Landowner Questions: How Does Drought Impact Trees?

By Kevin W. Zobrist, Associate Professor, WSU Extension Forestry, kevin.zobrist@wsu.edu

Drought, or the more generic term “water stress,” has been a big issue for trees in Western Washington, resulting in tree decline and mortality in forests, natural areas, and yards across the region.

Starting in 2012, we have had a series of record-setting summers in terms of heat and days without rain. This is taking a cumulative toll on trees.

Trees have an amazing ability to transport water from their roots all the way up to their tops, which may be over 100 feet in the air. The dominant theory of how this is done is called transpiration pull. A large portion of the stem of a tree is called sapwood, and this is a big plumbing system through which water (sap) flows upward.

The sapwood comprises many tiny “pipeline” pathways for water. Water flows into the tree roots through osmosis, flows up the sapwood, and makes its way into the leaves.

The leaves have tiny pores called stomata that open up for gas exchange to occur as part of photosynthesis. The pores open up to take in carbon dioxide, and oxygen and a little bit of water vapor are released in the process, known as transpiration.

Water transport in trees takes advantage of the fact that water molecules are polarized, which gives them a slight attraction to one another (cohesion) and other objects (adhesion). Surface tension, the “skin” on top of water, is an example of cohesion, and the way that water goes up slightly on the edges of a glass measuring cup (creating a “U” shape) is an example of adhesion.

As the leaf pores open up, the water molecule evaporates out. As it does, its cohesive nature “pulls” the water molecule next to it forward, which pulls the water molecule next to it, and so forth.

There is a continuous chain of water molecules, called the water column, that runs from the leaves down to the roots. They are stuck together by cohesion, and they cling to the sides of the narrow pathways in the tree by adhesion. So as a tree transpires, water molecules are pulled up the tree, hence the term “transpiration pull.”

The “pulling” by evaporation from the leaves creates a negative pressure that draws new water from the soil into the roots to replenish the bottom of the water column. As long as there is adequate water to be drawn in, all is well. In drought conditions, though, there may not be any soil water available.

As water continues to evaporate from leaf pores, especially on hot, dry days, the tension on the water column gets tighter and tighter because no new slack is coming in through the roots.

Under extreme water stress, the water column breaks at some point in the stem, which is called a cavitation. This can result in an air bubble, or embolism, in the pipe. The chain of water molecules is now disconnected between the roots and the leaves. If the tree is unable to repair the disconnect, that water pathway no longer functions.

drought impact 1
This conifer shows the typical outside-in and top-down dieback seen after drought stress. (Photo by Susan Hagle, U.S. Forest Service)

If there is a major failure in the collective water column, water can no longer flow beyond that point of failure. Everything above it dies, leaving the tree with a dead top. It could be that the water pathway to a particular branch failed, in which case that branch dies. The entire tree may also die due to water stress.

Water stress not only kills trees directly as described above, but also indirectly by causing them to be susceptible to other agents.

When trees don’t have enough resources (e.g. water), they become stressed. The tree has to prioritize how to use its inadequate resources because it can no longer maintain all functions.

One of the first two things a tree gives up is diameter growth. You can look at core samples from older trees and see spots where the diameter growth rings were particularly small, which may indicate that those were drought years.

The other thing the tree gives up first is insect and disease resistance. This leaves it more vulnerable to things like root disease and bark beetle attacks.

In Western Washington, bark beetles are not usually an issue with healthy trees, which can effectively resist them (and they know it). Rather, westside beetles are opportunistic, taking advantage of trees that either just died or are nearly dead from some other factor.

People are finding dead trees with beetles in them. The beetles are not the issue, though. Rather, the water stress was killing the tree, which attracted opportunistic insects looking for an easy meal.

