Off-Road Impacts of Mountain Bikes


Like any outdoor recreation participants, mountain bike riders will have impacts on the environmental conditions present, including the soils, vegetation, water, and wildlife. This review concentrates upon the soil and vegetation impacts, as the others relate more to the presence and overall level of use rather than its specific type. In a comprehensive review of physical impacts of outdoor recreation activities, Cole (1987) noted that recreationists altered soil and vegetation conditions in three main ways:

· Trampling by humans and packstock (e.g., horses).

· Collection and burning of firewood in campfires.

· Confinement and grazing of packstock (at campsites).

When considering the New Zealand situation, and making specific reference to mountain bike impacts, only those impacts related to "trampling" apply in most cases. The corresponding "roll-effects" of wheels could be termed "wheeling". The following sections summarise research on impacts from trampling by feet, impacts from wheel action (e.g., "wheeling"), and the comparative impacts from different activities (including mountain biking).

2.1 Physical Impacts of Trampling

When investigating the effects of trampling in an alpine setting in Sweden, Emmanuelsson (1985:66) described three types:

· Trampling outside of tracks, especially visible around hotels, huts, ski-lifts, riversides, viewpoints etc.

· Trampling on irregularly used tracks.

· Trampling on marked and formed tracks in regular use.

2.1.1 Impacts on undisturbed (non-tracked) surfaces

Much early research concentrated upon the specific impacts of trampling on different vegetation and soil types, under different environmental conditions (e.g., slope, aspect, rainfall, moisture content). This work most often involved experimental trials of trampling across sample quadrats in previously undisturbed vegetation and soils, and dealt with the impacts on the ecological and structural behaviour of these previously undisturbed environments. A brief synopsis of the findings from this work follows.

The early effects of trampling in this context were injury and destruction of susceptible ground-level vegetation. Some vegetation species and morphologies had greater capacity to survive trampling, so the species composition often changed along the route being formed. As further use damaged and removed vegetation the disturbance of the underlying soils increased. In the case of well-drained soils, this disturbance was most often compaction and reduced water infiltration-capacity. The greater occurrence of runoff increased the effect of erosive processes, particularly on slopes. In the case of poorly-drained and highly organic soils, this disturbance was most often structural deformation, leading to unconsolidated muddy areas. In both types of situation, when the damage made walking along the defined track more difficult, people tended to avoid the difficult areas by taking easier routes on either side. This behaviour resulted in the types of track widening noted in studies of alpine tracks such as Calais and Kirkpatrick (1986) in Tasmania; Bryan (1977) in Sweden; Bayfield (1973, 1985) and Lance et al. (1989) in Scotland; and Simmons and Cessford (1989) in New Zealand.

Research into the rate at which trampling causes changes in soil and vegetation conditions, has consistently found that the degree of impact is not related simply to the increase in use-levels :

"Perhaps the most important finding of these studies is the overwhelming evidence that the relationship between use and impact is curvilinear, with the greatest damage occurring with low use." (Kuss et al. 1990: 82), and:

"Compaction and erosion impacts are greatest at the early stages of use (Cole 1982, 1986). Thereafter, the negative impacts of additional use slow considerably (Stankey and Manning 1986)." (Cordell et al. 1990: 82)

The greatest proportion of trampling impact is represented by the initial damage and removal of vegetation, and the formation of unplanned bare earth tracks. Once these informal and unplanned tracks have developed, the same trampling processes which operate on management-defined tracks will apply. However, trampling processes operating on these unplanned tracks are likely to be more damaging, as these tracks would have developed without the careful consideration of track route, construction, and impact control usually undertaken on management-defined tracks. The remainder of the trampling discussion concentrates upon the ongoing impacts on tracks once they are defined (whether by formal or informal means).

2.1.2 Impacts on formed tracks

The primary environmental impacts associated with formed tracks arise through their initial construction. As noted by Cole (1987):

"It is difficult to define when trail impacts become problems because the majority of change is purposeful change caused by trail construction and maintenance ...

