Modeling Hemlock (Tsuga canadensis) Health During an Infestation of Hemlock Wooly Adelgid (Adelges tsguae) Christy Rollinson ‘08 Rufus Nicoll ‘09
About Hemlocks Hemlock stands are special because of their very dense shade that deters understory growth –Previously harvested for tannins Loss of hemlock has greatly altered local ecology
About HWA Reproduces parthenogenetically –As many as 400 adelgids for every adult Feed on ray parenchyma cells Primarily spread via wind or animals (birds, deer, people)
Infestation Background First found in Richmond, VA in 1950s Most likely came from China Currently affects most of the native range for hemlocks Hemlock mortality can occur within 5 yrs Biocontrol has been unsuccessful; chemicals are not feasible on a large enough scale to stop spread
Above: Treated (Healthy) Hemlock Below: Untreated (Unhealthy) Hemlock
Modeling Approaches Started with basic model of Hemlock growth –Cohort model Created model of an HWA population –HWA growth based on distribution of available foliage Flows out of each cohort based on HWA population
Key Stocks Hemlock Biomass (in m^2) –cohort model Hemlock Woolly Adelgid Population
Key Flows & Relationships New Hemlock Biomass: Logistic Growth HWA Reproduction: 1st order growth –Reproduction rate affected by age of growth
HWA Effect on the Hemlock Constant rate of impact per adelgid –Each adelgid affects a set amount of canopy Constant ratio of location of adelgid population location
More Relationships Converters to track live biomass, total dead biomass Vigor - overall tree health –Affects rate of growth and death
Left: Uninfested Hemlock Right: Infested Hemlock
Sensitivity: What’s Changeable HWA Infestation –Reproduction rate –Immigration rate –Impact Rate Hemlock –Biomass distribution –Biomass Growth Rate
How This Model is Useful Allows for visualization and explanation of basic process of HWA infestation –Educational Tool Basic model that can be expanded as more information and relationships are found –Possibly useful in research
What’s Next Model can be expanded to portray a stand of hemlocks and include migration of adelgids among trees Factors influencing HWA mortality and/or reproduction –over winter temperatures Other ecosystem characteristics –Soil nitrogen and pH, light availability