Healthy trees can even hold their own against root disease in some cases, compartmentalizing decay and keeping the disease at bay. If that tree becomes water stressed, though, it can no longer fight the disease and is overcome. We often see “pulses” of mortality in root disease areas during drought years.

drought impact 2
Pouch fungus is often a sign that a tree had a bark beetle infestation. The beetles are typically secondary pests, seeking out already-weakened trees. (Photo by Sandly Kegley, U.S. Forest Service)

Here is a real case study from Snohomish County.

I visited a landowner who had a bunch of recently-dead Douglas-fir trees. The first thing I noticed were white fungal conks all over the stems of the dead trees. Did this fungus kill the trees?

Then, I noticed pockets of insect frass in the bark crevices. I peeled back pieces of bark and found Douglas-fir beetles inside. Did the beetles kill the tree? Then I looked around the area and saw that it was in the middle of a laminated root rot pocket.

What happened was this: The trees in the area were suffering from root disease but were holding their own to some degree. A couple of drought summers tipped the balance in the disease’s favor, causing them to die. As they were dying (or right after), they were colonized by opportunistic bark beetles. The beetles bring in a fungus with them called pouch fungus, which causes white, pouch-like fungal conks to emerge from the beetle holes.

Water stress can set a whole chain of events into motion, and the stress is cumulative over time.

Some trees that endured the first few drought years finally succumbed in subsequent drought years, and we will see that continue. The result is a significant uptick in tree decline and mortality.

Some people are convinced that there is some sort of disease or insect epidemic that is wiping out trees left and right. Insects and diseases may indeed be involved, but they are not new or unusual. Rather, they are simply taking advantage of trees that are beginning to succumb to water stress.

Some people have found this too hard to believe. I’ve done several consultations where the property owner did not believe my explanation or did not think I was taking the issue seriously.

On the contrary, the repeated years of extreme heat and drought are a very serious issue right now. However, cutting down all your trees and burning them to stop the spread of some phantom agent, as some have done, is not the answer!

What is the answer? In some cases, there isn’t a good one. Some trees are going to continue to decline and die due to adverse summer weather, and there’s nothing we can do except let it play out. If these newly-formed snags do not pose a hazard, they will provide a huge benefit for wildlife.

Forty percent of our wildlife species require dead wood (standing or down). If the dead tree is a hazard, removal does not have to be all-or-nothing. Leaving the bottom 10-15 feet as a short snag will still provide important wildlife benefits while minimizing the potential for damage.

The best defense against the direct and indirect impacts of water stress is to maintain tree vigor, and there are two key ways to do that.

The first is to make sure the tree species is appropriately matched to the site. Where we see the most mortality (direct and indirect) from water stress is on certain “droughty” soil types. These tend to be gravelly soils that are excessively well-drained and dry out quickly.

Trees on these marginal soil types may do OK most of the time, but they quickly succumb in drought conditions. Planting more drought-tolerant species may be needed for these soil types. Your local Extension or Conservation District office can help you select appropriate species if you have sites where tree survival is poor.

The second key thing is density management. When trees are too crowded, they compete for water and nutrients and become stressed. In drought conditions, this competition becomes acute. When there is a very limited amount of available water, the more trees there are, the less water each will get. The WSU Extension Forestry program has educational resources to help you assess whether or not your trees are too dense.

Annual Summary of Insect and Disease Activity in Washington Released

By Glenn Kohler, Forest Entomologist, Washington State Department of Natural Resources, glenn.kohler@dnr.wa.gov

Each spring, the Washington State Department of Natural Resources and U.S. Forest Service publish a Forest Health Highlights report that summarizes forest health conditions and trends across Washington from the previous year. The 2018 report and previous year’s reports are available on DNR’s Forest Health website.

Information for the report is gathered through annual monitoring projects and special surveys in response to recent forest damage events that are conducted by DNR and the Forest Service. Examples include an annual aerial survey, insect trapping, baiting streams for the pathogen that causes sudden oak death and installing ground plots to monitor emerging forest health issues, such as bigleaf maple decline.

