Stump-Diameter Modelling of Diameter at Breast Height (DBH) for Selected Timber Species in Erukot Division of the Cross River National Park, Nigeria

Abstract

Regression models were developed and validated to relate stump diameter to diameter at breast height (DBH) for selected timber species in Erukot Forest, Oban Division of the Cross River National Park, Nigeria, and were subsequently applied to tree-volume estimation. The assessment covered three economically important species: Poga oleosa, Staudtia stipitata and Pycnanthus angolensis. Twenty sample plots were established through systematic line-transect sampling. Stumps of felled trees were identified and measured for stump height and stump diameter, whereas DBH was measured with a girthing tape. Relationships between stump diameter and DBH, and between DBH and tree volume, were analysed with regression models in SPSS Version 20. Model performance was assessed using the coefficient of determination, root mean square error, F-ratio, adjusted measures of fit and the Durbin-Watson statistic; Student's t-test and percentage bias were used for validation. Measurements were obtained from 31 P. angolensis, 26 P. oleosa and 27 S. stipitata trees. Their respective mean stump heights were 0.46, 0.49 and 0.43 m; mean stump diameters were 55.1, 47.9 and 45.3 cm; and mean DBH values were 40.3, 33.6 and 28.7 cm. The fitted equations indicated species-specific associations between stump diameter and DBH. Under comparable forest conditions, these relationships may provide a practical means of reconstructing the dimensions and volume of felled trees.