A distance-independent basal area growth model for oriental spruce (Picea orientalis (l.) Link) growing in mixture with oriental beech (Fagus orientalis Lipsky) in the Artvin region, North-East Turkey

Abstract

In this study, we developed an individual tree basal area growth model for oriental spruce (Picea orientalis (L.) Link) growing in mixture with oriental beech (Fagus orientalis Lipsky) in the Artvin, Turkey. In our modeling approach, the basal area growth variables were divided into 4 main groups, which included size, competition, site and mixture. The parameters of these variables were biologically consistent with general growth trends of forest growth models and we found them each to be statistically significant at the probability level of 0.05. Our model explained 62.4% of the basal area growth variation of oriental spruce with a standard error of 0.836 cm2. Furthermore, we found that the absolute and relative (%) biases and the root mean square error (RMSE and RMSE%) of the 5 year basal area growth of oriental spruce were 0.00823 cm2, 0.1353%, 0.8234 cm2, 33.39% respectively. We evaluated this model by plotting the biases with respect to considerable regressor variables. These graphical analyses of the model biases showed no meaningful and evident trend of bias values along with these independent variables. Our model provides a clear frame of reference for understanding about the individual tree basal area growth patterns of for oriental spruce growing in mixture with oriental beech. This model further shows that the parameter of the mixture proportion (BP) for oriental beech trees in the mixture model component was positive, indicating that basal area growth of spruce increases as the proportion of beech in the forest mixture goes up if other variables remain constant. This positive effect of admixture may be due to the facilitation process occurring in oriental spruce stands mixed with beech trees. The facilitation pattern of beech trees in mixing spruce stands suggests that poor or degraded forest sites may be improved by mixing beech trees for forest areas in this region. Key words: Oriental spruce, oriental beech, Individual tree basal area growth model, mixed species stands.