Extension of the Richards biological growth function has been applied extensively to describe height growth patterns or to develop site index equations. We introduce a linear filter model to achieve the same goals. Although derivation of the filter model involves more computation, it compares well with the Richards growth model in accuracy when applied to a black spruce (Piceamariana(Mill.) B.S.P) stem analysis data set from northern Ontario. The main advantage of the filter model, however, is that it can be used as a base-age invariant site index model.