Transpiration of Swartzia tomentifera in response to microclimatic variability in the central Amazon: the net effect of vapor pressure deficit

Abstract

Background: The distribution of precipitation in the Amazon region is changing in last decades, but it is uncertain if transpiration will decrease in response to changes in rainfall distribution. In this study we aimed to assess the effect of climatic variability on whole-tree transpiration (inferred from sap flow measurements). We collected climate data (precipitation, photosynthetically active radiation –PAR, relative humidity, vapor pressure deficit –VPD, and temperature) and measured sap flow rates (SFR) in trees of Swartzia tomentifera. The SFR was measured in the dry season and wet season using the heat ratio method. Results: Sap velocity was positively correlated with PAR and temperature, with partial correlation (rpart) of 0.33 to 0.57 (p < 0.001) during daytime, while at night rpart varied from 0.15 to 0.21 (p < 0.001). When the effects of PAR and temperature were controlled (i.e. after removing the effect of PAR and temperature on sap velocity and VPD), the net effect of VPD on sap velocity became slightly negative (rpart of-0.08 to-0.14, p < 0.01) during daytime. Maximum sap velocities were lower during the dry season (5.9 cm h-1) than in the wet season (9.8 cm h-1, p < 0.001). Conclusion: This study shows that tree transpiration can decrease in the dry season in the central Amazon, associated with an increase in temperature, PAR and vapor pressure deficit. A novelty of this study is to demonstrate that after adjusting for the effect of temperature and PAR, the effect of vapor pressure deficit on sap velocity can become negative.