MODELLING DIAMETER DISTRIBUTION IN EVEN-AGED AND UNEVEN-AGED FOREST STANDS

Five distributions, Weibull, gamma, Johnson SB, log normal and generalised normal, are compared in terms of their ability to model diameter data in even-aged and uneven-aged forest stands. Moments ratio diagrams of various statistical distributions are applied as a measure of the flexibility of the distribution in regard to their changes in shape. Some of the strengths and weaknesses of the distributions that have been used for describing diameter distributions in even-aged and uneven-aged stands are discussed. Data were obtained from 16 uneven-aged stands of mixed species located at Bukit Lagong Forest Reserve, Kepong, Selangor, and 9 even-aged stands of Acacia mangium species located at Kemasul Forest Reserve, Pahang. All the stands were from plantations and the ages ranged from 5 to 22 y. Lima taburan, Weibull, gamma, Johnson SB lognormal dan normal teritlak dibandingkan dan segi keupayaannya untuk membentuk model data garis pusat bagi dirian hutan seumur dan tak seumur. Gambarajah nisbah momen daripada pelbagai taburan Statistik digunakan sebagai satu ukuran kelenturan berhubung dengan perubahan dalam bentuk sesebuah taburan itu. Beberapa kekuatan dan kelemahan bagi taburan-taburan yang pernah digunakan untuk menggambarkan taburan garis pusat bagi dirian seumur dan tak seumur ini dibincangkan. Data garis pusat diperoleh daripada 16 dirian seumur dan tak seumur daripada pelbagai spesies terletak di Hutan Simpan Bukit Lagong, Kepong, Selangor dan 9 dirian seumur daripada spesies Acacia mangium terletak di Hutan Simpan Kemasul, Pahang. Kesemua dirian tersebut merupakan hutan ladang dan umurnya dalam lingkungan 5 hingga 22 tahun.