Thesis: As radioisotopes of iodine may concentrate in thyroid gland, 131I (t1/2 = 8.07 days) and 129I (t1/2 = 15.7 106 years) are of health concern. 129I is of major radioecological importance because it can potentially integrate natural biogeochemical cycle of its stable isotope (127I) due to its long half-life. Forests, characterized by their longevity, a high biomass turnover and a strong influence on hydrological and nutrients cycles, can intercept, recycle and accumulate a significant amount of pollutants and radionuclides released into environment. In this context, research developed in this thesis had the purpose of correct long-term prediction of iodine behaviour in forest ecosystems.Rainfall appears to be a significant input flux of iodine in forest soils and vegetation, depending on coastal distance and precipitation amount. Forest canopy modifies quantities and speciation of iodine initially present in rainfall. Humus would act as temporary iodine accumulation layer through its association with organic matter. However, humus layer also behaves as potential iodine mobilisation source by leaching and/or volatilization. This thesis has demonstrated that iodine levels in soils depend both on its atmospheric and litterfall inputs, but also on the soil's ability to fix iodine. Thus, environmental conditions characterized by organic matter accumulation and presence of metal (hydr)oxides in soils promote iodine retention. Thereafter, soil is the main iodine reservoir at the forest plot scale (~99.9%). Given small amounts of iodine in the trees (