Fall is Tree Thinning Time for Forest Landowners

Ponderosa pine stand thinned and pruned
Ponderosa pine stand thinned and pruned to prevent spread of low-intensity ground-level fires into the crowns. Photo: US Forest Service.

Fall is just around the corner and for those who own forested land, if you have not already done so, you may want to consider thinning some trees out. People who own forested property are often hesitant to remove trees for various reasons. Why should you thin? What are the advantages?

Forest health

Many people think of a forest as a stand of trees existing together in harmony. In reality, a forest, particularly a young forest, contains trees competing with one another for their life-sustaining resources: sunlight, water, and nutrients.

There’s even a priority list of sorts within individual trees. It varies depending on the species but, in general, the order in which resources are allocated is, from highest priority to lowest priority:

  1. Maintenance of respiration
  2. Production of fine roots
  3. Reproduction
  4. Height growth
  5. Diameter growth
  6. Insect and disease resistance mechanisms, and
  7. Storage

A dense stand of tall, thin lodgepole pine, is a good example of a stand where there are enough available resources to allocate up to priority number 4, height growth, but not enough resources to allocate much to priority number 5, diameter growth, or beyond. This lack of resources will affect overall forest health, as the trees will not be able to allocate resources to insect and disease resistance mechanisms.

What sort of insect and disease resistance mechanisms do trees have? Let’s use bark beetles as an example, since certain species of bark beetles can cause extensive tree mortality.

In most coniferous species a resin duct system produces oleoresin when the tree is wounded, such as a broken branch. Oleoresin is basically a mixture of essential oil (turpentine) and nonvolatile solids (rosin). Oleoresin is considered the primary defense of conifers against bark beetle attack. Beetles that attempt to attack a conifer that is in good health and capable of producing adequate, pressurized oleoresin are typically immobilized in the resin or killed by drowning in it. The chemical makeup of the oleoresin is important as well, as some of the volatiles released from the oleoresin are toxic to bark beetles.

Dense stands, which tend to grow slowly, are consistently associated with bark beetle infestations. The susceptibility of a stand to bark beetle infestations may be changed by reducing competition between trees; in other words, thinning. In western North America, thinning has long been advocated as a preventative measure to reduce or alleviate the amount of bark beetle caused tree mortality. Thinning improves tree vigor and growth. It also decreases the likelihood of bark beetle attacks on individual trees by allowing the site’s available resources to be concentrated on fewer stems, which means trees will have enough resources to allocate to priority number 6 (insect and disease resistance mechanisms).

Wildfire risk reduction

Successful fire exclusion over the past 60 to 70 years has resulted in greater stand densities and a change in species composition. In that span of time, many forests in dry ecosystems, such as eastern Washington, have transitioned from fire-adapted, open ponderosa pine stands to dense pine and Douglas-fir stands. In moist forests, the change has been from open stands of western white pine and western larch to relatively short, closed stands of grand fir, western hemlock and western redcedar. These changes have led to an increase in the occurrence of crown fires (fire that spreads from treetop to treetop), the most intense type of wildfire, and often the most difficult to contain.

Thin from below
Example of thin from below. Photo A: Before thinning. Photo B: After thinning. Photos: Michelle Ensminger.

 

Ponderosa pine, western white pine and western larch all tend to be tall and self-prune (the natural removal of lower limbs that don’t receive enough sunlight to survive). Western white pine and western larch have lower volume crowns and carry their crowns well above surface fuels compared to true firs, Douglas-fir, western hemlock, and western redcedar. Because of these attributes, western white pine and western larch do not carry crown fires well and tend not to create ladder fuels (fuels in the lower canopy that carry fire up into the crowns of trees). In contrast, stands dominated by true firs, Douglas-fir, western hemlock, and/ or western redcedar do not self-prune well. They tend to carry large branches low in the canopy and have relatively voluminous crowns. Stands dominated by these species usually support crown fires.