Because most of this is planned by management and accepted by the visitor, trail alteration becomes a serious problem only where it is unusually obtrusive (for example, where parallel ruts scar on alpine meadow), or where deterioration of the trail makes use difficult, and requires expenditure of large amounts of money and manpower for maintenance." (Cole 1987: 149)

Once a track route is clearly defined by managers, and usually a new "hardened" track surface formed, the process of subsequent trampling impact will continue with use. The management focus will now be more concentrated upon maintenance and cost implications of trampling rather than their environmental impacts. Most of the impacts occurring will do so almost immediately. As found with the rate of trampling change on undisturbed surfaces, most subsequent impacts on new management-defined tracks occurred in the initial "settling" period (Simmons and Cessford, 1989). Simmons and Cessford (1989) noted:

"Overall the intial effects of trampling on a new track may appear bad (e.g., loss of residual topsoil and vegetation). However, this change often leads to more stable soil conditions as the more compact underlying soils resist further damage. For example, "recent" soils formed on river gravels may lose surface soil with trampling, but subsequently provide ideal gravel walkways. The exceptions to this are where soils are poorly drained or become watercourses. Clearly local drainage conditions are important, since most damage to soils occurs when they are wet. Here the different properties of organic soils (e.g., peat) and mineral soils (e.g., sand/silt/clay) become important." (Simmons and Cessford, 1989: 58).

Once tracks are established, whether by formal or informal means, there are four main interrelated management problems arising from the ongoing trampling. Based upon the summaries of Cole (1985a, 1987), and the general finding of other studies, these track impact problems are:

· Excessive erosion from enhanced water flows and disturbed soil surfaces on sloping sections of track, or at drainage points across the track.

· Muddy stretches in water saturated sections of tracks, often including major soil structure disruption and widening of tracks.

· Development of multiple parallel tracks where the main track is harder to traverse than the adjacent surfaces (e.g., too rocky, muddy, wet etc).

· Development of informal tracks, including shortcuts on corners and switchbacks, and around focal sites such as huts, campsites and attractions.

The main questions, with regard to mountain bikes in particular, are the ways in which they contribute towards the occurrence of these impacts, and whether the impacts from mountain bikes are any greater than those generated by other users (e.g., walkers). While some research into possible relationships between recreational use and such problems has been done, almost none has been specific for mountain bikes. In a major review of the American situation (Keller 1990), only two specific studies of mountain bike physical impacts were noted. Neither was published in a form readily available to a wider management audience. In general, apart from anecdotal accounts of observations of environmental impacts by mountain bikes, managers have had to rely upon findings from research which has generally had a soil-science and botanical orientation. This work has concentrated upon the recreational trampling effects of walkers and horses, and the wheel-effects of motorised vehicles.

Overall findings of research related to the physical impacts of recreational use of trails have been summarised by Wilson and Seney (1994). The main points they emphasised were:

· The primary importance of rainfall intensity and slope gradient as key factors in explaining soil loss on trails.

· That soil properties such as structure, texture and moisture content determine the resistance to erosion, and play secondary roles.

Wilson and Seney (1994) concluded that trail degradation occurred regardless of specific uses, and that this was more dependent upon geomorphic processes than the types and amounts of activity. This reinforces the general finding that the type and degree of any track impacts vary more as a result of the environmental conditions of the tracks than on the types of uses present. In this context, clearly the most effective means of minimising impacts on tracks lies in the initial selection of the route, and ensuring that construction methods avoid situations conducive to impact development. As noted by Simmons and Cessford (1989):

"Settings with high rainfall, low drainage and a highly organic soil regime were identified as being most susceptible. It was considered that care in the route chosen for tracks was central to minimising use-induced impact. Where such susceptible settings could not be avoided, or impacts were occurring, careful track construction with an emphasis upon control of drainage was considered most important." (Simmons and Cessford 1989: 58)

However, in most cases managers are dealing with existing tracks which already traverse such areas. For these, the most effective means of minimising further impacts would involve "re-routing" some sections of track and "hardening" others. This in itself would create additional impacts, and considerable costs in maintenance (Cole 1987, Chavez et al. 1993). In this situation, managers are faced with a trade-off between the initial establishment costs, major one-off maintenance actions, and ongoing incremental maintenance demands in the future (Simmons and Cessford 1989).

Another option for managers would be to consider the amount and type of track use occurring, and to consider whether any management actions could reduce the development of impacts. Particular areas for attention would be the effects of user numbers, and different types of uses on track impact levels.

