Eco-hydrology of a Sahelian woody species : Faidherbia albida. Assessing root hydraulic redistributions in the agroforestry parkland of Sob, Senegal

Details

Supervisors

Vincke, Caroline ; Do, Frédéric

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en gestion des forêts et des espaces naturels, à finalité spécialisée

Abstract

This master’s thesis reports a study on the eco-hydrology of Faidherbia albida, a Sahelian woody species, in an agroforestry parkland in the semi-arid region of Fatick, Senegal. Although it is an emblematic species of the Sahelian zone with many advantages in agroforestry (Sileshi et al., 2020), its ecophysiology remains poorly documented. In particular, to the best of our knowledge, no study has been dedicated to the demonstration of the existence of root hydraulic redistributions(RHR) in this species, representing its capacity to bring water up, from the water table to the surface horizons, through its root system. Yet, F. albida presents many favorable conditions for the observation of this phenomenon (reverse phenology, phreatophyte with a dimorphic root system, deep water table and intense drying of the surface during the 8-month dry season). While there is growing evidence that RHR has significant potential to alter soil hydrology, there is still a lack of knowledge on how to accurately predict the conditions and the potential ecohydrological consequences of this phenomenon. This challenge is particularly pressing in water-limited environments, where as in Fatick, deeprooted plants can access the water table (Bleby et al., 2010). The objective of this study was therefore to observe for the first time RHR in F. albida in order to evaluate their spatial and temporal characteristics and to determine the climatic and edaphic conditions that trigger these periodic increases in surface water content. A two-month nternship was carried out at the 'Faidherbia-Flux' research station (groundnut basin, Fatick region) in the middle of the dry season to study the day-night fluctuations in soil humidity and the sap flow in F. albida's lateral roots. Different measurement methods were tested, by integrating intra-daily variability, seasonal variability, at different soil depths and different distances to the trees. This study reveals daily replenishment of surface soil moisture, coinciding with reversals in sap flow direction within F. albida's lateral roots. While the RHR process occurs across the entire study area, it exhibits substantial spatial and temporal variability. Notably, its intra-daily dynamics are irregular, with recharges not only limited to nighttime. Coarse surface roots detected by georadar appear to influence the amplitude and timing of soil moisture fluctuations. A marked shift in tree behavior and RHR dynamics was observed at high VPD (>45 hPa), characterized by a decrease in the upward trunk sap flow, causing phase shifts between the transpiratory flow and the root flow, as well as a shift in humidity peaks on a daily scale. The strong dependence of tree behavior on weather conditions, such as soil water recharge during the rainy season and maximum daily VPD during the dry season, raises concerns about potential climate change impacts on tree-scale water balances. These findings suggest that the quantities of water uplifted to surface layers are significant and that their quantification could enhance the accuracy of models predicting soil moisture (Fu et al., 2016), surface temperatures (Lee et al., 2005), transpiration rates (Zheng and Wang, 2007 ; Fu et al., 2016), Bowen ratio (Fu et al., 2016) and net ecosystem exchanges (Baker et al., 2008), particularly during dry periods.