Percentile-based Weibull diameter distribution model for Pinus kesiya stands in Benguet province, Philippines

Abstract

This study was conducted to develop a diameter distribution model for Pinus kesiya stands in Benguet province, Philippines, using the percentile-based Weibull model. The performance of the different equations was examined using evaluation statistics such as the fit index (FI) and root mean square error (RMSE). Results showed a FI range from 0.48799 (minimum diameter) to 0.94071 (95th diameter percentiles) and a RMSE range from 0.12622 (quadratic mean diameter) to 5.06882 (25th diameter percentile). To further evaluate the developed Weibull diameter distribution model, mean bias (Ē) in trees per hectare for each diameter class was determined, and the results showed that the highest over-prediction was found in the 18 cm diameter class, and that the highest under-prediction was found in the 12 cm diameter class. Using a site index equation and stem volume model developed for Pinus kesiya in the Philippines, a yield prediction system was created to predict the volume per ha (VPH) for each diameter class and, subsequently, the total volume of a stand. To evaluate the yield prediction system, the predicted mean VPH for each diameter class was compared to the observed mean VPH. Results showed that most of the under-predictions of the model were found in the 32–38 cm, 44–48 cm and 60 cm diameter classes. To assist forest managers in using this yield prediction system, an application guide was also created to predict the future yield of Pinus kesiya stands in Benguet, Philippines.

Keywords: growth and yield model, parameter recovery method, tropical forests, Weibull distribution