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Hollow / habitat trees per acre in northern mixed forests

Started by mn_timber, February 04, 2024, 10:26:29 AM

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We own and manage quite a few areas in north central Minnesota, consisting of Mixed Conifers (Red, White Pine, Spruce, Fir, Tamarack) and Mixed Hardwoods (Northern Hardwoods, Red Oak, Bur Oak, Lowland Hardwoods).   We are increasingly applying ecological silviculture prescriptions, and focusing on long term carbon storage and sequestration, general health, productivity, retention of high grade sawtimber for future value.  "Low grading" (opposite of high grading) is an expensive endeavor in the short run, because of the low value of what we harvest.  But we make that decision and we live with it and have a plan for economic sustainability by manufacturing and marketing natural character hyper local, Uber sustainable high end solid wood products direct to regional consumers and builders.   

This all is great in theory, and is kind of working in practice.  I at least feel comfortabe with what where doing when making ecological silvicultural harvest and tending operational decisions (especially timber marking).  But the piece that I feel we may be falling short on when we're low grading, is retention of an adequate number of habitat trees.  Rotten, hollow, standing, soon to be on the ground, etc.  Habitat for mammals as large as a black bear, as well as birds, rodents, insects.   How do I go about managing the quantity and quality of habitat trees?  Are there written guidelines, and if so, are they possible to integrate into the complex economic + ecological silviculture/forestry approach?

Ron Wenrich

Kudos for your mgmt planning.  I tried to do things like that a long time ago, but found it was hard to sell landowners on the idea.  They were more interested in today's return rather than tomorrow's.  I was able to weed out some of the undesirables in a sale while retaining some good crop trees.

I'm didn't do much for wildlife, but I do recall a rule-of-thumb of 1 den tree/acre.  I think a lot depends on the type of wildlife you're trying to attract and what's in the area.  You'll also need a few really large trees for the bears, and would need them spread out.

EDIT:  Looks like Ron Scott posted a higher number of den trees/acre.  Go with that.

One problem I found with hollow trees is that they were generally of a species you didn't want a lot of.  Diversity is good in forestry, but if the diversity is in a bunch of low quality wood, it won't pay for the mgmt costs.  Ones I used to find were black gum.  Good seed source and prolific. Those I would mark to be girdled and maintain a den feature without the seed source.

There is wildlife that needs habitat that don't use dens.  Leaving tops can benefit other critters.
Never under estimate the power of stupid people in large groups.


Does your inventory account for dead and dying in the categories that you expressed? How are you accounting for that carbon?

Knowing what you have and where it may be goes a long way into determining a desired future condition.

As a guide to the amount and distribution of potential habitat trees observed near your property, query the USFS FIA (Forest Inventory and Analysis) reports and data for that area. Somewhere regional averages are discoverable and can provide a target. 

An analytical approach would be to use existing satellite and aerial imagery and look for conditions similar to those you seek using remote sensing. Sentinel 2 data include bands specifically designed to measure plant stress. Indicies such as NDVI (Normalized Difference Vegetation Index) are calculated and be compared across your region, and across time using historical data for change detection. 

Great project to learn about remote sensing using open source software. Detailed work with specialized knowledge that requires some training on tools and techniques. Several hours of video tutorials will get you there. GIS with remote sensing is a fantastic skill for managing land. Extremely useful skill to make beautiful, colorful maps to impress the high end clients.  

Freelance Remote Sensing specialists love small one-and-done's in hopes for future projects. 

Expensive, extremely high resolution imagery and a skilled technician could identify individual trees with high confidence. This level of inventory might be appropriate for documenting compliance with certification services. 

Your question edited as a prompt for AI, or searched on should provide other insights. 

Carbon sequestered upon request.

Ron Scott

The general rule of thumb is to leave 3-5 den, snag, or cavity trees and mast trees/acre for wildlife.

The longer living species are preferred such as American beech, white oak, black cherry, ash, hickory, ironwood, hemlock etc. rather than the short-term species such aspen, paper birch, etc.

Hard and soft mast trees and shrubs are both favored for game and nongame wildlife species.


This page has a good chart on minimum diameter per species 

Also recommends:
  • In areas under uneven-aged management:
    • Retain a minimum of six live cavity trees and/or snag trees per acre, with one exceeding 18-inches DBH and three exceeding 12-inches DBH.
    • When lacking such cavity trees, retain live trees of these diameters with defects likely to lead to cavity formation.

    Keep up the good work!


Good for you on your plans! I think if you are trying to leave some snags for wildlife you are probably doing better than most other land managers. So I wouldn't focus on a number but on actually doing it and being selective about the trees (quantity over quality). 

With that said, I know the DNR did a study on fisher in northern MN and found that their preferred den tree was an aspen over 20-something inches DBH. Which I think is generally difficult to find. THat may very well be the results of fisher using what is on hand. For all I know they could prefer white pine but we don't have as many of those.

If I were in your position I would either leave good den trees or make some by girdling large aspen. And I would have a preference for the latter because quality den trees aren't common around here.
SAF Certified Forester


What is the progression of a girdled large aspen that ends up a quality den tree? Trying to put that together in my mind. Thinking you may know. 
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others


 I suspect that fisher couldn't care less about tree species, they just want a hollow tree with an entry hole. It has to be a challenge to manage a forest for both wildlife and quality timber production. But I applaud anyone for trying.


