A model is developed for predicting the location of major vegetation ecotones in eastern North America. The model is based upon the interaction of plant physiological characteristics, climate, and soil in order to be useful as an interactive boundary condition in atmospheric general circulation models. The model represents the relative competitive ability of different vegetation types by their annual net primary productivity as expressed through water use. At any given location, the vegetation type with the highest productivity is assumed to be dominant. Ecotones are located where competitive dominance shifts from one vegetation type to another. Productivity is computed as a function of annual evapotranspiration, which is a function of the length of the growing season, photosynthetic capacity, potential evapotranspiration, and soil moisture availability, among other things. When considering the coniferous boreal/deciduous forest ecotone, it is found that inherent physiological differences between conifers and deciduous trees lead to differences in productivity which are related primarily to temperature. The model predicts a reversal of deciduous/coniferous productive dominance at the latitude of the observed ecotone; that is, within the transition zone of mixed forest. The productivity model does not adequately explain the existence of the deciduous/southern pine ecotone. Soil chemistry apparently plays a role in the determination of this boundary. ¿ American Geophysical Union 1993