Thinning cannot alter all variables that influence fire behavior, but it can influence factors such as species composition, available fuel, fuel arrangement, fuel moisture and surface winds. The objective of thinning in wildfire risk reduction is usually to prevent or slow the spread of crown fire by reducing surface and ladder fuels. Thinning also raises the height of overstory crowns and breaks up the connectedness of crowns, which reduces tree-to-tree spread of crown fires.

Wildlife management

Species associated with fairly open canopies and an open forest floor may benefit from thinning treatments. Thinning a stand of trees increases the amount of sunlight reaching the understory, which stimulates the growth of grasses, wildflowers and native shrubs. Elk, deer, and moose will likely benefit from the increase in forage quantity and quality. Small mammals such as chipmunks and deer mice may increase in number, particularly after thinning in Douglas-fir and ponderosa pine forests. This may be advantageous to species of hawks, owls and eagles that prey on small mammals in open forests and small clearings. Although not often considered as part of the wildlife community, pollinators such as moths and butterflies may also benefit from changes in structural diversity as a result of fuel reduction treatments that increase the amount of light reaching foliage and the forest floor.

Timber production

If you are managing your forested land for future timber production, thinning will be an important part of your management plan. Thinning releases resources to the residual trees allowing them to allocate to their fifth priority, diameter growth, which leads to an increase in tree volume. This increase in diameter growth results in an increase in overall stand value.

Thinning techniques

The tools and methods by which thinning is implemented are quite varied, and can result in significantly different stand structures. The type of thinning you select may depend on your objectives and on individual stand characteristics, such as species composition.

When managing for forest health and fuel reduction, private landowners typically use the “thin from below” method. Thinning from below consists of removing trees from the lower canopy, leaving larger trees to occupy the site. This method mimics mortality caused by competition or surface fires and concentrates available resources on larger, healthier, fire-adapted trees, while removing the stagnant, unhealthy trees.

Thinning is best accomplished in the late summer and early fall if possible. At this time trees will be least susceptible to damage from the thinning operation and the populations of insects that may be attracted to the slash created will be low. Winter also is an acceptable time to thin, but can lead to soil compaction and erosion if done at the wrong time. Thinning in spring and summer is not recommended as it can attract insects such as bark beetles and can affect wildlife, particularly nestlings.

For more information about thinning your property, please visit the Forest Stewardship Program and the Landowner Assistance Center pages on the Washington State Department of Natural Resources website.

By Melissa Fischer, Forest Health Specialist, DNR Northeast Region, Washington State Department of Natural Resources

 

Resources to  learn more:

Fettig, C.J., Klepzig, K.D., Billings, R.F., Munson, A.S., Nebeker, T.E., Negron, J.F., and Nowak, J.T. 2007. The effectiveness of vegetation management practices for prevention and control of bark beetle infestations in coniferous forests of the western and southern United States. Forest Ecology and Management. 238: 24-53.

Graham, R.T., Harvey, A.E., Jain, T.B. and Tonn, J.R. 1999. The effects of thinning and similar stand treatments on fire behavior in western forests. U.S. Forest Service, Pacific Northwest Research Station. PNW-GTR-463.

Pilliod, D.S., Bull, E.L., Hayes, J.L. and Wales, B.C. 2006. Wildlife and invertebrate response to fuel reduction treatments in dry coniferous forests of the western United States: A synthesis. U.S. Forest Service, Rocky Mountain Research Station. RMRS-GTR-173.

Fuels Reduction and Improved Habitat: Try SLLOPPS (Snags, Logs, Legacy, Openings, Patches, Piles and Shrubs)

Overly aggressive thinning near Lake Chelan
This natural-looking setting near Lake Chelan is anything but natural. Overly aggressive thinning has left it without the snags, down logs, open patches and other features that wildlife need for habitat. Photo: Ken Bevis/DNR

Thinning and fuels reduction are necessary treatments in today’s overstocked dry forests. But sometimes, aggressive implementation of prescriptions can degrade habitats beyond what is really needed for fuels treatments. This article will make a few suggestions on ways to balance these objectives.

