Abstract
One of the most common and important tree characteristics used in forest management decision-making is tree diameter-at-breast height (DBH). This paper presents results on an evaluation of two growth functions developed to model stem diameter increases in individual Pinus occidentalis Sw. trees in La Sierra, Dominican Republic. The first model was developed in order to predict future DBH (FDM) at different intervals of time and the other for predicting growth, that is, periodic annual diameter increment (PADIM). Each model employed two statistical techniques for fitting model parameters: stepwise ordinary least squares (OLS) regression, and mixed models. The two statistical approaches varied in how they accounted for the repeated measurements on individual trees over time, affecting standard error estimates and statistical inference of model parameters. Each approach was evaluated based on six goodnessof-fit statistics, using both calibration and validation data sets. The objectives were 1) to determine the best model for predicting future tree DBH; 2) to determine the best model for predicting periodic annual diameter increment, both models using tree size, age, site index and different indices of competitive status; and 3) compare which of these two modeling approaches predicts better the future DBH. OLS provided a better fit for both of the growth functions, especially in regards to bias. Both models showed advantages and disadvantages when they were used to predict growth and future diameter. For the prediction of future diameter with FDM, accuracy of predictions were within one centimeter for a five-year projection interval. The PADIM presented negligible bias in estimating future diameter, although there was a small increase in bias as time of prediction increased. As expected, each model was the best in estimating the response variable it was developed for.. However, a closer examination of the distribution of errors showed a slight advantage of the FDM against the PADIM. Based on this, it is proposed that the FDM model is used to estimate future diameter and periodic diameter increment (growth) of P. occidentalis.
Highlights
One of the most common and important tree characteristics used in forest management decision-making is tree diameter-at-breast height (DBH)
This paper presents results on an evaluation of two growth functions developed to model stem diameter increases in individual Pinus occidentalis Sw. trees in La Sierra, Dominican Republic
Estimating periodic annual diameter increment (PADIM) with the mixed model procedure resulted in a model that includes D0, CCI, basal area of larger trees (BAL) and site quality measure (SI)
Summary
One of the most common and important tree characteristics used in forest management decision-making is tree diameter-at-breast height (DBH). This variable has numerous beneficial attributes, including being easy to measure (Zhang et al, 2004) and having strong correlations with other tree characteristics. Individual-tree diameter growth models are among the most basic and essential components of forest growth models (Sánchez-González et al, 2006). They allow one to project and describe the state of a tree at some future time. Compared with stand-class models, individual-tree models ensure reliable predictions for a wide range of tree sizes, sites and stand condition (Zhao et al, 2004). The only serious disadvantage to individual-tree distance-independent models is that they are incapable of predicting the growth of a specific individual tree with any reliability (low R2 values)
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