Biodiversity and spatial distribution of epiphytic ferns on Alsophila setosa Kaulf . ( Cyatheaceae ) caudices in Rio Grande do Sul , Brazil

The extractive exploitation of the tree fern Alsophila setosa Kaulf. alters forest formations and diminishes the availability of micro-habitat for epiphytes. A survey of epiphytic fern communities on A. setosa at 16 study sites in different forest formations in the State of Rio Grande do Sul, Brazil, documented the occurrence of 31 species representing 16 genera and six families. The greatest richness of species occurred in Polypodiaceae (39%) and in the genus Asplenium L. (22%). Habitual holoepiphyte was the predominant ecological category, representing 61% of the species. Similarity analysis demonstrated heterogeneity in the composition of the epiphytic ferns at the study sites and indicated that the vegetation type is not the main factor for floristic difference. The lowest total specific richness (5) was recorded for the seasonal deciduous forest site. The majority of the sites presented similar averages of phorophyte height and epiphyte richness per caudex. In 25% of the sites the height of the host plants presented significant correlation with specific richness. Considering that the majority of the epiphytes are habitual and that some of them occur exclusively or preferentially on tree ferns, the maintenance of these host plants in the vegetation is essential for the conservation of epiphytic species.


Introduction
Epiphytes constitute an important, yet little known, component of the biodiversity of tropical forests (Mucunguzi, 2007).Ferns compose a group of plants worthy of attention in the epiphytic environment, considering that about 2,600 species around the world are epiphytic (Kress, 1986).Among adaptive strategies that favour epiphytism in those plants, which can operate in several combinations, are: low light saturation point; poikilohydry; frond trichomes; wind dispersal of spores; high tolerance for instability of substrate nutrients (Müller et al., 1981;Benzing, 1987Benzing, , 1990;;Page, 2002); fine to filiform, long and ramified rhizome, besides reduced radicular system and small fronds (Dubuisson et al., 2003).
Tree ferns are host species that create a surface which acts as mechanical support to a diverse epiphytic flora, including bryophytes and vascular plants (lycophytes, ferns and angiosperms), some of them growing exclusively or preferentially on this substratum (Sehnem, 1977;Cortez, 2001;Windisch, 2002;Moran et al., 2003;Roberts et al., 2005;Schmitt and Windisch, 2005 ).However, tree ferns are subject to commercial exploitation, decreasing the availability of microhabitats for epiphytic species.
Among the tree ferns used for decoration and landscape gardening in the State of Rio Grande do Sul, Brazil, Alsophila setosa is currently being extracted from primary and secondary forests (Windisch, 2002).Populations are also being reduced by deforestation for lumbering and for the use of land for grazing or agriculture (Schmitt and Windisch, 2005).The plants grow up to ca. 10 m tall (Schmitt and Windisch, 2005) have a caudex with scars (from fallen fronds) on the basal portion and remains of stipe bases (with blackish spines) on the distal part.The growth of the plants seems to be limited by the forest canopy.In adult plants, the crown is up to 3 m long and has fronds with tripinnate-pinnatisect laminae (Schmitt and Windisch, 2006).
The study of plant communities is essentially comparative and the physiognomy of a vegetation stand is the result of its composition in terms of plant species or types (Focht and Pillar, 2003).The richness and composition of the epiphytic flora is influenced by the phorophyte (Heatwole, 1993;Medeiros et al., 1993;Benzing, 1995;Rudolph et al., 1998;Nieder et al., 1999;Roberts et al., 2005;Schmitt et al., 2005;Fraga et al., 2008), as well as by the vegetation and climate within the forest (Gentry and Dodson, 1987;Nieder et al., 1997).Alsophila setosa grows in pioneer formations, dense humid forests, mixed humid forests with Araucaria angustifolia, seasonal semideciduous forests and seasonal deciduous forests in South and Southeastern Brazil, between 20 and 1,800 m elevation.This study analyses the species richness and floristic composition of epiphytic ferns on A. setosa in different types of vegetation in the State of Rio Grande do Sul (Southern Brazil); the similarity of these epiphytic communities among different sites; the influence of caudex height on species richness and compares the species richness among distinct sites.

