Phenology-related water-use efficiency and its responses to site heterogeneity in rubber plantations in Southwest China

The survival strategy and ecophysiological process of rubber trees (Hevea brasiliensis), in which water-use efficiency (WUE) represents a vital indicator linking the carbon and water cycles, are mediated by phenology and can be severely affected by a low fruit-set rate and cold stress. However, the phenology-related WUE and its temporal variation as well as its responses to site heterogeneity in rubber plantations remain poorly understood. In this study, we used a four-year (2013-2016) continuous dataset of biometric survey, sap flow and phenology of rubber trees to examine the WUE in response to phenological changes and cold stress. The planting density and stand age, factors exhibiting a considerable difference and contributing to site heterogeneity, were explored to reveal their effects on the WUE of rubber plantations in Yunnan Province of Southwest China. Unexpectedly, the response of the WUE to phenological changes was characterized by the peak value (2.73 +/- 0.59 g C kg(-1) H2O) that occurred simultaneously with the fruiting period (P-f), and the WUE values during the P-f were significantly higher than that of other phenological periods (P < 0.01), indicating that the survival strategy adopted by the rubber trees to cope with the low fruit-set rate as well as cold stress during the defoliation period (P-d) and adapt to the local climate might be very effective. However, this effectiveness might be reduced to some extent, since the WUE was significantly affected by the planting density (df=2, F-value=11.51) and stand age (df=3, F-val-ue=22.84) indicated by one-way analysis of variance (P < 0.01). These findings can provide a deeper understanding for the adaptation strategies to adverse climate and the cultivation and management of rubber plantations.