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Efficient management of timber resources and wood utilization practices require accurate and versatile information about important characteristics of forest resources for evaluating the numerous management and utilization alternatives for timber resources. Tree height is considered one of the most useful variables along with stocking and diameter at breast height, in estimating forest stand wood volumes and productivity. Six nonlinear growth functions were fitted to tree height-diameter data of three major tree species in Western Mediterranean Region's forests of Turkey. The generalized regression neural network (GRNN) technique has been applied for tree height prediction, as well, due to its ability to fit complex nonlinear models. The performance of the models was compared and evaluated. Further, equivalence tests of the selected models were conducted. Validation showed the appropriatness of all models to predict tree height. According to the model performance criteria, the six nonlinear growth functions were able to capture the height-diameter relationships and fitted the data almost equally well, while the constructed generalized regression neural network (GRNN) models were found to be superior to all nonlinear regression models, in terms of their predictive ability.

Introduction The accurate estimation of stem taper and volume are crucial for the efficient management of the forest resources. Compatible segmented polynomial taper and volume equations were developed for Brutian pine (Pinus brutia Ten.), Lebanon cedar (Cedrus libani A. Rich.), Cilicica fir (Abies cilicica Carr.), Scots pine (Pinus sylvestris L.), and Black pine (Pinus nigra Arnold.). • Methods In this study, the Clark et al. (USDA For Serv Res Pap SE-282, 1991) segmented taper model was selected as this model is one of the most tested segmented models and has frequently been well ranked for its excellent performance. The data for each species were divided into two sets: the majority (about 75%) was used to estimate model parameters, and the remaining data (about 25%) were reserved to validate the models. The performance of the models was compared and evaluated by average bias, standard error of the estimate (SEE), and a fit index (FI).• Results The proposed model generally performed better than the other equations for the whole tree as well as for sections within the tree, based on the ten relative height classes examined. In addition, tree bole volume estimates were compared to other established tree bole volume estimation techniques.• Conclusion Based on our results, the taper equation of Clark et al. (1991) is recommended for estimating diameter at a specific height, height to a specific diameter, merchantable volume, and total volume for the species considered.

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