Regeneration responses to water and temperature stress drive recruitment success in hemiepiphytic fig species

Mechanisms for surviving water and temperature stress in epiphytes are essential adaptations for successful regeneration in forest canopies. Hemiepiphytes start their life cycle as epiphytes, eventually establishing aerial root connections to the ground. This strategy allows for greater light capture, while benefitting from minimized risk of fire, flooding and damage by terrestrial herbivores, but exposes the vulnerable seedling stage to heat and drought stress. However, the response to temperature and water stress during early regeneration in hemiepiphytes is not known. In this study, we tested the effect of temperature (15/5, 25/15 and 35/25 °C; day/night diurnal variation) and water availability, as substrate moisture (0.00, -0.20 and -0.35 MPa) and water vapor (18.5-99.5% relative humidity), on seed germination, seedling emergence and survival in six hemiepiphytic and nine non-hemiepiphytic Ficus species. Under high-temperature conditions (35/25 °C), hemiepiphytes had higher gemination and seedling survival, achieved peak germination slower and extended germination. Greater water stress (-0.35 MPa) in the growth substrate resulted in higher germination of non-hemiepiphytes; hemiepiphytes, in contrast, took a shorter time to complete germination, but had higher seedling emergence and survival. Hemiepiphytes germinated at 99.5% relative humidity more readily compared with non-hemiepiphytes. These findings provide the first comprehensive evidence that hemiepiphytic Ficus species are better adapted to drier and warmer conditions during the critical transition from seed to seedling. Through greater flexibility in achieving peak germination and duration of regeneration activity, hemiepiphytes modulate their recruitment process to be more resilient under abiotic stressors. This may allow them to be more successful in regenerating in forest canopies under ambient conditions that are transient. These results support previous work showing greater drought tolerance of hemiepiphytic Ficus species in larger size classes and extend this finding to show that physiological adaptations for drought and heat tolerance start from the early seedling emergence stage.