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  1. #1
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    Authoritative resource for protecting tree roots

    I hope somebody has already dealt with this question. I need an authoritative resource for how to protect tree roots.

    As the trail tread wears the tree roots become exposed. For years we have packed dirt around the roots and then rocks over that. Basically, we are replacing the soil lost or compacted on the trail tread. It seems to work fine and not harm the tree. But now some crackpot is claiming we are harming the tree. So I need some authoritative guidance on how to protect tree roots. Some one like USFS or similar would be best but I have not found anything.

    Can someone point me to such a resource?

  2. #2
    K&K
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    I'm just having fun posing as a research librarian, no professional expertise claimed.

    "Trail Construction and Maintenance Notebook" published by the National Forest Service here:
    https://www.fs.fed.us/t-d/pubs/htmlp...806/page09.htm
    This guide includes recommendations to cut the roots in some situations.
    "Remove roots that are parallel with the tread. They help funnel water down the trail and create slipping hazards."
    "Often, a sharpened pick mattock or Pulaski is used to chop away at the roots."
    Though the do offer other advice that doesn't include damaging the tree.
    "If roots are perpendicular to the tread, fairly flush, and not a tripping hazard, leave them.
    Route your trail above large trees. Building below trees undermines their root systems—eventually killing the trees."

    I was a bit surprised how often sources suggest cutting the roots and or removing the tree!

    A more tree-friendly approach is suggested in the following links:

    NMSU Cooperative Extension Service that suggests a rather elaborate solution that are aimed at protecting the tree, though it is in a yard situation.
    https://aces.nmsu.edu/ces/yard/2005/022605.html

    This link from a Pacific Northwest professional Arborist suggests that 2 - 4 inches of course cover is an appropriate approach
    https://www.arborilogical.com/articl...ver-with-soil/

    Several other credible sources echo the 2-4 inches of topsoil. All warn against adding more than that.
    https://www.isa-arbor.com/Portals/0/...m-planting.pdf
    https://www.mortonarb.org/trees-plan...-root-problems

  3. #3
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    Thank You! I had found the USFS link but not the others.

    What we have been doing is really close to the gravel and tiles from the NMSU article. But we are doing it with rocks found nearby.

  4. #4
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    Between all the natural wind, snow and other stuff that takes down trees, add in forest fires and logging, this is a concern? Are you in a old growth forest or protected species habitat? What state are you in? Seems the proof would be on the person claiming tree harm. Ugh.

  5. #5
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    Quote Originally Posted by leeboh View Post
    Seems the proof would be on the person claiming tree harm. Ugh.
    You would think so. But people like that can be ridiculously loud and annoying about the issue and using authoritative documents as OP is trying to do can convince a land manager to dismiss them even quicker. Otherwise, some land managers, at least, might be prone to acquiescing just to shut that person up.

    I've definitely noticed a trend in trailbuilding over the past 20yrs or so to give trees their space. Part of it is about protecting the trees from root damage, undermining, avoiding trees that would be a crash hazard, as well as avoiding undesirable corridor narrowing as the trees grow in girth. So small buffers tend to be given around bigger trees (at least as long as there's space to do so), and saplings are generally just removed. And places where large roots protrude tend to get armored, in part, to protect them, but also to limit the creation of a million alt lines, as well as limiting further erosion around them.

  6. #6
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    ^^^^ OK, some good points. CXagent, where are these trails? MA guy here, so many trees, so much forest everywhere. Most of the times our hand built trails are about the path of least resistance. Not cutting down mature trees for sure. Roots are everywhere here, being most of the trails are in a forest.

  7. #7
    since 4/10/2009
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    Quote Originally Posted by leeboh View Post
    Roots are everywhere here, being most of the trails are in a forest.
    Super common anywhere you've got shallow soil over a rocky base (so roots protrude whether there's a trail or not) and enough moisture to support tree growth. Lots of places like that, generally. I see that OP is in Austin, TX, and I've certainly seen the gnarled surface roots on the trails out that way, similar to what I see all over Pisgah now. Austin also has enough of a kook population that I can see the hyper-protective complaints about the trees being an issue.

  8. #8
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    I've worked a whole lot with this scenario. It's something that needs to be evaluated on a case-by-case basis. What I look for is what type of root mass am I dealing with? A large buttress root, smaller support roots, or fine feeder roots? Each gets a different approach.

