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Heavy metals: impacts and future effects on the forest

Laure Gandois (CNRS) takes the floor

What impact do heavy metals have on the forest? What does the future have in store?

Summary

Atmospheric contamination by trace metals

Trace metals (TMEs) are elements originating from the Earth's crust that are found in minute concentrations in the natural environment. They can be dispersed into the atmosphere through a number of natural processes such as forest fires, volcanic eruptions or marine aerosol emissions. However, since ancient times, man has also contributed to their dispersal by exploiting metals for industrial activities. This anthropic influence exploded in the 20th century in the northern hemisphere with the Industrial Revolution and the development of mechanised transportation. Natural TME cycles were altered at the global scale and their dispersal into the atmosphere over long distances contaminated areas that would normally have been free of this type of pollution.

Today, cadmium and lead emissions are decreasing in Europe, in particular thanks to the 2000 ban on lead additives in petrol and to improved industrial emissions-reduction techniques. This may not be the case for all metals, however. Some emissions, for example copper and nickel, seem to have stabilised while others are increasing (antimony, for example). Furthermore, though emissions are mostly decreasing in Europe, they are globally increasing at the planetary scale due to industrial development in Asia.

Monitoring atmospheric deposits

The EMEP Programme monitors and assesses the extent of long-distance TME atmospheric deposits in Europe. Deposition models are built from measurements taken at sites throughout the territory. In France, only two stations are equipped to monitor TME atmospheric deposits and both are part of the MERA Network. However, the BRAMM Network, which classifies mosses as TME bio-accumulators, covers the entire territory with its 528 sampling sites. Whenever possible, BRAMM sites are set up near RENECOFOR sites. This proximity makes it possible to compare mosses and lichens to evaluate their role as bio-indicators of TME deposits and to study how forest cover influences the recording of these deposits.

Indeed, forests are particularly sensitive to atmospheric pollution by metals. The canopy acts as an exchange surface for reactions with TMEs. Dry deposits accumulate there. The exudation of organic molecules through the canopy leaves can totally or partially dissolve this dry deposit, which can then create a solution with rainwater or other forms of precipitation. The canopy can also assimilate the elements, whether they are deposited in dry or wet form. These processes change the TME flux reaching the forest floor. Thanks to the CATAENAT Network, this effect can be directly observed by comparing deposit data recorded in the open air and under forest cover. For example, on average lead fluxes are five times as heavy under forest cover, while zinc fluxes are halved.

Metal dynamics in forest ecosystems

What happens to TMEs in the forest depends on which ecosystem compartment they are transferred to (soil, water, vegetation) - as shown in the figure below. Their mobility is linked at least partially to their potential role in the biosphere (some are important micro-nutrients like copper, cobalt, chromium, nickel or zinc; others like antimony, cadmium or lead play no known biological role). Their chemical properties also influence their mobility. In particular, their affinity with organic matter (both solid and dissolved) and their pH sensitivity control to a large extent how they are transferred within the soil layers and into the biosphere, and whether or not they accumulate in the forest soil. On an acidic plot, like SP 57 for example, trace metals will not accumulate in the soils because they will dissolve and be carried away in the water runoff whereas on a plot with base soils, like SP11, they will tend to accumulate.

Flow diagram of ETM in forest ecosystems. The balance sheet for the soil compartment is the result of the inflow (atmospheric input, litter fall, weathering) and output (drainage, export by vegetation). On the right: this report is presented for
Flow diagram of ETM in forest ecosystems. The balance sheet for the soil compartment is the result of the inflow (atmospheric input, litter fall, weathering) and output (drainage, export by vegetation). On the right: this report is presented for two plots RENECOFOR, SP 57 and SP 11. Cd: cadmium; Cu: copper; Cr: chromium; Ni: nickel; Pb: lead © Laure Gandois / CNRS

The impact of metals on forest ecosystems

Soil analyses on the RENECOFOR plots reveal the presence of metals dispersed through human activity, notably in eastern and northern France. However, contamination levels remain low. Current TME atmospheric deposits in French forests are moderate, and are representative of long-distance deposit levels in rural Europe. Most are of anthropic origin, but remain below critical charge thresholds - the maximum admissible quantities an ecosystem can support, according to current knowledge.

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