Assessment of atmospheric and soil water stress impact on a tropical crop: the case of Theobroma cacao under Harmattan conditions in eastern Ghana.

Abstract

In West Africa, Harmattan-induced atmospheric and soil droughts represent seasonally recurring hazards for Theobroma cacao L. agro-ecosystems. Under the influence of the Harmattan winds, precipitation is impaired and air humidity and temperature reach stressful levels. Climate change is causing an increase in temperature that will drive up the evaporative power of the atmosphere, risking to harshen both the soil and atmospheric stress. This would further threaten the viability of cacao cultivation in this region. To characterize the response of cacao trees to atmospheric and soil drought, we monitored two sub-plots, with and without irrigation, throughout one Harmattan season (November 2019 - March 2020) in the Eastern region in Ghana. For both treatments we recorded: sap flow velocity, photosynthetic active radiation (PAR) above and below the canopy, soil moisture, temperature, air humidity and daily precipitation. Leaf area index (LAI) was estimated from PAR measurements. To characterize drought responses of mature cocoa trees during the day and at the seasonal scale, we developed two boosted regression trees models (BRT) with the environmental variables measured. The atmospheric component of Harmattan-induced drought was found to affect the canopy to a similar extent as soil water stress, both causing a decline in LAI of 33%. This study confirmed the importance of soil drought but highlighted as well the crucial role of atmospheric drought for this species’ transpiration control. Soil and atmospheric water stresses did not have a synergistic effect on transpiration under the studied conditions. The BRT models identified LAI as one of the most influential drivers for sap velocity, which, in turn was sensitive to the interactive effect of both atmospheric and soil drought. Our results highlight that not only reduced precipitation but also increasing atmospheric drought is likely to negatively impact on cacao production in West Africa under increasingly dry conditions imposed by the influence of the Harmattan winds.