Time Lag of Stem Water Deficit in Response to Increased Vapor Pressure Deficit

Vapor pressure deficit (VPD) shows a gradually increasing trend that forces trees to produce ecological memory with time-delay characteristics to drought year by year. The reversible stem diameter variation of trees can serve as an indicator for tree water status. In the context of global warming, these variations can dynamically monitor the response of stem water to the change of environmental water, especially to VPD. In this study, stem diameter variations of Larix gmelinii Rupr. and environmental factors were measured at high temporal resolution. We analyzed the tree water deficit–induced stem shrinkage (TWD) response to VPD variations. The results showed that variation of TWD lagged behind that of VPD to a certain extent, by about 3–5.5 h. Environmental water supply of trees under natural conditions can be divided into water deficit and adequate water supply. The division of the two water conditions was helpful to study the correlation between TWD and VPD. Observing the response of changes of stem diameter to the time lag of VPD is helpful to accurately represent the relationship of changes between moisture in the tree body and moisture in the air and to identify the stress effect of the lag in a short period of time. Study Implications: Stem diameter changes with change in moisture in the environment and usually reversibly shrinks under water deficits. Such reversible change in stem diameter is closely related to change in vapor pressure deficit, which is an environmental factor. Tree water deficit–induced trunk shrinkage, an index reflecting the change in stem diameter affected by moisture, had a time lag correlation with vapor pressure deficit in terms of variation characteristics. The time lag of the tree water deficit–induced trunk shrinkage response to vapor pressure deficit was 3–5.5 h. Just as the phenomenon of ecological memory cannot be ignored in an observational study with the year as the time scale, the phenomenon of time lag should not be ignored during an observational study of moisture stress at small time scales.