Abstract:

Objective: To accurately quantify the response of the sap flow density of Platycladus orientalis plantations in the mountainous areas of Beijing to extreme soil drought, and to clarify its water use strategy in response to extreme drought stress, so as to provide a scientific basis for the coordinated and comprehensive management of forest and water in mountainous areas of Beijing under the background of climate change. Method: During the growing season of 2024 (May - October), the plantation of P. orientalis, the main local afforestation tree species, was selected in Wuzuolou Forest Farm in Miyun District, Beijing, and fixed a sample plot of 20 m×20 m was set up. The change characteristics of P. orientalis sap flow density in the growing season was continuously monitored by a thermal diffusion sap flow probe, and meteorological factors and soil moisture in 0?40 cm soil layer (expressed as relatively extractable soil water REW) were simultaneously observed, and the change characteristics of sap flow density and the response to environmental factors were analyzed. Result: 1) In the 2024 growing season, there was a continuous 73-days (May 1 to July 12) extreme soil drought, the REW was extremely low at 0.11 during the 73-days, accompanied by continuous high temperature and strong solar radiation intensity. Drought reduced the sap flow density of P. orientalis, maintaining a low level throughout the drought period, with an average value of 0.004 4 mL·cm^?2min^?1, which is significantly lower than the average sap flow density during non-drought periods in the growing season (0.020 2 mL·cm^?2min^?1) (P<0.01). 2) Drought stress diminished and no longer significantly affected the differences in sap flow density among weather types such as sunny, cloudy, and rainy days during the drought period (P > 0.05). Daytime rainfall during the drought period caused a temporary increase in sap flow density, resulting in a slightly higher average sap flow density on rainy days (0.0049 mL·cm^?2min^?1) compared to sunny and cloudy days, which were 4.3% and 16.7% higher, respectively. 3) Drought altereed the diurnal variation pattern and day-night ratio of sap flow density. During non-drought periods, the diurnal variation pattern of sap flow density on sunny and cloudy days follows a “unimodal” pattern, with the diurnal sap flow density accounting for over 95% of the total. However, during drought periods, the diurnal variation of sap flow density on sunny and cloudy days exhibits a “trough” pattern, and the diurnal sap flow density proportion significantly decreases to 44% and 51%, respectively. Daytime rainfall during drought periods can alter the diurnal variation characteristics of sap flow density, showing an increase with the increase in daytime rainfall. The diurnal variation pattern may also revert to a “unimodal” pattern, and the daytime sap flow density ratio gradually increases. 4) The sap flow density of P. orientalis was mainly affected by the meteorological factors (solar radiation intensity, vapor pressure deficit, precipitation) during the drought period. In non-drought periods, the sap flow density was affected by a combination of soil moisture and meteorological factors. Conclusion: Under extreme soil drought stress, P. orientalis significantly reduced its sap flow density and changed the diurnal variation characteristics of sap flow density. During the drought period, the sap flow of P. orientalis was basically not measured during the daytime, but the sap flow used to supplement the water storage of the tree was measured at night. The results showed that P. orientalis had strong drought resistance, and the diurnal variation mode and day-night ratio of sap flow density were changed by closing the stomata during the day to reduce the sap flow density to reduce transpiration water consumption and maintaining a high sap flow density at night to supplement the water storage of the tree, so as to form a higher adaptability to the drought stress environment. Therefore, for arid sunny slopes with thin soils that are highly susceptible to drought stress, planting drought-tolerant P. orientalis should be prioritized. Stand-density regulation and other silvicultural measures should be implemented to reduce canopy transpiration and stand evapotranspiration, thereby sustaining a fundamental forest water balance. This study elucidates the water-use strategies employed by P. orientalis under severe soil-drought stress, thereby providing a theoretical basis for developing climate-adaptive, forest-water-coordinated management schemes.

Key words: Platycladus orientalis, extreme soil drought, soil moisture, sap flow density, diurnal variation pattern