The report also summarizes recent wildfire activity, weather events and drought conditions that may affect forest health, and forest health initiatives such as the Forest Health 20-Year Strategic Plan for Eastern Washington. Maps, charts, photos, and brief descriptions make much of the information in the report accessible at a glance.

For those who want more detail, it includes links to other resources like maps and data and the contact information of forest health specialists.

Annual Insect and Disease Aerial Survey

An annual insect and disease aerial survey that covers the majority of Washington’s 22.4 million acres of forested lands provides much of the trend information in the report.

Every year since 1947, aerial observers have reported the location and intensity of damage by forest insects, diseases, and other disturbances across all ownerships of forestland in Washington.

Without aerial surveys, it would be impossible to track disturbance conditions over such a large area using ground-based methods. Aerial survey is also an important tool used to detect and map new outbreaks of native and exotic insects and diseases.

The total area mapped with some type of damage varies each year from a few hundred thousand to nearly 2 million acres.

Last year was the first time aerial observers in Washington adopted new federal data collection standards and used new software on digital mobile sketch mapping tablets.

When observers record a small area with tree mortality (fewer than 2 acres), they assign an estimate of number of trees affected. For larger areas with tree mortality, observers no longer estimate trees per acre as a measure of damage intensity. Instead, they now choose a “percent-class” value that estimates the percent of treed area affected.

2018 Aerial Survey Highlights

Smoky conditions and temporary flight restrictions around active wildfires in 2018 prevented observers from conducting any flights from early August to early September.

Fortunately, due to earlier-than-normal onset of damage signatures, observers were able to cover most of the survey area prior to poor visibility setting in across Eastern Washington. Improved visibility, favorable weather, and fewer flight restrictions allowed observers to complete the survey by the end of September.

In 2018, the aerial survey recorded some level of tree mortality, tree defoliation, or foliar diseases on 469,000 acres, similar to the 512,000 acres with damage in 2017.

The area with mortality from bark beetles was 235,000 acres. Mortality due to bear damage or root disease was mapped on 115,000 acres. Relative to 2017, tree mortality decreased for all major bark beetle species except fir engraver.

The area with conifer defoliation was 28,200 acres, primarily caused by balsam woolly adelgid and western spruce budworm. Approximately 16,300 acres had some level of disease damage, primarily larch needle cast and bigleaf maple decline.

It should be noted that disease damage is significantly underrepresented in aerial survey because symptoms are often undetectable from the air.

At 120,000 acres with damage, pine bark beetles made up the majority of bark beetle activity, but well below a recent peak of over 400,000 acres in 2009. The most significant damage occurred in northern Ferry County, eastern Okanogan County, and Chelan County.

Fir engraver caused mortality in true firs (Abies species) was recorded on 71,200 acres in 2018, the highest level since 2009. Recent drought conditions and effects of past defoliation by western spruce budworm are likely drivers of the increase.

An outbreak of Douglas-fir tussock moth has caused severe defoliation on approximately 1,900 acres in Kittitas and Chelan counties. This is the first observation of tussuck moth defoliation in Washington since 2012 and the first in Kittitas County since aerial surveys began in 1947.

Western spruce budworm defoliation, primarily in northeast Washington counties, decreased significantly to approximately 7,500 acres, the lowest level observed in the state since 1970.

A new outbreak of western hemlock looper has caused light to moderate intensity defoliation on approximately 870 acres in south Whatcom and north Skagit counties. This area experienced a similar sized outbreak in 2011-2012.

Larch needle cast damage in western larch was observed on approximately 4,900 acres, primarily in the central and south Cascade Mountains.

Crown discoloration and dieback in bigleaf maple was observed on approximately 6,100 acres, primarily in lowlands of southwest Washington and in the south Puget Sound area.

Maps and Other Aerial Survey Products available to the public:

Whether you are a regular user of aerial survey maps and data or just learning about what’s out there, check out some of the products now available.

If you have any questions about these products or need information about forest insects and diseases, please contact the DNR Forest Health Program at 360-902-1300 or email forest_health@dnr.wa.gov.

