Responses of leaf anatomy to light environment in the tree fern Cyathea caracasana (Cyatheaceae) and its application to some ancient seed ferns

The ecological interpretation of extinct plants requires that function-based ecological analogy be made between extant and extinct organisms. Particularly for non-angiosperms, such analogy has been reluctantly assumed. Using a non-angiosperm model, this study identifies a specific suite of characters that can be used to interpret light environments for ancient plants. The tree fern Cyathea caracasana is used as a living model because it offers structural and architectural analogy with ancient tree ferns and some Paleozoic free-standing arborescent pteridosperms. A variety of leaf anatomical and morphological measurements were made on 34 individuals growing in secondary forest and abandoned pasture at Reserva Natural La Planada, Nariño, Colombia. Leaf characters were correlated with site-light conditions quantified from hemispherical photographs. Spearman correlation coefficients show that blade size, petiole blade, and palisade / spongy mesophyll ratio are well correlated with light environment. Significant relationships were also observed in frond surface area, thickness of ultimate segment, epidermis thickness, and hypodermis thickness. Although some features may be influenced by nutrient and water status, a well-developed palisade layer and thick hypodermis are specific indicators of light environment that vary plastically in Cyathea caracasana. These data can be used to refine ecological interpretations of extinct foliage; Alethopteris- and Neuropteris- type medullosan pteridosperms of the Middle Pennsylvanian Westphalian) of Euramerica provide an illustrative example.