HEIGHT-DIAMETER RELATIONSHIPS FOR Eucalyptus grandis Hill Ex. Maiden IN MOZAMBIQUE: USING MIXED-EFFECTS MODELING APPROACH

Abstract

Equations express that height-diameter relationships are used to estimate tree heights that were not measured in the plots, as well as to calculate their volumes. In this study, we modelled height for Eucalyptus grandis Hill ex. Maiden stands using nonlinear mixed effect models in Mocuba district, Central Mozambique. Models were tested from 1414 trees measured, in 40 plots with dimensions of 20 x 20 m. Model one (M₁) was the best in relation to the others according to the evaluated criteria (R² = 0.9183; RMSE = 0.558; AIC = -1234.8; BIC = -1224.9). The inclusion of the variables dominant height (h₁₀₀, m), basal area (G, m².ha^-1) and mean basal area diameter (dg, m) in model 1 (M₁) within the scope of mixed effect model structures that involved both random and constant effect parameters (Model 7) provides better fitting and more precise predictions than those produced by the nonlinear fixed effect model structures. For calibration of random effects according to plot, height measurement of at least five trees is required (four dominant trees + one tree randomly selected in plot). Our model is site-specific and its application should therefore be limited to the stands with characteristics that were the basis of this study. Further works on recalibration, validation, and verification of our model using a larger dataset collected from a wider range of species distribution will be more interesting.