Habitat specificity

Habitat specificities were often reported without any quantitative measurements (Holttum, 1938; Page 1979a, 1979b; Cortez, 2001), with the exception of recent studies on epiphytes (Moran et al., 2003; Mehltreter et al., 2005) and terrestrial species (Poulsen et al., 2006). The role of mycorrhizae in ferns for mediating habitat specificity has been poorly studied. Despite the obligatory mycorrhizae of some fern groups (e.g., Lycopodium, Ophioglossum, Psilotum, Tmesipteris), only facultative mycorrhizae were reported for the sporophytes in a study of Hawaiian ferns (Gemma et al., 1992).

Terrestrial species

Poulsen et al. (2006) studied the floristic diversity of a plot of 1 ha of lowland Amazonian rain forest in Ecuador with an acidic mean soil pH of 3.33. The distribution and abundance of 29 fern species were correlated with soil calcium and sand content, and to a lesser degree with aluminum content. More studies of this type are needed on ferns at this geographical scale to allow for generalizations; the results of Poulsen et al. (2006) may not be applicable to sites with higher soil pH values, and where the calcium and aluminum contents could be less important to fern distribution. Edaphic niches of Polybotrya spp. in Northwestern Amazonia were best described by differences in soil texture, and cation content (Tuomisto, 2006).

Figure 8.2 (a, b) Vegetative reproduction and (c, d) leaf desiccation in some Mexican ferns. (a) Apical leaf buds of Asplenium alatum with young plant rooted in the ground. (b) Asplenium sessilifolium with completely developed plantlet at the leaf tip. (c) Desiccating leaf of Polypodium furfuraceum, bending pinnae inwards, exposing the lower scaly surface. (d) Desiccating leaf of Asplenium praemorsum folding pinnae accordion-like starting at the leaf tip.

Figure 8.2 (a, b) Vegetative reproduction and (c, d) leaf desiccation in some Mexican ferns. (a) Apical leaf buds of Asplenium alatum with young plant rooted in the ground. (b) Asplenium sessilifolium with completely developed plantlet at the leaf tip. (c) Desiccating leaf of Polypodium furfuraceum, bending pinnae inwards, exposing the lower scaly surface. (d) Desiccating leaf of Asplenium praemorsum folding pinnae accordion-like starting at the leaf tip.

Mangrove species

The three species of mangrove ferns in the genus Acrostichum are exceptionally tolerant to salt stress. They root in the soil and are often flooded, even though never completely submersed. The largest species, A. danaeifolium, is restricted to the Neotropics and is the least salt tolerant (Lloyd and Buckley, 1986; Mehltreter and Palacios-Rios, 2003). The smallest species, A. speciosum, from the Paleotropics seems to be the only obligately halophytic fern species (Kramer et al., 1995). The pantropically distributed A. aureum is ecologically intermediate

Table 8.3 Number of species with substrate specificity of four genera of Mexican rock ferns

Genus Limestone Gypsum Igneous rocks No preference

Argyrochosma 5 0 2 5

Cheilanthes 12 2 11 35

Notholaena 4 2 5 13

Pellaea 2 14 7

Data from Mickel and Smith (2004).

between the other two species. Other species that may grow in the coastal zones adjacent to the mangroves are Stenochlaena palustris in southeast Asia and Ctenitis maritima on La Réunion Island (Mehltreter, personal observation).


