Effects of water and phosphorus addition on the nutrition, flowering phenology, and sexual reproduction of Gymnopodium floribundum in a dry tropical Yucatan forest

Some trees in seasonally dry forests have developed adaptations to survive water and nutrient shortages. Some species even flower and renew their foliage in the dry season. However, this condition can establish trade-offs between growth and reproduction, so the availability of water and nutrients must be a determinant of photosynthesis and propagation. In this work, we experimentally evaluated the effects of the availability of water and phosphorus on the nutrition and reproduction of Gymnopodium floribundum, a tree species that shows simultaneous foliation and flowering during the dry season. Twenty adult trees were chosen, 10 of which were irrigated and supplied with phosphate fertilizer, leaving the remaining 10 as a control group. The roots of trees supplemented with irrigation and phosphorus presented higher concentrations of phosphorus. The percentage of nonstructural carbohydrates was low in both control and treated trees, suggesting limited production of photoassimilates. Trees with water and phosphorus experienced less water stress. Compared with the control trees, the irrigated and fertilized trees flowered more times, their flowering period was longer, and they produced more flowers. However, the number of seeds produced per one reproductive event did not differ between the treated and control trees, probably because the treated trees invested nonstructural carbohydrates in previous flowering events. These results suggest that water and phosphorus availability regulate the flowering phenology of G. floribundum. The nutrient cost of simultaneous flower and leaf production or limitation by compatible pollen explains the low seed production of this tropical tree.