Natural Ways to Kill a Tree

Girdling and Fire 

Stewards and managers of oak ecosystems know their biggest problem is the dark. Too many shrubs and trees. Our oaks and their thousands of interdependent animal and plant species represent more than 5 million years of evolution. These fire-dependent ecosystems comprise our richest wooded lands. But they’re losing species – dying a death of shade.

 

Knee-jerk environmentalism deplores killing any tree. Many well-meaning folks initially resist thinning. Other people argue that cutting trees is indeed natural. Certainly beavers do it. But in this post we’ll focus on two ways biodiversity stewards achieve standing dead trees – girdling and fire. 

 

A Very Short Case Study: 

One of the finest oak woodlands in northern Illinois was acquired at great expense by a conservation agency. Three decades later a study showed it to have ten times more trees - mostly not oaks. Plant and animal species, starved of light, were dropping out. The preserve was degrading. Occasional mild burns had not been enough for sustainability. Too late now, to solve the problem by fire alone. Time to cut or girdle. 

 

Part the First: Girdling

 

The basic principle behind girdling is simple: remove the phloem and leave the xylem. If you’re like me, you once knew the meaning of those words, but then forgot. The inside of a tree is the wood, the xylem, the stuff a xylophone is made of. Wood is important to a tree, but dead. The phloem is not the bark. It is a half-inch or so of living tissue that surrounds the xylem, and it has a crucial purpose. It brings the products of photosynthesis down from the leaves to nourish the roots. 

 

Most trees, if you cut them down, will put up lots of shoots, from what’s stored in the roots. (The roots send water and minerals up to the branches and leaves through the sapwood, the outer wood, just inside the cambium.) In time, those shoots will grow into new trunks.  But if you girdle most trees, the roots will sense that the top is still doing fine and doesn’t need more shoots. Yet the roots will starve. Is starve too harsh a word? The roots will age peacefully, go through the stages of life, and give up the ghost in tranquility. No one will hear the tree fall, because it will stand for many years as a bounty for woodpeckers, other hole nesters, beetles, and mushrooms. A smooth transition from living to life-giving – how beautiful is that!

 

Few understand either “the why” or “the how” of girdling. But some are learning. Shown here are stewards Monica Gajdel and Charlotte Ahern at Shaw Prairie in Lake County.

 

Th next photo shows the heart of Shaw Prairie, about 100 feet west. The major problem here is encroachment by aspen. With lack of fire, this one tree species can destroy a rare high-quality prairie as the network of tree roots launches an army of thousands of new trunks. Great effort for years had gone into driving the them back, but the roots of the large aspens around the periphery sent reinforcement energy to the dastardly invaders, and they continued to advance.  

When cut and herbicided, these death-dealing little aspens have survived thanks in part to resupply through root connections with the big aspens behind them. 

This post will focus mostly on four sites where Friends of Illinois Nature Preserves girdle for the good. No need for herbicide, quick and easy, safer for the stewards, good for wildlife, good for the ecosystem – girdling is often a best approach. 

 

We stewards and staff spend much of our winters cutting trees down and burning them in bonfires. This too is important and good. But look at the photo below and compute how much work was saved by girdling. Biodiversity loss is a crisis for the planet. Do we have time to waste?

The photo shows a piece of Langham Island Nature Preserve in Kankakee County. This unique island has long been known for incredibly rare plants. Native Americans likely burned it regularly for their own purposes, and a rare ecosystem remnant came along for the ride. Conservationists started to cut and burn in the 1980s. But they didn’t burn enough, and trees and brush grew so dense that most of what was special about the island faded out … or barely hung on. When Friends of Langham Island came on the scene in 2014, their biggest challenge was too much wood. They cut enough in a few key places to demonstrate that rare nature here could recover. So far so good. But, decades after restoration had begun, more than half the island still lay the gloom of unnatural shade. 

Belatedly, we girdled. I count thirty trees girdled in the photo. Compare that number to three trees we saved (bur and white oaks, marked with blue and white flagging). A shocking slaughter? But an oak savanna thrives best with fewer, scattered trees. The rare plants and animals here needed more light. Reproduction of the oaks requires more light. (In other parts of the island we left more shade – or cut all trees to restore prairie. There are many considerations, and variation itself is part of nature.)

One more dramatic fact about the photo above. We had marked those few young oaks years ago. By the time our work reached here, the one in the foreground was already dead. Shade kills. Let there be light. 

