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An xc trail in Hampshire County, MA.

Environmental Impacts of Mountain Biking Synopsis

By Jason Kahn

Featured Image: An mountain biking xc trail in Hampshire County, MA. (c) Jason Kahn

According to the online mountain bike trail database, Trailforks, there are currently  283,020 miles of 213,921 trails in 109 countries.  This list mostly includes approved trails on public and private lands.  The numbers in this database almost certainly under-estimate the actual number of trails and trail mileage.  It may be assumed that rogue or pirate trails may double the existing number of trails currently documented.

There are four types of mountain bike riding and therefore four types of trails. Each has unique characteristics and environmental impacts. These types of riding are: Cross Country(XC), Freeride, Down Hill(DH) & Bike Park/ Pump Track.

Cross Country trails are traditionally narrow, 12” to 24” in width, and meandering; the idea being that the trail is the goal.  As such the trail density may be quite high.  For example, 4 miles of trail can easily be constructed in 0.0078 square miles if desired.  The other end of the XC spectrum may have 1.5 miles of trail spread over 1 square mile.  The trail density is usually dependent on the size of the area available to the trail builder.  The trails tend to cross slopes with no more than a 5-8% slope so as to be ridden in both directions for maximum variety.  Trail building best practices, according to International Mountain Bike Association (IMBA) standards, allow for water to flow across trails instead of down the trail.  Where trails reverse direction (switchbacks), they tend to have water bars built into the trail before and after the switchback.  This prohibits water pooling which creates muddy conditions and erodes the trail tread.  XC style mountain bike trails have similar erosional impacts to hiking trails.  In fact, some studies have found the erosion due to hiking trails more detrimental due to bad practices.  Horse packing/equestrian trails are considered the most damaging.

A freeride trail in Bentonville, AR.

A freeride trail in Bentonville, AR. (c) Jason Kahn

Freeride trails are a combination of XC and DH trail-building and riding. They are wider than XC trails, 3’ to 6’. They are meant to be ridden in one direction, down, and are therefore more susceptible to erosion due to speed, braking, and landscape features such as slope.  Erosion may be limited by armoring the trail, placing rocks in the trail tread in areas that tend to be wet, or building bridges over these areas.  Most of these types of trails are on private land such as ski resorts that are trying to build a year-round tourist base.  Another feature of Freeride trails is berms, these are banked turns meant to be taken at higher speeds.  The berm is a built up turn usually constructed of down logs, rocks, and soil.  These features do cut down on braking and therefore erosion, but the construction of these features is more intrusive on the landscape.

Down Hill(DH) trails are traditionally 4’-8’ wide and meant to be ridden in one direction also.  They are designed for heavier bikes and are usually highly technical in nature.  Large rocks and roots typically punctuate these trails and can become erosional features.  Down hill race courses are commonly mud-filled and tend to channel water as a result of the slope.  Mud and rocks and roots are sought out by trail designers as technical features specifically for these types of trails.  Down Hill trails are usually associated with ski resorts for the same reasons as are Freeride trails.

Bike Park/ Pump Track trails vary in width from 2’ to 8’. They are usually built on flat ground and are commonly found on municipal properties, in towns and cities, similar to skate parks. They can be considered playgrounds for people on bikes.  Most have elevated features made of wood or metal and are completely synthetic in their design and construction.  From the standpoint of environmental impact, these are mostly a concern in terms of extraction of the resources, chiefly wood and dirt.

A bike park in Bentonville, AR.

A bike park in Bentonville, AR. (c) Jason Kahn

The effects of mountain biking trails on wildlife run the spectrum from benign to catastrophic. In some places, wildlife follow mountain bike trails, especially where the mountain bike trails followed existing game trails. In winter, less effort is needed by wildlife to travel during heavy snow periods if they follow trails.  However, this may favor some more opportunistic species, such as coyote, over shier species, like lynx.  If the trails are groomed for winter riding, they can be heavily traveled by deer and other browsers.  Again, though, in some places bike trails might be seen as attractive nuisances, if they favor opportunistic wildlife species over sensitive wildlife species.

On the other end of the spectrum are bike on animal collisions which can lead to animal fatalities.  Salamanders, frogs, snakes, chipmunks, squirrels, weasels, and rabbits have been run over by careless trail riders or riders traveling at high speed.  Larger animals can be reluctant to cross trails during daylight riding hours, but they are usually comfortable using and crossing trails at night.  This is evident when studying footprints and scat left on trails by coyote, bear, fisher, and bobcat.  On occasion, trail riders are injured by collisions with larger animals.  A trail rider in Rennselaer County, NY was injured when he collided with a black bear.  The bear swiftly recovered and fled the scene while the rider suffered minor abrasions and a bent front wheel.

