Characteristics of Stem Sap Flow of Two Poplar Species and their Responses to Environmental Factors in Lhasa River Valley of Tibet

Abstract

ObjectiveIn this study, the dry-wet seasonal variation characteristics of sap flow of the two afforestation poplar species in semi-arid Lhasa river valley, and the environmental impact factors were investigated in order to provide a theoretical base for species selection for vegetation reconstruction in the river valley. MethodThe Granier's thermal dissipation probe was employed to continuously measure the stem sap flow of Populus beijingensis and Populus szechuanica during the wet and dry seasons. At the same time, environmental factors, such as solar radiation (Q), air temperature (Ta), soil water content (SWC), and vapor pressure deficit (VPD), were recorded. ResultsThe main meteorological factors in the Lhasa river valley were quite different in wet and dry seasons. The Q, Ta, SWC, and VPD values during the wet season were higher than that in the dry season, and the average of them in the dry season wrer around 60% of those in wet season. In both seasons, the sap flow velocity in P. beijingensis was lower than that in P. szechuanica.The sap flux density of P. beijingensis was 71% and 58% lower than those of P. szechuanica during the dry and wet seasons, respectively. During the dry season, the sap flux density of two poplars declined by 65% and 76% compared with that in wet season. The main environmental factors of Q and VPD had less influence on the stem sap flow of P. beijingensisthan on that of P. szechuanica. ConclusionIn the Lhasa river valley area between dry and rainy season, the sap flux density of two poplar species all had obvious circadian rhythm. However, there were significant differences in peak value, daily average value, start-up time and the impact environmental factors between the two species, especially in the dry season.P. szechuanica, as native tree species, showed a stronger adaptability.