Soil texture moderates root functional traits in agroforestry systems across a climatic gradient

The diversification of agroecosystems with shade trees has a complex role in climate change adaptation. Multiple interactions among shade tree composition, heterogeneous soil conditions, and resulting microclimate modifications makes reproducible evaluations of agroforestry as a climate change adaptation practice challenging. In this study we systematically investigate soil water and nutrient acquisition strategies in cocoa (Theobroma cacao) along a climatic and diversity gradient in Ghana, West Africa. We adopted a functional trait-based approach to comparatively examine cocoa root strategies in monoculture or in agroforestry with a single species of shade tree (Terminalia ivorensis) across two precipitation regimes (optimal and suboptimal dry), and in contrasting edaphic conditions (sandy and loam). Variance decomposition indicated that shade trees explained 20 % of the variability in absorptive root trait covariation in cocoa. However, shade tree effects on trait expression were not systematic and depended on climatic and edaphic conditions. Notably, effects of shade trees were amplified on fine-textured soils, with significantly higher conservative trait values of cocoa absorptive roots when in agroforestry within a suboptimal precipitation regime. Transport root traits (root tissue density, diameter) associated with water and solute transport strongly varied among contrasting climate and edaphic conditions. Structural equation models indicated that soil texture played a critical role in regulating transport root trait expression. In agroforestry, clay content improved soil moisture levels, and, in suboptimal climate, fine root growth rates were positively affected by specific leaf area while being significantly controlled by soil texture. Results describe the importance of soil texture in controlling the effect of shade trees on cocoa cultivated in suboptimal precipitation regimes. Thus, agroforestry success as a climate change adaptation strategy in cocoa systems requires detailed assessments of crop strategies in different climatic conditions, as well as identification of soil-mediated filters on crop function.