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Hemlock’s Future:

Risk Maps, Management Strategies, and Preventive Thinning

Jake Moore, PA DEP, Mary Ann Fajvan, USFS Northern Research Station, Morgantown, WV, Andrea Hille & Jeanne Hickey, Allegheny National Forest, Susan L. Stout, USDA Forest Service Research & Development, Northern Research Station, Irvine, PA, Delaware, OH, and Durham, NH

With thanks to Amy Hill USFS FHP and Dick Cassagrande, University of Rhode Island

short version of the story is grim
Short Version of the Story is Grim

Lots of biocontrol trials, no forest triumphs

but there are a few bright notes
But there are a few bright notes…
  • In some cases, mortality can take as long as 15 years to become severe
  • Rate of spread seems slower in Allegheny SAF region than farther south ~ 8 km/yr vs. ~16 south of PA.
  • Serious seed collection efforts have been undertaken
  • We keep learning more and more about HWA and at some point, knowledge may be power
hemlock s future
Hemlock’s Future
  • Review risk factors for HWA susceptibility and vulnerability
  • Discuss stand and forest strategies to manage impact
  • Review a few VERY recent brighter notes
hemlock s past
Hemlock’s Past

Photo: D Luthringer DCNR

hemlock s present
Hemlock’s Present
  • Very high ecological importance
    • Provides unique habitat for many taxa
      • Year round cover – deer winter habitat and hiding
      • Branches to near forest-floor level
      • Influences stream temperature and environment – trout streams
  • Shade tolerance and form unusual among common associates – essentially no native species provides all of hemlock’s functions
  • Aesthetic/recreation value also high
management strategies
Management Strategies
  • Very well-coordinated interdisciplinary effort involving research, monitoring, biocontrol, chemical control, breeding, and silvicultural manipulations
  • USFS Northeastern Area Forest Health Protection in Morgantown, WV plays key coordination, monitoring, and education role
  • In addition to Allegheny Region, important work in New England & North Carolina
examples monitoring chem controls
Examples: Monitoring & Chem Controls
  • USFS FHP/ landowners in on-site monitoring near the advancing front
  • PA/NJ cooperating Landsat interpretation of imagery for detection
  • Imidacloprid - individual tree treatments – 146 sites in PA, for example
  • Dinotefuran– same family as - in use in NC
examples biocontrol
Examples: Biocontrol
  • Pseudoscymnustsugae (PT) was first and still most widely used
    • PA: 176,387 beetles released at 50 sites in 23 counties - recovered 642 beetles
  • Laricobiusnigrinus (LN): Primary in NJ now
  • Variety of others in various stages of testing
  • Aerially applied fungus in NC

PA DCNR Website

how can managers plan prioritize treatments
How Can Managers Plan & Prioritize Treatments?
  • Locate Hemlock
  • Develop metrics for
    • Susceptibility Risk
    • Vulnerability Risk
  • Prioritize values threatened
  • Assign treatments based values, risk, and cost

Ed Shipp Mon NF 1937

other management efforts
Other Management Efforts
  • West Virginia
    • Interagency effort started by Forest Health Protection & Monongahela NF
    • Expert knowledge to locate hemlock
      • Can separate conifer from deciduous with imagery, but difficult to distinguish conifers
    • Experts ranked sites based on their value for Natural Heritage, T&E Species, Recreation/High Use, and Riparian/Water Quality
    • Treatments assigned based on cost/benefit and current hemlock health
  • PGC developed priority process; also NFs of NC
allegheny risk map
Allegheny Risk Map
  • Conifer layer did not separate hemlock
  • Forest has a variety of other species composition data
  • Use literature to quantify risk
  • Jacob Moore, Slippery Rock
  • Andrea Hille & Jeanne Hickey, ANF
  • Develop a risk map for HWA for the Allegheny NF

Use existing data to create a hemlock GIS layer with uncertainty

    • Conifer cover
    • Geo-referenced stand inventories
    • Other stand data
  • Superimpose layers of known risk factors

J.S. Peterson @ USDA-NRCS PLANTS Database


Is polygon in conifer layer?

For polygons not in conifer layer but with

EH stand data, polygon IS probably hemlock

Stand data: FT? EH BA? Remarks?

For stands with

Non-EH data, polygon IS Non-hemlock

For polygons in conifer layer with

EH stand data,

Polygon IS hemlock

Probable non-

Hemlock are really polygons with no


For polygons in conifer layer even without EH data, polygon is probably hemlock unless stand type or remarks identify another evergreen

additional risk factors
Additional Risk Factors
  • Susceptibilty – Factors that affect the probability that HWA will infest hemlock in given polygon
  • Vulnerability – Factors that affect the rate of hemlock mortality in that polygon
susceptibility factors extensive literature search
Susceptibility Factors – Extensive Literature Search

Proximity to:

  • Nearest infestation (5, 10, 15 year buffers)
  • Major roads (172 m)
  • Streams (35 m)
  • Recreation destinations (1350 m. wind and 2 km birds)

USFS Forest Health Protection

susceptibilty ii
Susceptibilty II
  • Proximity to
    • Tree nurseries (72 km)
    • Private land
  • Wind direction – not mapped
  • Timber harvest sites – not mapped
susceptibility score
Susceptibility Score
  • Overlay buffers for each susceptibility factor on classified map
  • Assign Susceptibility Risk Score based on number of risk buffers that overlay the polygon
vulnerability risk factors
Vulnerability Risk Factors
  • Soil moisture – better on sites with low mortality
    • Developed Integrated Moisture Index for Allegheny NF
    • chicken or egg?
  • Winter temperature – Cold winters limit HWA population growth - Not mapped
  • Nutrient availability – High N trees accelerate HWA population growth - Not mapped
next steps
Next steps
  • Focus monitoring on high-susceptibility sites
  • Assign values to hemlock and probable hemlock polygons
    • Ecological – terrestrial and aquatic habitat, T&E species
    • Recreational
    • Historical
  • Prioritize Treatments on risk, value, cost
within stand risk
Within-stand Risk
  • Rentch, Fajvan, Evans, Onken
  • Based on long-term monitoring at Delaware WG
  • Degree of infestation most important factor
  • Intermediate trees 2.1 x more likely to develop decline than overtopped
  • LCR, CD, DBH also influenced probability of tree decline
  • Healthy hemlocks are not less susceptible to HWA
  • They do have a better chance of surviving longer (less vulnerable)
  • Thinning in advance of infestation may reduce vulnerability
  • Fajvan and Allegheny NF have installed pre-HWA thinning trials to formally test this hypothesis
end on a high note
End on a high note
  • Dr. Dick Cassagrande (U Rhode Island) and colleagues are testing “putatively resistant” hemlocks
  • Cuttings are rooted in nursery, then challenged with HWA
resistant hemlock
Resistant Hemlock?

Photos from D. Cassagrande

2009 hwa inoculation results
2009 HWA inoculation results

Slide from R. Cassagrande

2010 hwa inoculation results
2010 HWA inoculation results



HWA crawlers/cm)





Slide from R. Cassagrande