Helping Landowners Learn From Their Peers About Harvest Options

Northwest Natural Resource Group and Oregon State University are reaching out to forest owners for a voluntary study about timber harvesting methods to understand how they affect both financial and forest health outcomes.

The goal of this research project is to help landowners who are considering a timber harvest to learn from the experiences of others. There is limited information about the economics of commercial timber harvests that use thinning or uneven-aged management, and how those results compare with other harvest methods.

The researchers are looking to survey Oregon and Washington forest owners who harvested timber from their forest in the past five years (since 2014) and are willing to share information about the silvicultural methods and financial outcomes from these recent timber harvests. The survey asks detailed questions about the harvest techniques and equipment used, the volume of timber harvested, cost of the harvest work, and the harvest revenues.

The results of this study will be shared with forest owners through a variety of methods including articles, papers, and classes taught by NNRG, OSU, and partner organizations. Data in the study will remain confidential within the research team. Information will be aggregated so it cannot be traced to any individual ownership. Data will be collected from through June 2019.

If you are willing to share recent harvest information with NNRG and OSU, contact Lindsay Malone, one of the project researchers, at lindsay@nnrg.org. Lindsay can provide you with a copy of the survey.

Learn more about this research project at nnrg.org/thinning_study

Lindsay Malone, Director of Programs, Northwest Natural Resource Group, lindsay@nnrg.org

Tree Profile: Red Alder

Red alder (Alnus rubra) is the most common hardwood species in the Pacific Northwest.  For a long time, in fact, many people might have said that it was a little too common.  Having little monetary value, growing where nobody particularly wanted it, and often times out-competing more commercially viable species, red alder has had a long-standing reputation among tree farmers as an opportunity for chainsaw practice.

However, things have changed over the last few decades and alder wood can now be worth nearly as much as Douglas-fir.  Not to mention it can be managed on shorter rotations, adds nitrogen to the soil, and can tolerate wet sites where many other species grow poorly.  Coming from the Midwest, where alder only grows as a shrub, it has been very interesting to learn about the ecological and economic potential of alder for forest owners.  With this potential in mind, I thought it might be worthwhile to give a detailed profile of this tree and clear up some common misconceptions.

Distribution and Ecology

Red alder ranges in a long stretch along the Pacific coast from as far south as southern California to its northern extent in southeast Alaska.  Some patches of it can be found on north-facing slopes in northern Idaho and western Montana, but for the most part it enjoys low elevations west of the Cascade Mountains.  The best tree development occurs below 1500 feet in Oregon, Washington, and British Columbia (Harrington, 2006).

Under natural conditions, alder moves in first after a stand-clearing disturbance (fire, flood, large wind event, etc.), whichmakes it what forest ecologists call a “pioneer species”.  In unmanaged settings, it requires bare soil to establish and plenty of light to grow, so it’s rarely seen growing underneath an existing canopy.  It’s a fast grower, and achieves the majority of its height growth in the first two decades after establishment.  Additionally, alder is part of unique group of plants known as “nitrogen-fixers” that create symbiotic relationships with soil bacteria to transform atmospheric nitrogen for the plants use.  This allows it to establish on sites with limited nitrogen, such as new or highly disturbed soils.  Over time as natural succession continues, alder often gives way to shade tolerant species like western hemlock, red cedar, and Sitka spruce.  Typically living less than 100 years, alder has a relatively short lifespan.  So compared to other pioneer species like Douglas fir, which can often live to be 600 years old or more, alder is the “live fast, die young” tree of Pacific Northwest forests.

Silviculture and Management

It’s commonly known that red alder enjoys wetter soils.  Most forest owners can attest to this, as they’ve seen it take over their stream sides, ditches, and any other perennially moist sites.  While it’s true it is rarely found on drought-prone soils, it’s very important to recognize that it is also not a fan of overly wet, poorly drained soils like some species (i.e. ash, cottonwood).  For this reason, choosing a site where alder will grow into a valuable product can be difficult but is possibly the most important management step towards producing high quality logs.