(i) Use levels and impacts

Cole (1987) reviewed research on the effects of use volumes on the amount of physical impacts. As noted previously, the bulk of impact on unformed routes was generally found to occur at the initial lower use-levels. By the time higher use-levels were achieved, most of the site changes had already occurred. But in the case of formed tracks, much of this impact was incorporated into the process of constructing the track. With particular regard to research on these types of formed tracks, Cole (1987) stated:

"In sum, these results suggest there is little value, in terms of reduced impacts, in limiting the use of constructed trails." (Cole 1987: 157)

This statement does assume that the formation of the track route and surface is such that the users of it prefer to stay on it. Bayfield (1973, 1985) and others noted however, that where the track is more difficult to travel on than the adjacent vegetation and surfaces, track widening and multiple parallel tracks can arise. In general, Bayfield (1973, 1985) found that on relatively new tracks in the Scottish highlands, track widths increased with increasing use, but that on old traditional tracks, the widths appeared stable. This suggests that these older tracks had long passed of the type of "settling" phase proposed by Simmons and Cessford (1989) for newly constructed tracks. The widening and parallel track impacts in the alpine wetlands of the Tasmanian highlands noted in Calais and Kirkpatrick (1986) appeared, however, to be occurring continuously as the successive informal alternative routes themselves became extremely wet and muddy. This degeneration of the track setting did not necessarily reflect increases in use levels, although this would have increased the rate at which these impacts spread.

(ii) Different activities and impacts

When considering different types of activities, the main question is whether some are likely to cause disproportionately greater levels of impacts than others. Given that most tracks were developed with a tradition of walking use, mountain bikes, as a new form of user with a new array of impact types, may present a particular problem for managers concerned with maintenance of tracks as satisfactory recreational resources. The following sections address this issue by briefly discussing the specific physical impact effects of mountain bikes, and reporting on comparisons between these effects and those of the main alternative use types (e.g., walking).

2.2 Physical Impacts of Wheels - Mountain Bikes

The physical impacts of mountain bikes are often associated with those of motorised vehicles through the common element of both having wheels. Thus, in many ways, their types of impacts could be considered similar, although important differences arise due to differences in wheel loadings and power, since mountain bikes are lightweight and non-motorised. The key distinction between the physical impacts of mountain biking and other non-motorised trail activities (e.g., walking, tramping, running, horse-riding) lies in the unique effects of wheels on surfaces, relative to those arising from trampling by feet.

Studies of human trampling have been extensive and diverse. For example, the trampling motions of feet were described in Holmes (1979), the effects of different types of boot sole were compared by Kuss (1983), and the forces exerted on surfaces by walking were investigated by Quinn et al. (1980). Quinn et al . (1980) noted that damage from feet was caused first by the downward compaction forces from the heel early in the step, and then from rotational shearing forces from the toe at the end. The shearing action was found to be most important, particularly through soil deformation and "smearing" in wet conditions, and was found to be greatest on up-slope travel. Downhill walking was not investigated in the analysis by Quinn et al. (1980), but seperate work by Weaver and Dale (1978) and Weaver et al . (1979), found that downhill stepping (by foot and hoof) was more erosive than downhill motorbiking. This was due to the greater downward forces exerted through the heels in down-stepping. The importance of this distinction between downhill and uphill stepping was emphasised by Bayfield (1973), who found that although 20 percent fewer steps were taken on downhills than uphills, the erosive impacts of downhill stepping was still higher.

Wheels also exert compactive and shearing forces on surfaces, but their transmission of these forces to surfaces is different from that of feet. Soane et al. (1981) identified three types of forces exerted on soils surfaces by powered wheels. These included the downwards compaction force due to dynamic load on the wheel, the rotational shearing stress from the wheel torque acting around the axis, and vibration effects from the engine transmitted through the wheel. Clearly the latter does not apply in the case of mountain bikes.

Mountain bikes will exert downward force through their tyres, although the "mean ground contact pressure", which comprises the wheel load divided by the contact area (Soane et al. 1981, Smith and Dickson 1990) is likely to be less than that of heavier motorised vehicles, horses and heavily laden hikers. Weaver and Dale (1978) noted that motorcycles had least impact on downhill slopes, due to exerting lesser downward forces than hikers or horses. With the lower wheel loadings of mountain bikes, their impacts upon downhill slopes are likely to be much less than those from motorbikes. This does assume that the wheels continue to turn rather than skidding with hard braking. Such skidding can loosen track surfaces and move material downslope, and most significantly, promote the development of ruts which channel water-flow. The development of such ruts, which can promote erosive water-flows to a greater extent than by foot-step puddling, is the most distinctly unique "wheeling" impact. However, where skidding does not occur, impacts from the normal rolling effects of wheels would likely be less than those of foot steps.

