[Dynamics and influencing factors of stem diameter micro-variations during the growing season in two typical forestation species in the loess hilly region, China.]

Using DC3 high-resolution dendrometer and Granier-type thermal dissipation probes, we measured stem diameter micro-variations and xylem sap flow of two typical forestation species, Quercus liaotungensis and Robinia pseudoacacia, for a growing season in the loess hilly region of China. The main environmental factors (soil water content, solar radiation, air temperature and relative humidity) were monitored. The linkage between diameter micro-variations and transpiration water use were analyzed with respect to their responses to environmental factors. The results showed that the variations in stem diameter and sap flux density of both species had obvious diurnal rhythms. The maximum daily shrinkage was positively correlated with daily sap flux density. The micro-variation of stem diameter at the daily scale was affected by transpiration during the day. The maximum daily shrinkage of stem diameter was positively correlated and well fitted with transpiration driving factors (solar radiation, vapor pressure deficit, and the integrated variable VT). The difference in slopes of regression curves suggested that the daily variation of stem diameter in Q. liaotungensis was greater and more sensitive to meteorological factors than that in R. pseudoacacia. The sap flux densities of both tree species were higher during the period with relatively higher soil water content than that with lower soil water content. The difference of maximum daily diameter shrinkage between different soil water conditions was statistically significant in Q. liaotungensis, but not in R. pseudoacacia. These differences may be related to water use strategies, including transpiration regulation and stem water replenishment.