Study area
Floristic data of Alsophila setosa epiphytes were obtained from 16 sites (Figure 1) in the State of Rio Grande do Sul (Southern Brazil) as listed in Table 1.The climate is humid-temperate, with rainfall throughout the year (Moreno, 1961).Average yearly temperatures range from 14 to 20 °C, with the coolest months from 10 to 15 °C, with frost occurring frequently in the highest altitudes (Quadros and Pillar, 2002).

Locations and main environmental variables of study sites
The geographical coordinates and altitude were acquired by Global Positioning System (GPS).The annual precipitation data was obtained from the Climatic Information Bulletin, available on the database from the National Civil Defense Secretariat.The vegetational types followed the classification presented by the RADAMBRASIL Project (Teixeira et al., 1986), recognising the following types of forest: seasonal deciduous, seasonal semideciduous, mixed humid, dense humid, as well as pioneer formation (Table 1).

Floristic composition
At each site, ca.20,000 m 2 of forest area was surveyed to establish the species of epiphytic ferns on Alsophila setosa.The survey of the caudex surface was performed by direct observation of the plants or with the help of binoculars.The botanical classification followed the system proposed by Smith et al., (2006) and taxonomic identification was based on the specialised literature.Voucher specimens were deposited at the Herbarium Anchieta (PACA), at Universidade Vale do Rio dos Sinos, São Leopoldo -RS (Brazil).Presence-absence data of epiphytic species were subjected to Palaeontological Statistics -PAST Program in order to determine the floristic similarity among sites (by Ward's method for cluster analysis).In Ward's method, a Euclidean distance measure is the inherent algorithm and clusters are joined so that the within-group variance is minimised (Hammer et al., 2003).

Ecological categories
The epiphytes were classified according to the phorophyte, using the categories proposed by Benzing (1990): habitual holoepiphytes (mostly present in the epiphytic habitat), facultative (occurring in the epiphytic habitat as well as terrestrial plants), accidental (preferentially terrestrial) and secondary hemiepiphytes (germinates on/in soil but after contact with the phorophyte the basal part with radicular/ caulinar portions degenerates).

Phorophyte height and species richness
At each study site, 10 caudices were randomly selected among those with epiphytes (based on the previous records for floristic composition).The average values of species richness and caudex height were compared from distinct localities using the ANOVA test.In case of statistical differences of average values, the Tukey test was applied to distinguish group differences, at a 5% significance level.Linear regression analysis was used to evaluate the relationship between caudex height and species richness.Data analysis was performed as described by Vieira (1980) and Zar (1999) using the SPSS 16.0 programme installed at Universidade Feevale.

Results
Considering all sites, a total of 31 epiphytic fern species (Table 2) was recorded, representing 16 genera and six families.In species richness, Polypodiaceae (39%) was followed by Aspleniaceae (22%), Hymenophyllaceae and Dryopteridaceae (13%, each).Pteridaceae was represented by three species (10%) and Blechnaceae by a single species   Acronyms formed according to Table 1.
The highest total specific richness (16 species or 52%) was recorded for mixed humid forest in São Francisco de Paula (SfMHF) and seasonal semideciduous forest in Sapiranga (SaSSF).The lowest total specific richness (5 or 16%) was recorded for seasonal deciduous forest in Salvador do Sul (SsSDF).Except for seasonal semideciduous forest in Riozinho (RiSSF), Polypodiaceae presented highest species richness in all sites.
The genera Asplenium and Trichomanes were among those with highest richness in nine study sites.The species presenting generalised occurrence were Trichomanes angustatum Carmich.and Blechnum binervatum (Poir.)C.V.Morton and Lellinger, found in all sites.Campyloneurum nitidum C.Presl.occurred in all sites except in EsSSF, while Vittaria lineata (L.) Sm. and Pecluma pectinatiformis (Lindm.)M.G.Price were not found in four sites (Table 2).
The distribution of the total epiphytic species in different ecological categories (Table 2) showed the dominance of habitual (HAB) holoepiphytes (61%), followed by accidental (ACI) holoepiphytes (23%), facultative (FAC) holoephytes (13%) and secondary (HMS) hemiepiphytes (3%).The family presenting only habitual holoepiphytes was Hymenophyllaceae.Polypodiaceae presented the largest number of habitual holoepiphytes.The category of secondary hemiepiphytes is only represented by Blechnum binervatum, which usually germinates on the ground and grows up on the caudex as a hemiepiphyte.However it was also observed germinating directly on the caudex.
At all of the 16 sites, the habitual (HAB) holoepiphytes represented the ecological category with highest specific richness (often > 50% of the total).At six sites accidental (ACI) holoepiphytes were not found and at five of them this category presented the lowest specific richness.Except for Asplenium claussenii Hieron.and A. inaequilaterale Willd., all accidental holoepiphytes occurred in a single site (Table 2).
Floristic similarity analyses segregated two large groups.The first group (A) was formed only by two geographically    close sites, but with distinct vegetation.The second group (B) included the remaining sites and reflects the natural heterogeneity of the floristic composition in these habitats.Two subgroups can be recognised, one formed only by two close sites presenting different vegetation (Ba), and the other (Bb) where all seasonal deciduous forest sites (SDF) appear together (Cb) (Figure 2).
The average height of the phorophytes in 14 sites was similar.The lower average height was found for seasonal semideciduous forest in Sapiranga (SaSSF), and the highest for seasonal deciduous forest in Santa Maria (SmSDF).The majority of sites (13) presented similar average richness of epiphytes per caudex.In only 25% of the sites the height of the host plants presented significant relation with specific richness (Table 3).

