Persistent organic pollutant cycling in forests

Owing to their toxicity, persistence and capacity for long-range atmospheric transport, persistent organic pollutants (POPs) are internationally regulated. However, forests can uptake and sequester POPs from the atmosphere, acting as a filter as they are transported to the poles as part of the so-called grasshopper effect. In this Review, we summarize POP (and polyaromatic hydrocarbon) cycling and distribution in forests, and discuss the environmental factors that impact POP fates. Pollutants are taken up by foliage and transported to the forest floor, where they can be stored in the litter layer or leach further into the soil. Typically, soil organic carbon content, temperature and latitude are the most important factors influencing POP distribution and storage, with boreal and tropical forests accumulating the greatest POP concentrations. Forest fires and deforestation, however, threaten the ability of forests to sequester POPs, with the former also anticipated to increase production of POPs and polyaromatic hydrocarbons through combustion. In order to better estimate the burden of POPs in the environment, greater large-scale and long-term observations are required in all forests, particularly in tropical regions and the Southern Hemisphere. Persistent organic pollutants are transported globally, but there is evidence that forests act as a filter by sequestering these pollutants. This Review examines the role of forests in persistent organic pollutant cycling, including their storage in foliage and soil, and the impacts of global change.