Analysis on the transpiration response of Japanese cedar (Crytomeria fortunei) and influencing factors after expansion of moso bamboo (Phyllostachys edulis)

Moso bamboo (Phyllostachys edulis) expansion into adjacent forests has been reported to alter the local water cycle. Transpiration is the most important component of the water budget. However, the changes in transpiration within Japanese cedar forests following moso bamboo expansion, as well as the primary factors driving these changes, remain unclear. In this study, we conducted an investigation in a mixed plantation of Japanese cedar and moso bamboo, a pure Japanese cedar forest, and a Japanese cedar forest after the removal of moso bamboo, from July 2017 to June 2018. We measured leaf area index (LAI), root biomass (Root), and sap flow density (Js). The annual transpiration of Japanese cedar significantly decreased from 521.48 mm in the pure Japanese cedar forest to 293.34 mm in the mixed plantation of Japanese cedar and moso bamboo. Likewise, the leaf area index decreased from 5.26 to 3.96, and the root biomass decreased from 252.78 g/m2 to 160.24 g/m2. In Japanese cedar forest, the transpiration in summer (189.47 mm) is greater than the sum of transpiration in autumn and winter (143.36 mm). The partial least squares path modeling (PLS-PM) analysis demonstrated that the decrease in root biomass was the direct cause of the reduced transpiration, while the decrease in leaf area index primarily led to the decline in root biomass. The results imply that the expansion of moso bamboo changed the above-ground canopy structure of Japanese cedar through the competition of above-ground mechanical damage, which reduced the leaf area and photosynthetic capacity of Japanese cedar, and then affected the below-ground root biomass, finally leading to the reduction of transpiration and the variation of ecohydrological process.