FERNS IN AN ANGIOSPERM WORLD: CRETACEOUS RADIATION INTO THE EPIPHYTIC NICHE AND DIVERSIFICATION ON THE FOREST FLOOR

The Cretaceous period saw dramatic shifts in the functional plant biology of the Earth's flora. This was an episode of explosive plant radiations that produced our modern angiosperm-dominated world. This period was also a time of great Pteridophytic diversification and experimentation and saw the rise of most extant fern lineages. Prominent among these was the widespread appearance of epiphytic species. In modern tropical forest canopies, epiphytic ferns often represent some of the most abundant taxa and can dominate epiphyte biomass. Yet from most perspectives, the epiphytic habitat is vastly different from the forest floor. What happened during the Cretaceous that led to such widespread epiphyte fern diversification? What are the traits that allow extant ferns to flourish in this dry, nutrient-poor, bright habitat? How did terrestrial ferns cope with this new habitat? We review a number of functional aspects of fern biology to include both gametophytic and sporophytic physiology. It is possible that angiosperms engineered conditions that lead to aseasonal tropical forests. Data suggest that ferns may have reduced water use efficiency. If so, then wetter forests may have facilitated fern diversification. Successful epiphytic species would have required major modification in the gametophyte generation to include indeterminate growth, extreme stress tolerance, and an outcrossing breeding system. Terrestrial species also radiated at this time and may have relied on unique aspects of photobiology to take advantage of low-light terrestrial habitats. The study of fern ecology has reached a fevered pitch. Continued investigation will no doubt reveal some of the most exciting changes in our understanding of this remarkable lineage.