Gametophyte autecology

Multiple factors clearly act to control the distributions of fern game-tophytes and studies have demonstrated the importance of disturbance and edaphic factors (Farrar and Gooch, 1975; Cousens, 1979, 1981; Cousens et al., 1985, 1988; Peck, 1980; Peck et al, 1990; Greer and McCarthy, 1999) and plasticity (Greer and McCarthy, 1999) on the distribution and survival of temperate gametophytes. Data on tropical fern gametophytes suggest that there may be considerable differences among different functional groups. For example, both gametophyte ontogeny and morphology vary tremendously among species of different functional groups (e.g., epiphytic and terrestrial) (Atkinson and Stokey, 1964; Nayar and Kaur, 1971). A common observation is that there are fundamental differences in morphology and potential longevity between the gametophytes of epiphytic and terrestrial species (Dassler and Farrar, 1997, 2001; Watkins et al., 2007a). Thus, there is likely to be considerable variation in gametophyte ecology especially as it relates to different fern functional types.

Fern gametophytes are not simply transient members of their environment. Early studies have produced either anecdotal or laboratory-based data to suggest that if fertilization is prevented, fern gametophytes can be extremely long lived. Longevities of 12 years have been reported in some laboratory-reared gametophytes (Walp, 1951). Watkins et al. (2007a) have shown that the gametophytes of tropical ferns, especially those of epiphytic taxa, can live for years in natural

Figure 9.3 Kaplan-Meier survivorship curves of the gametophytes of five species over a 25 month study period. No data were collected during months 4-7 and 15-24. Open symbols indicate terrestrial species and closed symbols indicate epiphytic and hemi-epiphytic species. Campyloneurum brevifolium (Lodd. ex Link) Link and Vittaria lineata (L.) Sm. are epiphytes that commonly occur in the inner canopy and in high-light understory habitats. Lomariopsis vestita E. Fmyn. is an abundant hemi-epiphyte that occurs in the understory of both primary and secondary forests throughout the study site. Danaea wendlandii Rchb. f. is a terrestrial species typical of more stable understory habitats of primary forests. Pityrogramma tartarea (Cav.) Maxon is an abundant species often found in disturbed sites such as trail and road sides and on the root balls of recently fallen trees.

Figure 9.3 Kaplan-Meier survivorship curves of the gametophytes of five species over a 25 month study period. No data were collected during months 4-7 and 15-24. Open symbols indicate terrestrial species and closed symbols indicate epiphytic and hemi-epiphytic species. Campyloneurum brevifolium (Lodd. ex Link) Link and Vittaria lineata (L.) Sm. are epiphytes that commonly occur in the inner canopy and in high-light understory habitats. Lomariopsis vestita E. Fmyn. is an abundant hemi-epiphyte that occurs in the understory of both primary and secondary forests throughout the study site. Danaea wendlandii Rchb. f. is a terrestrial species typical of more stable understory habitats of primary forests. Pityrogramma tartarea (Cav.) Maxon is an abundant species often found in disturbed sites such as trail and road sides and on the root balls of recently fallen trees.

settings. Provided that fertilization and more importantly, disturbance, is limited, many terrestrial species can also live for years (Figure 9.3).

Increased longevity generally results in increased gametophyte size. Gametophytes of many square centimeters that can produce dozens of sporophytes in space and time are not an uncommon occurrence in tropical forests (Watkins, personal observations). Even in the case of many temperate species, gametophytes have been shown to over-winter (Pickett, 1914; Sakai, 1980; Farrar and Gooch, 1975; Sato and Sakai, 1981). Such observations call for fundamental revision of the traditional view of gametophyte longevity and sporophyte production. Although the view that gametophytes are short lived and produce a single sporo-phyte may be applicable to many temperate and tropical terrestrial species, this perspective should be reconsidered in the broader context of fern and lycophyte diversity and evolution.

The role of habitat disturbance provides another example of differences between epiphytic and terrestrial gametophyte species. Recent studies by Watkins et al. (2007a) have confirmed earlier studies (Peck, 1980; Peck et al., 1990) showing the general dependence of terrestrial gametophytes upon disturbance for the creation of new habitats whereas epiphytic species appear to be negatively impacted by disturbance. Epiphytic gametophytes maintain significantly lower densities in more highly disturbed sites than do terrestrial species (Watkins et al., 2007a). In contrast, terrestrial species rely on disturbance that produces a bare, litter-free habitat for gametophyte establishment and growth (Cousens et al., 1985), yet such sites are inherently unstable and likely to be short lived. Mortality rates of most terrestrial gametophytes are directly linked to microsite instability that frequently results in habitat collapse and wholesale loss of game-tophyte populations (Peck et al., 1990; Watkins et al., 2007a). Epiphytic gametophytes produced significantly lower densities in more highly disturbed sites compared to terrestrial species. These observations suggest that the gametophytes of epiphytic and terrestrial life forms have evolved radically different ecological strategies. It is important that these two functional groups be considered separately in attempts to understand species ecology.

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