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Abstract of thesis entitled
The Biomonitoring of Heavy Metal Pollution in the Wood and Leaf Chemistry
of Urban Trees in Hong Kong
no Ching Yee (Christina)
for the Degree of Master of Philosophy at the University of Hong Kong
in January 1999
This study is to investigate the applicability of using trees grown in a humid subtropical climate as biological monitors in detecting and tracing environmental changes such as metal air pollution in Hong Kong. Ficus microcarpa (Chinese Banyan), Aleurites moluccana (Candlenut Tree), Delonix regia (Flame of the Forest) and Bomhax malaharicum (Red Kapok) are sampled from urban areas as the polluted samples and from country parks as the control samples. The foliage and stem core samples are analyzed with inductively coupled plasma atomic emission spectroscopy (ICP-AES) to find out their concentrations of Pb, Cd, Cr, Ni, AI, Ba, P, Cu, Mn, Zn and Ca. Results are then compared with the air pollutant records collected by the Environmental Protection Department since the 1980s, and the local air metal pollution levels reckoned from traffic intensity data.
Many elemental concentrations of the foliage and young wood samples of the four species form significant coefficients either with the air pollutant records or with nearby traffic intensity, or with both, showing their potential in biomonitoring present-day metal loading. Biomonitoring with foliage samples of F. microcarpa, A. moluccana and B. malaharicum is more promising than with D. regia. Wax-coated leaves of F. microcarpa and hairy-surfaced A. mo/uccana and B. malabaricum have greater metal retaining ability
than the small leaflets of D. regia. Besides, the higher leaf longevity of evergreen F. microcarpa and A. mo/uccana also means longer period of biomonitoring. The tree stem core chemistry of the four species cannot serve as chemical history recorders to reveal the missing temporal and spatial metal loading history of urban Hong Kong. Elements are present in high concentrations in old woody stem sections formed in a relatively clean era, the first few decades of this century. The significant coefficients between certain young and old stem sections point to heartwood formation as an explanation.
In some cases, the elemental concentrations of the control samples are not the lowest among their own sampled species, showing the influences of the different elemental bioavailabilities in the alkaline roadside soil in city areas and the acidic natural country park soil, and the different stress regimes of urban trees and country park trees.