescence development. Stem gallers consistently reduced both stem growth and reproductive output. Insect herbivory was consistent over time, so fireweed patches with heavy insect damage one year were likely to be heavily damaged in subsequent years, whereas browsing was uncorrelated over years. Thus interactions between insect and ungulate herbivores change over time and space in these subalpine fireweed populations.