Abstract:

Objective: This study aims to quantitatively study the transpiration of Larix gmelinii var. principis-rupprechtii plantations on different slope aspects in response of meteorological conditions, soil moisture, and forest canopy structure in the semi-arid area of Liupan Mountains in Ningxia, which is an important water source area on the Loess Plateau of China, so as to provide a theoretical basis for the integrated forest-water management in this region. Method: From May to October 2023, a fixed plot of 20 m×20 m of L. gmelinii var. principis-rupprechtii plantation was set in the middle of typical slopes with different slope aspects (northwest slope NW50°, north slope N0° and northeast slope NE30°) in the small watershed of Diediegou. The sap flow density, meteorological parameters (represented by potential evapotranspiration , PET), and the soil water content of 0–60 cm soil layer (represented by the relatively extractable soil water , REW) were monitored continuously, and the canopy leaf area index (LAI) was regularly measured, to analyze the response of stand transpiration to PET, REW, and LAI. Result: 1) During the growing season (May to October) in 2023, the average daily transpiration of the stand on the north slope was the highest (0.93 mm·d^?1), which was 0.34 and 0.2 mm·d^?1 higher than that on the northwest slope (0.59 mm·d^?1) and on the northeast slope (0.73 mm·d^?1), respectively. During the soil drought period (REW < 0.45), the stand transpiration rates on different slopes were close with each other and showed no significantly difference (P>0.05). 2) The daily stand transpiration rapidly increased first and then slowed down and gradually stabilized with rising REW and LAI, with an exponential function, while the daily transpiration rapidly increased first and then slowly increased and then decreased slightly after a certain peak with rising PET, with a binomial function. 3) The response of daily transpiration to PET was significantly stronger during non-drought periods (REW > 0.45) than that during drought periods (REW < 0.45), showing that the increasing rate of transpiration with rising PET was greater during the non-drought periods than that during the drought period, and the increasing transpiration rate on the shady slope was significantly higher than that on the half-shady slopes. During the drought period, the stand transpiration was limited by soil moisture, showing that the transpiration increased rapidly with rising REW, and the increasing rate of transpiration on the half-shady slope was significantly higher than that on the shady slope, indicating that the response of transpiration on half-shady slopes to soil drought was more sensitive than that on the shady slope. Based on the analysis of contribution ratios of various individual factors to the deviation degree of stand transpiration from the average state of the growing season, it was found that REW was the dominant factor causing the transpiration difference during the drought period, with an average contribution ratio of 10.21%, and the most obvious limitation effect on the half-shady slope (11.8%). During the non-drought periods, LAI was the dominant factor causing the transpiration difference, and the limitation effect by LAI became weaker as the slope aspect deviated more from the due north. Conclusion: During the continuous soil drought period, the transpiration of L. gmelinii var. principis-rupprechtii plantations is significantly limited by soil drought stress, and the soil drought limitation is lower on shady slope with relatively good soil moisture than that on semi-shady slope. Considering the more frequent and severe drought events in future, site-specific forest management should be carried out according to slope aspects. The result of this study can provide a theoretical basis for developing an integrated forest-water management planning with climate change adaptability.

Key words: Larix gmelinii var. principis-rupprechtii, stand transpiration, slop aspects, drought, soil moisture, potential evapotranspiration