Discussion
The 31 species on Alsophila setosa represent approximately 10% of the total of fern species cited by Falavigna (2002) for the State of Rio Grande do Sul.This result shows the importance of this single host species as a microhabitat for epiphytic ferns in the forest environment.
A previous work by Schmitt and Windisch (2005) on the epiphytic ferns in a tract of seasonal semideciduous forest in the State of Rio Grande do Sul reported 14 species on Alsophila setosa.In the present study, the sampled sites within this vegetation revealed 22 species showing low similarity among sites and indicating heterogeneity of the epiphytic fern flora.The total of 31 species indicates that the surrounding environment affects the biodiversity of the  Polypodiaceae, with a larger number of epiphytic species in the study sites (and 39% of the total) is also the family with the highest richness of epiphytes worldwide (Madison, 1977;Kress, 1986;Benzing, 1990), as well as in the neotropical region (Gentry and Dodson, 1987).The genera Asplenium and Trichomanes, which presented the largest number of species in this study, are considered the richest (more than 100 epiphytic species) on a worldwide basis by Madison (1977) and Kress (1986).
Morphological adaptations of epiphytic species from the Trichomanes genera such as a branched, fine to filiform rhizome, with reduced radicular system and frond size (Dubuisson et al., 2003) contributed to the occurrence of T. angustatum in all studied sites.Other adaptations such as poikilohydry (Benzing, 1987;1990), fleshy rhizome, nidular habit (Waechter, 1992), trichomes (Müller et al., 1981) and scales, probably account for the generalised occurrence of Campyloneurum nitidum and Pecluma pectinatiformis.The wide distribution of these three species and of Blechnum binervatum and Vittaria lineata suggest that they present high ecological plasticity.On the other hand, although the accidental holoepiphytes were considered the second largest group, they were found (except for two species) on a single site in our study, probably due the lack of proper vegetative/biological adaptations.
The distinction between accidental holoepiphytes and facultative ones is not clear cut, and their classification may vary according to the author or local predominant environmental conditions.Rumohra adiantiformis (G.Forst.)Ching, for example, here treated as a facultative epiphyte, was considered by Kersten and Silva (2002) as accidental holoepiphyte in a mixed humid forest in the State of Paraná (Brazil).The classification in ecological categories in the present study is based on the most frequent situation observed in the sampling sites.
The reduced total number of species and the lower average richness observed for seasonal deciduous forest in Salvador do Sul (SsSDF) agree with the observations of Sehnem (1977;1979) that in Southern Brazil the seasonal deciduous forests (SDF) present low fern richness.According to this author, the low specific richness can be due to the lack of connections between the seasonal deciduous forest and dense humid forest.
The floristic similarity analysis demonstrated the heterogeneity of epiphytic ferns in study sites, and indicated that the vegetation type is not the main contributor for this difference.The relevant factors in defining habitats within sites were not evident.Micro-climatic differences were likely to account for much of the different epiphytic assemblages among study areas, and disturbance history, animal interactions and dispersal limitations are also factors that can influence the distribution of species (Roberts et al., 2005).