Lithophytes (petrophytes) are plants that grow primarily on rocks and boulders. Species that grow on rocks along rivers and stream banks often reproduce vegetatively by apical leaf buds, e.g., Asplenium (Figure 8.2a, 8.2b). Rock ferns are often good indicators for the underlying chemistry of the substrate. For temperate zones, there are dozens of examples of ferns that only grow on igneous rocks (e.g., granite), metamorphic rocks (e.g., serpentine), or sedimentary rocks (e.g., sandstone, gypsum, and limestone). In some genera such as Asplenium these substrate specificities change in newly formed hybrids compared to their parental taxa, e.g. A. adulterinum is restricted to serpentine rocks, although it is the allopolyploid hybrid between calciphilous A. viride and A. trichomanes which grows on silicate rocks (Kramer et al., 1995). Although in tropical humid zones limestone is scarcely found because it erodes quickly, some species are typically restricted to this habitat, especially those in the genera Argyrochosma and Cheilan-thes (Table 8.3). Mickel and Smith (2004) report rock substrate specificities for 50 out of 110 species of Argyrochosma, Cheilanthes, Notholaena, and Pellaea with more or less equal proportions occurring on limestone (23 species) or igneous rocks (22 species). Consequently, these species are restricted to mountain ranges with these substrates and may be less common and more frequently endangered than more widespread, substrate-generalist species. Over the last twenty years, some ferns have been reported to hyper-accumulate heavy metals, especially arsenic, cadmium, copper, and zinc (Table 8.4) and, thus, may be good indicators of soil type. Recently, some fern species, especially Athyrium yokoscense (Nishizono et al., 1987) and Pteris vittata (Ma et al., 2001), have been used as phytoremediators to manage heavy-metal contaminated soils.

Table 8.4 Heavy metal hyper-accumulating fern species


Life form

Accumulated heavy metals

Pityrogramma calomelanos5 Azolla caroliniana6 Azolla filiculoides2 Marattia spp.3 Pteris vittata4 Athyrium yokoscense1

terrestrial aquatic aquatic terrestrial terrestrial and lithophytic terrestrial

1Nishizono et al., 1987;

3Kramer et al., 1995;

5Francesconi et al., 2002;

6Bennicelli et al., 2004.


Ferns are the third most species-rich group of epiphytes after orchids and bromeliads in the New World. The three fern families with the greatest number of epiphytic species are Polypodiaceae, Hymenophyllaceae, and Aspleni-aceae, all with more than 50% of the family occurring as epiphytes. Vittarioid ferns (Pteridaceae) are entirely epiphytic (Gentry and Dodson, 1987).

In comparison to terrestrial ferns, epiphytes must grow in substrates that have a lower nutrient availability and water retention capacity (Benzing, 1990, 1995), and they must cope with large seasonal and daily changes in humidity, as well as an enormous variety of potential hosts (Kramer et al., 1995). Apparent adaptations to prevent water loss include such characteristics as simple or pinnate leaves, thick leaf texture (e.g., Niphidium), an often dense cover of scales (e.g., Polypodium, Elaphoglossum), water storing rhizomes (e.g., Davallia), and leaf succulence together with crassulacean acid metabolism (e.g., Pyrrosia). For humus accumulation and water storage, some ferns have large nest-forming rosettes (e.g., Asplenium) or specific niche-forming leaves (e.g., Drynaria, Platycerium) or leaf bases (e.g., Aglaomorpha). All of the latter group are restricted to the Paleotropics, with the exception of one species of Platycerium (P. andinum) and perhaps Niphidium spp. with large, but morphologically undifferentiated leaves. This nearly complete restriction of humus accumulating fern life forms to the Paleotropics has been interpreted as competitive exclusion by bromeliads, which are absent in the Paleotropics (Kramer et al., 1995). Some other fern species (Solanopteris spp. in the Neotropics, Lecanopteris spp. in the Paleotropics) have ants collecting nutrients for them. These ferns have thick hollow rhizomes that are inhabited by ants. The plants form roots within the interior of the rhizome to take up the nutrients that the ants bring in (Wagner, 1972; Gomez, 1974; Walker, 1986; Gay, 1991).

The concentration of gemmae-forming gametophytes in mainly epiphytic groups such as grammitids (Polypodiaceae), Hymenophyllaceae, and vittarioids (Pteridaceae) is often interpreted as an adaptation of epiphytism (Page, 1979b; Farrar, 1990; Dassler and Farrar, 2001).

