In an ideal world, timber harvests are premeditated activities, scheduled years, sometimes decades, in advance as part of a carefully considered forest management plan. Sometimes, though, stuff happens that's outside of our control. For some woodlot owners, that list of "stuff" has grown to include the hemlock woolly adelgid. The appearance of this invasive pest can force decisions on whether it makes sense to harvest hemlock relatively quickly in order to salvage some value from trees that will soon be lost. While it presents difficult decisions, there also can be opportunities to make choices that will ultimately improve the health of the forest and protect it against future pests and disease.
The hemlock woolly adelgid attacks eastern, western and Carolina hemlock. It can be identified through the "cottony" white wax filaments produced by the females.
Photo courtesy of the U.S. Forest Service.
Hemlock woolly adelgid is not a new phenomenon in the U.S., but its presence is growing. It was first reported in 1924 in the northwestern part of the country. In 1951, a separate population was detected in Virginia. Scientists now believe this introduction in the East came directly from Japan rather than spreading across the U.S.
Over the years, the pest has been confirmed in Oregon, California, Virginia, Delaware, Maryland, New York, Pennsylvania, New Jersey, Connecticut, Rhode Island, West Virginia, North Carolina, South Carolina, Georgia, Maine, Vermont, New Hampshire, Tennessee and Kentucky. "Its range extends pretty much to the southern extent of the hemlock's range all the way up to southern Maine right now," says David Jackson, forest resources educator with Penn State Extension. "I think it's just a matter of time before it moves further north into the mountains of New England."
He estimates that the hemlock woolly adelgid now covers nearly half of the hemlock's historical range. In Pennsylvania alone, the pest is now found in 49 counties. Jackson says that recent mild winters have allowed the hemlock woolly adelgid to spread further north and also into higher elevation forests: "Researchers have found that severe cold will knock it back, but I think we're going to continue to see it spread as long as we see warmer, milder weather." The pest attacks eastern hemlock, as well as western hemlock and Carolina hemlock, he adds.
For woodlot owners, this heightens the importance of scouting. "The only thing you can do to monitor for hemlock woolly adelgid is to be looking for the white waxy filament that the female covers herself and the eggs with. It looks like cotton, but it's actually wax," Jackson explains. "You'll see that generation in the spring and early summer. You just have to grab the boughs of your hemlock trees and flip them over and look at the underside. When they're heavily infested it will be very obvious - you can't miss it, really." The hemlock woolly adelgid can appear anywhere in the tree, and if it's high up in the tops it can be difficult to spot. It also might be found close to the ground on young seedlings, where it's easy to spot.
Adelgids are typically found near the base of the needles, where they use piercing-sucking mouthparts to penetrate the needles. "They're removing the fluids that the tree is translocating, so they're ultimately desiccating the tree," states Jackson. "The crowns will start to thin; you'll get a lot of needle drop and a discoloration of the crowns where it doesn't have the nice bright green foliage it should."
Depending on the level of infestation, the health of the tree and other factors, it may take some time for the hemlock to actually die. "I've seen trees hang on for five years in some cases," says Jackson. He says weather plays a large role in determining how long trees can survive. A real cold winter may stunt adelgid populations for a time, while mild weather may allow them to build up; and drought or other stressors on the tree can also make them more susceptible.
The hemlock harvest at Penn State Extension's woodlot was about more than salvaging economic value. It also provided an opportunity to improve overall forest health and wildlife habitat. For example, trees with little or no commercial value were marked with a "W," for Wildlife Tree, and retained on the site, particularly if they had holes in them or provided mast such as nuts or fruit.
Jackson recently saw firsthand what woodlot owners in many areas are experiencing when the hemlock woolly adelgid appeared in a 103-acre forest managed by Penn State Extension for educational purposes. "It really hit us hard - we really got devastated," says Jackson. "You really feel helpless. There are insecticide treatments you can do, but it's really not practical in a woodlot setting when you're looking at hundreds and hundreds and hundreds of trees." (Researchers are working on some biological controls, notably small nonnative beetles that eat the adelgid.)
Though no harvest had been planned in the Penn State Extension forest, the decision was made to cut a 15-acre area of infested hemlock in order to salvage some value from the timber before it died. "We were holding off and holding off, and finally we decided that if we were going to get anything out of this area we were going to have to make a sale," he explains. Hemlock being a softwood, the decision must be made in relatively short order to preserve the quality of the timber.
