The bark of the branches of holm oak (Quercus ilex L.) for a retrospective study of trace elements in the atmosphere

Tree bark has proved to be a useful bioindicator for trace elements in the atmosphere, however it reflects an exposure occurring during an unidentified period of time, so it provides spatial information about the distribution of contaminants in a certain area, but it cannot be used to detect temporal changes or trends, which is an important achievement in environmental studies. In order to obtain information about a known period of time, the bark collected from the annual segments of tree branches can be used, allowing analyses going back 10-15 years with annual resolution. In the present study, the concentrations of As, Cd, Co, Cu, Fe, Mn, Ni, Pb, V and Zn were measured by atomic emission spectrometry in a series of samples covering the period from 2001 to 2013 in an urban environment. Downward time trends were significant for Cd, Pb and Zn. The only trace element showing an upward time trend was V. The concentrations of the remaining six trace elements were constant over time, showing that their presence in bark is not simply proportional to the duration of exposure. This approach, which is simple, reliable and widely applicable at a low cost, allows the "a posteriori" reconstruction of atmospheric trace element deposition when or where no monitoring programme is in progress and no other natural archives are available.