Effects of water availability on floral traits and pollinators in a columnar cactus endemic to the Atacama Desert, Chile.

Drought frequency and intensity are increasing with climate change. The effects of drought on plants and their pollinators require further study, especially in arid environments. In three populations distributed along an aridity gradient, we investigated how aridity is related to corolla size, floral display, stamen and stigma size, flower phenology, and fitness of the cactus Eulychnia breviflora Phil., endemic to the Atacama Desert. We also evaluated pollinator richness and visitation rate. In addition, in a dry year we conducted a water addition experiment in the field to evaluate more precisely the effects of water availability on floral traits, pollinators and cactus fitness. With increased aridity E. breviflora flowers were smaller but the plants displayed more flowers, and whereas the flower bud stage was shorter, flowering was delayed and extended. Pollinator richness and visitation rates were similar across populations. Water addition resulted in increased seed number, but fruit set was similar. No floral traits or pollinator parameters were affected by water addition. Overall, the results showed that variation in floral phenotype and phenology with aridity in this cactus is only partly related to water-saving strategies and that pollinator service is unaffected by aridity. The experiment showed that floral traits are unresponsive to water availability in the short term. However, the increased seed set following water addition suggests that cacti may capitalize on rain pulses typical of deserts, prioritizing resource allocation towards seeds. Further research is needed to understand the complex interactions between environment, floral traits, pollinators, and fitness in desert-adapted plants.