[Sap flux density in response to rainfall pulses for Pinus tabuliformis and Hippophae rhamnoides from mixed plantation in hilly Loess Plateau].

Thermal dissipation probe (TDP) was used to continuously measure the sap flux density (Fd ) of Pinus tabuliformis and Hippophae rhamnoides individuals in hilly Loess Plateau, from June to October 2015, and the environmental factors, i.e., photosynthetic active radiation (PAR), water vapor pressure deficit (VPD), and soil water content (SWC), were simultaneously monitored to clarify the difference of rainfall utilization between the two tree species in a mixed plantation. Using the methods of a Threshold-delay model, stepwise multiple regression analyses, and partial correlation analyses, this paper studied the process of Fd in these two species in response to the rainfall pulses and then determined the effects of environmental factors on Fd . The results showed that, with the increase of rainfall, the response percentages of Fd in both P. tabuliformis and H. rhamnoides increased at first but then decreased; specifically, in the range of 0-1 mm rainfall, the Fd of P. tabuliformis (-16.3%) and H. rhamnoides (-6.3%) clearly decreased; in the range of 1-5 mm rainfall, the Fd of P. tabuliformis decreased (-0.4%), whereas that of H. rhamnoides significantly increased (9.0%). The lower rainfall thresholds (RL ) of Fd for P. tabuliformis and H. rhamnoides were 6.4 and 1.9 mm, respectively, with a corresponding time-lag (τ) of 1.96 and 1.67 days. In the pre-rainfall period, the peak time of Fd of P. tabuliformis converged upon 12:00-12:30 (70%), while the Fd of H. rhamnoides peaked twice, between 10:30 and 12:00 (48%) and again between 16:00 and 16:30 (30%). In the post-rainfall period, the peak time of Fd of P. tabuliformis converged upon 11:00-13:00 (40%), while that of H. rhamnoides peaked twice, between 12:00 and 13:00 (52%) and again between 16:30 and 17:00 (24%). Among the environmental factors, the rank order of factors associated with the Fd of both P. tabuliformis and H. rhamnoides was PAR>VPD, before rainfall. However, the rank order of factors influencing the Fd of P. tabuliformis was PAR>VPD>0-20 cm SWC (SWC0-20 ), whereas this order was different for H. rhamnoides: SWC0-20 >PAR >VPD, after rainfall. This mixed plantation of P. tabuliformis and H. rhamnoides trees had a high stability of water utilization.