Within the understory of wet tropical forests there is a fairly constant air humidity (i.e., from the base of the trunk to about 3 m above ground). Epiphytic ferns abound as lower trunk epiphytes, especially Hymenophyllaceae, and may be the dominant plant group (Zotz and Buche, 2000; Mehltreter et al., 2005). In the drier and exposed canopy, bromeliads and orchids are apparently better adapted to the extreme changes of temperature and humidity (Johansson, 1974; Kelly, 1985; Benzing, 1990).

The first fern epiphytes (e.g., Botryopteris forensis) are known from the Carboniferous, where they grew on marattiaceous tree ferns of the genus Psa-ronius (Rothwell, 1991). For contemporary epiphytes, three main host groups are available as substrate: tree ferns, gymnosperms (e.g., pines), and woody angiosperms. One species usually found growing on tree ferns is Polyphlebium capillaceum (syn. Trichomanes capillaceum) (Mickel and Beitel, 1988; Moran et al., 2003; Mehltreter et al., 2005; Ebihara et al., 2006; (see Table 8.5, Figure 8.3b). It develops especially on the trunk bases when the tree fern develops its adventitious roots forming a continuous root mantle up to half way to the top. This root mantle has a high water retention capacity and porosity because of its entangled roots, which makes it a favorable substrate for many epiphyte species (Heatwole, 1993; Medeiros et al., 1993; Mehltreter et al., 2005), including recently derived fern clades (e.g., Elaphoglossum, Polypodium) that diversified along with the angiosperms (Schneider et al., 2004). However, most modern epiphytes are not restricted to tree ferns and also grow on a wide range of angiosperm hosts. Nevertheless they may be very abundant on tree fern trunks (Table 8.5), which they can cover entirely, for example Asplenium harpeodes, Blechnum fragile (Figure 8.3c), Elaphoglossum lonchophyllum (Figure 8.3a), and Terpsichore asplenifolia (Figure 8.3a) in Mexican cloud forest (Mehltreter, personal observation). On the other hand, some epiphytic ferns do not occur on tree ferns, perhaps because of the acidic pH and high tannin content of the root mantle (Frei and Dodson, 1972), e.g., Elaphoglossum peltatum (Table 8.5, Figure 8.3d). Another example is Vittaria isoetifolia on La Réunion Island, which is only found on angiosperm trees. Its short rhizome may have problems attaching to the surface of the root mantle or even may be overgrown by the latter. Horizontally extended branches of angiosperm trees allow the large leaves of V. isoetifolia to hang down freely and avoid touching the

Table 8.5 Epiphytic ferns with significant host specificities for tree ferns or angiosperm trees in order of their frequency on tree ferns; data are percentages of presence on host trunks

Tree ferns Angiosperm trees

Elaphoglossum decursivum1 95 30

Blechnum fragile1 90 0

Trichomanes capillaceum2 89 0

Pecluma eurybasis1 65 20

Dryopteris patula1 50 0

Elaphoglossum stenoglossum1 50 10

Blechnum attenuatum3 41 4

Campyloneurum sphenodes1 40 0

Hymenophyllum elegans1 35 0

Asplenium auriculatum1 35 5

Asplenium serratum1 30 0

Elaphoglossum petiolatum2 29 4

Trichomanes reptans2 18 67

Asplenium nitens3 2 24

Vittaria isoetifolia3 0 16

Elaphoglossum peltatum2 0 19

1Moran et al., 2003;

2Mehltreter et al., 2005;

3Mehltreter, unpublished data from La Réunion Island.

vertical trunk surface where they would be exposed to competition with other epiphytes.