A common phrase to hear among some foresters and tree farmers is that “anywhere Doug-fir will grow, alder will grow too”.  While there is truth to this, it’s not the entire story.  Just because alder will grow on a site, doesn’t mean it will grow well.  Table 1 shows some basic characteristics of a good alder site.

Table 1.  Characteristics of good and bad alder sites
Traits of Good Alder Sites Sites to Avoid Planting Alder
Elevation less than 1500ft Bogs or marshes
Lower slopes, flood plains, benches Upper slopes
West to east slopes South to southwest aspects
10-30% slope Frost pockets
Moderately well to well-drained soil Sandy, excessively drained soils
Silt loam, silty clay loam, clay loam, silty , clay Poorly drained soils
Soil depth greater than 30″ Sites with excessive weed competition
Depth to summer water table <10′ Exposed or windy sites
Source:  Dobkowski, 2006

If you’re interested in the financial return of growing alder, the best prices come from trees with large-diameter boles and few lower branches.  Getting alder to grow this way requires planting densely.  The competition with nearby trees forces alder to grow upwards quickly rather than growing laterally and creating a thicket of branches.  Recommended planting density is between 540 – 680 trees per acre (or a spacing of 9’ x 9’ or 8’ x 8’), although some plant lower where natural regeneration is likely.  And while you don’t need bare soil to plant alder trees, you will likely have to control competing vegetation.  A critical metric for successful alder management is the first year of growth.  Alder can reach 50% of its maximum height within 15 years, with the fastest growth rates occurring in the first 5-7 years (Harrington, 2006).  So if your trees aren’t shooting up out of the ground something is wrong, and it’s likely your site.  This early growth is critical to get out from under other competing vegetation (i.e. Scotch broom), which can stunt growth and kill seedlings.

How you manage your alder after it is established is going to depend a little on your goals.  Are you in it for timber?  Are you interested in its wildlife benefits? Are you looking for smaller logs to fuel your mushroom growing operation?  Heck, maybe you’re just growing it just because you like the look of it.

Alder requires a live crown ratio (the proportion of the stem with living foliage) of about 50% to maintain vigorous growth, where trees like Douglas fir can get away with 40% or even 30% at times.  Since it grows so fast, alder crowns in a densely planted stand can get below 50% as early as 7-10 years, depending on the site quality.  This is where your management goals come in.  If you’re not cash-driven and want to take a more hands-off approach, thin heavily and leave it be.  You can remove about 70% of the stems (down to around 150 trees per acre).  This will keep you from having to do another thinning down the road and you will still produce a fair amount of volume at the time of final harvest.  If you want more volume, thin lighter to about 250 trees per acre (or about 50% of the stems).  This will give you a little more volume to play with for a final harvest and possibly a commercial thinning before then.  The basic gist here is to plant alder densely to minimize the retention of lower branches and thin hard to maintain crown ratios.  The main difference between managing alder and managing conifer species is that you have to do it earlier and more frequently, which requires actively monitoring your stands.  If you play your cards right, choose the right site, and don’t miss your thinning windows, you could have an alder harvest as early as 30 years on really productive ground.  A quick, but important, note: don’t leave valuable alder logs on the ground after cutting.  Alder decays quickly.  So if you’re going to harvest, make sure you have a plan to get them to the mill quickly.