  

Let’s review in more detail why girdling may be a good approach:

• Herbiciding may be needed for many purposes, but we prefer to minimize it in the natural ecosystem. Girdling uses the trees' natural processes to replace herbicide.
• Especially when there are thousands of invading stems close together, there is a risk that enough herbicide to kill the invaders will kill many of the surrounding grasses and wildflowers also. We worry about that risk most in the finest, rarest areas.
• In many preserves, old standing dead trees are missing. They're an important part of a natural ecosystem. To thrive for wildlife, woodlands and savannas often need more standing dead trees than they have today. 
• There is urgency. Most nature preserves don’t have either the budget or staff that they need. They never will. The needs are too great. Bigger budgets, more staff, and more volunteers are needed –  and more efficient methods. It’s not good enough to argue that “we’ll just keep plugging away.” More and more priceless biodiversity is being lost day after day, year after year, decade after decade. Girdling speeds the recovery.
• It’s safer. You girdle and leave before the limbs and trees fall. In most cases, the tree never does fall. Instead it disassembles: little branches drop first, then fungus rots, and finally, dried out, standing or fallen trunks burn up during a controlled fire. 

While “the old snag” is standing, the ecosystem appreciates the diversity of fungi, bacteria, lichens, grubs that eat wood, and woodpeckers drilling holes. Animals that raise families in those holes include flying squirrels, great crested flycatchers, eastern bluebirds, wood ducks, screech owls, chickadees, titmice, wrens, white-footed mice, all the woodpeckers of course, and much more. 

 

How to

 

It is by far easiest and most successful to girdle during the “period of bark slip” – late spring and early summer. At this time, the cambium is growing fastest, the phloem will pop off with a little help. (The cambium is the thin, weak, creative layer that divides the xylem from the phloem – and which generates both.) 

 

With a saw or axe, make two cuts around the trunk about six inches apart.
Then with a hammer or the back of an axe, knock out the phloem, which may even pop off in one piece if the bark is “slippy” enough. Do not try to shave off the phloem, even though it might seem easier, because it’s then difficult not to leave strips of connecting phloem or cambium, in which case the trunk will recover, and you’ll have wasted your time. 

 

Special cases

 

Aspen is an especially good tree for girdling because any other approach is likely to result in monstrous re-sprouting from far-flung roots, no matter how much herbicide you apply. In my experience, isolated stands of large aspens were entirely killed with no re-sprouting by simple girdling.  If the aspen already have a lot of root sprouts, this approach does not kill them. But it does prevent the large trees from over-riding the herbicide. Someone might also ask: “Why kill aspen at all? It’s a beautiful and native tree.” That’s true. In large preserves it may play an important role in the fire dynamic. But for some remnant savannas and prairies it has become a lethal invader and needs control.

 

Black locust, silver poplar, and tree-of-heaven don’t respond well to girdling. Unlike most trees, for unknown reasons, they put up massive root-sprouts. These species can be better controlled by regular cutting and stump treating, or by frilling, in which you make deep cuts into the tree and apply herbicide. Even then, you may get lots of resprouts, which then will be best controlled by foliar spray.  

 

Buckthorn can be successfully girdled, but it’s rarely worth it. Girdled buckthorns often put up a circle of sprouts under the girdle, which can be knocked off easily, and then the tree dies. But buckthorns are often so dense that all the dead and falling trees make such a mess that it’s hard to facilitate ecosystem recovery, so we cut and burn them. 

 

Follow up

 

In years following the girdling, sometimes no follow up is needed. The tree dies; the limbs gradually fall off; the trunk bit by bit flakes and crumbles; and the remains are just nature. In other cases, the dead tree catches fire during a controlled burn, and it goes out in a blaze of glory. 

 

But dead trees can need clean-up. All trees die, sooner or later, and may become a problem. If a girdled tree presents a danger to a trail or other heavily used area, most managers would cut it before it starts to weaken, as with any dead tree.  

 

If a great many trees girdled at the same time fall and make a colossal mess at the same time, as sometimes happens, at least two approaches are possible. One approach is to cut them all up and burn then in bonfires, as we might have done in the first place. This takes time, but there are still advantages. The dry wood is lighter and burns faster and cleaner. Herbicide expense and damage were avoided. And the woodpeckers and their friends had a great time for years. Another approach is to just let them rot or burn and, in the meantime, explain to horrified people what’s going on.   

 

Part the Last: Kill by Fire

 

Killing big trees with fire takes a hotter burn than most sites get. It’s easy to manage a fire that will top-kill saplings. It’s more difficult to manage a fire powerful enough to kill large invading, over-dense trees. Many burn bosses successfully do it, on rare days when they get the right weather. Over the millions of years that prairie and oak woodland biodiversity evolved, the fires that spread and raged most extensively over the landscape occurred on days too hot, dry, and windy for most fires today. That's why we sometimes have to cut and girdle. 