If you live in a dry-but-forested area, such as eastern Washington, and are planning a thinning or harvest on your forestland, here’s a simple habitat acronym for you and any contractors you hire to keep in mind: SLLOPPS, which stands for snags, logs, legacy, openings, patches, piles and shrubs. Incorporating these seven features into your project will help reduce future risks of wildfire and insect infestation while promoting a healthy natural habitat for beneficial wildlife.

Natural Forests

In its natural state, the dry forest ecosystem experiences frequent low-intensity fires. This cycle of periodic fire results in tree stands dominated by large, old trees and, generally, not over-stocked with smaller trees and other growth as many stands are today.

Historic photos of eastern Washington and Oregon show classic stands of old ponderosa pine (and some Douglas-fir) with riders on horses and wagons cruising through the open, grassy understory. These conditions did not occur everywhere, but the prevalence of ground fire at 7- to 15-year intervals ensured that these stands seldom suffered crown fires. Individual tree vigor was strong thanks to reduced competition for resources. Thus, fire disturbance helped maintain these forests.

These stands contained large standing dead trees as well, and some enormous down logs that could survive low intensity fires. Regeneration was often patchy, resulting in numerous openings and areas of dense regeneration that might flash out in the next fire. Many shrub species are fire adapted, and after burning would either re-sprout in clumps, or sprout from seed in the soil, creating a vigorous shrub understory.

Wildlife species, such as white headed woodpeckers and flammulated owls, are adapted to this open forest and its plentiful snag and log habitats and rich understory of shrubs.

Human Actions

Native Americans are believed to have played a significant role in the fire ecology of the inland Northwest. Their activities led to the landscape-shaping fires that produced the stands encountered by the early European settlers to this region. Also during this time, lightning fires often would burn until season-ending weather events such as snowfall.

Logging (until very recently) in these dry forests usually removed the large, excellent quality trees. This was economically advantageous but ecologically unfortunate, as these trees would have been the survivors of the fires. Without recognizing what we were doing, we removed the backbone of the dry forest habitat.

The biology of dry forest tree species involves producing large numbers of seeds to give a chance for a few to survive the inevitable fires. Fire suppression efforts that began in the early 20th century inevitably led to the dense stands that we see on the landscape today.

Now, we are aggressively thinning across the landscape, where funding, motivation and political will let us. Unfortunately for wildlife, caution over “fire safe” and “forest health” can lead us to produce stands that are simply “too clean” and “parked out” to serve as quality wildlife habitats.

In this article, I will discuss seven tools — snags, logs, legacy, openings, patches, piles and shrubs (SLLOPPS) — that can provide some habitat diversity while addressing the issues associated with overstocked stands and tree mortality due to stress and insects.

Prescription for Habitat Diversity

SNAGS: Some of the most important habitat features in any forest are made of dead wood; specifically, standing dead trees (snags) and down logs. Live trees with dead portions of their stems and branches can also fill this role. Insects reside in the dead wood, often feeding on fungi, while woodpeckers, nuthatches, chickadees and other birds feed on these insects. Cavities created by woodpeckers during regular nesting and courtship behavior can provide homes for secondary cavity species such as bluebirds or flying squirrels. Because many of these species are voracious feeders on insects, including some that are forest pests, their presence helps to keep the forest healthy but only if suitable habitat is provided so that they can occupy territories for feeding and nesting.

recently thinned stand with several wildlife features
This recently thinned stand on private land displays the key elements (snags, down wood, piled chipped materials, etc.) that make it useful wildlife habitat. Photo: Ken Bevis/DNR

DNR’s cost share thinning projects target dangerous fuels which are generally woody stems less than 3 inches in diameter. These smaller stems will carry fire quickly and spread flames into crowns. Larger wood, which ignites more slowly and creates less flash hazard, can be left for habitat and soil enrichment.

Snags should be greater than 10 inches in diameter at breast height (dbh) in order to provide opportunities for large excavators, such as the hairy woodpecker or flicker, to create cavities. Natural snag densities vary tremendously, so the best policy for habitat is to maintain all snags greater than 10 inches dbh, and protect them from firewood and timber harvests. Forest practices laws in Washington state require 2 wildlife trees per acre; although this is likely not a biologically optimum number, it can serve as a target for forest management. Following this rule could include creating 2 snags per acre where they do not exist. Optimum snag densities are closer to between 12 and 16 snags per acre but in managed forests this is a hard number to reach.

