The impact of air pollutants on trees has been a research interest for decades. Nevertheless, uncertainty exists about the potential effects of the pollutant concentrations occurring at present, for example, in Central Europe. This uncertainty results from an incomplete understanding of the mechanisms by which air pollutants and other environmental factors influence woody plants. This review presents examples of the mechanisms of ozone and NO2 impacts at low concentrations on birch and poplar. The question of whether the responses found in the ecophysiology and structural differentiation of the trees reflect acclimation to these pollutants rather than injury is given particular attention. An attempt to interpret the response of carbon allocation to ozone and NO2 by one explanatory model is presented. The capacity of air pollutants to change carbon allocation may affect interspecific competition and, by this, the species composition and the dynamics in the development of forest ecosystems.