 

Bonus photos and details

Below, a former landscape of prairie, fen, and sedge meadow had been over time invaded by clumps of box elders, buckthorns, cherries, and the like: 

Friends of Kishwaukee Fen Nature Preserve first made a plan,

 then cut the buckthorns out of the way last winter. 

For the bigger trees, this summer Ben Davies (right) saws parallel cuts through the phloem. 

Next, like Tom Ziomek (left) he'll use a hammer to knock out the sawed strip. 

After quickly girdling the bigger trees in June, the stewards went on to other priorities. During this summer, they are herbiciding the understory buckthorn seedlings and other invasives. This fall, they’ll re-plant the prairie that once grew here, which is expected to start off well in 2023 under a “nurse crop” of thinning trees. When the trees are dead and dried, they'll be burned on some cold winter day. Standing dead trees in a prairie are not a plus. 

 

Notice above that some girdles are bright, and some are dark. The fresh girdles stand out shockingly (or “beautifully,” according to some). Trees girdled a week earlier have turned darker. 

 

At least five elements stand out in the above bonus photo from Langham. 1) To the left rear, you can see how dense the honeysuckle and other brush had been. 2) To the right rear, you can see the one old oak that grew in an open habitat. 3) All the others are “pole trees” which will never make a natural ecosystem here. If you blow up this photo, you can see four small bur or white oaks marked with blue-and-white flagging; they’re much smaller than the pole trees and would have died soon without help from stewards. Some oaks flagged a few years earlier died before cutting or girdling liberated them. The surviving oaks are unnaturally tall, thin, and weak. Opening up this area by girdling gives them a better chance to grow stronger trunks and broader limbs, as such oaks normally would. But if they start to break or fall over, we may cut the tops (above deer-browse level) and let them fill out as would be more natural for an oak. 4) Some understory grasses and wildflowers have begun to recover; mostly they are woodland plants that have invaded during the time of shade; other species are being seeded in from more open parts of the island; a few species are recovering from the fire-liberated seed bank (but they won't survive if it's too dark). This more gradual process may lessen the chance that aggressive species like tall goldenrod will take over. 5) Stewards Molly Bilderback-Ulrich and her sister Ally can do this powerful work efficiently … while others pull invasive weeds and gather seeds. They found that 3 people could girdle 23 trees in 2 hours.   

 

On a great  2018 tour of Braidwood Savanna, Will County Forest Preserves’ land manager Floyd Catchpole showed us the above sedge meadow. It had been degrading in the shade of invading silver maples. Staff not only girdled the maples shown above but also many over-dense oaks in the sand savanna visible behind the meadow. 

 

Pole sugar maples girdled among bur and white oaks in Somme Woods. Note the almost complete lack of  grasses and wildflowers.

 

Young but big cottonwoods around a pond that had been in an open savanna landscape in a 1938 aerial Somme Woods photo. These were girdled by staff with chainsaws.

 

The options and decisions can get complicated. Consider this Somme Prairie Grove photo: 

Should this beautiful shagbark hickory have been “a keeper” or “a girdler”? Trees are far apart here, and shagbark is a natural part of an oak woodland. It would not be unreasonable to retain this tree and let the fires decide if it should go. In high fuel areas, hot controlled burns often kill hickories. But this area is receiving intensive restoration. Sadly, the original vegetation at this spot was killed utterly by dense shade, cleared years before. Restoration depends on painstakingly gathered rare seed. We don’t want to waste it. The 1839 Public Land Survey shows this area to be on the line between prairie to the west and savanna to the east. Thus, any conservative surviving fungi, invertebrates, bacteria etc. would be adapted to prairie or savanna - not woodland. 

 

The above photo looks east. The girdled hickory and the bigger red oak next to it are relatively young trees here. The two trees in the background tell the real story. They have huge horizontal limbs reaching west. They stood at the western edge of trees here; the hickory stands in former prairie. The next photo will show our same hickory, but looking west. 

In this direction, all was prairie. Those tall, thin trees grew that way among others in the absence of fire (most since cleared by stewards). They are not adapted to the biodiversity restoration under way here. They've been left in place for now because there were higher priorities. Perhaps over the years and decades they’ll remain and develop an okay relationship with the gradually improving prairie and savanna around them. Perhaps not. We don’t have extra time to think and plan for them at the moment. But this foreground tree was adjacent to an area of recovering high quality savanna. Fuller light was needed. As the hickory dies, we will scythe the dense woodland sunflower and tall goldenrod around it, to save many semi-smothered species that are struggling to gain a toehold. (They include wide-leaved panic grass, yellow pimpernel, baneberry, and rue anemone to the savanna east – and Leiberg’s panic grass, cowbane, Canada milkvetch, and Culver’s root of the savanna-edge prairie to the west.)   

