Tree Barks as Environmental Biomonitors of Metals - The Example of Mercury

Tree tissues still occupy a relatively small niche as bioindicators when compared to other more classical bio substrates such as mosses and lichens. Tree bark, because of its structural porosity, is potentially very efficient for the accumulation and retention of aerosol particles, making this substrate an excellent indicator in air pollution monitoring studies, especially for heavy metals. Mercury (Hg) is possibly the ideal element to be monitored through barks because a) generally, it is scarcely bioavailable in soils; b) its presence in bark is mainly ascribable to atmospheric transport; c) it could be retained in tree bark both physically and chemically. Recent studies in the Monte Amiata (central Italy) area documented the ability of Pinus sp. barks to take up from the atmosphere extremely high levels of Hg. We present here the results of a reconnaissance study conducted in the urban area of Firenze (Florence; Italy), documenting the sensitiveness of Pinus sp. barks in recording small variations of Hg concentration in atmosphere. The study includes a former industrial zone, where anomalous concentrations of Hg in soils and atmosphere are still present. Barks sampled in this area show Hg concentrations statistically higher than in other zones of Firenze, lending further support to the concept that tree barks may be useful tools for biomonitoring the airborne Hg levels.