These fern species (Lacey Tree Fern — Cyathia cooperi, Giant Creek Fern — Pneumatopteris sogerensis) are growing well despite the current dry spell. They have some things in common which might explain why they survive here while other plants don't. They are slow growing with a tough trunk or thick rhizome. Trunks store nutrients and water so these plants can cope with extended dry conditions. They also provide a flexible anchor for when the creek is in flood (and other plants are carried away). And, being slow growing, these species can "make do" with the filtered light they receive. So there is probably an intersection of conditions that results in a narrow habitat that is just suitable for these species.