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.
以黄土丘陵区油松-沙棘混交林为研究对象,运用热扩散式探针(TDP)于2015年6—10月对油松和沙棘的树干液流密度(Fd)进行连续观测,同步测定了光合有效辐射(PAR)、水汽压亏缺(VPD)和土壤水分(SWC)等环境因子,分析两树种对降雨利用的差异.采用Threshold-delay 模型、多元回归分析和偏相关分析方法,研究两树种Fd对降雨的响应过程,并确定环境因子对Fd的影响.结果表明: 随着降雨量递增,两树种Fd的最大变化量都先上升后降低;其中0～1 mm降雨范围内,油松Fd(-16.3%)和沙棘Fd(-6.3%)都明显降低;1～5 mm降雨范围内,油松Fd(-0.4%)降低而沙棘Fd(9.0%)明显升高.油松和沙棘Fd对降雨响应的最小降雨阈值(RL)分别为6.4和1.9 mm,滞后时间(τ)为1.96和1.67 d.降雨前油松Fd峰值集中在12:00—12:30(70%),沙棘Fd峰值分别集中在10:30—12:00(48%)和16:00—16:30(30%);降雨后油松Fd峰值集中在11:00—13:00(40%),沙棘Fd峰值分别集中在12:00—13:00(52%)和16:30—17:00(24%).降雨前影响油松和沙棘Fd的环境因子大小顺序为PAR＞VPD;降雨后影响油松Fd的环境因子大小顺序为PAR＞VPD＞0～20 cm SWC(SWC0～20),影响沙棘Fd的环境因子大小顺序为SWC0～20＞PAR＞VPD.油松-沙棘混交林对水分利用的稳定性较高.
Keywords: Threshold-delay model; environmental factor; mixed plantation; rainfall pulses; sap flux density.