Vol. 4/1, pp. 13–27 in Plant Ecology, © Urban & Fischer Verlag, 2001 Evolution and Ectomycorrhizas:their role in forest ecosystemsunder the impact of acidifying pollutants WSL Swiss Federal Research Institute for Forest, Snow and Landscape Research, 8903 Birmensdorf,Switzerland; e-mail: email@example.com The physiologically active lateral rootlets of all main trees in temperate forests arecolonised by ectomycorrhizal fungi, forming so-called ectomycorrhizas. These symbi-otic organs are the sites of exchange of nutrients, mainly P and N, provided from thefungal partner, and C from the host. Emerging from the ectomycorrhizas, fungal hy-phae exploit the soil for the mobilisation and absorption of water and nutrient ele-ments. By doing so, they connect the tree roots intimately with the soil and provideanchorage. The deposition of acidifying pollutants into forest ecosystems is a poten-tial threat to the health and vitality of forest trees because it leads to the acidificationand eutrophication of forest soils. Pollutants are also a threat to the functioning of ec-tomycorrhizas. Increased N concentrations in the soil lead to enhanced fungal N up-take and storage, and to enhanced N transfer to the host plants, and therefore tohigher plant biomass of above ground parts. In consequence, there is a decrease of Callocation to the plant roots. This in turn leads to reduced ectomycorrhization, and toreduced production of external mycelia and fruiting bodies. Soil acidification leads toenhanced availability of Al, heavy metals, and radionuclides in the soil, all of whichcan be toxic to plants and fungi. Reduced growth of roots and hyphae are amongstthe first symptoms. In ectomycorrhizas, the hyphae of the fungal tissues contain vac-uolar polyphosphates which have the ability to bind Al, heavy metals, radionuclidesand N. These electronegative polymers of phosphates represent an effective storageand detoxifying mechanism which otherwise is lacking in roots. Therefore, ecto-mycorrhizas have the potential to increase the tolerance of trees to acidifying pollu-tants and to the increased availability in the soil of toxic elements.