The affinity between the epiphytes found in mixed humid forest and in the seasonal semideciduous forest (SfMHF and SaSSF) or in the seasonal deciduous forest (MaMHF and ApSDF), confirms the continuity of these forests cited by Rambo (1961), Sehnem (1977) and Marchiori (2004) and the occurrence of some fern species is common as presented by Sehnem (1977Sehnem ( , 1979)), in the State of Rio Grande do Sul.The TcPF site (pioneer formation in Três Cachoeiras) probably has climatic conditions comparable to the MsDHF site (dense humid forest in Morrinhos do Sul) which is located only some 6.5 km away, on the North coast of the State.This explains the presence of typical dense humid forest (DHF) epiphytic species in pioneer formations (PF) and the great similarity among these two sites.
The present results indicate that once Alsophila setosa is established in the vegetation, the type of forest exerts little influence on the average richness of epiphytes on the caudices.The average richness of similar epiphytic ferns on A. setosa may be related to the fact that space and colonisation time availability are similar, because these host plants also had, in a general way, the same mean average height among the studied sites.Based on the yearly frond production rates and the total frond vestiges on the caudex, Schmitt and Windisch (2006) estimated the age of 4 m tall Alsophila setosa sporophytes at 28-34 years, in secondary forest formations in the northeastern part of the State of Rio Grande do Sul.
The maximum number of epiphytic fern species found by Heatwole (1993) on Blechnum palmiforme (Thouars) C. Char (5) and by Schmitt et al. (2005) on Dicksonia sellowiana Hook.( 6) was lower than the maximum for Alsophila setosa (7).The averages of 3.1 species on B. palmiforme (Heatwole, 1993) and 2.46 species on D. sellowiana (Fraga et al., 2008) are within the range observed on A. setosa.Heatwole (1993) and Schmitt et al. (2005) discovered a correlation between epiphytic species richness and the height of tree ferns, such as occurred in four sites in the present study.Longer caudices (older plants) may present different conditions of microclimate and water supply along their extension, favouring spatial and temporal distribution of the epiphytes.Nieder et al. (1999) indicated that old, structurally diverse trees with many different microhabitats host many different epiphyte species.Yeaton and Galdstone (1982) considered that the period of time during which the substrate is available constitutes a major factor for the colonisation of the phorophytes.
Some of the epiphytic species in this study were observed exclusively or preferentially on tree ferns by previous authors: Asplenium mucronatum C.Presl, A. scandicinum Kaulf., A. harpeodes Kunze, Pecluma truncorum, Trichomanes anadromum, T. angustatum and T. polypodioides (Sehnem, 1977;Waechter, 1992;Cortez, 2001;Sylvestre, 2001).Extraction of adult plants and deforestation are threatening the existence not only of the tree fern species but also of microhabitats for epiphytic fern species, some of them exclusively (or mostly so) found on this kind of substrate.Therefore, additional research on the biology of the tree fern as well as of the epiphytic species is urgently needed as their interactions are still poorly understood.

Table 2 .
Epiphytic ferns on Alsophila setosa Kaulf.at 16 study sites in the State of Rio Grande do Sul, Brazil, and respective ecological categories.

Table 2 .
Continued... epiphytic ferns on A. setosa.Habitual holoepiphytes such as Trichomanes anadromum Rosenst., T. polypodioides L., Microgramma tecta (Kaulf.)Alston and Pecluma recurvata (Kaulf.)M.G.Price were found on A. setosa only in dense humid forest, mixed humid forest or pioneer formations.The assembly of several distinct vegetation types presenting heterogenic environmental conditions contributes to the establishment of species with distinct ecological requirements.Dendogram of the epiphytic fern communities of the 16 study sites in the State of Rio Grande do Sul, Brazil.Acronyms formed according to Table 1.

Table 3 .
Average phorophyte height and richness of epiphytic ferns on Alsophila setosa Kaulf.; linear regression equations and correlation coefficients (R 2 ) from the relation between phorophyte height and richness at 16 study sites in the State of Rio Grande do Sul, Brazil.