Abstract:

Objective: This study aims to investigate the growth of average tree height and diameter at breast height (DBH) of Larix principis-rupprechtii plantation stands and the growth responses to site factors and stand structure in the small watershed of Diediegou located in the semi-arid area of Liupan Mountains in Ningxia of northwest China, and thereby providing a theoretical basis for tree growth prediction and sustainable forest management. Method: In 2018 and 2019, 42 and 57 sample plots of L. principis-rupprechtii plantations were selected respectively, to investigate the stand structural characteristics (tree height, DBH, forest age, density, etc.) and site factors (slope aspect, soil thickness, etc.). In addition, the data from 93 sample plots surveyed between 2003 and 2017 in the Liupan Mountains were also collected for this study. Based on the data from all these 192 plots, the response patterns of tree height and DBH growth to the influencing factors such as forest age, density, slope aspect, and soil thickness were analyzed, and the growth models of tree height and DBH were established and validated using 2/3 and 1/3 of the dataset. Result: The tree height and DBH exhibited distinct response patterns to forest age, density, slope aspect, and soil thickness. Both tree height and DBH increased rapidly with rising forest age until to 20 years and thereafter increased slowly. With rising tree density, the tree height and DBH initially remained stable, but then decreased gradually after exceeding the thresholds of 2 000 and 1 500 trees·hm⁻² and decreased with accelerated rate after exceeding the thresholds of 3 000 and 2 300 trees·hm⁻², respectively. As the slope aspect deviated more from due north, the tree height and DBH initially remained stable until to the threshold of 80° and thereafter decreased rapidly. With rising soil thickness, the tree height and DBH firstly increased and then stabilized when the soil thickness exceeded 90 and 80 cm, respectively. Based on expressions of three types of forest growth models (multiplicative coupling, site index, site factors) in response of tree height and DBH to forest age, tree density, slope aspect, soil thickness, and site index, the coupling models were established and the model parameters were fitted. The determination coefficients of the three models of tree height were 0.804, 0.902, and 0.806, and the corresponding NS coefficients were 0.793, 0.903, and 0.762, respectively. The determination coefficients for the three models of DBH were 0.846, 0.803, and 0.834, and the corresponding NS coefficients were 0.837, 0.840, and 0.818, respectively. All the three models performed satisfactorily compared with the observed data. Conclusion: In the semi-arid area of Liupan Mountains, the growth of tree height and DBH of L. principis-rupprechtii plantations has different responses to the main influencing factors, i.e., firstly increases quickly and then increases slowly with rising forest age, firstly stabilizes and then decreases with rising tree density and slope aspect, and firstly increases and then stabilizes with rising soil thickness. All the multi-factor coupling models of three types (multiplicative coupling, site index, and site factors) exhibit high accuracy. However, considering the undetermined distribution pattern of the site index, the accuracy differences and application difficulty and structural complexity of models, it is suggested to use the multi-factor coupling model for predicting the tree height and DBH growth of L. principis-rupprechtii plantations in the study region.

Key words: Larix principis-rupprechtii plantations, tree growth, forest age, stand density, site index, coupling model