An empirical model of yield-density relationships is discussed. The model has three main features: 1) it can provide a satisfactory theoretical description of the yield-density curve at low densities where there is no intra-specific interference, 2)it can describe a wide range of yield-density types, including linear, non-asymptotic, asymptotic and parabolic relationships, and 3) the parameters in the model are ecologically meaningful. The model is applied to analyse the intra-specific interference stress a...