We present the first study on the reproductive phenology and above-ground and below-ground trait relationships of Andira humilis, an endemic underground tree from Brazil. We studied an A. humilis population in caatinga remnants. The phenology seasonality was assessed using circular statistics (monthly frequency) based on herbarium vouchers. We used PCA on functional traits and climate to examine their relationships, and linear regression to assess the effects of mean rainfall and temperature on phenophases. A. humilis exhibits seasonality, flowering in the dry season and fruiting in the rainy season. Functional traits (low-wood density and high-water storage) enable flowering during water stress. Temperature drives fruiting, while flowering is independent of rainfall or temperature. Fruiting is less seasonal than flowering, likely due to extended maturation periods. This study highlights the role of A. humilis functional traits in adapting to extreme water stress in dry forests.