Abstract
In this study, an individual tree crown ratio (CR) model was developed with a data set from a total of 3134 Mongolian oak (Quercus mongolica) trees within 112 sample plots allocated in Wangqing Forest Bureau of northeast China. Because of high correlation among the observations taken from the same sampling plots, the random effects at levels of both blocks defined as stands that have different site conditions and plots were taken into account to develop a nested two-level nonlinear mixed-effect model. Various stand and tree characteristics were assessed to explore their contributions to improvement of model prediction. Diameter at breast height, plot dominant tree height and plot dominant tree diameter were found to be significant predictors. Exponential model with plot dominant tree height as a predictor had a stronger ability to account for the heteroskedasticity. When random effects were modeled at block level alone, the correlations among the residuals remained significant. These correlations were successfully reduced when random effects were modeled at both block and plot levels. The random effects from the interaction of blocks and sample plots on tree CR were substantially large. The model that took into account both the block effect and the interaction of blocks and sample plots had higher prediction accuracy than the one with the block effect and population average considered alone. Introducing stand density into the model through dummy variables could further improve its prediction. This implied that the developed method for developing tree CR models of Mongolian oak is promising and can be applied to similar studies for other tree species.
Highlights
Tree crown size is an important variable that is commonly involved in growth and yield models used as decision-support tools in forest management [1,2,3]
The used data were obtained from a total of 118 permanent sample plots (PSPs) established in Mongolian oak natural forests allocated in Wangqing Forest Bureau of northeast China (123° 560–131°040 E, 43°050–43°400 N) (Fig 1, Table 2)
These PSPs were nested within 15 blocks, out of which 12 blocks with 100 PSPs are located in Tazigou forest farm and the other 3 blocks with 18 PSPs in Jincang forest farm, that were randomly located in the study area to represent the stands with different site conditions
Summary
Tree crown size is an important variable that is commonly involved in growth and yield models used as decision-support tools in forest management [1,2,3]. Tree CR is defined as the percentage of crown length from the base of live crown to the tree top to its total height. Tree CR is a useful indicator of vigor [4,5,6,7] and stand density [8]. It is a variable of interest in the management of many non-timber resources including recreation forests and wildlife habitats [9]. Developing accurate tree CR models is necessary, which allows forest managers to accurately predict tree CR
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