Research into Effect of Mountain Bikes on Seed Dispersal

, June 26, 2017

The growth of mountain bike parks as permanent pieces of infrastructure brings to questions their ecological impact. Since tracks divide different parcels of land, the question of seed dispersal and natural systems is brought to mind. Back in September 2014, Fabio Weiß of Universtiy of Freiburg published research which looked at the role that mountain bikes have on seed dispersal in the southern Black Forest.

Between June and August 2014, Weiß monitored a forest trail on the outskirts of Freigburg, choosing a point on the northern side of the Kybfelsen (820m). The trail leads through managed forest and with a small variety of trees including Beech (Fagus sylvatica), Douglas fir (Pseudotsuga menziesii), Spruce (Picea abies), Fir (Abies alba) and Scots pine (Pinus sylvatica). The trail was slightly sloped and held a mix of different surfaces common for trails in the area. It featured hardpack, partly lose soil, coniferous litter, broad-leaf litter, small amounts of gravel and stony passages.

The bicycle was ridden with a speed between 10 km/h (~6.2 mph) and 15 km/h (~9.3 mph) representing average uphill speed (personal experience). The pick-up area had the size of 213cm by 50 cm; 213 cm long to allow each wheel a full rotation within the pick-up area to reach maximum exposure of the tyres, 50 cm to ensure that the rider entirely hits the pick-up area with both wheels, even when crossing it not completely straight. 12500 seeds of five species were evenly distributed in the pick up area. The seed density per species was ~0.23 seeds · cm-2, yielding a total seed density of ~1.17 seeds · cm-2.

Measurements were taken at a distance from the pick-up area. The distances were 0m, 5m, 10m, 20m, 50m, 100m, 200m and 500m for semi-wet conditions (see 2.4.) and 0m, 5m, 10m, 20m, 50m and 100m for wet conditions

Black Forest Canopy (Daniel Rauber)

A 2012 Bergamont Contrail LTD full suspension mountain bike was used to ride through the pick-up area. It featured a 120mm front- and rear-suspension. The bicycle was running 26 inch wheels equipped with 2.25 inch (wide) tyres. During all the testing, both tyres were run with an air-pressure between 22 psi (~1,5 bar) and 26 psi (~1,8 bar). With a contact area of 45mm, the bike weighed 13kg bike, with the rider adding a further 63kg.

The following five species were chosen for the experiment: Pastinaca sativa, Onobrychis viciifolia, Vicia villosa, Sinapis alba, Anthriscus sylvestris, which I will refer to by their generic names in this thesis. The seeds of Anthriscus are elongate and slim. Pastinaca seeds are relatively large and flat (<1mm). Sinapis and Vicia represent spherical seeds, while Vicia are larger than Sinapis. The shape of Onobrychis can be described as roughly rounded with an uneven surface. Average weight determined by weighing 100, then dividing to get the average weight of a seed of a species and calculating the average for a single seed.

Onobrychis (20.6 mg), Pastinaca (6.3 mg), Sinapis (4.7 mg ), Vicia (6.3 mg and Anthriscus (2.8 mg).

I used daylight UV-active bright pink pigment to colour-mark the seeds (Figure 2.3.). For all species except Pastinaca I additionally used Paraloid B72 (15% solution in Ethylacetat) as fixer like suggested by Lemke et al. (2009). This brought the advantage that the pigment would not wear off due to water or mud during the experiment. This was especially important for the longer distances and the runs in wet conditions. When using the pigment in connection with B72 it was crucial to ensure that single seeds would not stick to each other, which would have changed overall results.

A pilot study suggested that seed dispersal by mountain bikes is negligible in entirely dry conditions. Within ten runs, undertaken using the described methods and material, there was no seed attached after 5 meters in any of the runs. There was also no sign of seed movement within the first five meters.

To rectify this, a semi-wet pursued – puddle added 213cm · 50cm. It was initially wetted with 2 litres of water and re- wetted after each run with approximately 0.25 litres using a spray bottle wet condition On these dates it had rained between 20 and 22mm in the 48 hours prior to the testing.