This is a short, sweet, and fairly rosy summary of even-age alder management, so I encourage you to check out the sources at the end of this article to get more in-depth information on alder silviculture and potential pitfalls.  Additionally, I know it is likely that the majority of landowners reading this are not sitting on an empty plot waiting to be planted.  Instead many are dealing with existing, and likely decadent, stands of alder.  These stands are common throughout western Washington and are a result of intense logging without replanting in the mid-20th century.  Intervening at this point is complicated and depends largely on your goals as the landowner.  If you are purely cash-driven, it can often be the most efficient choice to simply cut down the decaying alder and start over.  Thinning in these older stands provides little benefit to tree growth since crown ratios are typically small and trees are reaching the end of their lifespan.  However, many landowners aren’t motivated by income and instead are driven by a love of wildlife, aesthetic appearance, or recreation and consequently may be interested in developing more complex forest systems.  Your options for management can range from light-intensity thinning of the alder overstory to underplanting shade-tolerant species, or even doing nothing at all.

However, it is important to note the risk of doing nothing.  On occasion, the dense shrub layer that often exists in pure alder stands can inhibit regeneration of shade-tolerant conifers below and leave you with a thicket of shrubs after the alder dies.  To learn more about options for existing alder stands, try the WSU publication Management Options for Declining Red Alder Forests (PDF) by Kevin Zobrist.

It is also important to note that because it can tolerate wet sites, many existing stands of alder may be in Riparian Management Zones (RMZs).  It goes without saying, but your management choices are much more limited in these areas.  However, some options, such as converting alder to longer-living conifers (AKA a “hardwood conversion”), allow for management in these areas.  Contact the Washington DNR Small Forest Landowner Office to learn more.

Market and Uses

In the first half of 2018, the market value of red alder in the Washington coast market area has averaged around $720 per mbf (1,000 board feet).  Compare that to Doug fir, which has averaged about $745 this year. Clearly the market has changed since the 1990’s when alder was regularly worth less than $250/mbf.  And better yet, it’s showing no signs of slowing.  While the timber market is and always will be volatile and unpredictable in the long run, it seems that prices for quality alder saw logs are going to be competitive for the foreseeable future.

Alder is used for a variety of end products because of its wood characteristics and availability in the marketplace.  It has a reputation for being easy to machine, glue, and finish.  While it ranks low in durability and hardness, it is prized for its grain and appearance.  It’s often used for veneer, furniture, cabinets, paneling, doors, and food dishes.  Lower quality logs are often used in pulp products like tissue and paper.

Alder has many uses beyond traditional wood products.  Small diameter logs serve fantastically for mushroom growing material. The bark contains salicin, which is a chemical similar to aspirin that can be used to cure headaches and other
maladies. Native Americans often used alder for dyes, baskets, medicine, and even used the dried bark as soup thickener.

In terms of wildlife, birds welcome the fact that alder seeds stay on the tree throughout the fall and winter, when food is scarce.  Additionally, the dense shrub layers provide forage and habitat for a multitude of wildlife species.  Deer and elk also love the leaves, twigs, and buds (much to the frustration of tree farmers).

Conclusions

Alder is the best bet for a forest owner interested in hardwood management and its potential is remarkable both because of its vigorous early growth and ability to add nitrogen to the soil between conifer rotations.  If you’re a landowner and you’re hesitant to commit to growing alder, it’s understandable.  It has a long history.  And ultimately red alder is less forgiving than species like Douglas fir, so it may not be the right choice for an absentee landowner.  But many tree farmers have experienced planting conifers in an area, only to have them overtaken by naturally established, faster-growing alder.  In my mind, these are sites predestined for alder production and a great way for all types of forest owners to dip their toes in alder management (provided they are not in RMZs!).  After all, why fight it?  Now that it’s worth as much as other species, why not take advantage of what the site is doing already?

Learn More:
http://cru.cahe.wsu.edu/CEPublications/EM003/em003.pdf
https://www.fs.fed.us/pnw/publications/gtr669/pnw_gtr669b.pdf
https://www.fs.fed.us/pnw/olympia/silv/hardwoods/alder_plantation_establishment.pdf

Works Cited:

Dobkowski, A. (2006). Red Alder Plantation Establishment: Site Selection, Site Preparation, Planting Stock, and Regeneration. In R. Deal, & C. Harrington, Red Alder: A State of Knowledge (pp. 87-94). Portland, OR: U.S. Department of Agriculture – Pacific Northwest Research Station.