As stewards watch results and adjust the plan (as the ongoing process here is beyond detailed predictions), we may work to facilitate prairie in this spot (no shade) or savanna (some shade). The decision may depend in part on whether especially significant species here seem to need more of one habitat or the other. As the world turns. But we’ll be inspired to do our best by the success of older restoration on the back side of the low glacial hill you see on the right. There, young bur oaks thrive among very high-quality herb vegetation. Thus, two last photos, the first taken on the other side of that low hill, in an area of restored high quality, looking west:

 

Second, a photo taken on this side of that hill, on the edge of the bur oak woodland:

This was all bare ground under buckthorn. The herb flora restoration started "from scratch." All that was here were the old oaks, the soil biota, and some invertebrates. 

None of the herb species identifiable in in the above two photos were in this area when the restoration began. Such recovery of ecosystem health is what inspires us stewards.  

 

Bonus eight-second video

Okay, it's goofy, but fun, and what we managed to record. Emma Leavens asks a significant question: Instead of whacking off the phloem, she tries pulling it off, and she asks Molly if that's okay. 

Molly assures her that it is. Technically, it's "shaving" the phloem off that tends not to work. When people try that, filaments tend to survive through which the phloem can grow back; a year later you'll have a whole new phloem, and the problem shade will stay the same. What you want is a clean break at the weak cambium layer. That's why it's generally best to whack it during the period of bark slip ... so it pops off or peels away. 

Bonus diagrams and a sort of a cartoon 

old drawings by S. Packard 

from a 1991 Nature Conservancy stewards handbook

Additional info on the sites

Kishwaukee Fen Nature Preserve:

https://woodsandprairie.blogspot.com/2021/09/introducing-fen-in-need.html

https://woodsandprairie.blogspot.com/2021/10/kish-fen-kick-off-sunday-oct-17.html

https://woodsandprairie.blogspot.com/2022/03/controlled-burn-at-kishwaukee-fen-march.html

Langham Island (Kankakee River Nature Preserve):

https://woodsandprairie.blogspot.com/2014/08/survivor-langham-island.html

https://woodsandprairie.blogspot.com/2014/10/myth-or-miracle-unexpected-news-of.html

https://woodsandprairie.blogspot.com/2019/10/degradation-and-redemption-at-langham.html

https://woodsandprairie.blogspot.com/2020/08/langham-island-rebirth-of-rebirth.html

https://woodsandprairie.blogspot.com/2020/09/september-12th-langham-island-update.html

https://woodsandprairie.blogspot.com/2020/11/langham-island-update-and-plans-nov-5.html

Shaw Woods and Prairie (Skokie River Nature Preserve):

https://woodsandprairie.blogspot.com/2021/09/the-diversity-and-promise-of-shaw-woods.html

https://woodsandprairie.blogspot.com/2021/11/anatomy-of-new-community.html

https://woodsandprairie.blogspot.com/2022/03/burn-at-shaw-prairie-march-21-2022.html

https://woodsandprairie.blogspot.com/2022/05/may-14-2022-shaw-woods-what-we-did-and.html

The Somme Preserves:

Somme Prairie Grove

https://woodsandprairie.blogspot.com/2017/07/july-9-2017-restoration-tour-of-somme.html

https://woodsandprairie.blogspot.com/2018/02/the-somme-prairie-grove-experiment.html

https://woodsandprairie.blogspot.com/2020/10/a-celebration-of-vestal-grove-study.html

 

Somme Woods

https://woodsandprairie.blogspot.com/2017/12/the-unexpected-discovery-of-somme-woods.html

https://woodsandprairie.blogspot.com/2018/02/principles-of-somme-woods-conservation.html

https://woodsandprairie.blogspot.com/2018/04/breeding-bird-revival-after-habitat.html

Acknowledgements

Fundamental credit goes to the stewards and staff and interested supporters of the land-owning and conservation agencies:

All sites - Illinois Nature Preserves Commission

All sites - Friends of Illinois Nature Preserves

Braidwood Dunes - Will County Forest Preserves

Kishwaukee Fen - Village of Lakewood

Langham Island - Illinois Department of Natural Resources

Shaw Woods and Prairie - Lake Forest Open Lands Association

Somme Preserves - Cook County Forest Preserves

Girdling diagram by Paul Nelson from the Tallgrass Restoration Handbook (Packard and Mutel, eds.) 

Credit for proofing and editing go to Rebeccah Hartz, Jess Sladek, Eriko Kojima, Andrew Rosulek and Molly Bilderback-Ulrich.