Tree species as tools for biomonitoring and phytoremediation in urban environments: a review with special regard to heavy metals
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Trees play an important role for the improvement of environmental quality in urban areas. The improvement of microclimate, runoff mitigation, carbon storage and sequestration, noise reduction, air purification through removal and fixation of pollutants in leaves, stems, and roots are ecosystem services provided by urban greening. Additionally, the capacity of certain tree species as bioindicator or to take up contaminants has to be taken into account. Presented here is a review that focuses on 9 ornamental tree species commonly planted along urban streets in Central Europe. Their potential role for bioindication and phytoremediation was assessed. Due to physiological and morphological characteristics and to the intrinsic tolerance to several stress factors, some species seem particularly promising as an indicator for the state of the urban environment or to decrease the amount of specific pollutants. It must be pointed out that intrinsic species properties (e.g., tolerance and/or bioindication capacity for a specific contaminant) can help planners create an effective monitoring net in strategic areas of a city or to detect single contaminants representative of a specific human impact. In particular, Betula pendula and Robinia pseudoacacia can be considered ideal, low-cost candidates for phytoremediation. Due to their high hardiness, pollution tolerance, and their characteristic as pioneer species, both species might additionally be taken into account as biomonitors, or for their foliar trapping capacity. Tilia cordata is also suitable for phytoremediation in urban environments due to its foliar trapping capacity that can provide valuable information on airborne pollutants.