Tree leaf wettability as passive bio-indicator of urban habitat quality

This study evaluated the effect of urban habitat quality on the wettability of tree leaves. We measured leaf wettability of five common tree species, i.e., Alnus glutinosa, Acer pseudoplatanus, Betula pendula, Quercus robur and Sambucus nigra, in semi-natural and industrial urban habitats in the city of Gent (Belgium). Possible seasonal variation was taken into account by measuring in late spring and in late summer. Drop contact angle (DCA), and height over width ratio were measured on the abaxial and the adaxial leaf surface as proxies for leaf wettability. The relative standard deviation for the height over width ratio was higher than for the DCA, so that only the latter was considered further. Habitat type significantly influenced leaf wettability: the DCA values of Q robur leaves were significantly lower in the industrial than in the semi-natural areas, in both June and September while, for S. nigra, the DCA was in both sampling events significantly higher in the industrial areas. For the adaxial leaf side, the differences between the considered habitats were more pronounced in June than in September. The adaxial DCA of A. pseudoplatanus was significantly higher in June compared to September, while the opposite held for abaxial values of A. glutinosa. We conclude that leaf wettability is potentially a good indicator to point out differences in urban habitat quality, but selection of the most sensitive tree species and the appropriate time of measuring is an important prerequisite. (C) 2011 Elsevier B.V. All rights reserved.