Tools and Links

• The Hemlock Woolly Adelgid (HWA) Website
• Map of Current HWA Distribution

Management

• Silvicultural options for hemlock given HWA - Harvard Forest
• Guidelines to Minimize the Impacts of Hemlock Woolly Adelgid

Forest Type Descriptions
Hemlocks are part of a number of forest types which include:

• World Wildlife Fund
• Appalachian-Blue Ridge forests
• Appalachian mixed mesophytic forests
• NatureServe
• Hemlock - Northern Hardwood Forest
• Hemlock - Beech- Oak Forest
• Hemlock - Spruce - Hardwood Forest
• Hemlock - Hardwood Swamp
• US Forest Service
• Eastern Broadleaf Forest
• Adirondack-New England Mixed Forest-Coniferous Forest
• Central Appalachian Broadleaf Forest--Coniferous Forest

General Resources

• Article on spotting HWA in Maine

Discussion ^

A recent exotic invader, the hemlock woolly adelgid (HWA), Adelges tsugae, threatens to drive eastern hemlocks (Tsuga canadensis) to extinction (Orwig et al. 2002). HWA induced hemlock mortality affects wildlife (Ross et al. 2002, Tingley et al. 2002), nutrient cycling (Jenkins et al. 1999, Yorks et al. 2003, Stadler et al. 2005, Cobb et al. 2006), and allows other alien species to invade (Orwig and Foster 1998, Small et al. 2005). HWA challenges managers to preserve forest values in the face of the potential loss of eastern hemlock from the ecosystem.
There are some hopeful signs for eastern hemlocks, particularly in the northern portion of its range. HWA is spreading much more slowly that previously estimated. It is moving about 5 miles a year (8 km/yr) in the northern portion of hemlock's range and (20.8 km/yr) 13 miles a year in warmer regions (Evans and Gregoire 2006). Infested hemlock are surviving longer than the 4 to 14 years previously estimated (Orwig and Kittredge 2005). At least three different predator beetles and a fungus have been released to control HWA (Reardon and Onken 2004). Researchers are also studying the possibility of breeding resistant eastern hemlocks (Bentz et al. 2002) or planting resistant species (Evans In press).

Silviculture ^
Contrary to the precautionary approach of ecological forestry, fears of complete hemlock mortality have incited pre-emptive salvage logging in New England (Kizlinski et al. 2002). Hemlock stands with low HWA densities and non-timber values such as aesthetics and wildlife habitat should be preserved (Ward et al. 2004). Maintaining hemlock stands will tend to increase regional diversity because of hemlock mortality and pre-emptive salvage logging (Kizlinski et al. 2002). Hemlock reserves protect the unique character of stands with deep shade and little understory vegetation. Where patches of hemlock advanced regeneration exist, harvests can encourage these seedlings through group selection to create small gaps (Marshall 1927). Hemlock seedlings are able to reach the canopy if not out-competed by faster growing hardwoods. If the gap is closed by lateral expansion of canopy dominants, hemlock will increase growth for the life of the gap and then await another release event (Oliver and Stephens 1977). The creation of gaps less than half the dominant tree height in diameter will permit hemlock release but may not be sufficient for them to reach the canopy (Hibbs 1982). Gaps of a tree height in diameter should allow hemlock advanced regeneration to reach the canopy while maintaining enough shade in the gap to exclude most hardwood competition.
In stands where HWA densities are high, crown thinning is a way of staggering the departure of hemlock from the stand and reallocating growing space to other species. Group selection can augment regeneration of other species in a hemlock crown thinning by providing more light in gaps. Gaps from patch selection cuts release sufficient growing space for a new regeneration cohort to initiate in areas where HWA populations are higher and a mix of species is present.

References ^

Bentz, S. E., L. Riedel, M. Pooler, and A. M. Townsend. 2002. Hybridization and Self-Compatibility in Controlled Pollinations of Eastern North American and Asian Hemlock (Tsuga) Species. Journal of Arboriculture 28(4):200-205.

