Temporal variations in transpiration of Vitellaria paradoxa in West African agroforestry parklands

Abstract

Lack of data on water use of key species of drylands constitutes an obstacle to understanding their role in hydrological processes in this environment. To elucidate seasonal variation in water consumption by Vitellaria paradoxa, the dominant species of parklands of the semi-arid areas of West Africa, we’ve measured its transpiration using heat ratio method (HRM) and seven potential explanatory variables. Sap flux was found to be significantly different among years with 0.64, 0.59 and 0.67 L h−1 dm−2 in 2008, 2009 and 2010, respectively. Sap flux was significantly higher in the dry (0.73 L h−1 dm−2) than in the wet season (0.53 L h−1 dm−2). Nighttime sap flux during dry season (0.48 L h−1 dm−2) was significantly higher than that of the wet season (0.20 L h−1 dm−2) and it contributes on average to 26% of daily sap flow with a maximum reaching 49%. The mean transpiration rate per tree was 151 L day−1 and all measured variables except rainfall and soil water content were significantly correlated with sap flux. These correlations were stronger (higher R value) during the rainy than in the dry season. Vapor Pressure Deficit (VPD) explained the highest proportion of sap flux variation and their curve was of parabolic type (R2 = 0.54) indicating that V. paradoxa can probably down-regulate its canopy conductance beyond a certain threshold of VPD, which is about 3 kPa in the present study. Future studies should investigate such hypothesis as well as the impacts of the variation of V. paradoxa transpiration due to climatic variables on hydrological cycles.