Development of Mixed-Effects Individual-Tree Diameter Increment Model for Casuarina Equisetifolia Considering the Effects of Tree-Size Diversity, Tree Density Reduction, and Climate

ABSTRACT Based on the data provided by the 6th (1998), 7th (2003), and 8th (2008) National Forest Inventory (NFI) in Hainan Province, southern China, we developed an individual-tree diameter increment model considering tree-size diversity, tree density reduction, and climate for Casuarina equisetifolia. Since the data was longitudinal and had a nested structure, we used a linear mixed-effects approach to construct the mixed-effects model based on sample plot effects. And we applied the method of 10-fold cross-validation to test the basic model without random effects and final mixed-effects model. The results indicate that natural logarithm of initial DBH (logDBH), sum of basal area of trees larger than objective tree (BAL), soil thickness (ST), Gini coefficient of DBH diversity for residual trees (GCres), and mean coolest month temperature (MCMT) had significant impact on the individual-tree diameter increment for Casuarina equisetifolia. Comparing with basic model, the final mixed-effects model performance was greatly improved. In the model validation, the mixed-effects model also showed a better fitting goodness. The individual-tree diameter increment models of Casuarina equisetifolia developed in this study will provide a good basis for estimating and predicting growth of Casuarina equisetifolia forests over larger areas.