LEGACY: Big trees are the backbones of dry forest ecology. Keep large trees, including defective ones. They will produce more cones and branch surface area than younger stems, provide perches and nest sites, and will become future dead wood.

LOGS: Logs can be treated the same as trees by emphasizing the protection for all large pieces by preventing them from being piled or burned, and by leaving them in place. Scattering tops and large pieces of unmerchantable wood across treated units are additional desirable actions to improve habitat.

OPENINGS: Wildlife also benefits from openings—areas within the forest where all, or nearly all, of the overstory trees are not present. These openings allow shrubs and grasses to flourish as wildlife forage. Natural meadows are the best candidates for these areas, but openings also can be embedded within stands to allow big game animals to feel secure and to provide habitat for other wildlife associated with edge habitats.

PATCHES: Denser habitats made up of young conifers and shrubs provide quality habitat for many species, such as feeding or nesting songbirds, and as browse and cover for big game. Retaining small patches of trees in thinning units can provide this habitat, but it requires forethought and follow through. Before thinning, mark areas from 30 to 50 feet in depth, and at least the same distance in length, or preferably longer. These areas should be left unthinned, (or thinned lightly), in order to maintain shrubs, trees and other mid-level vegetation while providing cover for large mammals such as deer, elk and bear. These patches should be configured across forest units so as to break long-sight distances, and staggered at distances of 200-300 feet apart.

PILES: Wood piles can be left as distinct habitat elements and act as surrogates for down wood. They will provide cover for many species of wildlife. The best approach to creating piles for wildlife involve placing at least three to five layers of larger logs that are crisscrossed, or laid lengthwise in triangular arrangements. When covered with a few layers (about 2 to 3 feet deep) of fine branches, the pile will provide habitat with small interior spaces. Habitat piles also can be used as a non-burning solution for managing slash. Place piles constructed for wildlife away from overhanging trees so that if a pile should catch fire it will not act as a ladder fuel to the crowns. It’s best to provide these wildlife piles at a rate of two to three per acre, preferably in clusters away from roads, trees and structures. Because these piles are not intended as sources of firewood they should be marked for retention after the thinning work is done but before other brush or slash piles are burned.

SHRUBS: Many shrub species provide excellent fruit and vegetation for many types of wildlife. Ask your local U.S. Conservation District office which shrubs are the best for your area. Elderberry is always a good choice, as is most anything else with “berry” in the name.

Putting it All Together

A general rule of thumb for 10 to 15 percent of the project area to be made up of one, or all, of these desirable wildlife habitat elements. Providing patches of habitat for cover, or around a feature such as a snag, can provide much in the way of habitat diversity and reduce the potential impact of thinning projects on the diversity of animal and plant species that live in your forest.

If done thoughtfully, thinning projects that maintain snags, logs and shrubs a sufficient distance from overstory trees will provide quality habitat while improving the health and resilience of dry forest stands. Work closely with contractors and be very specific as to where these habitats are to be provided. Thinning will increase resilience to both fire and insects through improved individual tree vigor, which in turn benefits many wildlife species. Risk of catastrophic loss of entire stands can be avoided with good projects too. And that benefits wildlife in the long term.

Case Study: Swauk Pines, Kittitas County

In 2015, Suzanne Wade of the Kittitas County Conservation District (KCCD) partnered with private landowners at Swauk Pines, a new 50-acre development near Cle Elum made up of 3- to 8-acre parcels in a dry pine forest. The Taylor Bridge fire (2012) came very close to this area and created significant motivation for landowners, some of whom had already built residences while others were in the planning stages, to reduce their wildfire risks while maintaining wildlife habitat.

Swauk Pines development before forest-thinning
BEFORE: Swauk Pines development near Cl Elum before forest-thinning treatments. Note the many small (less than 8” diameter) trees, trees with low branches and the heavy woody understory–all prone to spreading fire. Photo: Ken Bevis/DNR.

