Variation in Tree Fern Stipe Length with Canopy Height: Tracking Preferred Habitat through Morphological Change

Cyathea caracasana is a common open-habitat tree fern in the Andes of Colombia. In full sun, stem growth rates are high (up to 2 cm/month) and individuals regularly produce spores. However, even the fastest growing ferns are overtopped by woody angiosperms after 10 to 15 years of natural forest regeneration. As individuals are overtopped, C. caracasana produces nearly vertical fronds with long stipes (commonly over 3 m) apparently to place the photosynthetic surface into the canopy. We compared stipe length and blade length and width among individuals growing in open sites and in the understories of two regenerating forests: one with a canopy of 20-25 m, and one with a canopy of 5-8 m. Stipes and blades were shortest in open habitat and longest in the low-canopy forest. Ferns in the high-canopy forest had intermediate measurements. Despite the change in frond length, the number of primary pinnae per-frond did not differ among the habitats sampled. This suggests that elongation cues are received late in the development of the frond. This conclusion is supported by a positive relationship between stipe length and the distance of the fern meristem below the canopy. Because both understory populations show stipe elongation relative to open-hapitat ferns, the cue to elongate is likely a low red/far-red wavelength ratio of the light received by the apical meristem. Extraordinary elongation is probably made possible by extra carbon resources available to low-canopy plants, which still have leaves in full sun. This sense and response mechanism allows individual plants to produce elongated fronds as their apical meristems are overtopped. Functionally, the long-stiped plants remain in full sun even after they are overtopped, thus they "track" their preferred, open habitat.