Arboriculture & Urban Forestry 37(2): March 2011 Arboriculture & Urban Forestry 2011. 37(2): 45–50 45 Root Growth Response of Platanus orientalis to Porous Pavements Justin Morgenroth Abstract. An experiment was established to determine the effect of porous pavement on underlying root growth. An augmented factorial arrangement of pavement profile designs and pavement types was installed and fifty Platanus orientalis seedlings were evenly distributed to control plots or one of four treatments. Treated plots were characterized by either porous or impervious pavement pads measuring 2.3 m × 2.3 m, and underlain by either fine sandy loam or a gravel base and compacted subgrade, reflecting two pavement profile designs. Following two growing seasons, root abundance was categorized by diameter and depth. Results suggest root abundance is greater, especially at shallow soil depths, under pavements. Pavements designed with a compacted subgrade and gravel base only exacerbated shallow root growth, though they could decrease total root abundance. Finally, porous and impervious pavements affected root abundance and distribution in similar ways, dismissing the use of porous pavements to promote deeper rooting. Key Words. Abundance; Biomass; Diameter; Distribution; Oriental Plan; Permeable; Pervious; Road; Sidewalk; Soil Compaction; Street Tree. The root systems of different tree species have varying architec- ture, and though some species have a deep tap root that penetrates vertically into the soil, root systems are typically shallow and wide- spreading. It is generally accepted that most roots grow in the up- per 30 cm of soil, and that they spread well beyond the crown (Gil- man 1990). This architecture ensures stability and optimal access to water and minerals (Perry 1982). Unfortunately, in urban envi- ronments, shallow root growth conflicts with overlying pavements (Kopinga 1994; Nicoll and Armstrong 1998). As roots expand radi- ally, they deform the soil above them, placing tensile stress on the upper surface of overlying pavements (Nicoll and Coutts 1997). While pavements are strong in compression, they are weak in ten- sion, as a result, underlying root growth leads to eventual pave- ment failure. These conflicts negatively impact both pavements and trees, often necessitating the repair or replacement of both. It’s im- portant to recognize that not all pavement damage is due to under- lying roots; engineering faults and underlying soil type can result in cracking as well (Sydnor et al. 2000). Standard pavements are designed to be impermeable for structural purposes, but if cracked, can expose underlying soil to atmospheric conditions, such as pre- cipitation and relatively high oxygen concentration. D’Amato et al. (2002) found that significantly greater root growth was located beneath existing cracks, and suggested that increased soil aeration beneath the crack resulted in greater root growth. In what could be considered a positive feedback loop, root growth can cause pave- ment failure and pavement failure can promote root growth. Unlike structural pavements, some pavements are designed to be perme- able to air and water; these are called porous pavements. While it had previously been suggested that porous pavements may be a solution to conflicts with roots (Barker 1988), high permeability may result in improved soil conditions for root growth and hence, increased incidence of conflict. To understand how different pave- ment types and profile designs affect underlying root growth, this experiment’s objective was to contrast root growth in open grown trees with those surrounded by porous and impervious pavements. METHODS Study Site and Experimental Design A comprehensive description of the study site and experimental design was previously described by Morgenroth and Visser (2011), nevertheless, an abbreviated description is provided herein. The experiment was located at the city council nursery on the outskirts of Christchurch, New Zealand (Lat: -43.493, Long: 172.437). Fifty, one-year-old, bare root oriental plane (Platanus orientalis) seedlings were randomly assigned to plots in an augmented fac- torial experiment consisting of controls and four treatments; trees were split evenly among treatments. The treatments were based on the combination of pavement type (2 levels: porous, impervi- ous) and pavement profile design (2 levels: +/- subgrade com- paction and gravel base). The resulting four treatments were im- pervious concrete pavement (IP), impervious concrete pavement with compacted subgrade and gravel base (IP+), porous concrete pavement (PP), and porous concrete pavement with compacted subgrade and gravel base (PP+). The distinction between the two levels of pavement profile design is related to the preparation of the profile below the pavement surface course. The profile design of IP and PP plots was characterized by a concrete pavement sur- face course installed over leveled topsoil, whereby the profile de- sign of IP+ and PP+ plots included a concrete pavement surface course installed over a leveled gravel base and compacted sub- grade. Soil penetration resistance in the uppermost 30 cm of soil in plots, following treatment installation, differed significantly ©2011 International Society of Arboriculture
March 2011
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