By Adrian Hagarty
Nestled between the Rappahannock Parking Deck and University Dr. is a sliver of sloping, grassy land populated by trees and shrubs reintroduced over the years since the lot was first built. Forested zones are a crucial form of land cover due to their role in carbon sequestration and support of biodiversity (Ostrom & Cox, 2010). Though the trees present by Rappahannock are sparse, they have the potential to reduce surface runoff, recharge groundwater sources, and improve soil absorption as well as moisture (Benegas et al., 2021). Woody plants surpass herbaceous species in nutrient and water uptake, though the latter act as crucial filtration systems by soaking up particulate matter brought down the slope via storms (Duan, 2021). Both flora are crucial for their role in maintaining slope stability and reinforcing the soil, which is especially important given the topography (Li et al., 2021). With the parking lot at the bottom of the hill and the street at the top, the land by Rappahannock is at risk of multiple forms of environmental degradation from drivers who use the area to commute or park their vehicles. Parking lots can raise the temperature for nearby residential areas through heat absorbed and released from asphalt throughout the day, and vehicles can leave oil, gasoline, and other harmful residue which threatens waterways when washed away by storms (Cotrone, 2022). Furthermore, when the lot was initially built, the removal of vegetation and topsoil increased the zone’s vulnerability to erosion (EPA, n.d.). There are storm drains present at the site, which means flooding is unlikely – however, the water entering these drains is still at risk of carrying eroded pollutants and sediments, which are eventually destined to enter local watersheds.
The street, parking lot, and the hill in between them form a singular system which involves the people and vehicles using the area, as well as the plants in the forested strip, the latter of which serves as a buffer. Buffer zones are critical for pollution control and protection of nearby land, and are one of the most commonly used best management practices, especially for reducing nonpoint pollutants such as stormwater runoff (National Academies of Sciences, Engineering, and Medicine, 2000). Small forests with low commercial value but strong community drive have been assessed as possessing major potential for management and protection (Ostrom & Cox, 2010). George Mason University has, in the past, already taken measures to restore the area. The Green Patriots Reforestation, a $21,000 initiative, was first passed in 2013/2014 to restore the area, but was only partially completed (Mason University Sustainability, n.d.). In the 2020/2021 academic year the project was picked up again, this time through two smaller initiatives that added an additional handful of trees and shrubs (Mason University Sustainability, n.d.). Continuing these efforts would be effective in involving the Mason community as well as restoring an important natural resource on campus.
One study evaluating the safety factor of sloping landscapes determined that the stability of vegetated land was ranked at 49.6%, compared to a 12.1% rating for non-vegetated counterparts (Li et al., 2021). This significant difference held true even when wind was involved as a factor, and it was found that although shrubs held better resistance to wind due to smaller canopies, trees were better at enforcing soil stability due to their root systems (Li et al., 2021). At Rappahannock, the three major current plant groups are short grasses, shrubs, and small trees. Out of all combinations tested, shrubs and trees actually had the lowest collective safety factor — grass and trees, in contrast, had the highest (Li et al., 2021). Grasses are effective due to their faster growing period, and can provide stability in the early stages while trees are being established (Li et al., 2021). Notably, the study used Setaria viridis, a much taller grass than typical lawn species. Therefore, it is recommended that for future efforts, George Mason considers taller native species, which will not only require less maintenance but should perform better at erosion control. A diversity of plants yields the most effective result for filtering pollution (National Academies of Sciences, Engineering, and Medicine, 2000). However, it is crucial to choose species which work well together.
Grasses should be a considerable focus. Though adding more trees would increase the benefits they provide, crowding them too densely may have setbacks. In fact, the optimum tree cover theory states that intermediate densities in at-risk soils are more effective at maximizing groundwater recharge (Benegas et al., 2021). It is more important to properly care for the trees already present, and deal with a problem currently putting them at risk — mulching. When done right, deposition of mulch can increase soil organic carbon, enhance organic matter storage, and encourage improved forest productivity (Sun et al., 2021). However, to reap these benefits, mulch must be integrated correctly. Several of the trees present by Rappahannock have been mulched via the “volcano” method, which places mulch in a sizable dome under the tree, overlapping the bark and causing a myriad of issues.
A tree’s bark is dead, dry tissue which serves to protect from pests and dehydration, but when mulch is piled up against the base of the tree, the bark trapped beneath is subject to damage and even rot (Boggs, 2020). Furthermore, certain mulches, such as those made of bark, become compacted over time and interfere with oxygen uptake (Boggs, 2020). To breathe, the tree grows secondary roots that extend into the mulch – these may then be dehydrated or may even strangle the tree by wrapping back around the trunk in a “stem girdling” formation (Boggs, 2020). Mulch can even act as a water repellant as it decomposes and dries (Boggs, 2020). All of these factors may induce stress and make the tree vulnerable to pests, disease, and rot.
An example of what not to do: “volcano mulching”
In order to have an effective pollution and run-off control system, healthy trees are a must-have. Mulching properly is simple — the depth should be shallow, no more than 2-3 inches, instead of a heaping pile (Boggs, 2020). Keeping mulch away from the trunk is also crucial to avoid bark and root issues.
The reforestation of the land by Rappahannock has made progress since 2014, but continuous restoration should be pursued to maximize the benefits of the area as a means of pollution and stormwater runoff control, slope protection, and as a green site on the Fairfax campus.
References
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Boggs, J. (2020). Treemergency: Mulch Volcanos are Erupting in Landscapes! Ohio State University.
https://bygl.osu.edu/node/1494
Cotrone, V. (2022). Green Parking Lots: Mitigating Climate Change and the Urban Heat Island. Penn
State Extension. https://extension.psu.edu/green-parking-lots-mitigating-climate-change-
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Duan, Y., Tang, J., Li, Z., Yang, B., Yan, Y., Yang, Y. (2021). Vegetated Buffer Zone Restoration Planning
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