198 tion and water percolation and storage as well as create zones of failure, which help fragment the soil, form aggregates, and decrease resistance for further root growth. Roots form macro- pores by creating compressive and shear stresses when grow- ing through the soil matrix (Goss 1991). Radial pressure ex- erted by growing roots compresses adjacent soil (Dexter 1987), which enlarges existing pores and creates new ones. Bartens et al. (2008) demonstrated that live roots can create channels through compacted soils and vastly increase water infiltration, although flow may be greater once roots die and decay (Mitch- ell et al. 1995). As root decay occurs, tissue remnants and as- sociated microflora coat pore walls, which may enhance wa- ter transport efficiency (Barley 1954; Yunusa et al. 2002). Tree roots aid in soil aggregate formation Aggregate stability, an indicator of soil structure, results from soil particle rearrangement, flocculation, and cementation; it is mediated by soil organic carbon, biota, ionic bridging, clay, and carbonates (Bronick and Lal 2005). Rhizosphere soil has been found to have greater aggregate stability than nonrhizosphere soil (Angers and Caron 1998), and is influenced by rhizosphere de- position as well as a number of root system attributes, including root length, mass, density, size distribution, turnover rate, and hy- phal growth (Caravaca et al. 2002). Dorioz et al. (1993) observed that adsorption of water by roots promoted reorganization of the clay, characterized by oriented and compacted clay particles, and that this environment was very rich in root mucilage. “The outstanding effect of the rhizosphere on soil structure can be re- lated to the rhizosphere as being the privileged site for growth for a wide range of microorganisms at various sizes, each of them organizing the material at its own scale” (Dorioz et al. 1993). Tree roots can directly enhance aggregation by releasing a va- riety of compounds that have a cementing effect on soil particles (Bronick and Lal 2005). For example, polysaccharides from root tips can penetrate and impregnate surrounding soil up to 50 µm while bacteria polysaccharides penetrate less than 1 µm (Dorioz et al. 1993). Research suggests that the root exudate polygalat- uronic acid (PGA) stabilizes soil by increasing strength of bonds between particles and decreasing wetting rate of soil via water repellency at the soil surface (Czarnes et al. 2000). Tree roots also indirectly contribute to soil aggregate formation and stabil- ity because their exudates are a food source for soil organisms, which in turn release their own exudates that contribute to soil aggregation (Tisdall et al. 1978). These exudates are also a food source for earthworms (Angers and Caron 1998), which create macropores as they burrow through the soil (Edwards et al. 1989). Soil strength and stability Tree root systems form part of a complex matrix that can sta- bilize soil and reduce erosion, both important contributions to environmental sustainability. Soil inhabited by plants dries more quickly due to transpiration; as a result, the soil has greater shear and tensile strength and the root/soil tangential resistance to slipping will be increased (Waldron and Dakes- sian 1982). Lower soil water content resulting from the pres- ence of plants may also help soils resist compaction (Horn and Dexter 1989; Lafond et al. 1992). Deep-rooted woody vegeta- tion extracts more water from greater soil depths than grassy vegetation (Bethlahmy 1962; Rogerson 1976; McColl 1977). ©2010 International Society of Arboriculture Day et al.: Tree Root Ecology in the Urban Environment This deep water extraction and resulting wetting and dry- ing cycles can cause shrinkage and strengthening of the soil. In addition to drying soil, tree roots increase soil stability via mechanical reinforcement (Waldron and Dakessian 1981; Wal- dron and Dakessian 1982; Abe and Iwamoto 1986; Mamo and Bubenzer 2001a; Mamo and Bubenzer 2001b; Wynn and Mo- staghimi 2006). Construction of highways and other infrastruc- ture alters the natural terrain, often resulting in steep, barren slopes that pose a landslide hazard. Tree roots have been used as tools for slope reinforcement, either alone (Norris 2005), or in combination with engineered approaches (Naoto et al. 2008). Although herbaceous vegetation may provide more immediate cover and soil stabilization, woody plants may provide greater reinforcement strength. In a study comparing the shear resistance of soil inhabited by different plants, alfalfa and grass had a more immediate effect on sheer resistance than yellow pine, but the older pine roots were clearly superior to young alfalfa roots, and shearing resistance was proportional to the number and diam- eter of pine roots (Waldron and Dakessian; Waldron et al. 1983). Trees can also play an important role in stream bank stabili- zation (Docker and Hubble 2008; Pollen-Bankhead et al. 2009). In urban areas, stormwater runoff results in widely fluctuat- ing water levels in streams, leading to channel erosion and im- paired water quality (Schoonover et al. 2006). An in situ study of vegetated stream banks showed that an increase in the vol- ume of roots with diameters of 2–20 mm was correlated with reduced soil erodability (Wynn and Mostaghimi 2006). Wynn et al. (2004) compared root distribution and density in stream banks inhabited by both herbaceous and woody vegetation. Their findings suggest riparian forests may provide better pro- tection against stream bank erosion than herbaceous buffers. Hydrology Impervious surfaces, soil compaction, and stormwater drains prevent dispersed infiltration of stormwater in the built environ- ment, decreasing groundwater levels and stream baseflow (Kaye et al. 2006). Even unpaved urban soils can have much reduced infiltration rates compared to undeveloped land (Gregory et al. 2006). In vegetated areas, only 5%–15% of rainwater runs off the ground and the rest evaporates or infiltrates into the soil, whereas about 60% of rainfall in urban areas is exported through storm drains (Bolund and Hunhammar 1999). Older stormwater sys- tems are often connected to sewers and when these stormwater systems overflow, untreated sewage pollutes surface waters. Even if storm drains are not connected to sewers, stormwater is still concentrated and not allowed to infiltrate in a dispersed fash- ion, thereby reducing the influence of plants and soil on water chemistry and increasing stream temperatures when stormwa- ter is directly deposited into surface waters (Kaye et al. 2006). Urban trees are well recognized as effective tools for mitigat- ing urban runoff (Xiao et al. 2000; Xiao and McPherson 2003), but the specific role of the root system is largely unrecognized. Root systems aid in dispersal of stormwater into the soil by guid- ing stormwater along root channels, playing a primary role in base flow (Dasgupta et al. 2006; Johnson and Lehmann 2006), aiding in water infiltration (Bramley et al. 2003; Bartens et al. 2008), and absorbing water (Wullschleger et al. 1998; Szabo et al. 2001). In addition, hydraulic lift by tree roots may improve survival of other plant species in dry climates, thus enhancing the contribution of
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