    Exposed roots are one thing but the most critical issue is actual physical damage to a tree root. In many cases, again depending on circumstances, removing a root and doing so properly by cutting with a sharp saw and then burying the distal cut end in the soil can be a viable solution. It's not a lot different than removing a branch in the canopy. Continuous impact damage and exposure of a root can create an entry point for pathogens that might possibly translocate to the trunk and create potentially life threatening situations but there is no "one-size-fits-all" fix. As an Arborist, it's helpful that I can be out on builds as we encounter these situations. We can easily taylor an appropriate fix based on the given circumstances. If damage has occurred to a root, in many cases, it can be more beneficial to remove the root as tissue damage along a roots surface can not necessarily be repaired by the tree's compartmentalization process.

    Covering exposed and undamaged roots is perfectly acceptable. We generally don't want to cover them with an impermeable material. We need to encourage the natural gaseous exchange between soil and atmosphere. 4" inches of soil cover is usually adequate if it fits the situation. I have used armoring with native rock sitting on top of a layer of soil over the root, I've used 70# decorative pavers and backfilled with soil or gravel and incorporate it as a high spot for a grade reversal. So many thing to be done that work effectively but just need to be considered on an individual basis.

    Fly me down there...I'll look at the root!

  9. #9
    FatBike Fiend
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    Alaskan trail builder here... I deal with this issue a lot as we tend to have really shallow, spreading root systems due to cold subsoils. I love to use interesting-looking trees as anchor points for turns, and as other aesthetic and functional features such as gateways, chokepoints, and as viewpoint framing. I try to locate most of my trails on side slopes of varying steepness for drainage purposes and have found that when one excavates a full-bench trail on a steep hillside through a stand of shallow-rooted trees, you end up cutting almost all the roots that protrude into the trail prism. In fact, the tread surface usually ends up almost completely root-free because it's below the root line in the slope profile. All too often, this means the tree will die a long lingering death over the course of a few or more years and end up crashing down onto the trail during a windstorm -- which doesn't do wonders for the intended aesthetics or function.

    I've also noticed that tree root structures tend to mirror the branches of the crown; the further from the trunk you prune, the less damage you do to the tree, so I try to route the trail outside the drip line of the branches whenever I can which gives the tree a good chance at survival. Mature trees are also much less forgiving of severe root pruning than younger, actively growing trees. This pretty much precludes using mature trees as anchors near the radius points of anything other than large radius turns, though. At these spots, it's often better to save a smaller, younger sapling and let it grow into the desired anchor feature or use a large rock if available.

    Also, different tree species have different tolerances to root burial. Up here, birch and white spruce roots will slowly suffocate with anything over around 4" of soil added on top of them while aspens and cottonwoods will often tolerate several feet or more of soil piled on top. So if you need a tree as a feature close to the trail, locate the trail near a cottonwood or aspen.

    Bottom line is that a good many of the trees adjacent to the new trail are going to end up dying, so the best strategy is to pick and choose the most desirable trees, take measures to preserve them and not worry so much about the run of the mill trees. And to answer your question, no, I haven't run across much literature on the subject.

  10. #10
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    Thanks for all of the responses. I will try to respond properly. If I miss your question, it was not intentional.

    The trails are in and around Austin Texas. Or soils are typically thin, primarily clay soils a few inches over a rocky base layer. Many trees have spreading root systems that attempt to stay above the rock layer. Those trees blow over in storms. Larger (older) trees have a lot of roots above the rock layer plus a lot of roots penetrating the rocks thru cracks. These trees are well anchored.

    There are several problems with the exposed roots -
    1) The indicate how much soil has been lost on the trail since they are buried just off the trail. Land managers don't like soil loss.

    2) A lot of our trees are oak trees. Exposed oak roots get damaged and become an entry point for oak wilt. Once oak wilt starts in an area - just about all oak trees in the area will die. Better to protect the roots than lose a bunch of trees.

    3) The City has a criteria for impact to a tree root zone for trees 19 inches in diameter and up. Most trees less than 2" diameter we removed. Most invasive species we remove. Even invasive trees above 5" diameter we have to have approved before removal. At this point the City usually agrees with our assessment or has only minor modifications.

