196 Adverse Soil pH While some instances of slightly lower pH in forested lands near urban cores have been documented (Pouyat et al. 1995), disturbed urban soils are rarely too acidic for satisfactory tree growth. Instead, soil alkalinity is a more common consequence of urbanization and therefore a more common impediment to tree health. The use of concrete and other calcareous construc- tion materials is nearly universal in urban areas and the removal of topsoil and horizon mixing facilitates the increase in soil pH. In Hong Kong, China, soils sampled from 100 locations around the city core had a mean pH of 8.68 (Jim 1998). Sampling of soil pH in the top 10 cm of mineral soil around the Virginia Tech central campus in Blacksburg, VA, by students during laboratory exercises in horticulture and forestry classes taught by two authors of this review revealed soil pH is always above 7.0 and as high as 8.3, whereas nearby relatively undisturbed sites has surface soil pH of 5.9–6.2 and nearby disturbed road- side ditches a pH of 6.8–7.3 (Harris et al. 2008). A study of six urban landscapes in Moscow, ID, and Pullman, WA, found av- erage pH ranges from 6.64 to 7.32 (Scharenbroch et al. 2005). At higher soil pH, many tree species suffer from micro- nutrient deficiencies (B, Cu, Fe, Mn, and Zn) because these nutrients exist in insoluble forms that are unavailable to the plant (Mengel and Kirkby 2001). Availability of P is also re- duced in alkaline soil. Elevated pH may also alter the compo- sition and abundance of endomycorrhizal fungi that inhabit soil (Porter et al. 1987), which could influence root system colonization and therefore nutrient uptake capacity. On the other hand, soil alkalinity also reduces the solubility of cer- tain elements such as Al and Pb, which are toxic to tree roots. Sensitivity to alkalinity-induced nutrient deficiencies differs among tree species. In even slightly alkaline soils, sensitive spe- cies such as Quercus palustris (pin oak) and Quercus phellos (willow oak) may develop interveinal chlorosis in response to Fe and Mn deficiency while others remain unaffected [e.g., Ulmus americana (American elm) and Platanus × acerifolia (London plane)] (Dirr 1998). Root adaptations have been identified in some tolerant species that enhance Fe uptake, one example being the production of a specialized enzyme to reduce Fe (Moog and Brüggemann 1994). An evaluation of olive tree cultivars and root- stocks indicated that tolerance of calcareous soils was conferred by the rootstock rather than the scion (Alcántara et al. 2003). Because of the ubiquity of alkaline soils in urban settings and the varied sensitivity of tree species to these soils, lists have been published to assist practitioners in selecting tree species and cultivars that tolerate particular soil pH levels (e.g., Apple- ton and Chaplin 2001; Bassuk et al. 2009). These lists are based partly, although certainly not exclusively, on practitioner experi- ence since research reports are limited on many trees. In orchard trees, a clear asymptotic relationship is apparent between extract- able Fe in the soil and leaf chlorosis: leaf greenness increases rapidly with increasing extractable Fe until a maximum level is reached, at which point the relationship levels off (de Santiago et al. 2008). However, in some urban trees, iron deficiency chlo- rosis has not shown a strong relationship with soil pH (Watson and Himelick 2004) and therefore likely not with the associated variable of extractable soil Fe either, although this last relation- ship has not been reported. A host of root system stresses – in- cluding root severance can negatively affect Fe uptake by urban tree roots. This has real consequences for urban trees since Fe ©2010 International Society of Arboriculture Day et al.: Tree Root Ecology in the Urban Environment or Mn deficiency impairs photosynthetic capacity (Abadía et al. 1999), which may diminish tree growth and stress tolerance. Salt Contamination Salt contamination of soils can stunt or kill tree roots depending upon species sensitivity, environmental variables (soil physical and chemical properties, precipitation, light intensity, tempera- ture), duration and timing of exposure, and severity of contami- nation (Headley and Bassuk 1991; Bernstein and Kafkafi 2002). Salt contamination can arise from meltwater or spray from de- icing salts (Kayama et al. 2003), from saltwater intrusion into groundwater, from sea salt blown ashore in coastal areas, or even from repeated applications of sewage sludge (Usman et al. 2004). De-icing salt is a common soil contaminant in cold- er climates. NaCl is the most widely-available, cost-effective material for de-icing streets, sidewalks, and parking lots, al- though other formulations such as CaCl2 In Denmark, high road salt concentrations were found in soils within 2 m of roadways, but quickly dissipated at greater dis- tances (Pedersen et al. 2000). When precipitation is abundant, salt does not persist in the top layers of soil and eventually leaches down to subsoil horizons and groundwater (for a review of the environmental effects road salt, including effects on vegeta- tion, see Priority Substances List Assessment: Road Salt 2001). Because of its agronomic importance, salt stress has been the subject of considerable research. Nonetheless, the physiological mechanisms for tolerance are varied and complex and likely rep- resent expressions of multiple genes as well as other adaptive re- sponses (for reviews, see Cheeseman 1988). Root growth is usu- ally less sensitive to salt stress than shoot growth, resulting in a higher root:shoot ratio in salt-stressed plants (Cheeseman 1988). However, in landscape situations, tree roots can be subjected to acute salt shock when large amounts of roadside deicing salt are applied (Headley and Bassuk 1991). High levels of salinity impose two types of stress on roots; first, osmotic stress results from lowered water potential in the soil solution (desiccation), and second, ionic stress results from changes in concentrations of specific ions in the soil solution and inside growing tissues (toxicity). Root systems vary in their ability to tolerate salts; tol- erant species may be able to selectively exclude salt ion uptake (Lloyd et al. 1987). However, few generalizations can be made. For example, in a study of grafted Citrus spp. (lemon trees), sa- linity reduced growth of some rootstocks more than others and in some cases physiological stress was governed primarily by toxic levels of Na+ and K2 CO3 and Cl- in leaf tissue (Gimeno et al. 2009). Salinity can also alter the symbiotic relationship between the roots of woody plants and mycorrhizal fungi, but this is not well understood at this time (Tian et al. 2004; Porras-Soriano et al. 2009). Because of the economic importance of salt tolerance in food crops, research is quickly identifying plant mechanisms of salt tolerance and their genetic control (e.g., Papdi et al. 2009). Trace Elements and Heavy Metals Numerous trace elements are essential or beneficial for plant function, including B, Cu, Fe, Mn, Mo, and Zn (essential); Cl and Ni (sometimes essential); and Co, I, Na, Si, and V (beneficial) (Marschner 1996; Mengel and Kirkby 2001). However, all these elements can be toxic when their concentrations are too high (Hagemeyer and Breckle 2002). Heavy metals are commonly are used.
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