The effects of mountain bike trails on soil and vegetation depend on the type of trail being built.  XC trails usually result in minor soil compaction and initial increased soil erosion.  Best practices can usually minimize soil erosion on any type of trail, but XC trails are usually sustainable. In almost all trail building scenarios large trees 6” or larger are considered natural features, so the trail is routed around the tree for aesthetic purposes. Freeride and DH trails are usually not as careful about vegetation avoidance.  Higher speeds and the turning radius at those speeds may require some trees to be removed.  Best practice will have trails reverse slope direction to scrub speed by a temporary uphill turn.  This will allow for less vegetation to be removed.

A freeride trail in Bentonville, AR.

A freeride trail in Bentonville, AR. (c) Jason Kahn

The new trend in DH and Freeride trail building is more problematic.  Machine built trails use small excavators, usually 4’ wide, tracked machines with an articulating cab that controls a 12-18” wide bucket.  These machines are less likely to avoid trees and roots.  On trails that traverse slopes, these machines sever roots and remove larger amounts of soil which can lead to increased erosion and topple trees that are leaning up or down slope due to the loss of a stabilizing root structure.  These machines have been known to create 25 miles of trail in an eight-month period.  Hand-built trails, due to their more carefully chosen line, may take up to 4 years to build a single trail of 12 miles.

The use of machines to build trails has for the most part been restricted to ski resorts and municipal properties.   These trails are usually constructed by professional trail crews trained in best practices, but the size and slope of the trails usually make best practices difficult to adhere to.

The effects of mountain bike trails on water are similar to those of hiking trails.  Depending on the slope, climate, and vegetation there can be increased turbidity and/or siltation, if the trail’s location is close to a stream. This increase in turbidity and siltation will have an adverse effect on water quality and breeding habitat for aquatic species, including trout and salmon. Best practices would keep any trail from paralleling a stream within 25’ of its banks.  If a trail crosses a stream, best practices would mandate a bridge constructed of wood harvested from within the area and armored with stone on the entrance and exit from the bridge.

An xc trail in Bentonville, AR.

An xc trail in Bentonville, AR. (c) Jason Kahn

The newest threat to the environment from mountain bike trails is the emountain bike (eMTB).  These electric powered pedal assist bicycles are heavy, and they may attain faster speeds for longer periods of time than a traditional mountain bike.  Many conservationists have noted the increased distances a rider may travel in a day with an eMTB.  While this is true in most cases, it’s fairly common for trail riders on traditional mountain bikes to cover 30-60 miles a day which is similar for eMTB’s.  The real threat to the environment due to eMTB’s is the influx of new trail users who wouldn’t normally ride a mountain bike due to the difficulty.  With this new influx comes the possibility that the riders of eMTB’s may not have the ethic of most mountain bike riders.  That ethic says:  Do not ride trails in muddy conditions.  Do not skid into turns. Yield to uphill riders. Do not harass wildlife.

The new eMTB’s, in fact, seem to become almost indistinguishable from motorbikes.  Newer eMTB’s have motors with 750watts of power and can travel at speeds of 28 MPH.  It’s also touted that with minor adjustments to the motor, speeds of 46 mph are attainable.  This has become such a contentious issue that the International Mountain Bike Association (IMBA) chose to oppose changes in the wording of the 1964 Wilderness Act that would allow for mountain bikes in Wilderness areas for fear it would become the slippery slope that allowed for any motorized vehicle to lawfully gain access to protected Wilderness areas.  IMBA should support keeping all roadless areas free of mechanized recreation vehicles, but at least they took the right position on upholding existing language in the Wilderness Act. In the age of Donald Trump and his appointed heads of the Department of Interior and Department of Agriculture, it’s easy to connect the dots and see what are the motivations of the agitators who want to modify the Wilderness Act.  Mountain bikers should not fall in with that selfish lot.

Many states have already adopted rules, regulations, and restrictions on ebikes on public land.   It will be up to both conservationists and mountain bikers to encourage their state lawmakers to restrict these eMTB’s from public lands. They should be allowed on public roads, but not on foot trails or mountain bike trails and never off-trail.

An xc trail in Granby, CT.

An xc trail in Granby, CT. (c) Jason Kahn

As more cities and towns adopt mountain biking as an economic engine to boost tourism and economic development, it should be noted that most of these cities and towns are concentrating the trail construction on privately owned and municipal lands.  Trails are kept close to town where riders may ride right from the center of town and return to enjoy the food and amenities the town has to offer. Examples of this model are Kingdom Trails of Burke, Vermont; and the city of Duluth, Minnesota, which claims to have 100 miles of trails within the city limits.  Another example of this model is the city of Bentonville, Arkansas, which claims almost 100 miles of trails that can be ridden from the city and most of which are located in peoples’ backyards and formerly misused city-owned land that has been repurposed for mountain biking.