    4) We really don't see many trees next to a trail die. I have to point out that we usually remove more dead trees building a trail than we have die near the trail in 5 to 10 years. I can't say I have much experience with trail more than 10 year after we built it. We do have a fair number of trails that are over 20 years old. I can't think of any old trails that are not very well shaded by large trees.

    5) I agree with the comment by Oh My Sack - it is best to do a case by case assessment and decide what to do from there. However, we don't have that option. I have several landscape architects to use for reference but I don't (yet) have an arborist. Want to move to Austin? We have some great trails here. And more all the time.

  11. #11
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    I forgot several items I meant to include -

    We don't get to do much bench cutting. When you hit solid rock at 2 inches below the surface, cutting a bench and removing a trapezoidal chunk of 'earth' like in the IMBA Trails book does not happen. Since we rarely remove much soil, we rarely cut or remove many roots.

    When we do remove roots, we try hard to never remove large roots. Since the City regulations have already limited how close to the tree the trail can be, there are not many roots near the surface. Most of the roots I was asking about are on trails that have existed for decades that are now showing a good bit of soil loss.

    By City regulations, we rarely cut of fill more than 4 inches of soil. All of this makes our trails fairly safe for trees. Most of the impact is due to soil compaction and / or soil loss.

    There are plenty of "kooks" with their own opinions they want to spread to anyone and everyone. You might have heard of this example - https://www.cxmagazine.com/austin-zi...rosser-opinion

    That is a good example of the problem we face. Someone (USAC) went thru all of the City processes, got City approvals including walking the course with City personnel who were to approve the course - and then still were faced with the last day of the race being cancelled / postponed when yet another outside person who had no involvement with the approvals complained. That is the reason I want to have documented, authoritative data in hand before this comes up again.

  12. #12
    Dirt Monkey
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    If we run into roots larger than 3" diameter for "trees of ecological value" we build a roller over them. Otherwise they are removed or incorporated into the trail dependent on the difficulty rating of the trail. We have an Oak Wilt problem in our area so minimizing damage to roots of that species is something we are mindful of. It's usually beetles feeding on wounded trees which spread Oak Wilt and MTBs are really low on the list for tree wound causes.

    In my 10 years of building in deciduous forest environments, I've never seen a mature tree killed by a trail passing over its roots. Most common causes for tree death I've encountered are: being blown over by wind, succumbing to disease, or just old age.

  13. #13
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    I dont mean to ruffle feathers, but bike trails 100% definitely have a negative impact on tree roots, especially where there is erosion and bike tires in direct contact with the roots. If you are looking for a professional source to tell you otherwise, you arent going to find one. If you want some good information, read through the ANSI A300 text. Oaks especially, are extremely sensitive to root zone disturbances. I have seen lots of oaks succumb to this.

    Two possibilities for rectifying the situation include building an elevated boardwalk with intermittent posts or “sleepers” contacting with the ground over the root zone, or simply rerouting the trail. I am not an arborist but i am a landscape architect and i deal with arborists on tree protection plans regularly.
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  14. #14
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    Quote Originally Posted by the_joe View Post
    I dont mean to ruffle feathers, but bike trails 100% definitely have a negative impact on tree roots, especially where there is erosion and bike tires in direct contact with the roots. If you are looking for a professional source to tell you otherwise, you arent going to find one. If you want some good information, read through the ANSI A300 text. Oaks especially, are extremely sensitive to root zone disturbances. I have seen lots of oaks succumb to this.

    Two possibilities for rectifying the situation include building an elevated boardwalk with intermittent posts or “sleepers” contacting with the ground over the root zone, or simply rerouting the trail. I am not an arborist but i am a landscape architect and i deal with arborists on tree protection plans regularly.
    No doubt, any compacting forces can and will have an effect on a trees root system. As I said in my post above, it's really case by case. If roots were not exposed, I would much rather see single track in closer to the trunk than out in the drip zone of the canopy. I can easily mitigate compactive effects by mulching or armoring a single track over larger support or buttress roots whereas it's much harder to control those force's effects on the fine root mass that is doing all the work in the dripline and beyond.

    It doesn't have to be prohibitive, just properly planned and managed.

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