Examining the reciprocal property of base-age invariant site index models

The reciprocal property of published base-age invariant site index models was examined through simulation. Various tree heights and ages were combined as input and were alternatively assigned to (H1, t1) and (H2, t2) in the models to predict heights. Numerical methods were required because some of the models did not have an inverse function. The difference between the two predicted heights with the same input assigned to (H1, t1) and (H2, t2), respectively, was computed to reflect reciprocity; as such predicted heights would be identical if the models were reciprocal. Examination of the differences through graphs and summary statistics indicated that some models possessed numerical reciprocity, although they were analytically nonreciprocal, whereas the others did not. An understanding of the reciprocal property of the models would improve their application in growth and yield studies.