Conditions in urban environments place trees under numerous stresses including compacted soil, soil moisture extremes, and reduced soil fertility. Polluted air is another stress that contributes to the decline of urban trees. Air pollution may cause short-term (acute) damage, which is immediately visible, and long-term (chronic) damage, which can lead to gradual tree decline. Long-term damage may predispose trees to other disorders, making diagnosis difficult. The major phytotoxic (toxic to plants) air pollutants, in decreasing order of severity, are: oxidants [ozone and peroxyacetyl nitrate (PAN)],sulfur dioxide , and particulates

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