Stemflow chemistry of urban street trees

pH and ion concentrations (Na⁺, K⁺, Mg²⁺, Ca²⁺, NO₃^-) in the stemflow of the evergreen broad-leaved tree, Ilex rotunda, planted in the median strip of a highway and nitrogen oxides concentration in the air in an urban site were compared with a suburban site in Fukuoka city, Japan. The annual average of the nitrogen oxides concentration in the air was higher and NO₃^- concentration in the stemflow at the urban site was higher or similar compared with the suburban site. However, the annual average of pH in the stemflow at the urban site was higher than at the suburban. The annual average cation concentrations in the stemflow at the urban site were higher than at the suburban except Na⁺. In particular, K⁺ and Ca²⁺ were higher throughout the measurement period. Therefore, higher pH in the urban stemflow was probably due to neutralization by higher concentrations of K⁺, Mg²⁺ and Ca²⁺.