Jackson and his colleagues had to weigh the stumpage value of the harvested timber against its ecological value in the forest. The reality is that hemlock is a relatively low-value timber. If it were a higher-value species, the decision on whether and where to harvest might be a little different. "With hemlock, the economic incentive to harvest isn't great, so the decision has to be made about whether the monetary value is worth the disturbance you're going to see in the forest," says Jackson. "Often it depends [on] where it's growing and how easily you can get access to it. Every situation is different."
At the Penn State Extension forest, a network of forest roads was already in place, making for easy access, and a buyer was available, so it made sense to harvest hemlock in areas where streams and springs wouldn't be impacted. While the decision to harvest had to be made relatively quickly, the harvest itself was thoroughly thought out. The main goal was to generate some revenue from the timber sale while still protecting the wildlife habitat and water quality of the woodlot. For example, one affected section contained a number of springs and small drainages, so that was excluded from the harvest. "We put it in a streamside management zone and decided we just didn't want equipment in there," says Jackson, pointing out that this is just the type of scenario facing many woodlot owners. "It helps us show people the importance of riparian buffer zones."
A harvest forced by hemlock woolly adelgid also presents an opportunity to consider wildlife habitat. "You'll have a lot of opportunities to create cavity trees or insect feeding trees by leaving dead standing trees," he notes. Dead, downed woody debris and standing "snag" trees make good wildlife habitat, Jackson points out. "We marked a lot of trees that had holes in them already as well as other dead standing trees in order to provide nesting or feeding sites for wildlife," he explains.
This photo shows a good boundary of the cut on the right and uncut area on the far left. The marked trees are to be left. "A total of 12 different tree species were marked to be left," explains David Jackson, forest resources educator with Penn State Extension. "These trees will help reseed the area. Most will be harvested in five to seven years once the next forest is in place."
Photos courtesy of David Jackson/PSU Extension unless otherwise noted.
Hemlock harvests under these circumstances do present an opportunity to consider regeneration: which species will be coming in and how you can impact the makeup of the forest. While Jackson says he has heard some reports of a few trees showing resistance and surviving heavy adelgid infestations, seedlings are impacted along with mature trees, so it's difficult if not impossible to regenerate the forest back into hemlock. "It doesn't discriminate between 500-year-old hemlocks or tiny little seedlings," he says.
In the case of the Penn State Extension forest, the hemlock harvest provided an opportunity to improve the overall health of the woodlot by not only cutting the hemlock, but also getting rid of some lower-quality trees of other species and leaving select trees in place. "We left some really big, beautiful oak behind. In fact, we left 12 different species behind on the site that was harvested," says Jackson. "We're trying to really promote a diversity of species so we don't end up in the same situation again by having an area that's all hemlock or all ash. You really open yourself up to being attacked by one exotic insect that way." The opportunity to promote this type of diversity might be as good a reason as economic gain to conduct a hemlock harvest in the face of the adelgid, he concludes.
If dying hemlocks are left in place without a harvest, these trees, even with almost no crown, will provide some shade. "So you're apt to regenerate something that's shade-tolerant that way. Unfortunately, in Pennsylvania, we end up seeing a lot of black birch, which certainly doesn't provide the same value of shading streams or cover in winter that hemlock does," says Jackson. "We're not seeing evergreens coming back; we're seeing deciduous hardwoods coming back." Some woodlot owners may opt to try to plant or encourage natural regeneration of spruce or northern white cedar, for example, to take the place of hemlock, he adds.
Because hemlock is a relatively low-value timber, the decision to harvest and salvage some value is not always easy. At Penn State Extension's forest, for example, there was good access and an available buyer. In other cases a harvest may not make economic sense.
Finally, woodlot owners with hemlock stands don't necessarily have to wait until the hemlock woolly adelgid appears in order to act. It may make some sense to conduct a preemptory harvest. "There have been some studies showing that if you can do it well in advance of the adelgid, thinning hemlocks can help," says Jackson. Hemlocks tend to grow in dense, dark groves, which makes them perfect for shading and cooling streams. However, the competition ultimately hampers the health of the trees. Thinning hemlocks and freeing up the most vigorous one can help the remaining trees grow and expand their crowns, making them as healthy as possible in order to perhaps survive attacks from the adelgids once they arrive. "It may be something that can help them fight it off," Jackson says.
Patrick White is a freelance writer based in Middlesex, Vt. Over the past 10 years, he has covered a wide range of agricultural operations around the Northeast. He is always on the lookout for unusual stories.