It should be noted here that the compaction forces will only be contributing to impacts if they occur off formed tracks. Most tracks are constructed to provide a consolidated and compacted surface, which allows easy travel for users. Where tracks are soft and wet, the effect of downward forces will be less a case of compaction, and more one of soil smearing and deformation.

Mountain bikes will exert shearing forces from the torque applied to the rear wheel in particular. The front wheel is essentially "un-powered". When the shear strain of the soil is exceeded, particularly in wet conditions or on unconsolidated surfaces, "wheel-slip" occurs. In motorbikes this can be generated on level surfaces and uphill sections over considerable distances by high acceleration and loss of most traction. Motorbikes were found to have their greatest erosive effects on uphill sections (Weaver and Dale 1978; Weaver et al. 1979). Mountain bikes cannot generate the power to match the degree of torque generated by motorbikes, and rotational wheel-slip for them can only occur on extremely wet or unconsolidated surfaces. Usually, the occurrence of wheel-slip means the rider must dismount and walk, unlike motorbikes which can apply more power to maintain way until better traction is achieved. This power difference provides motorbikes with a far higher capacity for sustained wheel-slip and its associated gouging effects. Both motorbikes and mountain bikes can have downhill shearing effects through loss of lateral traction and side-slipping, although this is more likely in extremely wet conditions, on uncompacted surfaces, or due to poor braking practices. As becomes apparent in the next section, the downhill effects of mountain bikes, where they have their greatest erosive potential, are not greater relative to those of other activities (e.g., walking).

2.3 Impact Comparisons for Different Activities

Specific research on the physical impacts of mountain biking is rare, with the little which has been done not being readily accessible. Only one study which included mountain biking in a comparative assessment of impacts was available - Wilson and Seney (1994). The extensive review of mountain bike issues by Keller (1990) discussed two other American studies, which were not widely available. Other work has included comparisons of impacts from different activities such as hikers, horses and motorbikes (Dale and Weaver, 1974; McQuaid-Cook, 1978; Weaver and Dale 1978; Weaver, et al. 1979; Price, 1985; Summer, 1986).

While these comparative studies did not include mountain bikes, they did find that the degree of physical impacts increased from hikers through to motorbikes and horses. However, it was also found that these activities had impacts in different ways. Wilson and Seney (1994) summarised the most comprehensive of these studies (Weaver and Dale 1978) thus:

"Motorcycles moving uphill established a narrow rut which increased the velocity and sediment transport capacity of trail runoff. The development of this linear channel was a direct result of the imprint of the tyre and the torque applied by the motorcycle which then led to increased erosion. However, motorcycles moving downhill, when torque is not needed, caused less erosion than hikers and horses, which tend to loosen soil when descending a steep trail because greater forces are applied when decelerating and moving down a steep trail." (Wilson and Seney, 1994: 78)

The general consensus from these comparative studies was that the trampling impact was greater on slopes than on level sites; on wet rather than dry surfaces; and that it tended to be greatest for hikers and horses moving downslope, and motorbikes moving upslope. However, as noted in Section 2.2, mountain bikes lack the weight and torque generating capacity of motor bikes. On this basis, mountain bikes should have far less impact than motorbikes. Jenkins (1987) concluded that while detailed research results were not available, it was obvious that the impacts from mountain bikes were far less than those of motorbikes, four-wheel drive vehicles, and horses; and that on consolidated tracks the degree of impact was similar to that of hikers. Keller (1990) reviewed two studies which compared mountain bike impacts to those of other activities, and found that on the basis of the impact indicators used, the impact effects of hikers and mountain bikers could not be distinguished. Keller (1990) and Chavez et al. (1993) both cited the overall findings from a detailed study by Seney (1990), who stated:

"It was difficult to distinguish bicycle impacts from hiker impacts on the measurements of sediment yield, water runoff, trail micro-relief changes and soil density changes." (Keller 1990: 18)

In addition, Wilson and Seney (1994) noted that:

