Hemlock woolly adelgid and sudden oak death: Contrasting drivers of ecosystem change

Background/Question/Methods

Adelges tsugae (Hemlock woolly adelgid) and Phytophthora ramorum (cause of sudden oak death) are examples of exotic organisms causing regional-scale decline of dominant or foundation tree species. Epidemiological understanding can greatly improve predictions of the degree and nature of ecosystem change following outbreak of these organisms or other insects and pathogens. Host range, transmission sources, and apparent competition (increased dominance of species not directly impacted by outbreak organisms) are central to predicting patterns of outbreak and the magnitude of ecosystem change. I use the A. tsugae outbreak in Connecticut eastern hemlock (Tsuga canadensis) forests and P. ramorum outbreak in California tanoak (Notholithocarpus densiflorus) forests to illustrate that epidemiological and ecological drivers determine the magnitude and direction of ecosystem change following insect and disease outbreak. A. tsugae is a host specific insect pest while P. ramorum is a generalist pathogen with a broad host range. In both outbreaks, spread and ecosystem impacts are greatly influenced by the prevalence of host trees which support reproduction or sporulation.

Results/Conclusions

A. tsugae herbivory causes modest changes to foliar chemistry but host mortality results in significant increases in soil N availability, nitrification, and decomposition rates. P. ramorum causes modest changes in litterfall chemistry and soil N availability but no changes in cycling rates. For both organisms, the magnitude of these changes are driven by the amount of host biomass, the prevalence of hosts which support spread, and characteristics of post-outbreak communities. P. ramorum occurs in mixed species forests where other overstory species range from immune to supporting high levels of sporulation without suffering mortality; consequently, apparent competition among trees and positive feedbacks on pathogen populations emerge in many P. ramorum impacted forests. For P. ramorum, patterns of host mortality, pathogen spread, and accumulation of large woody debris are greatly determined by the prevalence of tanoak and California bay laurel (Umbellularia californica), a highly competent source of sporulation that does not suffer mortality. In contrast, host-specific A. tsugae is likely to experience negative feedback because of host specificity and eventual decline of eastern hemlock populations. In both examples, tree species that can capitalize on newly available resources have the largest and longest-lasting influences on ecosystem processes.