Most of the development was treated in a cost share project in which the KCCD worked closely with the thinning contractor to incorporate SLLOPPS principles into the forest treatments. These treatments included retaining large snags and logs, and including shrub patches. A bird survey was conducted before the project began to identify where to create open patches attractive to nesting birds.

Results

As a result of the strategic approach to forest thinning, habitat quality was maintained, fire risk was reduced, and forest health improved large. Homeowners were asked to take responsibility for the areas immediately around their houses. This project is an excellent example of successfully implementing multiple objectives.

Swauk Pines after thinning
AFTER: Swauk Pines after thinning to remove brushy overgrowth and most of the trees less than 8 inches in diameter. Note the retention of snags and patches of shrubs for wildlife. Photos: Ken Bevis/DNR

Including these habitat elements in thinning projects is only the beginning. Vegetation always grows back so the job of maintaining the levels of fuels acceptable to individual landowners is an ongoing task that will need to be revisited every few years.

Thinning and fuel reduction projects are crucial to help our forests survive the current rounds of drought and devastating wildfire. Including habitat elements in these projects is not only possible but an additional benefit of meeting our fire and forest health objectives.

For more information or to schedule a site visit to your forest property, please contact the DNR Small Forest Landowner Office. For information or assistance with habitat, contact DNR Landowner Assistance Wildlife Biologist, Ken Bevis at Ken.Bevis@DNR.wa.gov

By Ken Bevis, DNR Landowner Assistance Wildlife Biologist, Ken.Bevis@DNR.wa.gov

 

New Pests of the Understory

In a time of world trade and global movement of people and products, hitchhiking insects are becoming more and more common. In the past 20 years, almost 60 exotic insect species have established in Washington state. Some of these hitchhikers can become serious agricultural and forestry pests. The risk continues to grow as global markets continue to expand.

A 2010 study led by Julieann Aukema, a forest ecologist with the National Center for Ecological Analysis and Synthesis in Santa Barbara, California, estimated that there is 32 percent risk that a wood boring insect more damaging than the emerald ash borer will be introduced into the United States in the next ten years. In addition to exotic insects that can cause significant economic impacts to agriculture and natural resources, there are a number of species affecting the natural and cultural ecosystems. The following are a few examples of newly introduced insects that are, or likely will, impact the forest understory and those that rely on it.

Viburnum leaf beetle

Viburnum leaf beetle adult.
Viburnum leaf beetle adult. Photo: T. Murray/ Washington State University
Viburnum leaf beetle larvae.
Viburnum leaf beetle larvae. Photo: T. Murray/Washington State University

The viburnum leaf beetle (VLB), Pyrrhalta viburni, was first discovered in Washington state in Whatcom County in 2004. Since then, it has spread down to King County. Recent collections of VLB have been made in Spokane. VLB overwinters in its egg state in the stems of last year’s new viburnum growth. Larvae hatch when the first leaves unfold in spring. Damage caused by feeding larvae is very distinctive and won’t be confused with any other feeding damage on viburnums. After feeding, larvae migrate to the soil to pupate for a few weeks. Adults emerge and continue to feed on foliage causing additional damage. Adult beetles feed, mate and lay eggs until first frost. Viburnum plants are not able to tolerate multiple defoliation events over consecutive years. The native Viburnum edule, high bush cranberry, is susceptible to attack. Many wildlife species rely on high bush cranberry for a reliable food source. To learn more about the viburnum leaf beetle in Washington state.

