Crys Moosman
Grinnellians like to take the path of least resistance. Give
them an article to read, and they’ll skim the abstract, introduction, and
conclusion. Put a heavy door in front of them and they’ll use the handicap
button to automatically open the door. Build them a sidewalk to get from the
East campus residence halls to the JRC, and they’ll walk straight across the
lawn. This is exactly how the various social trails around Grinnell College’s campus have formed over
the years, and with each new step, the ground becomes slightly more disturbed.
With disturbance comes altered vegetation cover and soil compaction, both of
which play a significant part in determining the richness and distribution of
the trailside plant communities. If you go out to the social trails on campus,
you’ll be able to see the difference between the trailside vegetation and the
vegetation growing five meters away - the trail is patchy, with tendrils and
leafy clumps scattered among clover-speckled grass, while the distant
vegetation is made up of thick clover-speckled grass (Figure 1).
Trailside
vegetation variation as a result of trampling is not just a phenomenon that
occurs on college campuses in rural Iowa. The impacts of trampling are present
in urban woodland pathways, national park mountain trails, and grassland trail
systems, to name a few (Littlemore
and Barker 2001; Nepal
and Way 2007; Potito and
Beatty 2005). In attempts to gain a better understanding of the effect of
trail usage on trailside vegetation, many researchers have completed vegetative
analysis of trail corridors. One such
experiment was completed by Aaron Potito and Susan Beatty (2005) in public
grassland trail systems in Boulder County, Colorado. In this experiment, Potito
and Beatty examined the establishment patterns of trailside vegetation on four
recreation trails of various usage levels and ages in hopes of better
understanding how exotic and ruderal species abundance and density is influenced
by trampling. To determine the presence
of ruderal,
exotic,
non-ruderal, and native plant species, they set up multiple transects parallel
to each trail. For each transect they identified present plant species and
determined the percent coverage of each species.
What could
parallel transects alongside these hiking trails possibly show about the impact
of trampling on species establishment? Multiple things, it turns out. First,
the trailside has more bare ground and a greater presence of exotic and ruderal
plants than ground 25 meters or more from the trail. Potito and Beatty found
that the percent
coverage of ruderal species decreased from 15% near the trail to 5%, 25
meters away from the trail (Figure 2). Similarly, exotic species exhibited 37%
coverage on the immediate trailside and 10% coverage 25 meters away. Native and
non-ruderal plant coverage did not differ, but trailside species were more
interspersed with exotic and ruderal plants than species away from the
trailside. The trailside had 10% more bare ground than spaces away from the
trail. Below, figure one from Petito and Beatty’s piece provides a visual
representation of the percent coverage of ruderal, exotic, non-ruderal, and
native species.
Figure 2. Percent cover of different vegetation classifications
for trailside transects versus transects away from trailside for established
trails (**significant at 0.01 level using one-tailed t-tests). Error bars represent the 95% confidence interval for the
means. (Adapted from Petito and Beatty 2005).
Second,
trail sides had altered species
richness. There was a 35% reduction in native species and a 46% reduction
in non-ruderal species in addition to a 24% reduction in the overall presence
of plants along the trailside. The number of ruderal and exotic species present
did not differ from the edge of the trail to 25 meters beyond the trail. Potito
and Beatty explain this by proposing that trail corridors reorganize local
vegetation in a manner that favors ruderal and exotic plants over non-ruderal and
native plants, thus decreasing the abundance and richness of native and
non-ruderal plants alongside the trail.
The last
notable thing Potito and Beatty determined in their observations was the
increased rate of establishment of ruderal and non-native plants along trails.
This suggests that non-native and ruderal plants are more capable of growing in
disturbed and compacted soils than native and non-ruderal plants. It also
explains the reduced presence of non-native and ruderal species away from the trailside
- where there is minimal disturbance, there is reduced opportunity for these
plants to preemptively outcompete the native and non-ruderal species.
Let’s jump
back to Grinnell College, where my lab partner, Vishva, and I are trying to
figure out how the student-made social trails around campus are affecting the
surrounding vegetation. Upon a quick scan of the compacted trails, we
identified five different plants: turf grass, clover (Trifolium repens), broadleaf
plantain (Plantago major), knotgrass (Polygonum arenastrum), and dandelion (Taraxacum officinale). Both broadleaf
and knotgrass are considered a weed. The turfgrass and clover were abundant
both on the immediate trailside and everywhere else on the lawn. But knotgrass
and plantain seemed to be abundant only near the trails, and usually within a
half a meter of the center of the trail (Figure 1). To try to gauge the vegetation
coverage of trails, Vishva and I measured plant coverage every five meters
using 2 meter (200 cm) transects that were perpendicular to the social trail.
All five species previously identified were accounted for in our transects.
Turf grass and clover were found along many transect points, from 5 cm to 200 cm.
On the other hand, invasive plants were never found beyond 175 cm. This suggests
that knotgrass and broadleaf are able to establish themselves well when the
ground has been disturbed, and are less successful in less trampled sections of
the lawn.
Though our research is still in progress, both our
preliminary research and the research completed by Potito and Beatty implies
that in order to successfully reduce the abundance and success of invasive and
ruderal species, management practices need to encourage narrow trails and
minimal trampling beyond the edges of the trails. Places similar to the
grasslands in Boulder County can encourage users to stay on defined trails.
Meanwhile, Grinnell College can encourage its students to use the sidewalk was
constructed for the purpose of student walking.
References
Littlemore, J. and Barker, S. (2001). The ecological response of forest ground flora and soils to experimental trampling in British urban woodlands. Urban Ecosystems. 5: 257.
Nepal, S.K., Way, P. (2007). Comparison of vegetation conditions along two backcountry trails in Mount Robson Provincial Park, British Columbia (Canada). Journal of Environmental Management. 82:240–249.
Potito, A.P. and Beatty, S.W. (2005). Impacts of recreation trails on exotic and ruderal species distribution in grassland areas along the Colorado Front Range. Environmental Management. 36:230.
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