Characteristics and factors driving the variations in bark thickness of major woody plants in China

Bark is an important protective tissue and vegetative organ of woody plants. Variations in bark thickness have a direct effect on plant growth and development, forest community structure, and terrestrial ecosystem function. However, the characteristics of these variations in tree bark thickness (BT) and adaptation to environmental factors remain unclear. In this study, we established comprehensive BT traits and environmental factor dataset for 75 major woody plant species in China and analyzed the spatial pattern and factors driving BT traits in the major woody plants of China. As results, the BT of major woody plants in China varied significantly with spatial location (latitude and longitude) (P < 0.001). A significant difference was observed in the relative age bark thickness among different plant types (gymnosperms and angiosperms) and life forms (evergreen and deciduous) (P < 0.001). Significant differences in relative bark thickness (RBT) were detected among the different bark phenotypes (smooth and rough) (P < 0.05). However, no significant difference in the relative bark volume was observed among the three groups (plant types, life forms, or bark phenotypes). All BT traits had particular phylogenetic signals. The Mantel test results showed that all BT traits except RBT were correlated with the environmental factors (Mantel's P < 0.05). The redundancy analysis revealed that the spatial variation in BT traits was significantly affected by the annual mean temperature, the mean diurnal range, and altitude (P < 0.05). The results of variance decomposition showed that the BT traits were affected by spatial factors (20 %), climatic factors (8 %), and their interaction (8 %), while the effects of soil factors could be ignored. Our results demonstrate that the biotic factors affecting the BT of woody plants in China were the different plant types, life forms, and bark phenotypes, while the abiotic factors were the spatial and climatic factors and their combined effects.