Lily leaf beetle

Lily leaf beetle adult.
Lily leaf beetle adult. Photo: E. LaGasa/Washington State Dept. of Agriculture

The lily leaf beetle (LLB), Lilioceris lilii, was discovered in Washington state just outside of Seattle in Bellevue during the spring of 2012. Thus far, LLB has only been found in Bellevue, Seattle and Issaquah. Adult beetles are very conspicuous as scarlet red beetles. Adults overwinter in protected areas and move to feed, mate and lay eggs on emerging true lilies (Lilium spp.) and fritillaries (Fritallaria spp.) in the spring. Eggs are laid in irregular rows on the underside of the lily leaves. Once eggs hatch, beetle larvae feed on the lily foliate and developing flower buds. Larvae cover themselves in excrement and other debris as a defensive tactic and superficially resemble slugs. Two key native species in the Pacific Northwest that are likely susceptible are the tiger lily, Lilium columbianum, and the chocolate lily, Fritillaria lanceolate. Learn more about the lily leaf beetle in Washington state.

Azalea lace bug

Azalea lace bug adult
Azalea lace bug adult. Photo: T. Shanan/Oregon Dept. of Agriculture

The azalea lace bug, Stephanitis pyrioides, was first discovered in Seattle, King County, in 2008. The following year, it was identified in Oregon. Lace bug nymphs emerge from eggs in the spring. Having a piercing-sucking mouthpart, the nymphs feed by removing the liquids from plant leaves creating a stippled or bronzed burn on the leaf surface. Distinctive tar spots appear on the undersides of leaves as evidence of their presence. Adult lace bugs are quite attractive with a clear, lacy appearance. In the Pacific Northwest there will be multiple generations per year. Azalea lace bugs are causing significant damage and mortality to landscaped azaleas and rhododendrons in both the Seattle and Portland areas. What is most concerning about this newly introduced insect is the degree of damage it can cause and the expanded host ranges documented in the Pacific Northwest. Jim LaBonte from Oregon Department of Agriculture has found damage on huckleberry and salal in addition to other native plant species. Learn more about the azalea lace bug in the Pacific Northwest.

Spotted winged drosophila

Spotted wing drosophila.
Spotted wing drosophila. Photo: E. LaGasa/ Washington State Dept. of Agriculture

The spotted winged drosophila (SWD), Drosophila suzukii, is a significant new pest to many small fruits and has had a major impact on blueberry, raspberry and cherry production in regions of the Pacific Northwest. SWD was first discovered in 2009 in Seattle, just shortly after its detection in California the previous year. Since then, SWD has spread across the continent. SWD adults overwinter in protected areas. When berries and other food resources become available in spring, SWD adults lay eggs into ripening fruit using an ovipositor—an appendage—with a saw-like edge. The ability to egg-lay in under-ripe fruits has made this fruit fly a serious pest. Being a fruit fly, SWD has a high reproductive capacity and fast generation time. Populations can build rapidly. Larvae feed on the flesh of fruit and quickly cause the fruit to rot. Larvae pupate outside the fruit and emerge as adults to repeat the process.

In 2013, SWD was found infested huckleberries at high elevations in the Indian Heaven Wilderness Area of the Gifford Pinchot National Forest. Almost 50 percent of the huckleberries picked turned to be infested by SWD. Since 2013, SWD has consistently been collected from infested huckleberries in high elevations (5100 feet) in remote areas. SWD was able to disperse successfully in nooks and crannies of the Mount Adams and Mount Hood forests very rapidly. SWD has likely done so in other forests where huckleberries are common.

The economic impact to agriculture and natural resources of new pests is the focus for research and investments; there are few resources available to understand the impact on natural and cultural systems. The significance of these new pest introductions into natural areas has yet to be fully realized. To put it in perspective however, humans have harvested huckleberries from the Indian Heaven Wilderness Area for almost 10,000 years without experiencing wormy, rotten berries until now.

By Todd A. Murray, Director, WSU Agricultural and Natural Resources Extension Program Unit
tmurray@wsu.edu

Spring Brings Out Pine Engraver Bark Beetles

Pine engraver frass
Pine engraver frass seen on trunk of infested treed. Photo: Brytten Steed, USDA Forest Service, Bugwood.org

As temperatures begin to warm up, the pine engraver bark beetle will once again rear its ugly head. The pine engraver is a very small (1/8 to 3/16 inches long) bark beetle that attacks small diameter pine trees (2-8 inches DBH*) or the tops of large pine trees.

