Arboriculture & Urban Forestry 36(5): September 2010 found in urban soils. They persist in the environment and can ac- cumulate over time to levels toxic to plants. Besides industry, ve- hicular traffic is the main source of metal pollutants. The highest levels occur near roads (Jim 1998) and levels decrease with dis- tance from the roadside (Birch and Scollen 2003; Fakayode and Olu-Owolabi 2003). Although modern regulations have reduced Pb emitted from vehicles, it persists in the environment and may remain elevated in roadsides. Zinc from tires is another major con- taminant associated with vehicular traffic (Roberts et al. 2006). Excessive concentrations of trace elements or heavy met- als cause phytotoxicity through several mechanisms, including changes in cell membrane permeability, interference with meta- bolic processes, and replacement of essential ions (Patra et al. 2004). In roots, metals inhibit growth by interfering with cell di- vision or cell elongation (Hagemeyer and Breckle 2002). These negative effects on roots may translate directly to negative effects on aboveground physiological function. For example, Hg toxic- ity symptoms of spruce seedlings such as decreased transpira- tion and lowered chlorophyll content were attributed primarily to root injury (Godbold and Hutterman 1988). Enhanced lateral root formation and compact, dense root branching habit have been observed in response to increasing concentrations of Pb, Zn, Mn, Cd, and Cu (Kahle 1993; Hagemeyer and Breckle 2002). It is thought that injury to the root apex by metals diminishes api- cal dominance, thereby increasing lateral root primordia. Lead also interferes with root hair formation. For example, root hair formation in Fagus sylvatica (European beech) was strongly in- hibited by Pb at a concentration of 44 ppm and was completely eliminated at 283 ppm (Kahle 1993). Although a reduction in root hair density is an adaptive response for decreasing absorp- tion of heavy metals, absorption of water and nutrients will also likely be reduced. In addition, nutrient uptake may be further reduced because of direct ion competition from heavy metals. For example, Kahle (1993) found lower nutrient concentrations in roots of numerous tree species exposed to heavy metals due to both reduced uptake and increased membrane leakage. Thus heavy metals commonly found in urban areas may both reduce root exploration of the soil and restrict uptake of nutrients and water. For a discussion of heavy metal threshold concentra- tions that reduce root growth, see Kahle (1993). Metal phyto- toxicity is tempered in soils with high pH, CEC, clay content, and organic matter because these conditions lower metal bio- availability (for reviews, see Kahle 1993; Sieghardt et al. 2005). Tolerance of heavy metals Plant tolerance of heavy metal toxicity varies among species and genotypes, and tolerance of one metal does not imply tolerance of all metals. Because of their relatively long life span, trees can ac- cumulate large amounts of toxic elements when growing on con- taminated soils. Moreover, they often lack the morphological and physiological adaptations possessed by herbaceous plants that regulate internal concentrations of toxic trace elements (Hage- meyer and Breckle 2002). Heavy metals are likely not uniformly accumulated in the root system. Violina et al. (1999), for exam- ple, found that Pb concentrations in grapevine (Vitis spp.) were highest in fine absorbing roots and much lower in older, woody roots. Trees that can survive on metal-rich sites may rely on phe- notypic plasticity, which enables roots to avoid areas of high contamination (Lepp 1991; Turner and Dickinson 1993; Hage- meyer and Breckle 2002). On the other hand, tolerant ecotypes of 197 some genera, such as Betula spp. (birch) and Salix spp. (willow), may exhibit multiple survival strategies, including synthesis of phytochelatins that immobilize metal ions within the plant, rapid root turnover, and metal ion exclusion (Kahle 1993), and can be- come dominant species on metal contaminated sites (Gallagher et al. 2008). Salix spp. are frequently employed in phytoreme- diation of soils, where plants are selected for their ability to ac- cumulate heavy metals or other contaminants from the soil and later harvested and safely disposed (Pulford and Watson 2003). Organic Pollutants and Pesticides There are a number of synthetic organic compounds (commonly pesticides and industrial compounds/by-products) that are poten- tial pollutants in urban settings, and some may persist in the envi- ronment. Toxic levels of industrial organics usually are a concern on sites that have historic industrial activity, but may also occur at accident “hotspots” such as along roadways and railways. Some pesticides can have a negative impact on nontarget soil organisms (Bunemann et al. 2006) and may therefore adversely affect root growth. Mycorrhizae, for example, are sensitive to certain pes- ticides, particularly fungicides. Container-grown Liriodendron tulipifera (tulip-poplar) inoculated with arbuscular mycorrhizal fungi and subsequently soil-drenched with benomyl fungicide had reduced growth and mycorrhizal colonization compared to their non-drenched counterparts (Verkade and Hamilton 1983). ROOT CONTRIBUTIONS TO ENVIRONMENTAL SUSTAINABILITY below the reach of crop roots; increase water infiltration and stor- age; decrease loss of nutrients to erosion and leaching; decrease soil acidity; and improve soil biological activity (Young 1997). Tree roots have the potential to positively influence soil quality, hy- drology, and biogeochemistry in urban settings. More specifical- ly, the roots of trees improve soil physical properties; maintain or enhance soil organic matter, N2 fixation, and nutrient uptake from Soil Structure There are many factors in the urban environment that contrib- ute to degradation of soils and in particular, soil structure (see Compacted Soil as a Permeable Impediment). Thus, the potential of tree roots to influence soil structure is of considerable inter- est. Tree roots are primary contributors to the development of soil structure and, in the longer term, soil formation. This new appreciation of the influence of roots on soil is redefining and enlarging our concept of rhizosphere: the area where soil inter- acts directly with living roots (Richter et al. 2007). Tree root contributions to soil structure not only affect plant growth, but a host of other soil functions that provide ecosystem ser- vices such as stormwater runoff mitigation through enhanced soil permeability (Bramley et al. 2003; Bartens et al. 2008). Tree roots form soil macropores Tree roots aid in improving soil structure in several ways. One of the most significant plant-induced changes in soil structure is the formation of continuous macropores (i.e., channels) by pen- etrating roots (Angers and Caron 1998). A large proportion of pores formed by roots fall into the macropore range (>30 µm) (Gibbs and Reid 1988). These macropores facilitate soil aera- ©2010 International Society of Arboriculture
September 2010
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