I don't know much about aspen progressing to den trees but I do know that of all the species we have in northern MN, not many consistently make good den trees. Of the species we consistently have up here, cedar basswood and maples are probably the most consistent to find hollow. 

Aspen can grow large enough to become hollow and suffer from eutypella canker often enough that it should provide a good start to a den tree. I don't think fisher care about the species either, they just need a large enough cavity. 

In the larger picture, I think we are sometimes too set in our ways to make management choices happen. For example, the Forest Service had (and probably still has) a policy that any marked tree on a timber sale had to brought to the landing. When my friend worked for Pioneer forest he said they had similar policy but marked trees had to be girdled, dropped or brought to the landing. 

Coming back to snag/wildlife/den trees, sometimes I think we just need to make them. Girdle the tree and walk away. Let nature take over.
SAF Certified Forester


Along these lines, Pileated Woodpeckers often excavate nest cavities in aspen. No doubt, in part because aspen is soft/easy to excavate. The one and only nest that I ever found was in an aspen. Ruffed grouse will eat aspen buds.


When I mark trees on Forest Service land or private property I will intentionally leave good den/ wildlife trees as I find them. Often I will paint a W on the ground so the auditor knows why I left it. Most of the time they wouldn't be good for anything other than firewood if that. Loggers don't want to cut a hollow 24"+ sugar maple or basswood, it's just wasting their time. Big basswood tends to get hollow here like a gun barrel, if you bang the tree with a hatchet(blunt side) you can usually tell if it is hollow or not.


Summer of '92, I was on a USFS National Forest in Oregon participating in a demo for removing Pacific Yew when it was all the rage for harvesting yew for its bark after discovery of natural ingredient from the bark, "paclitaxel". This led to the cancer drug Taxol. 
Yew harvesting techniques were being designed and developed in haste at that time. While on this demo, forest rangers explained the timber harvesting guidelines in effect at that time. Trees were marked for harvest, but dead trees had to be left standing, and several perfectly good trees has to be left cut and left on the ground to rot, as well as some good trees left standing to eventually die and become wildlife trees. Not sure if this was a plan for the harvesting on just that Forest or not. Didn't sit well with some of the eco enviro participants in the group of attendees. 
Soon after this rage to rush the harvesting of the yew bark, artificial production of Taxol saved the yew. 
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others


I have heard of intentionally leaving "future" wildlife trees, but that doesn't make much sense to me. If you leave a tree long enough they're all candidates. The east side of the Chequamegon- Nicolet NF has been girdling a lot of red oak, but only to try to stop spreading the oak wilt.


Just so you know. Most bear dens I have found are under old hardwood stumps and some under blowdown with raised roots. Found one of those blow down ones last fall and very active. Dens I find are usually surrounded by young thickets. They don't need standing trees. Last summer I found 2 bear dens on one lot. One recent, one older. Also found the bear, which I had to scare off and he never came back. :D I've also found bears around rock outcrops. I remember one time being at the bottom of rock outcrop and happened to look up, Mr bear was looking down at me. I'd say he was 20 feet higher than where I stood. You are being watched. :D

I have a section of woods and neighboring woodlot owner where the woodpeckers thought they died and went to heaven. The aspen are all dying and falling down and only 30 years old. Softwood, ash, red maple, white birch will soon be the new forest. White birch lives 200 years up here. I've cut a lot of aspen on me that are pecked in the upper third and often cankered. The only aspen I see making it fast 40 years are large tooth. Years ago we cut trembling aspen twice that age, big wood. The difference was that was seeded after fire, the current stuff is suckered regrowth off roots. Doesn't live as long.
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))


I am big in leaving dead trees standing. In fact I kill many trees and leave. My preferred method of killing is drilling 3/8 holes about 3-4" apart at about waist height all around a tree. Drill about an inch deep at a downward angle so that herbicide remains in the hole. I use 20% glyphosate for the herbicide. Best done in July to November.

I kill primarily Aspen and Red Maple. Tho in areas where I want to thin and open up I will also kill sugar maple and Ironwood. Usually The Sugar Maple and Ironwood are small, larger ones used for firewood. These add to all the dead Ash that EAB has killed.

I have had Pileated woodpeckers nesting in dead aspen in the last 2 years. I usually hear them making the nest then after that its easy to find.
They don't reuse the cavities but make a new one every year.

I would say that the amount of dead trees per acre is very high in the areas I have thinned. In the neighborhood of 2-3 doz/acre.


I suspect there is no hard and fast rules about "habitat" trees, because it's going to depend on your actual goals. If you want wildlife / wood production / carbon sequestration or something else. 

I think the right answer is "some". More nest trees than the area can feed won't get you more owls / woodpeckers / bats than the area can support. At the other extreme, having none means many of those critters have to move out. 

Recommended numbers are going to depend on local conditions and management objectives. 
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)



My plan is to keep thinning for firewood needs and leave lots of seed trees and potential log trees for future, many of them will without a doubt go over mature and be cavity trees. Mainly fir with scattered aspen. I've got lots of spruce, but it will be 70 more years before they have much size. I target trees of poor form and vigour for firewood. There's all kinds of trees that will carry on for seed, logs and wildlife. I don't clearcut and I can't even thin it fast enough for what little is needed for firewood. Nothing much growing up here is high value, no matter how perfect it looks, there's no end in sight of supply plus you're competing with government low ball prices. And they are clear cutting wood every 30-40 years now. A lot of bigger wood on trucks now is what they left behind before and mostly low grade. 
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

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