Measurements of transpiration from savannah shrubs using sap flow gauges

Abstract

Sap flow gauges were used to measure transpiration from the shrub component of a 7-year-old agricultural fallow in Niger, West Africa. The aim of the study was to provide the data necessary to develop and validate multi-source models of evaporation from semi-arid land surfaces by measuring transpiration from a single component of the vegetation. The vegetation at the study site was a type of savannah, consisting of scattered shrubs, with an understorey of annual grasses and herbs. Sap flow measurements were restricted to Guiera senegalensis J.F. Gmel., the dominant shrub species. Sap flows were measured throughout the wet season of 1990 (June–September), using sap flow gauges operating on a constant power heat balance principle. Sap flow rates measured in individual stems were strongly related to leaf area and net radiation. Strong positive correlations were observed between the hourly mean flow rates measured in different stems. Daily transpiration, T, averaged across a 0.25 ha sample plot was compared with daily total evaporation (measured by eddy correlation), E. T was calculated by scaling up from sap flow measurements in a sample of stems to the whole plot by using the frequency distribution of stem diameter in the plot, and assuming that sap flow in each stem was proportional to its basal cross-sectional area. E varied from about 2 mm day −1 at the start of the wet season to 4–6 mm day −1 during August and September, when maximum leaf area and soil profile water content were observed. E, a measure of evaporation from all plant and soil sources, always exceeded T. At the end of two dry periods in June and July, when soil evaporation and transpiration from the herb understorey probably totalled about 0.5 mm day −1, E was only 0.7–0.9 mm day −1 greater than T, suggesting that the scaling procedure adopted gave reasonable estimates of bush transpiration. Over the whole wet season, transpiration from G. senegalensis accounted for 35% of the total wet season rainfall of 454 mm.