Cultivar and tree size, but not climate, are principal factors affecting stem quality of Populus tomentosa plantations in the North China Plain

Abstract

In order to explore the management strategies for cultivating and improving the stem quality of Populus tomentosa plantations under the background of climate change, this study focuses on P. tomentosa plantations over 10 years old in the North China Plain. Using linear mixed models and ordered logistic models, the impacts of cultivar, tree size, stand age, competition, and climate on the stem quality of P. tomentosa (including crown base height, tapering, branching grade, and straightness grade) were analyzed. The study found that: cultivar significantly affected all stem quality indicators (P < 0.05). Compared to other cultivars, the P. tomentosa f. yixianensis had a 23 % increase in branch height, an 8 % reduction in taper, and the risk of having poorer branches and stem form decreased by 96 % and 80 %, respectively. In addition, taller and bigger-diameter trees had better external stem quality. The impacts of competition-related indicators on stem quality were inconsistent: reduced canopy openness could improve stem quality by enhancing light competition, however, increased tree density increased the risk of deteriorating branching and straightness grade by 1.2 % and 0.9 %, respectively. Among all factors, cultivar and individual tree size had the greatest relative importance for various stem quality indicators, followed by competition-related factors, while stand age and climate factors have no significant impact on P. tomentosa stem quality (P > 0.05). Currently, climate change has little impact on the external stem quality characteristics during the cultivation of P. tomentosa plantations. Management strategies for stem quality can focus on cultivar selection and competition regulation. It is worth noting that increasing tree density in the North China Plain may not necessarily improve stem quality of P. tomentosa plantations, so caution is needed in the process of regulating competition.