I also tested how two different profile patterns of the tyres affect attachment and detachment of seeds. There were two different tyre models used: a MAXXIS ‘Ardent’ (2.25 inch) and a MAXXIS ‘Advantage’ (2.25 inch). Profile depths were 3mm for the ‘Ardent’ and 3,5mm for the ‘Advantage’. The ‘Ardent’ is advertised as ‘do-it-all-tyre’ and to be categorized as All Mountain tyre. The ‘Advantage’ is listed as XC tyre (MAXXIS official website). After five of the ten runs for each distance, I switched the tyre models between front and rear wheel. This was necessary to ensure that differences in overall results would not be caused by one tyre being permanently on the front wheel and the other one being on the impellent rear.

A questionnaire with 10 questions was designed. The survey took place mainly at a mountain bike festival in Freiburg on 18th and 19th of May 2014 and on trails in the Blackforest close to Freiburg between June and July 2014. Several riders were surveyed via email. The majority of participants were local.

First a simple linear model in which lov(distance) stands for the proportion of seeds left on the mountain bike at a distance. The variables a and b represent the the mean proportion of seeds attached to the mountain bike at d=0 and the detachment rate:

lov(distance) = −(b∗distance)+a

Of the 12500 deployed seeds a mean of 301.1 (± 23.4) for semi-wet conditions and 65.9 (± 5.1) for wet conditions became initially attached to the mountain bike tyres. In one case there were 459 seeds counted. The factor for the contact area would be 1.0, given the situation that the rear wheel exactly follows the line of the front wheel; 2.0 if it takes a completely different line. It is rather unlikely that one of these scenarios dominated during the field trials. Therefore, I set the factor to 1.5, representing a more realistic intermediate scenario.

In total, 90% detachment thresholds derived from the best fitting model were 16 meters in semi-wet conditions and 15.5 meters. 500m and 100m, respectively for last seeds.

Most of these seeds were attached to the downtube or the underside of the bottom bracket. I also discovered seeds underneath the saddle, on the fork, on the chain and the rims. Within the 70 overall runs in semi-dry conditions I counted 28 seeds on the bike frame. For the 50 runs in wet conditions there were 21 seeds found. Resulting from these figures the chance to have any seed attached on the bike other than on the tyres was 40% in semi-wet conditions and 42% in wet conditions for one run.

65 mountain bikers in survey. The mean distance until the next cleaning of the bicycle is 70.15 ± 4.1 km, which equates 2.48 average rides. The dispersal distance, on the other hand, was considerably shorter than expected in the beginning. While seeds on shoes are dispersed up to 5 km, predictive models even suggest 10 km (Wichmann et al., 2009), the seeds in my experiment reached maximum distances of 500m in semi-wet conditions and 100m in wet conditions, respectively.
Limitations of speed.

Is this long-distance dispersal – Pastinaca seed to become attached to the tyre when being exposed to it was 39.6% attach and detachment rates increased with reduction in weight.

Socks and clothing also collect – 500 sew per sock (mount and pickering 2009). Mountain bikers putting down shoe and collecting seeds. There are many riding styles, which substantially differ in riding speeds, terrain choice and material.

Propagules (seeds, spores, whole individuals, plant parts). Some species are capable of actively dispersing their seeds (Autochory). Others rely on vectors to facilitate dispersal (Allochory). linear trails change hydrology.

Passive seed dispersal has been categorized into Anemochory (dispersal by wind), Hydrochory (dispersal by water), Zoochory (dispersal by animals) and Anthrochory (dispersal by humans) = human-mediated dispersal (hmd). The morphological dispersal syndrome (MDS) names the fact that many plant species specialize on one or more mechanisms which they rely on in terms of seed dispersal (Higgins et al., 2003).

One study remarks that hiking boots are able to transport seeds up to 5000m (Wichmann et al., 2009). quantifying the seed-dispersal by mountain bikes, little research when compares with clothing, shoes and cars soil erosion, vegetation damage and trail degradation (Pickering et al., 2010b). In the United States 43.3 Million people rode a mountain bike in 2000 (NSRE, 2000), aided by advancing technology which allows for greater variety of terrain and year-round riding in all conditions.

Mountain biking unique – high range and high degree of area-permeation
During their survey among mountain bikers in 1995 Morey et al. (2002) remarked that riders with suspension systems on their mountain bikes tended to prefer single trails and challenging terrain more than those without. During that time only the minority of mountain bikes (~40%) was equipped with suspension (Morey et al., 2002).