Incorporating stand density effects and regression techniques for stem taper modeling of a Larix principis-rupprechtii plantation

Abstract

Stem form is the shape of the trunk, differs among tree species and mainly affected by stand density factor. Accurate taper equations are crucial for estimating the stem diameter, form and tree volume, which is conducive to timber utilization and sustainable forest management and planning. Larch (Larix principis-rupprechtii Mayr.) is a valuable afforestation species under large-scale development in North China, but no study on the effect of density on its stem taper has been reported yet. The dataset included 396 analytical trees from 132 standard plots of larch plantation in Saihanba, Hebei Province. Based on 12 different forms of models, we explored the optimal basic equation for plantations and the effects of the stand density, basal area, canopy density and different forms of stand density on the prediction accuracy of the variable-exponent models. The variable-exponent taper equation that includes Sd (stand density) was constructed by using nonlinear regression, a nonlinear mixed effect model and the nonlinear quantile regression method. The results indicate that the Kozak’s 2004 variable-exponent taper equation was the best basic model for describing changes in the stem form of larch plantations, and the density factor in the form of Sd improved the prediction accuracy of the basic model. Among the three regression methods, the quantile regression method had the highest fitting accuracy, followed by the nonlinear mixed effect model. When the quantile was 0.5, the nonlinear quantile regression model exhibited the best performance which provides a scientific basis for the rational management of larch plantations.

Biomass allometric models for Larix rupprechtii based on Kosak’s taper curve equations and nonlinear seemingly unrelated regression

“…Some studies demonstrated that the Kozak (1988) and Kozak (2004) variable exponent taper equations could describe the shape of stem for some tree species ( Corral-Rivas et al., 2007 ; Lumbres et al., 2017 ). However, the Kozak (2004) variable exponent taper equation is flexible and easily used ( Xu et al., 2022 ). It has been proven to be applicable to many species in different countries ( Huang et al., 2000 ; Klos et al., 2007 ; Jiang and Liu, 2011 ; He et al., 2021 ).…”

Under-canopy afforestation after 10 years: assessing the potential of converting monoculture plantations into mixed stands

“…The total operating area of the forest farm is 94,000 hm 2 , with 73,000 hm 2 designated as forest land, including 57,000 hm 2 of planted forests and 16,000 hm 2 of natural forests. The forest coverage rate is 80%, the total forest volume is 5.025 million m 3 , and the average annual growth rate is 9.7% ( Xu et al., 2022 ). The main tree species include L. principis-rupprechtii , Picea asperata , Betula platyphylla , and P. sylvestris ( Wu et al., 2023 ).…”

Developing the Additive Systems of Stand Basal Area Model for Broad-Leaved Mixed Forests

“…An increase in SDI will gradually increase the tree species-level and stand-level basal area in the P. davidiana × B. platyphylla broad-leaved mixed forests (Table 4), eventually stabilizing the SBA at a higher density [76]. The number of trees per hectare and SDI are common indicators reflecting stand density in different forest types [43,79], with the la er extensively employed in the construction of a forest growth and harvest model [80,81]. The results of the present study showed that the incorporation of SDI achieved increased accuracy in predicting tree species-level and stand-level basal area (Table 1).…”