The Impact of Thinning on the Growth and Yield of Pinus caribaea at Buhindi Forest Plantation, Sengerema District, Tanzania
Abstract
Proper thinning reduces competition among the residual trees, enabling them to grow larger, straighter and ensuring that the species achieves its economic potential at the rotation age. However, limited empirical data exist on how thinning influences the growth and yield of Pinus caribaea under operational plantation conditions in Tanzania. This study assessed the impact of thinning on the growth and yield of P. caribaea at Buhindi Forest Plantation (BFP), Sengerema District, Tanzania. Data on thinning history, diameter at breast height (Dbh), and tree height (from five sample trees per plot, including large, medium, small, and two fattest) were collected from 11 compartments selected purposively across two age classes of 8-13 years and 16–21 years, representing first and second thinning schedules. The five trees per plot was intended to capture within-plot variability in tree size and ensure accurate calibration of height-diameter models for estimating heights of trees measured for Dbh only and dominant height. These trees were measured within a total of 243 circular plots of a radius of 9.78m, systematically distributed within the selected compartments. The Wilcoxon Signed-Rank test was used to determine whether the measured growth and yield parameters differed significantly from the scheduled values in the standard yield table. Results revealed that in comparison with the scheduled values in the yield tables, inadequate thinning were found to reduce mean diameter by 24.7-39.6%, basal area by 22.1-45.0%, volume by 41.3-59.1%, and mean annual increment by 59.6-73.3%. In contrast, dominant height, though without statistical differences, deviated from the scheduled values by -9.1-+9.9% meaning that the measured heights were in some cases lower and in other cases higher than expected values. The study concludes that poorly executed thinning limits the ability of the residual trees to fully utilise site potential, resulting in diminished growth rates and reduced timber yields and ultimately affecting long-term revenue collections. The study recommends strict adherence to thinning schedules to enhance stand development and productivity at BFP.
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