Upon emergence in the spring, the pine engraver typically infests fresh slash, wind throw, or snow-damaged trees. The adults mate and build “galleries” under the bark and the female begins laying eggs. Within 4-14 days, the eggs will hatch into larvae and feed on the phloem (living, inner-most layer of bark). After 10-20 days, the larvae will pupate and emerge as adults 10 days later. Overall, it takes about 40-55 days for the pine engraver to complete development from egg to adult. The new generation of adults produced will begin fresh attacks following emergence. If slash is available, this will be preferentially infested. If no slash is available, the beetles will begin attacking nearby live, standing trees. By mid- to late-August another generation will be completed in this material. The generation that emerges in August typically seeks out places to hibernate for the winter, but sometimes makes a feeding attack prior to hibernation, where trees are attacked and the phloem eaten, but no brood is produced.

Signs of pine engraver infestation include reddish-colored frass (refuse and excrement) on the outside of the bark, Y or H-shaped galleries under the bark and/or woodpecker damage on the surface of the bole. The pine engraver is often associated with the western pine beetle and the red turpentine beetle.

Typical y-shaped pine engraver gallery.
Typical y-shaped pine engraver gallery. Photo: USDA Forest Service Region 6 Pacific Northwest Archive, USDA Forest Service, Bugwood.org

The pine engraver can outbreak, and outbreaks can include hundreds of trees. Outbreaks typically occur as a result of drought, overcrowding, and/or the creation of slash, windthrow, or snow damage. Thinning dense pine stands can help mitigate the potential of a pine engraver outbreak by increasing the availability of water, sunlight, and nutrients for the residual trees. This enhances their vigor and allows for more defensive capabilities, such as increased resin flow.

The timing of slash creation is crucial. It is best not to create slash from January through July. Slash created during this time period does not have enough time to dry out prior to the several flights that the pine engraver may carry out each year. Slash created in fall or early winter will usually dry by spring and will be unsuitable habitat. If slash must be created during high risk months, there are several disposal options that may help reduce the risk of infestations that lead to outbreaks.

Pine engraver-infested trees with slash pile circled in red

Following the creation of slash, the best way to mitigate outbreak potential is by burning slash piles. This of course cannot always be accomplished due to dry, fire-weather conditions. Another option is called the “green chain” where you continuously make slash throughout the entire pine engraver flight period so that the beetles continue to infest the slash (which they prefer) rather than the standing, live trees. At the end of the season these piles can be burned. A third option would be “pile high and deep”, in which case you would create a very large slash pile (10-20 feet in width, length, and height). This method can be effective because the first generation of pine engraver emerging from the outer portion of the slash pile will likely continue moving deeper into the pile where branches are still moist and viable. Again, at the end of the season these piles can be burned. Although these methods have been shown to be effective in some cases, please remember, bark beetles can be unpredictable and may still choose to attack your live, standing trees.

pine engraver-infested trees
A stand of pine engraver-infested trees with a slash pile of diseased wood circled in red.

Some other methods of mitigation include completely removing slash from the site. Just be sure not to move it somewhere that may cause damage to someone else’s property. You may also debark larger slash, but this method is hard work and very time consuming if you have a lot of slash. Debarking is best when only one or two trees have been felled. Another alternative is to chip slash, but remember, even though the beetles cannot live and breed in the chips, they will be attracted to the area by the volatiles (chemicals trees give off that can attract bark beetles) that are emitted from the chips and the beetles may start attacking standing trees in the area around the chips. This is also true of the “lop and scatter” method. You can lop all slash greater than three inches in diameter into small pieces and scatter these pieces in a sunny area to dry out the phloem rendering the slash unsuitable for pine engraver infestation, but again, the volatiles emitted may attract beetles to the area and they may then attack your standing trees since the lopped up slash is not viable.

Finally, limit pruning activities to October through December if possible, as pruning wounds also release volatiles that may attract pine engraver. And never stack freshly cut wood next to live, standing trees during high-risk months.

*DBH = Diameter at breast height or diameter of the tree’s bole at 4.5 feet.

Read more in the US Forest Service’s Management Guide for Pine Engraver