In conclusion, dozens of epiphytic ferns exhibit host specificity but few are restricted to one host group. Tree ferns are generally a better substrate for trunk epiphytes than angiosperm trees, but there are important exceptions, for which we only have speculative explanations. Specific needs of the gametophyte, the life form of the sporophyte, pH, content of tannins, and structural differences of the root mantle of tree ferns may be involved. Moreover, fern epiphytes specific to angiosperm trees may have been overlooked (Callaway et al., 2002), because these may be restricted to regions where no tree ferns occur. For example, palm trees are especially good substrates when their leaf bases stay attached to the trunk (Zotz and Vollrath, 2003). Their epiphyte communities have been studied mainly in cultivated species. In Costa Rican oil palm fields, Nephrolepis spp. and Phlebodium spp. commonly occur at palm leaf bases (Mehltreter, personal observation). Interestingly, tree fern skirts formed by old leaves that stay attached to the

Figure 8.3 Habitat specificities of Mexican trunk epiphytes on tree ferns (a-c) or on angiosperm trees (d). (a) Elaphoglossum lonchophyllum with entire leaves and Terpsichore asplenifolia with pinnate leaves on Alsophila firma. (b) Trichomanes capillaceum on Alsophila firma. (c) Blechnum fragile on the root mantle of Dicksonia sellowiana. (d) Elaphoglossum peltatum on the tree stem of Quercus spp.

Figure 8.3 Habitat specificities of Mexican trunk epiphytes on tree ferns (a-c) or on angiosperm trees (d). (a) Elaphoglossum lonchophyllum with entire leaves and Terpsichore asplenifolia with pinnate leaves on Alsophila firma. (b) Trichomanes capillaceum on Alsophila firma. (c) Blechnum fragile on the root mantle of Dicksonia sellowiana. (d) Elaphoglossum peltatum on the tree stem of Quercus spp.

trunk were found to inhibit the colonization by larger epiphytes and climbers (Page and Brownsey, 1986).

8.4 Synthesis of current perspectives

Unexpected phenological patterns suggest that we cannot easily draw general conclusions because of limited observations and little quantitative data. We need more detailed quantitative field data across wider geographical and taxonomic scales to understand the fascinating phenology and habitat requirements of ferns. For horticultural purposes, both issues should be of some concern, when species are known to be difficult to cultivate (for example, gram-mitids (Polypodiaceae), Hymenophyllaceae, and Gleicheniaceae). For conservation purposes, specific habitat requirements may restrict some fern species geographically, and may be responsible in part for their endangered status. Hyper-accumulators of heavy metals may be more common in ferns than known until recently and can play an important role in the future as bioindicators and for phytoremediation.

8.5 Future goals and directions

Within the field of ecological research of ferns and lycophytes, there are at least three areas on which future studies should be focused.

(1) Long-term research, especially in the tropics where ferns are most diverse and abundant.

(2) Broad-scale studies within a wide range of species at different latitudes, altitudes, and habitats.

(3) Multi-disciplinary approaches of ecology, physiology, biochemistry, morphology, systematics, and genetics, which combine molecular methods and field experiments.

Whatever approach is selected, quantitative approaches are preferable over observational and merely qualitative studies. Field studies should be comparative or experimental to answer specific questions, for example temperature and soil optima for new species of horticultural interest.

8.6 Importance of long-term studies

Most results of phenological studies depend heavily on climatic conditions at the study site during the years of observation. If these have been exceptional, extrapolations and general conclusions cannot be drawn without the risk of committing significant errors.

For this reason long-term studies are particularly important. Moreover, for conservation purposes it is critical to document complete life cycles of ferns to improve our understanding of demographic processes (see Chapter 9). Long-term growth measurements allow calculation of the ages of individual plants, and determination of quantified survival rates for better estimations of population turnover, which are fundamental values for conservation management.

Our actual understanding of fern phenology is restricted to a few species and even fewer locations. Only studies on a wider geographical scale will allow us to understand, for example, how phenological patterns change within and among species at different latitudes and altitudes. This knowledge is fundamental for addressing future challenges, such as understanding the possible consequences of global warming on ferns. Will tropical fern communities benefit from a warmer climate or will they decline because of a possibly longer dry season? Some species that now produce only one or two fertile leaves per year may discontinue doing so when climatic changes affect their development (Sharpe, 1997; Mehltreter and Palacios-Rios, 2003). Over the long term, this will significantly reduce the reproductive success and increase the chance of extinction of local populations, when these cannot recover from a spore bank.


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