Examining the interactive effects of neighborhood characteristics and environmental conditions on height-to-diameter ratio of Chinese fir based on random forest

In forest dynamics modeling and estimation, the nonlinear relationship between response and explanatory variables is often concerned, while the interaction among variables is neglected. Height-to-diameter ratio (HDR) is an important indicator for evaluating tree stability and performance. Based on the data from 112 forest in-ventory plots across Hunan province, China, we modeled the HDR of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) by using random forest and examined the interactive effects of neighborhood variables (neigh-borhood competition, density and mean diameter) and environmental conditions (dominant height, topography and climate) on the HDR of Chinese fir. Neighborhood competition contributed the most to HDR variation and affected HDR positively. Dominant height and elevation also had a strong influence on HDR. Furthermore, the changes in HDR were jointly dominated by neighborhood features and environmental conditions. For example, HDR varied from 50.5 to 80.3 under the interactive influence of neighborhood competition and dominant height, but varied from 56.8 to 70.2 under the influence of neighborhood competition alone. The relationship between HDR and neighborhood variables might be mediated by environmental conditions, while neighborhood variables did not change the overall trend of HDR responses to environmental factors. When the dense Chinese fir stand is located at low elevation (<500 m) or rich sites (>20 m), frequent and systematic thinning should be carried out in the early stages of forest development. Our findings can provide guidance for managing Chinese fir forests to improve their stability.

Automated summary

In this study, the random forest method was used to model the height-to-diameter ratio (HDR) of Chinese fir based on data from 112 forest inventory plots in Hunan province, China. The interactive effects of neighborhood variables (neighborhood competition, density, and mean diameter) and environmental conditions (dominant height, topography, and climate) on HDR were examined. The results showed that neighborhood competition had the greatest contribution to HDR variation and had a positive effect on HDR. Dominant height and elevation also had a strong influence on HDR. The findings of this study can provide guidance for managing Chinese fir forests to improve their stability.