Abstract:

Objective: This study aims to investigate the dynamics of stem sap flow and time lag in Populus tomentosa after windthrow and restoration, analyze the influence of environmental factors on the hysteresis bias of windthrown trees, and verify whether root damage drives the shift in hysteresis bias. Method: The study focused on P. tomentosa that had been windthrown and restored. Trees were assigned to mild (0°–45°), severe (45°–90°) windthrow groups, and a control group (CK). Sap flux density (SFD), solar radiation (R), and vapor pressure deficit (VPD) were monitored. Hysteresis parameters, including lag time and loop area, were calculated, and a time lag bias index (BiasT) was constructed to characterize hysteresis bias. A separate validation experiment was conducted to investigate the effects of root damage on hysteresis parameters. Result: 1) Over the growing season, cumulative transpiration in the mild and severe windthrow groups decreased by 53%–57% compared with CK, their peak SFD was lower than that of CK, and their responses to environmental factors were more sluggish than that of CK. 2) Monthly variation significantly affected lag time (P<0.05), while the degree of windthrow had no significant impact on the duration of the duration of time (P>0.05). The duration of lag time between SFD and R and BiasT were greater at the beginning and end of the growth season and less in the mid-season. An opposite seasonal pattern was observed for the duration of lag time between SFD and VPD, which was longer in the mid-season and shorter at the beginning and end of the growth season. 3) Path analysis indicated that the lag process of SFD was basically controlled by the inherent lag relationship among environmental factors. The larger the R, the closer the SFD peak approached R peak, however, the larger the VPD, the more the SFD peak deviated from VPD peak. 4) BiasT values (0.54, 0.45) in the mild and severe windthrow groups were significantly higher than that in CK (0.11, P<0.001), indicating a delayed response of SFD to R and water use strategy shifted from radiation-driven to transpirational pulling-driven. 5) In the validation experiment, BiasT increased from 0.11 to 0.44 two days after root damage, and was significantly different from CK (P<0.05). 6) There was no significant difference in water consumption between the mild and severe windthrow groups (P>0.05). The severe group’s second-year DBH increment decreased by 41% (P<0.05 vs. control), while the mild group increased by 71% (P>0.05 vs. control), indicating a decoupling between growth and water consumption. Conclusion: Windthrow substantially reduces transpiration and water consumption in P. tomentosa, resulting in a lasting hydraulic carry-over effect. Root hydraulic damage may drive the shift in hysteresis bias. BiasT can assess hydraulic impairment. Mildly windthrown trees can be retained; severely windthrown trees require a trade-off: removal or restoration.

Key words: triploid Populus tomentosa, windthrow and restoration, sap flux density, hysteresis bias, hydraulic damage