Responses and driving force analysis of typical arbor and shrub sap flow to patterns of rainfall in semi-arid areas

• Elucidating distinctions in vegetation transpiration across different rainfall categories in semi-arid areas and their mechanisms. • Proposing a method to define the vegetation transpiration threshold under varying soil water content levels. • Characterizing dynamic relationships between potential evapotranspiration, stomatal conductance, and soil moisture in governing sap flow, especially during transitions from adequate to limited or supersaturated conditions. Understanding plant transpiration responses to rainfall in semi-arid regions is vital for grasping climate change impacts on ecohydrological processes. This study categorized 105 rainfall events in Horqin Sandy Land (2018–2019) into three types: Category I (short-term, light rain), Category II (short-term, heavy rain), and Category III (long-term, moderate rain). It focused on Caragana microphylla Lam . (a sand-fixing shrub) and Populus L. (arbors), analyzing sap flow (SF), photosynthesis, and meteorological data. SF rates in both species were primarily influenced by net solar radiation (Rn), air temperature (Ta), relative humidity (RH), and vapor pressure deficit (VPD) (P < 0.05). The analysis indicated that potential evapotranspiration (PET) can be used as a composite influencing factor to explain the changes in SF. The results showed that under different rainfall types, the changing trends of SF and PET of Populus L. were generally consistent, However, as rainfall intensity increased, the effect of PET on SF of Populus L. weakened. Conversely, the effect of PET on the SF rate of Caragana microphylla Lam . increased with the increase of rainfall intensity. Notably, whether the SWC at the two test sites is effectively recharged or oversaturated due to rainfall is also an important factor affecting SF. Specifically, the SF of Caragana microphylla Lam . is generally affected by the combined effects of PET and stomatal conductance (Gs); when SWC is less than the field capacity, the inhibitory effect of Gs is obvious; when SWC exceeds field capacity, the inhibitory effect of Gs is significantly reduced, and the SF of Caragana microphylla Lam . is mainly controlled by PET. In contrast, as the SF of Populus L. increases from the field capacity to the saturated water holding capacity of SWC, the influence of PET on the SF of Populus L. weakens, and the influence of Gs on SF becomes more prominent.