Examining the complementarity in belowground water use between different varieties and ages of Arabica coffee plants and dominant shade tree species in an organic agroecosystem

Traditional Arabica coffee plantations in which coffee is grown under diverse shade tree cover are increasingly recognized as a sustainable agricultural practice because of their ecological and socioeconomic benefits as well as their resilience to climate change. In addition, organic coffee production has expanded in the last decades due to premium market prices. However, the extent to which coffee and shade trees show positive (complementary) or negative (competitive) root interactions for belowground water sources in traditional agroforests under organic management is largely unknown. The present study investigated relative and absolute source contributions to water uptake of coffee and shade trees under progressive decline of soil water availability during the dry season in an organic coffee plantation characterized by dense and diverse canopy shade. We used the MixSIAR Bayesian stable isotope mixing model to determine proportional use of belowground water sources for three Arabica coffee varieties/ages and five dominant shade tree species. Corresponding absolute uptake amounts by the whole coffee crop and the whole shade tree canopy were calculated using transpiration estimates obtained from micrometeorological and stomatal conductance measurements. The absence or potential presence of competition for soil water in the studied coffee plants was investigated using a combination of stomatal conductance measurements and modeling, supported by leaf and root hydraulic trait data. Results show that the observed relative and absolute plant water uptake patterns provide evidence of vertical complementarity in belowground water use between coffee and shade trees during the dry season. Findings show that the main water source for coffee was the near-surface soil (< 5 cm depth; 44–56 %), with no distinction among coffee varieties and ages. Our data suggest that dew was an important water input to the near-surface soil and to the coffee water cycle. Soil water limitation during the driest period of the study was observed in the youngest coffee plants (5–20 years old), but not in the oldest (∼80 years old). Deeper soil water (60–120 cm; 45–69 %) was the main source for the shade trees, but important differences were observed among species, showing the potential of minimizing competition for water through tree species selection. Finally, our study shows that water uptake from groundwater sources increased with increasing tree size.