Estimation of height–diameter curves through multilevel models with special reference to even-aged teak stands

Abstract

The use of a multilevel model for estimation and prediction of height–diameter curves in planted teak stands is discussed in the context of analyzing data from a stratified two-stage sample survey. Plantations selected randomly from different age groups within each Territorial Division in the State of Kerala, India, served as first stage units; circular plots along a transect within each selected plantation formed the second stage units. Girth at breast-height was measured on all the trees within the selected plots and height was measured on a sub-sample of trees within each selected plot. Plantations, plots within plantations, and trees within plots formed the three levels in the multilevel model for the height–diameter relation. The overall height–diameter model included fixed effects of age and mean diameter of the stands and also random effects on the deviations of the expected values of the parameters of the height–diameter curve at the plantation and plot levels. Differences in the height–diameter curves among the different Territorial Divisions were also investigated. The age along with the mean diameter of the stands could explain only a small part of the variation in the height–diameter curves. Random plantation and plot effects were prominent, but predictable using height measurements on a few sample trees from plots in individual plantations. The models are useful in generating accurate (localized) predictions of tree height which would eventually lead to better tree volume predictions and evaluation of site quality.