Pattern and driving factor of intense defoliation of rubber plantations in SW China

摘要

The pattern of intense defoliation of rubber trees (Hevea brasiliensis) in Southwest China is assumed to mainly be attributable to drought stress. However,thishas never beenproven, and the pattern remains unclear. Our objectives were to reveal the driving factor and pattern of intense defoliation, and to explore the characteristics of sap flow density (Js) and relate them to climatic factors before, during and after the intense defoliation period. We continuously measuredJsand related climatic factors in a rubber plantation in Southwest China from 2013 to 2016 and found that (1) the cold stress (<10 °C) was more relevant to intense defoliation than drought stress, and after encountering cold stress the rubber trees might enter dormancy for approximately 50 days during thedefoliationperiod; (2)Jsshowed a self-regulating ability to respond to high air temperature (Ta) and water vaporpressuredeficit (VPD) with atime lag of approximately one hour during all periods except during thedefoliationperiod; (3) the sensitivity ofJstoTaweaken (slope = 0.14,r = 0.17,P > 0.05) during thedefoliationperiod, but it rebounded back to a relative high level (slope = 1.44,r = 0.55,P < 0.01) during therefoliationperiod which was similar to that during the remaining periods; and (4) the relationship betweenJsandTawas generally positive, while bothSWC5andSWC100exerted inhibitory effects onJs, and the interactive effects amongTa,SWC5andSWC100onJswere significant during therefoliationperiod (P < 0.01). Based on these findings, we evaluatedJsas a crucial physiological indicator of the processes underlying important phenological transition periods, and the cold stress rather than drought stress was likely the driving factor of the pattern of intense defoliation occurred in Southwest China rubber plantations.