These models have chosen to leave open public land reasonably untouched and concentrate most of the trails close to or within the town or city.  In these places, a rider can park his or her car and then can ride for days without having to drive anywhere.  This idea is spreading to new trail development in most towns and cities that are trying to draw tourist dollars by becoming mountain bike destinations.  Kingdom Trails has a user fee that is used to help pay the property taxes of landowners that allow trails on their property.  This cooperative association has allowed landowners to keep their land intact instead of selling it to developers.  This is beneficial to both the residents of Burke, VT and the wildlife that live in and pass through the area.  If this model of mountain biking carefully concentrated on private and municipal lands spreads, it could also lessen pressure to open public wildlands to mountain bikes.

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Scott swanson - January 8, 2021

A look at George Weurthner’s article on the impacts of mt biking would expand Jason’s idea of wildlife impacts beyond simply colliding with bears and salamanders on the trail. Wildlife displacement results from any trail use, but is much greater from mt biking. Glad to see some figures re the number of trails and their mileage, though the average of 1.3 miles per trail seems suspect. Also glad to see a guesstimate of private trails, any mention of which is usually left out of discussions about mt biking. Singletracks mag says there 13,000 miles of illegal trails in California alone, not including private land and parks. Where we !ive on a private inholding inside USFS land we’re surrounded by illegal bike trails that the FS refuses to do anything about. Not only are there miles of pirate trails, but bikers poach any trail they want, including in Wilderness areas. The most effective way to lessen the impacts of mt biking is for bikers to ride where they are allowed to ride and no where else. This kind of arrogance is unique to mt biking and portends the greatest impact not only to the land, but the sport itself.

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    Jason L Kahn - January 15, 2021

    Scott,
    Thank you for the expansion upon the theme of the article. It’s true that mt. bikes do have greater impact on wildlife. Weurthner’s article does however equate mt. bikes with ATV’s and ORV’s in terms of their impact on wildlife. But, the paper it’s based upon states no reason why the 3 activities are lumped together. They are lumped together nonetheless. While I admit mt. bikes have an adverse effect on wildlife, and I have no quantifiable evidence to the contrary I find the comparison a false equivalency.
    As to the pirate trails penetrating Wilderness Area and National Parks. That is disturbing to hear. Being a mt. biker myself I feel that proper education of trail users and advocacy for responsible trail use by everyone would go a long way to limit the damage. Lastly most mt. bike trails will vanish from usable recognition, in temperate climates anyway, if left along for about a year. So the damage is reversable if the trail is left alone.

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Out n About - January 10, 2021

Jason, Thanks for the useful synopsis of recreational impacts. I was a bit put off, however, by your brief mention of the issue of bicycles in federally designated Wilderness.

First, IMBA has a number of reasons for not stepping up, yet, for more active representation of backcountry cyclists, and the advent of e-bikes is just one complication. You also advance the commonly stated slippery slope concern that e-bikes are increasingly indistinguishable from non-motorized bicycles and, presumably, will pose an impossible management challenge. But what’s even more difficult is telling the difference between a responsible hiker, horseman, skier, snowshoer or boater and a hiker, horseman, skier, snowshoer or boater traveling without a permit, in violation of their permit, starting illegal fires, leaving trash, traveling off trail, etc. Yes, management and enforcement are perennial problems across all user groups, so why single out bicycles for total exclusion?

You also state that all roadless areas (beyond federal Wilderness?) should be free of “mechanized recreation vehicles,” but you make no mention of the mechanized recreation vehicles that are currently allowed in Wilderness such as skis, snowshoes, boats with oar locks, hiking poles, etc.

Lastly, you point with great suspicion at the congressional and agency sponsors and supporters of the legislation intended to establish – or restore – the possibility of bicycle access to some Wilderness lands. But is that really a good enough test? Did you, for example, “connect the dots” and oppose the recent Boulder White Clouds Wilderness designation on the grounds that it too had congressional sponsors no less “selfish” than those supporting the bid to allow local land managers to make determinations about bicycle access on their Wilderness lands?

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    Jason L Kahn - January 15, 2021

    Out n About,
    Thanks for the thoughtful and thought provoking questions. Your first question about machines in Wilderness Areas. That’s a matter of supplemental power. True all of the items you mentioned are machines. As are saws, axes, knives and hanging a bear bag from a tree. Cook stoves arguably have an internal combustion component to there operation. So there is another example.
    Your question about the Boulder White Cloud Wilderness is a a bit more pointed. May I suggest that the sponsors of the Wilderness bill were acting in support of protection of habitat and the wildlife that call that area home. The loss of the trail system north of Sun Valley is problematic, but less that 4% of the US is protected at the level of Wilderness. That gives us mountain bikers the other 96% to ride bikes. Remember you and I are still permitted to enjoy the Wilderness Area, we just can’t bring our bikes. When Jeremy Bentham founded the idea of Utilitarian thought, he stated “the greatest happiness of the greatest number should be the guiding principle of conduct” Although it’s often assumed he refers to the greatest number of humans, he never once mentioned humans in his doctrine.

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