Research articleThe influence of layout on Appalachian Trail soil loss, widening, and muddiness: Implications for sustainable trail design and management
Introduction
Nearly all protected natural areas globally have recreational trails as an essential infrastructure component to accommodate outdoor recreation activities and visitor access, supporting a diverse array of opportunities and experiences that include nature study, hiking, camping, mountain biking, and horseback riding (Leung et al., 2018). In the U.S. alone there are more than 193 500 miles of trails on federal lands and 42 500 miles on state lands (American Hiking Society, 2015).
Unfortunately, many of these trails predate protected area designations; they follow historic, pioneering, logging, firefighting, and transportation routes that were not designed for long term sustainability or use as recreational infrastructure. As such, these “legacy” trails frequently deteriorate under heavy use, diminishing their function to provide quality recreational activity and access. In contrast, sustainably designed recreational trails can remain in stable condition while accommodating intensive long-term use with appropriate maintenance (Marion and Wimpey, 2017). With sound design, proper construction and occasional maintenance, sustainable trails effectively protect natural resources by concentrating recreational traffic and impacts along narrow impact-resistant trail treads (Wimpey and Marion, 2010).
Recreational traffic and natural processes can degrade trails, decreasing their utility and requiring costly maintenance and rehabilitation work (Leung and Marion, 1996). Trail treads are vulnerable to lateral migration, braiding, soil loss, muddiness, trail widening, and compositional changes to trailside flora, including the introduction of invasive species (Marion et al., 2016; Pickering et al., 2007). Soil loss is perhaps the most significant of these impacts because natural soil regeneration is extremely slow, soil cannot be easily replaced by managers, and once waterborne, soil causes stream sedimentation and degrades aquatic insect and fish habitats (Kidd et al., 2014; Marion et al., 2016; Olive and Marion, 2009). Furthermore, unmitigated erosion results in treads with deep ruts or exposed rocks and roots that impair trail travel and exacerbate tread widening and muddiness (Marion and Wimpey, 2017). Trail widening from trailside traffic greatly increases the areal extent of human impact and can contribute to additional erosion and tread drainage problems (Dixon et al., 2004; Wimpey and Marion, 2010). Trail muddiness can also lead to widening and the proliferation of visitor-created trails when hikers attempt to bypass muddy areas (Leung and Marion, 1996). The environmental degradation associated with trails is a threat to the ecological conservation goals of protected natural areas world-wide.
Although local climate, soils, and vegetation influence the rate and severity of trail degradation, designers can minimize the influence of these factors by selecting sustainable trail alignments relative to topography. Experienced trail professionals and scientific studies identify two influential factors: a trail's grade and its orientation relative to local topography (Marion and Wimpey, 2017). Several studies have found a strong positive relationship between trail soil loss and trail grade (Farrell and Marion, 2001; Olive and Marion, 2009; Wallin and Hardin, 1996). A few studies have also found that trails routed directly up slopes, called direct-ascent or fall-line trails, are particularly susceptible to degradation from waterborne erosion and widening (Hesselbarth et al., 1996; IMBA, 2004; Marion and Wimpey, 2017; Tomczyk and Ewertowski, 2015). The close alignment of these direct-ascent trails with the natural direction of surface flow promotes channeling water; once incised, it is extremely difficult to drain water off and away from the trail's tread. Conversely, side-hill trails that travel across slopes have one side-slope lower than the trail tread, which facilitates drainage across and off the trail (Marion and Leung, 2004; Marion et al., 2016) and inhibits trail widening behavior (Wimpey and Marion, 2010).
The limited geographic scope of past trail studies limits the applicability of their findings in dissimilar settings. Protected area managers need improved science-based guidance on how to design, construct, and maintain sustainable trails able to support the intended types and amounts of traffic while remaining in good condition. This research investigates the influence of trail layout on three core types of degradation using an exceptionally large and environmentally diverse dataset from the Appalachian National Scenic Trail.
Section snippets
Trail soil loss
Soil loss as measured in trail studies is largely caused by water erosion, though wind can remove tread soils in dry climates, and soil can be compacted or displaced downhill or laterally (Marion and Wimpey, 2017). The rate and severity of soil loss is influenced by trail alignment relative to topography and environmental attributes, tread substrates, climate, tread maintenance actions, and use-related factors including amounts and types of use (Leung and Marion, 1996; Olive and Marion, 2009).
Study area
The Appalachian Trail (AT) is the United States’ first and most popular National Scenic Trail. It is marked with white blazes for 3524 km through 14 states from Springer Mountain, Georgia to Mount Katahdin in Maine. An estimated 3 million people visit the trail annually and the number of hikers attempting a northbound thru-hike has more than doubled in recent years, from 1460 hikers in 2010 to 3735 in 2017. The AT is restricted to hiking, except for a short section open to horses in Great Smoky
Trail soil loss
One-way ANOVA testing confirmed that the severity of soil loss varies significantly with trail grade (F = 21.3, p < .0001, df = 3), with a post-hoc Student's t-test revealing significant increases between each category of trail grade (Fig. 2). A one-way ANOVA test examining the combined influence of trail and landform grade on mean maximum incision found significant differences between groups (F = 8.1, p < .0001, df = 9), and a post-hoc Student's t-test identified the greatest soil loss
Soil loss
The findings of this study reinforce an existing consensus that steep trails are prone to severe soil loss (Cole, 1983; Fox and Bryan, 2000; Marion and Wimpey, 2017; Nepal, 2003; Olive and Marion, 2009). Maximum incision values increase with trail grade across the diverse AT dataset, and the steepest trail segments are the most severely incised (Fig. 2, Fig. 3, Table 2). Also supported by other studies is the finding that fall-line trails are highly vulnerable to soil loss (Fig. 3) and that TSA
Conclusions
This study sought to investigate and integrate trail sustainability findings related to the three core forms of trail impact: soil loss, muddiness, and widening. Findings confirm that rolling side-hill trail alignments with steeper side-slopes and landform grades are the most sustainable trail alignments. Severe soil loss is likely on steep trail grades, particularly for trails aligned closely to the fall-line, which rapidly incise and make the diversion of water from their treads exceedingly
Credit Author Statement
Fletcher Meadema: Methodology, Formal Analysis, Writing – Original Draft, Writing – Review & Editing, Visualization. Jeffrey Marion: Conceptualization, Methodology, Formal Analysis, Writing – Original Draft, Writing – Review & Editing, Visualization, Supervision, Project Administration, Funding Acquisition. Johanna Arredondo: Methodology, Formal Analysis, Writing – Review & Editing. Jeremy Wimpey: Conceptualization, Methodology, Formal Analysis, Writing – Review & Editing, Supervision, Funding
Acknowledgments
We thank Chris Carr for collaboration and assistance in sampling, protocol development, and fieldwork, and Brian Peterson, Dylan Spencer, Kaitlin Burroughs, Mary-Ellen Burnette, and Mitch Rosen for their dedicated assistance collecting field data. This study was funded by the US National Park Service, with guidance, collaboration, and contracting support provided by the Appalachian Trail Conservancy. Use of trade, product, or firm names is for descriptive purposes only and does not imply
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