Ecological significance of seed traits in the genus Macaranga

According to the regeneration niche hypothesis, the transition from seeds to seedlings is critical in determining plant distribution patterns. This initial stage of recruitment may be strongly influenced by seed mass via effects on the sensitivity to light quality and soil moisture availability for germination, and via susceptibility to predators and pathogens. In this study ten Macaranga species (Euphorbiaceae) varying in seed mass and adult crown illumination were used as a study system to test hypotheses linking seed mass to light and soil moisture requirements for germination, investment in seed chemical and physical defense, and seed persistence in the soil. Empirical measurements of germination and seed persistence in the soil were made at Lambir Hill National Park in Borneo, and coupled with analyses of the physical and chemical defense characteristics of seeds. I found that seed mass in Macaranga was significantly correlated with germination response to light quality represented by the ratio of red to far-red light. Previously reported opposing relationships between seed mass and light requirements for germination between temperate (negative relationship) and tropical taxa (positive relationship) were found to be robust, indicating contrasting selection on germination responses across biomes. In contrast, seed mass was not associated with soil moisture requirements for germination in Macaranga and two additional pioneer species. Nonetheless, soil moisture requirements for germination of species from Lambir were higher than those previously reported for species from seasonal tropical forest. Macaranga also exhibited strong interspecific variation in physical and chemical defensive traits. Overall, there was no evidence for a trade-off between investment in seed physical versus chemical defense; however, seed physical defenses were more important than chemical defenses in predicting the duration of seed persistence in the soil. While phenolic concentrations used in chemical defense were generally lower in seeds than leaves, certain phenols were expressed in higher fraction in seeds than leaves, and may be important in defending seeds from natural enemies that primarily attack seeds. Finally, among Macaranga species, variation in soil moisture requirements for germination and in the investment in seed physical defenses was most strongly associated with adult crown-illumination. Interspecific differences in germination requirements and defensive traits found in this study emphasize the importance of regeneration niche differentiation as a contributing factor to species coexistence in diverse plant communities