Exhaust fumes, industrial fumes, and other pollutants in urban areas can cause an increase of heavy metal toxins within the soil environment that supports tree populations (Chen et al., 2005; Manta et al., 2002; Markiewicz-Patkowska et al., 2005; Wei & Yang, 2010). Heavy metals such as lead, cadmium, aluminium and mercury, are incredibly toxic to tree species even at low quantities, and can therefore inhibit vigour and bring about health implications as a result (Banu Doganlar et al., 2012; Day et al., 2010).
Heavy metals induce morphological and physiological changes in plants, including the inhibition of root and shoot development. The acidification of the soil by such heavy metals combined with their impact on tree vigour and function means that trees may not be able to sustain existing mass and may suffer dieback consequentially (Boudot et al., 1994; Horst et al., 2010).
A reduction in root hair density is an adaptive response for decreasing absorption of heavy metals, though absorption of water and nutrients will also likely be reduced as a result. In addition, nutrient uptake may be reduced further due to direct ion competition, as research has identified lower nutrient concentrations in roots of numerous tree species exposed to heavy metals – this is considered to be due to both the reduced uptake from reduced root hair density and increased cell membrane leakage induced by heavy metal ion abundance within the soil (Day et al., 2010).
Thus, as heavy metals are commonly found in urban areas, both root exploration in the soil and the uptake of nutrients and water will be adversely impacted, in turn impacting upon the tree’s ability to generate and sustain mass.
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