Black pine (Pinus nigra) bark as biomonitors of airborne mercury: sampling and analytical suggestions for minimising methodological biases.

Tree tissues still have a relatively small niche for heavy metals biomonitoring when compared to mosses and, in particular, lichens ([1] and reference therein). Tree barks are, in principle, excellent adsorbents of airborne pollutants, including toxic metals [2-5]. As emphasized by [6], there are contradictory data and opinions concerning the adequacy of barks as reliable bioindicators of atmospheric pollution (cf. [7]). The skepticism of some researchers is based on: a) the need for a deeper knowledge into mechanisms of bark interaction with air pollutants, and b) limited data regarding variability in bark pollutants, which complicates the assessment of the actual environmental pressure on a given area (see [5] and references therein for a comprehensive review). About b), there is obviously a need for a commonly accepted sampling procedure to make the results more comparable and, hopefully, more reliable. The present study aims to outline some factors that may minimize sampling biases in the analysis of mercury (Hg) in barks. In this work, barks of black Pine (Pinus nigra J.F. Arnold) trees from the Abbadia San Salvatore area (Mt. Amiata region, Southern Tuscany, Italy) and the underlying soils were sampled in summer 2016. In this area two industrial activities, past Hg mining and ongoing geothermal energy production, affect local atmospheric Hg levels [8-9].