Forestry For the Birds: Variable Density Thinning

///Forestry For the Birds: Variable Density Thinning
Forestry For the Birds: Variable Density Thinning 2017-03-30T12:55:00+00:00

Black throated blue warbler (Dendroica caerulescens)

Management Goals

  • Improve forest productivity and future timber quality, with diameter goals of 16-22” for most species
  • Diversify forest structure (canopy layers, spatial arrangement of trees and features such as standing dead trees) to enhance habitat for priority songbird species
  • Promote the ecological processes and species associated with the Northern Hardwood forest


This forest is adjacent to one of the earliest homesteads in the area, but was probably never cleared for farm fields. It was likely a woodlot used by the farmers, providing firewood and possibly maple sap.

Small amounts of timber harvesting continued through the early 1900s, and around 1960 the stand was logged heavily. Since then, it has remained mostly undisturbed, despite heavy cutting of the surrounding forest in the 1980s.

Stand Description

With good growing conditions and relatively little disturbance, this stand represents one of the more mature hardwood stands in the Spitzer Forest. Soils are deep and rich (less acidic, with better nutrient retention) compared to other parts of our forest; this allows species such as sugar maple and white ash to thrive, and as well as less common trees such as basswood and butternut. But the site is also somewhat wet, requiring careful skid trail layout. Located near the Tripp Hill summit, the terrain in this area is gradually sloping with a mostly northern aspect.

FFTB Variable Density Thinning (Stand 18)

What is Variable Density Thinning?

A conventional, even-aged thinning leaves a consistent spacing of trees across the stand, and most of those trees will be the same age. Variable Density Thinning modifies that strategy by leaving undisturbed pockets of forest, removing clusters of trees where appropriate, and cultivating healthy trees of multiple ages. The resulting forest is more structurally diverse and closer to the condition of a natural forest ecosystem. Undisturbed areas provide dense cover and protect sensitive soils, small canopy openings create a flush of new understory growth, and high-quality trees are given space to grow in size and value. With nearly immediate habitat enhancement and benefits to long-term productivity, this technique was a great match for our management goals.


Show Me the Money!

Funding from the NRCS made this project possible, and even profitable. Without NRCS funding, we would have incurred a cost of approximately $45 per acre (a price tag that should pay for itself in future timber values). 

NorthWoods Staffing: -$3,436 

Logging Income: $2,247

NRCS Funding: $7,163 

Net Income: $5,974 

*In the time since this project was completed, NRCS programs have transitioned toward funding only pre-commercial and non- commercial forestry. Commercial timber harvests such as this demonstration site may no longer meet meet NRCS cost-share criteria. Please contact your local NRCS office to find out whether your project is eligible for funding.

Stand Data

Area Treated: 26 acres   Date Treated: Winter 2012-13

Pre-Harvest Post-Harvest
Basal Area: 122 sq ft/ac 85 sq ft/ac
Acceptable Growing Stock (AGS) 47% 55%
Mean Diameter 9.9″ 11.9″
Snags (12″ + DBH) 2/ac 4/ac

…what’s that machine back there?

The equipment used for cutting trees on this job is known as a “cut-to-length harvester.” With a sophisticated sawing head mounted to a rugged excavator-like vehicle, a skilled operator can fell, limb, measure, cut to specific lengths, and pile logs for collection with a skidder or forwarder… all from the safety of an enclosed cab. Wide tracks reduce the equipment’s ground pressure, allowing it to “float” on softer soils. Working during winter when soil is mostly frozen also helps to prevent rutting.

Foresters for the Birds

As a forest landowner, do you choose to grow timber or manage wildlife habitat? Audubon Vermont has developed a toolkit of assessment methods and silvicultural recommendations to do both! 

The Foresters for the Birds project provides information to foresters, and this harvest at NorthWoods is one of the first ten sites to ground-truth the management recommendations.

More info available at:


Tips For Success

  • A knowledgeable forester will be indispensable as you plan a Variable Density Thinning! He or she will be able to tell you which trees will grow well in the future, and which trees can be harvested now; the forester will probably also be the person to mark trails and trees to guide the logger. Be sure to talk through your management goals (what you want your forest to look like in the future, influenced greatly by the type of harvesting you do now) with your forester. 
  • Choose a logger with equipment that’s appropriate for the size and condition of your forest. Large, heavy machinery is best suited to large acreages and firm, dry ground. Small, lightweight machines or draft animals will cause less damage at lower cost on small jobs or wet soils. 
  • If wildlife habitat enhancement is one of your goals, go back in a few years and see if it worked. It might take time for the birds and beasts to make their way into your forest, but they’ll appreciate the work you’ve done! NorthWoods, Audubon Vermont and the NRCS are conducting post-harvest bird and habitat surveys to measure these impacts in this forest.

Target Species

black-throated blue warbler

Black-throated blue warbler

these birds rely on a dense vegetation less than 6 feet tall for nesting. This layer will develop in areas under canopy gaps where saplings and shrubs will begin to grow.


eastern wood-pewee

Eastern wood-pewee

the relatively dense canopy (at least 80% cover) retained during this Variable Density Thinning is beneficial for the eastern wood-pewee, as are the open mid-story and small canopy gaps found in this stand.



Yellow-bellied sapsucker –

with a penchant for creating nest cavities, this species will benefit from the snags (standing dead trees) and large aspens retained during the harvest.