Coffee and shade trees show complementary use of soil water in a traditional agroforestry ecosystem

Globally, coffee has become one of the most sensitive commercial crops, being affected by climate change. Arabica coffee (Coffea arabica) grows in traditionally shaded agroforestry systems in tropical regions and accounts for ∼70 % of coffee production worldwide. Nevertheless, the interaction between plant and soil water sources in these coffee plantations remains poorly understood. To investigate the functional response of dominant shade tree species and coffee (C. arabica var. typica) plants to different soil water availability conditions, we conducted a study during near-normal and more pronounced dry seasons (2014 and 2017, respectively) and a wet season (2017) in a traditional coffee plantation in central Veracruz, Mexico. For the different periods, we specifically investigated the variations in water sources and root water uptake via MixSIAR mixing models that use δ¹⁸O and δ²H stable isotope composition of rainfall, plant xylem and soil water. To further increase our mechanistic understanding of root activity, the distribution of below-ground biomass and soil macronutrients was also examined and considered in the model as prior information. Results showed that, over the course of the two investigated dry seasons, all shade tree species (Lonchocarpus guatemalensis, Inga vera and Trema micrantha) relied, on average, on water sources from intermediate (>15 to 30 cm depth: 58± 18 % SD) and deep soil layers (>30 to 120 cm depth: 34±21 %), while coffee plants used much shallower water sources (<5 cm depth: 42±37 % and 5–15 cm depth: 52±35 %). In addition, in these same periods, coffee water uptake was influenced by antecedent precipitation, whereas trees showed little sensitiveness to antecedent wetness. Our findings also showed that during the wet season coffee plants substantially increased the use of near-surface water (+56 % from <5 cm depth), while shade trees extended the water acquisition to much shallower soil layers (+19 % from <15 cm depth) in comparison to drier periods. Despite the plasticity in root water uptake observed between canopy trees and coffee plants, a complementary use of soil water prevailed during the dry and wet seasons investigated. However, more variability in plant water sources was observed among species in the rainy season when higher soil moisture conditions were present and water stress was largely absent.