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

Abstract

-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. Plant populations respond to environmental variation in several ways. When the environment fluctuates infrequently relative to the life span of individuals, adaptive segregation may produce differential response to environmental cues among separate populations. Consider variation in light environment. Sunadapted populations of Impatiens capensis L. showed enhanced growth in response to changes in the ratio of red (600-700 nm) to far-red (700-850 nm) light relative to shade-adapted populations (Dudley and Schmitt, 1995). Similar sun/shade segregation in growth response to red/far-red (R/FR) has been observed in many angiosperm species in open (as compared to understory) communities (Morgan and Smith, 1979). In contrast, when the environment varies frequently relative to the life span of individuals, adaptive segregation cannot occur and individuals must rely on morphological and physiological flexibility (plasticity) (Thompson, 1991; Sultan, 1993; Ackerly and Bazzaz, 1995; Arens, 1997). In such cases, plants may change form or physiology to suit new conditions, for example switching from sun-leaf to shade-leaf anatomy (Arens, 1997). Alternatively, individuals track or follow their preferred habitat as it moves. Conventionally, habitat-tracking in plants has been applied to intergenerational migration (Davis et al., 1986; Webb, 1987; Davis and SaThis content downloaded from 157.55.39.128 on Mon, 18 Jul 2016 03:58:47 UTC All use subject to http://about.jstor.org/terms AMERICAN FERN JOURNAL: VOLUME 90 NUMBER 1 (2000)