Modeling Hemlock (Tsuga canadensis) Health During an Infestation of Hemlock Wooly Adelgid (Adelges tsguae)

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Modeling Hemlock (Tsuga canadensis) Health During
an Infestation of Hemlock Wooly Adelgid (Adelges
tsguae)

• Christy Rollinson 08
• Rufus Nicoll 09

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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

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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)

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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

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Below Untreated (Unhealthy) Hemlock
Above Treated (Healthy) Hemlock
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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

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Key Stocks

• Hemlock Biomass (in m2)
• cohort model
• Hemlock Woolly Adelgid Population

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Key Flows Relationships

• New Hemlock Biomass Logistic Growth
• HWA Reproduction 1st order growth
• Reproduction rate affected by age of growth

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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

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More Relationships

• Converters to track live biomass, total dead
  biomass
• Vigor - overall tree health
• Affects rate of growth and death

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Left Uninfested Hemlock
Right Infested Hemlock
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Sensitivity Whats Changeable

• HWA Infestation
• Reproduction rate
• Immigration rate
• Impact Rate
• Hemlock
• Biomass distribution
• Biomass Growth Rate

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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

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Whats 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