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.

在西非，Harmattan 引起的大气和土壤干旱代表可可可可的季节性反复危害L. 农业生态系统。在哈马坦风的影响下，降水减少，空气湿度和温度达到压力水平。气候变化正在导致温度升高，这将增加大气的蒸发能力，有可能使土壤和大气压力变得更加严峻。这将进一步威胁该地区可可种植的生存能力。为了表征可可树对大气和土壤干旱的响应，我们在加纳东部地区的一个 Harmattan 季节（2019 年 11 月至 2020 年 3 月）监测了灌溉和不灌溉的两个子地块。对于这两种处理，我们记录了：树液流速、冠层上方和下方的光合有效辐射 (PAR)、土壤湿度、温度、空气湿度和日降水量。叶面积指数 (LAI) 是根据 PAR 测量值估算的。为了表征成熟可可树在白天和季节性尺度上的干旱响应，我们开发了两个增强回归树模型 (BRT)，其中测量了环境变量。发现 Harmattan 引起的干旱的大气成分对冠层的影响程度与土壤水分胁迫相似，两者都导致 LAI 下降 33%。这项研究证实了土壤干旱的重要性，但也强调了大气干旱对该物种蒸腾控制的关键作用。在研究条件下，土壤和大气水分胁迫对蒸腾作用没有协同作用。BRT 模型将 LAI 确定为最有影响力的 sap 速度驱动因素之一，其中，反过来，对大气和土壤干旱的交互影响很敏感。我们的研究结果强调，在受哈马坦风影响而日益干燥的条件下，不仅降水减少，而且大气干旱加剧，都可能对西非的可可生产产生负面影响。