Search
Browse by Subject
Contact Information

Northern Research Station
One Gifford Pinchot Drive
Madison, WI 53726
(608) 231-9318
(608) 231-9544 TTY/TDD

Hemlock Woolly Adelgid

Silvicultural Management

Research Issue

[photo:] Hemlock tree after thinning: see stumps around it.Thinnings are designed to reduce stand densities in fully stocked and overstocked hemlock-hardwood stands and hemlock-hardwood-white pine (Pinus strobus L.) stands.  When a stand is fully stocked, it contains the maximum sustainable amount of foliage given fixed site resources.  Fixed site resources are those resources that cannot be manipulated silviculturally.  Thinnings reallocate fixed resources among fewer stems, increasing the amount of light, water, and nutrients per tree.  Foliar quantity is affected because of modifications to the branch size – foliage weight relationship and the number and (or) size of branches within the crown.  Healthier hemlocks can tolerate high densities of hemlock woolly adelgid (HWA) better than can trees of low vigor and hence may survive or persist longer during an infestation.  However, because thinning affects foliar nutrient content, measuring the changes in foliar nutrients in thinned and unthinned stands before and during HWA infestation may indicate whether silvicultural manipulations make hemlocks more or less attractive to HWA attack.

Our Research

  • Determine whether thinning mixed hemlock-hardwood stands improves the health and growth of hemlock trees before the arrival of HWA.  Healthier hemlock trees may have higher survival rates during an HWA infestation.
  • Monitor ecological changes in forest structure as an HWA infestation progresses in thinned and unthinned stands.

Expected Outcomes

Silvicultural thinning guidelines that aim to reduce stand densities, reallocate resources, improve hemlock health, and increase growth rates across a range of stand types and structures.  The goal is to thin stands prior to HWA invasion, as part of an integrated approach to reduce hemlock’s vulnerability to HWA especially when combined with biological and chemical controls. 

Research Results

Each study area is a minimum of 60 acres, and half of each stand was not harvested and serves as controls. There are currently 9 study areas in three states (Pennsylvania, Massachusetts, New Hampshire), with the earliest harvests occurring in 2006 and the most recent in 2010.  All sites are geographically similar in that they typically experience periods of extreme winter temperatures that can be lethal to HWA.  The combined effects of climate and silvicultural treatment may serve to periodically decrease adelgid populations and increase hemlock survival. Long-term monitoring of tree health and eventual HWA infestation, are essential to document whether thinning practices are effective in reducing stand vulnerability.

Pre-harvest and post-harvest (every 5 years) sampling includes measurements of stand density and structure; residual tree stems and crowns; understory vegetation.  Many State, Federal, and university cooperators are involved in vegetation data collection and annual HWA monitoring.  To date, the only study area with HWA present is located on the Quabbin Reservation, near Belchertown, MA.

Five-years after harvest, findings from three stands on the Allegheny National Forest indicated that residual trees in thinned treatment areas grew an average of 1-inch more in stem diameter compared to trees in un-thinned areas. In addition, foliar chemistry in these stands was not influenced by thinning over the same period. 

Publications

Fajvan, M. A. 2008. The role of silvicultural thinning in eastern forests threatened by hemlock woolly adelgid (Adelges tsugae). In: Deal, R.L., tech. ed. Integrated restoration of forested ecosystems to achieve multiresource benefits: proceedings of the 2007 national silviculture workshop; 2007 May 7-10; Ketchikan, AK. Gen. Tech. Rep. PNW-733. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: 247-256.

Research Participants

Principal Investigator

  • Mary Ann Fajvan, Research Forester, U.S. Forest Service, Northern Research Station

Research Partners

  • Richard Turcotte, Supervisory Entomologist, USDA Forest Service, State and Private Forestry
  • Andrea Hille, Forest Silviculturist, USDA Forest Service, Allegheny National Forestry

 

Last Modified: 05/05/2015

About this Research Area

Science theme: Forest Disturbance Processes

Science Topic: Invasive Species


About Hemlock Woolly Adelgid
Featured Publication

Fajvan, M. A. 2008. The role of silvicultural thinning in eastern forests threatened by hemlock woolly adelgid (Adelges tsugae). In: Deal, R.L., tech. ed. Integrated restoration of forested ecosystems to achieve multiresource benefits: proceedings of the 2007 national silviculture workshop; 2007 May 7-10; Ketchikan, AK. Gen. Tech. Rep. PNW-733. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: 247-256.