Cobb, R. C., D. A. Orwig, and S. Currie. 2006. Decomposition of Green Foliage in Eastern Hemlock Forests of Southern New England Impacted by Hemlock Woolly Adelgid Infestations. Canadian Journal of Forest Research 36(5):1331-1341.

Evans, A. M. In press. Growth and Infestation by Hemlock Wooly Adelgid of Two Exotic Hemlock Species in a New England Forest. Journal of Sustainable Forestry 26(3).

Evans, A. M., and T. G. Gregoire. 2006. A Geographically Variable Model of Hwa Spread. Biological Invasions 8(Online First):1-14.
DOI: 10.1007/s10530-006-9039-z

Hibbs, D. E. 1982. Gap Dynamics in a Hemlock-Hardwood Forest. Canadian Journal of Forest Research 12:522-527.

Jenkins, J. C., J. D. Aber, and C. D. Canham. 1999. Hemlock Woolly Adelgid Impacts on Community Structure and N Cycling Rates in Eastern Hemlock Forests. Canadian Journal of Forest Research 29(5):630-645.

Kizlinski, M. L., D. A. Orwig, R. C. Cobb, and D. R. Foster. 2002. Direct and Indirect Ecosystem Consequences of an Invasive Pest on Forests Dominated by Eastern Hemlock. Journal of Biogeography 29(10-11):1489-1503.

Marshall, R. 1927. The Growth of Hemlock before and after Release from Suppression. Harvard Forest Bulletin 11:1-43.

Oliver, C. D., and E. P. Stephens. 1977. Reconstruction of a Mixed-Species Forest in Central New England. Ecology 58(3):562-572.

Orwig, D. A., and D. R. Foster. 1998. Forest Response to the Introduced Hemlock Woolly Adelgid in Southern New England, USA. Journal of the Torrey Botanical Society 125(1):60-73.

Orwig, D. A., D. R. Foster, and D. L. Mausel. 2002. Landscape Patterns of Hemlock Decline in New England Due to the Introduced Hemlock Woolly Adelgid. Journal of Biogeography 29(10-11):1475-1488.

Orwig, D. A., and D. Kittredge. 2005. Silvicultural Options for Managing Hemlock Forests Threatened by Hemlock Woolly Adelgid. Pages 212-217 in B. Onken and R. Reardon, editors. Proceedings of the 3rd Symposium on Hemlock Woolly Adelgid. USDA Forest Service, Ashville, NC, USA.

Reardon, R., and B. Onken. 2004. Biological Control of Hemlock Woolly Adelgid in the Eastern United States. FHTET-2004-04, USDA, Forest Service, Forest Health Technology Enterprise Team, Morgantown, WV.

Ross, R., L. Redell, and R. Bennett. 2002. Mesohabitat Use of Threatened Hemlock Forest by Breeding Birds of the Delaware Water Gap National Recreation Area. Pages 353-354 in B. Onken, R. Reardon, and J. Lashomb, editors. Hemlock Woolly Adelgid Symposium. USDA Forest Service and Rutgers University, East Brunswick, NJ.

Small, M. J., C. J. Small, and G. D. Dreyer. 2005. Changes in a Hemlock-Dominated Forest Following Woolly Adelgid Infestation in Southern New England. Journal of the Torrey Botanical Society 132(3):458-470.

Stadler, B., T. Müller, D. Orwig, and R. Cobb. 2005. Hemlock Woolly Adelgid in New England Forests: Canopy Impacts Transforming Ecosystem Processes and Landscapes. Ecosystems 8(3):233-247.

Tingley, M. W., D. A. Orwig, R. Field, and G. Motzkin. 2002. Avian Response to Removal of a Forest Dominant: Consequences of Hemlock Woolly Adelgid Infestations. Journal of Biogeography 29(10-11):1505-1516.

Yorks, T. E., D. J. Leopold, and D. J. Raynal. 2003. Effects of Tsuga Canadensis Mortality on Soil Water Chemistry and Understory Vegetation: Possible Consequences of an Invasive Insect Herbivore. Canadian Journal of Forest Research 33(8):1525-1537.