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. Key pointsThe forest filter effect describes the uptake of atmospheric persistent organic pollutants (POPs) by foliage and their transport to the forest floor via litterfall, throughfall and the erosion of wax and particles.The global forest can store more than 100Gg of POPs, delaying their long-range atmospheric transport.POP and polyaromatic hydrocarbon concentrations tend to be higher in European forest soils than on other continents.Deforestation and afforestation caused by human activities and climate change can substantially influence the terrestrial stock of POPs.International strategies and regional and/or global models of POP fate should consider the impacts of climate change and forest fires on POPs cycling.

Automated summary

Persistent organic pollutants (POPs) are internationally regulated due to their toxicity and environmental impact. Forests act as a filter for POPs, absorbing and storing them from the atmosphere, but factors like soil organic carbon content and climate can influence their distribution and storage. Deforestation and forest fires threaten the ability of forests to sequester POPs, impacting their global cycling. Regular large-scale observations in forests are needed to better understand the burden of POPs in the environment.