Harrington, C. A. (2006). Biology and Ecology of Red Alder. In R. Deal, & C. Harrington, Red Alder – A State of Knowledge (pp. 21-54). Portland, OR: U.S. Department of Agriculture – Pacific Northwest Research Station.

Patrick Shults, Extension Forester, Washington State University, patrick.shults@wsu.edu

Does Chipping Pine Attract Bark Beetles?

Lop and scatter method of slash disposal
Lop and scatter method of slash disposal. Photo: USDA

As many forest landowners already know, pine slash has a tendency to attract pine bark beetles. There are several recommendations for slash disposal to mitigate this problem. Two common methods — lop and scatter, and chipping — focus on eliminating bark beetle habitat while retaining the nutrients contained within the slash on site.

The lop and scatter method involves cutting slash into smaller pieces (less than 12 inches in length, less than 3 inches diameter) and scattering the pieces throughout the site. Chipping involves running slash through a chipper and scattering it throughout the site, similar to lop and scatter. While there is no chance that bark beetles can infest the chips, it is still possible for bark beetles to infest the pieces in the lop and scatter method. The chances of the beetles being able to successfully produce brood in these pieces is pretty low, as they will likely dry out before the larvae are finished developing. Both methods are a reasonable substitute to piling and burning when burning is prohibited, BUT…

Anytime you cut pine, you are releasing volatiles. Volatiles emitted from pine contain chemicals called monoterpenes. Some monoterpenes have been shown to attract bark beetles and are often included as part of the lure when trapping for bark beetles.

female bark beetle
A female bark beetle eating the phloem of a tree. Photo: USFS

If you are choosing between lop and scatter and chipping, which is best? You would think that chipping would be the better choice since there is no chance that bark beetles can infest the chips. But a study by Fettig et al. (2006) showed that chipping in ponderosa pine stands attracted bark beetles more so than the lop and scatter method, particularly in the spring. Why would this be? As it turns out, directly following treatment chipping releases much larger quantities of attractive monoterpenes than the lop and scatter method. If large quantities of attractive monoterpenes are released at a site where the slash has been chipped and bark beetles arrive, where are they going to go? They cannot infest the chips. They are going to go to the standing, residual trees. Although the act of thinning should increase the health and vigor of the residual trees, it will take at least a year for those trees to respond, therefore, you may wind up with bark beetle infested leave trees.

So, does this mean you cannot chip? Not necessarily. Some landowners prefer to begin thinning treatments on their property in the spring; it is finally warm (but not too warm), the snow is melting off, and they are ready to go outside and get things done. Other landowners find that they have to thin in the spring for various reasons (the snow is too deep during the winter; fire season often prohibits the use of machinery in the summer; contractors cannot fit everyone in during the bark beetle “off” season). Unfortunately, as Fettig et al. (2006) found, the response of bark beetles to high concentrations of volatiles released from chips is more significant in the spring because a large segment of the beetle population is most active during this time.

If you plan on chipping, it would be best to avoid treatments in the spring. The best time to chip (or do any sort of management that releases pine volatiles for that matter, including pruning), would be in late summer (late July/early August) through early winter (December). When producing chips, avoid piling them and absolutely do not pile chips at the base of any residual trees.  Spread the chips out in the sun if you can. The quicker they dry out, the quicker the volatiles will dry out, and the less overall bark beetle risk there will be.

By Melissa Fischer, forest health specialist Washington State Department of Natural Resources, Northeast Region, melissa.fischer@dnr.wa.gov

References

Fettig, C.J., J.D. McMillin, J.A. Anhold, S.M. Hamud, R.R. Borys, C.P. Dabney, and S.J. Seybold. 2006. The effects of mechanical fuel reduction treatments on the activity of bark beetles (Coleoptera: Scolytidae) infesting ponderosa pine. Forest Ecology and Management. 230: 55-68.