Arboriculture & Urban Forestry 35(3): May 2009 Arboriculture & Urban Forestry 2009. 35(3): 135–141 135 Soil Moisture and Aeration Beneath Pervious and Impervious Pavements Justin Morgenroth and Graeme D. Buchan Abstract. Pervious paving has been increasingly installed in urban areas as a stormwater management strategy. Pervious pavements exhibit similar strength to impervious analogues, but are highly permeable to air and water. These functional characteristics have led to speculation that pervious paving, used instead of impervious paving, could benefit urban trees. Given that permeability to air and water will have a direct effect on the soil environment, this paper describes research that explores the effect of pervious pavement on underly- ing soil physical conditions. Results indicate that while soil moisture and aeration dynamics differ greatly beneath paved and unpaved surfaces, differences are usually insignificant between pervious and impervious paving. If urban trees do benefit from overlying pervi- ous paving relative to impervious paving, it is probably not a consequence of soil moisture or aeration. The results challenge existing theories and contribute to the understanding of how pervious pavements affect the moisture and aeration dynamics of underlying soils. Key Words: Concrete; Permeable; Porous; Road; Sidewalk; Water. Urban areas are characterized by a high concentration of im- permeable surfaces; pavements are most pervasive, cover- ing more than half of all land in highly developed urban areas (Ferguson 2005). A recent paradigm shift has resulted in the proliferation of pervious pavements. This is especially true in the United States where the Clean Water Act and other regula- tions enforced by the Environmental Protection Agency neces- sitated new methods for stormwater management. These regu- lations require decreasing surface runoff and treating water at the source, both of which are achieved by pervious pavements. Though pervious paving is proliferating, research detail- ing its impact on the surrounding environment is lacking. A number of untested theories are liberally quoted in literature concerning the direct impact of pervious paving on the under- lying soil environment, and its indirect effect on urban trees. Tennis et al. (2004) concluded that pervious pavement is “ideal for protecting trees in a paved environment” and Fer- guson (2005) suggests that it can “increase the longevity of trees by improving moisture and oxygen relations.” Though these sources provide no experimental evidence, their as- sumptions appear logical as normal tree growth and func- tion require adequate soil water and aeration (Larcher 2003), both of which are allegedly enhanced by pervious pavements. This paper presents data collected following the first year of a two-year experiment, representing the first phase of a larger experiment. While the overall aim of the re- search is to better understand the relationship between per- vious paving, soil physical conditions, and tree growth, the data presented herein are limited to the effects of overlying pervious pavement on underlying soil. It is expected the per- meability of pervious paving, relative to impervious paving, will result in differing soil moisture and aeration dynamics. METHODS Study Site The experiment is located on a large parcel of city council land in Christchurch (Lat: 43°493’S, Long: 172°437’E), the larg- est city in New Zealand’s South Island. The top meter (3.28 ft) of soil is a fine sandy loam (Raeside 1974) overlying a de- posit of sand and gravel, a remnant of the alluvial outwash de- posited by an ancient glacier (Brown and Weeber 1992). The climate is temperate, with mean daily maximum tempera- tures ranging from c. 10°C (50°F) in July, to 21°C (70°F) in Janu- ary (McGann 1983). Occasional dry northwesterly winds occur during spring and summer, when temperatures can reach 30°C (86°F) and relative humidity can drop to 20%–40% (McGann 1983). Rainfall ranges from 600–700 mm (24–28 in) annually and is generally evenly distributed throughout the year, with a tendency for slightly higher early winter precipitation (McGann 1983). Uncharacteristically, the summer of 2008 was character- ized by intense precipitation. Approximately 15% of average an- nual precipitation fell during a single week in February 2008. Site Preparation and Experimental Design In July 2007, the site was cultivated to remove the existing turf and ensure uniform physical conditions to 30 cm (11.8 in) depth. The mean sampled bulk density following cultivation was 1.26 Mg/m3 (2123.8 lb/yd3 ticle density of 2.65 Mg/m3 (4466.7 lb/ yd3 ). Given this density, and an estimated par- ), the total porosity of the soil is c. 52.5%. Following soil preparation, a fully ran- domized, complete block experiment was installed comprising fifteen plots [each 230 cm x 230 cm (90.5 in x 90.5 in)] ran- domly assigned to one of three treatments: 1) Control–the soil was left unpaved and persistent grasses and clover were peri- odically sprayed with glyphosate and glufosinate-ammonium; ©2009 International Society of Arboriculture
May 2009
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