Daily and seasonal patterns of stem diameter micro-variations in three semiarid forest species in relation to water status and environmental factors

Understanding the relationship between stem diameter micro-variation (SDMV) and stem water status and revealing the mechanism regulating stem water are essential for elucidating environmental adaptation and water use strategies in forest trees. Here, we report the results of the year-round monitoring (November 2018–October 2019) of SDMV and xylem sap flow obtained using high-resolution dendrometers and Granier-type thermal dissipation probes, respectively, in mature trees of three species in the semiarid Loess Plateau region: Quercus liaotungensis, Platycladus orientalis, and Robinia pseudoacacia. The year-round trend for the variations in stem diameter can be divided into phases of a decreasing period and a relatively stable period in the non-growing season, and a progressive rise and fluctuating plateau throughout the growing season. The observed interspecific differences in the time to full recovery from the water deficit state corresponded to species phenology and soil moisture conditions. These differences were manifested as the responses of sap flux density (F_d) and tree water deficit (TWD) to variations in soil water content during the growing season. Q. liaotungensis implements stomatal regulation early to avoid drastic water loss in the trunk, P. orientalis maintains a relatively stable F_d at the expense of increased TWD under soil drought, and R. pseudoacacia maintains relatively low stomatal conductance under soil drought with an increase in TWD. The results revealed that SDMV is determined by the stem water status and species-specific strategies for water use, which could provide beneficial information for forestation practices in semiarid regions.