Modeling the height-diameter relationship of planted Pinus kesiya in Zambia

Abstract

Plantations of non-native tree species cover over 50,000 ha in Zambia of which Pinus kesiya accounts for 36% of the main tree species. In order to quantify the growing stock of P. kesiya, updated and reliable height-diameter (h-d) models are required. Therefore, the aim of this study was to develop appropriate h-d models for P. kesiya that are consistent with current forest management practices in the country. Using 7691 measured pairs of diameter and height of P. kesiya from four different forest sites, the following questions were investigated; (i) do frequently used h-d models accurately estimate height of P. kesiya in Zambia?; and (ii) what h-d models account for site and rotation in the country? Predicted h from the frequently used h-d models (Näslund, Modified Logistic and Hossfeld) was significantly lower than the measured h (p ≤ 0.001). This suggests that frequently used h-d models for pine significantly underestimates tree height of P. kesiya in Zambia. The deviations associated with these models varied but generally increased with tree diameter. In this regard, country level h-d model developed using the Weibull function, and tested in this study proved to be more accurate than the frequently used models in predicting tree height of P. kesiya in Zambia. Furthermore, site and rotation were found to influence the overall accuracy and reliability of the country level model (p ≤ 0.001) suggesting the need to refine this model in order to account for site and/or rotation effect. Therefore, the h-d models that are specific to site and in some instances to rotation were derived and increased the accuracy and precision in estimating tree height of P. kesiya in the country. These locally developed h-d models are recommended for future forest plantation inventories, management planning and as a basis for modeling h-d relationships for other Pinus species in the country.