"The multiple comparisons test results further clarified the roles of the different treatments and in particular showed that horses and hikers (hooves and feet) made more sediment available than wheels (motorcycles and off-road bicycles) on prewetted trails and that horses make more sediment available on dry plots as well." (Wilson and Seney 1994: 86)

At the current stage of research knowledge, it has not been established that mountain bikes have greater impact than hikers. Wilson and Seney (1994) do note that further research into the different impacts of mountain bikes and hikers is necessary. It is obvious that mountain bikes do have some different types of impacts . While they cannot usually generate the uphill erosive channelling found for motorcycles, they can have a similar effect on downhill slopes, most particularly when the surfaces are unconsolidated and wet, and/or the bike is ridden badly. Kellor (1990) noted:

"...down hill mountain bike travel has the greatest potential for environmental impact to the trail (caused by skidding and poorly executed braking)." (Keller 1990: 19)

As noted in Section 2.2, this is a type of impact unique to wheeled vehicles, and is the major source of impact potential unique to mountain bike use. As stated by Keller (1990):

" Land managers and other trail users often point out that bicycles create a linear track, compared to hikers and horses, who leave behind distinct foot or hoof tracks - like pockets - in the soil. A linear track tends to promote channelling of water, as opposed to puddling. The concern that bicycles will create channels, gullies, or troughs, in the trails, leading to trail erosion, is legitimate." (Keller 1990: 21)

While this acknowledges that mountain biking can cause unique impacts, it does not recognise that this effect would depend particularly on having wet soils, or on the occurrence of repeated skidding. Wetter soils are generally associated with low-lying areas with poor drainage rather than slopes, and skidding from braking on downhill slopes is often a result of inexperienced riders. These points are included to illustrate that the occurrence of erosive impacts will vary according to site conditions and rider behaviour . Chavez et al. (1993) cited research showing the inappropriate riding behaviour of those who rode around log waterbars on a trail, thereby widening the trail and compromising the effectiveness of the waterbars in controlling water flows. This type of behaviour has parallels with the general track-widening behaviour demonstrated by walkers, as described in Section 2.1 (e.g., Calais and Kirkpatrick, 1986; Bryan, 1977; Bayfield, 1973; Lance et al. 1989). In other studies of user behaviour, horses were found to create deeper and wider tracks than walkers (Dale and Weaver 1974), although the effects were more localised to the track, and fewer informal parallel tracks were formed (Weaver and Dale 1978; Price 1985). Walkers and horses in particular were also more likely to short-cut corners than motorbikes (McQuaid-Cook 1978; Price 1985).

At this point, it is important to put physical impacts such as trampling and wheeling into perspective. The two main studies of manager attitudes toward impacts found that over 70 percent of managers mentioned track and campsite impacts as being most important (Godin and Leonard 1979; Washburne and Cole 1983). As noted by Cole (1985a):

"Trail damage is a problem in most wilderness areas and more money is invested in mitigating this impact - primarily in the form of maintaining and relocating trails - than any other." (Cole 1985a: 149)

However, such impacts are generally localised and confined to strips alongside tracks and around focal points such as huts, campsites and viewpoints (Cordell et al. 1990). For instance, Price (1985) cited a study of a heavily used backcountry area in Banff National Park, which estimated only 0.035 percent of its area consisted of bare soils along tracks. As noted in this review, most of this type of track development is likely to have been undertaken by managers, and most of the initial impacts would have been incorporated into the track construction process. Any subsequent impacts would be more dependent upon the choice of track route and construction methods, than the types of use received (e.g., walking or mountain biking). The limited amount of research available provides no conclusive evidence that subsequent impacts on susceptible sites would be any greater from more mountain biking use, than they would be from more walker use. As stated by Ruff and Mellors (1993):

"To date, however, there has been little solid evidence to suggest that mountain bikes are any more damaging to bridleways than many pairs of feet or horses hooves though in some cases they can contribute further to problems caused by over-use. The major problem would appear to stem from perceptions of the countryside and hence that mountain bikes are not an acceptable form of countryside recreation." (Ruff and Mellors 1993: 105)

The conclusion of this statement by Ruff and Mellors (1993) represents an alternative focus for the debate on mountain bikes in off-road (track) settings, into the area of social perceptions, and the role these play in how both physical and social impacts are perceived. The remainder of this review deals with research related to this area.