Using past growth to improve individual-tree diameter growth models for uneven-aged mixtures of Pinus sylvestris L. and Pinus nigra Arn. in Catalonia, north-east Spain

In this study, growth models of individual tree diameter for uneven-aged mixtures of P. sylvestris and P. nigra in Catalonia were developed using a past growth index as site descriptor. These models were compared to an earlier model, based on the same data, that did not use past growth as a predictor. The growth index was calculated as a ratio of the measured and predicted past growth of sample trees within a given stand, the predictors pertaining to an average site. The models for future growth were based on 7 982 and 5 673 observations, and the models for past growth on 1 997 and 1 686 observations for P. sylvestris and P. nigra, respectively. Due to the applied Snowdon correction, the biases for the diameter growth models were zero. The relative RMSE values were 56.3% for the earlier P. sylvestris model and 52.0% for the new model, and 48.7% for the earlier P. nigra model and 47.1% for the new model. The accuracy of stand-level predictions (10-year change in basal area of living pines in the inventory plots) was better for the new models (Bias % = 2.79 and RMSE % = 34.79) than for the earlier ones ( Bias % = 3.27 and RMSE % = 41.54). The results indicate that the new models adapt to specific stand conditions better than models that omit past growth measurements.