Modelling dominant height growth of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in Portugal

The first plantations of the North-West American conifer Douglas-fir were established in Portugal in 1904. Investigations into growth and yield patterns were started in 1969. Since then others have carried out further studies. This study includes data from the previous studies and covers the whole range of site conditions where Douglas-fir grows in Portugal. Dominant height data from 87 stem analysis trees from 12 sites were used to model dominant height growth of Douglas-fir using the McDill-Amateis, Chapman-Richards and Lundqvist-Korf growth functions. The Chapman-Richards and Lundqvist-Korf growth functions were used in their integral and difference forms. For the evaluation of the candidate growth models' performance, three steps were adopted: (1) all the candidate growth equations were fitted with the data available from stem analysis from 87 trees, eliminating growth equations with non-logical and non-biological consistency and poor statistical properties; (2) the remaining growth equations were cross-validated using two sub-samples of the stem analysis data and selection of the growth equations with the best statistical results; (3) validation, using the whole data set from stem analysis for fitting growth models and the stand data for calculation of the prediction errors, was carried out. Out of the nine models evaluated five were rejected in the first step and two in the second step. The two best models had similar results in the third step and were compared with previous Douglas-fir site index curves. The McDill-Amateis function performed best.