Year class coexistence or competitive exclusion for strict biennials?

We consider a discrete time model of semelparous biennial population dynamics. Interactions between individuals are modelled with the aid of an ``environmental'' variable I. The impact on and the sensitivity to the environmental condition is age specific. The main result is that competitive exclusion between the year classes is possible as is their coexistence. For moderate values of the basic reproduction ratio R 0 there is a strict dichotomy: depending on the other parameters we either find competitive exclusion or coexistence. We characterize rather precisely the patterns of age specific impact and sensitivity that lead to either of these outcomes.