INTRODUCTION

As for the flora of Silent Valley, only the angiosperms received much attention in the past (Manilal, 1988). Even among them, except orchids, no comprehensive study was taken up on the epiphytic flora of the region. Other groups like Pteridophytes, Mosses, Fungi, Lichens and Algae were collected and studied very sparingly. However, some studies were conducted on the pteridophytes of Silent Valley (Nayar and Geevarghese, 1993). Studies on the tropical forest ecosystem will remain incomplete until the specific roles played by the epiphytes are evaluated.

OBSERVATIONS

The authors have collected 124 species of vascular epiphytes from Silent Valley besides nonvascular ones belonging to mosses and lichens.

Of the 124 species of vascular epiphytes collected, 50 species are ferns and fern allies; 58 species are orchids and 16 species are other epiphytic angiosperms.

Ferns and fern allies

Epiphytic ferns are not abundant in Silent Valley though some species like Araiostegia pulchra, Asplenium crinicaule, A. formosum, Crypsinus montanus, Lepisorus nudus, Mecodium exsertum and Vittaria elongata are widespread. Some taxa like Antrophyum plantagineum, Elaphoglossum beddomei, Grammitis pilifera, Lepisorus nudus are common in the northern region of Silent Valley. The southern region is inhabited by species like Lepisorus nudus, Mecodium exsertum, Pyrrosia po rosa and Vittaria elongata. Epiphytic ferns enjoy better distribution in the central region of Silent Valley viz. Parathod, Poovanchola, Poochappara etc. A few like Antrophyum sp., Microsorium punctatum, Pyrrosia adnascens prefer low altitude and riparian trees. Others like Araiostegia pulchra, Asplenium normale, A. phyllitidis, Elaphoglossum beddomei, Loxogramme involuta, Oleandra musifolia etc. are restricted to deeply shaded dense forests. Species like Microgonium bimarginatum and species of Hymenophyllaceae prefer lower surfaces of tree trunks in the evergreen forests. Leptochilus bahupunctika and L. decurrens grow as semiepiphytes. Some of the epiphytic ferns like Davallia bullata, Leucostegia immersa and Pyrrosia spp. are restricted to the upper canopy of the forests. Some terrestrial ferns like Nephrolepis sp. are growing as epiphytes on tree trunks. This is due to the high humidity of the area. Some epiphytic ferns like Elaphoglossum beddomei, Microsorium punctatum, Phymatosorus lucidus and Selaginella involvens are capable of growing on shaded rock surfaces besides on tree trunks that are their natural habitat. Nearly all the epiphytic fern taxa possess adaptation to withstand drought. Thus a majority of them have abscissile fronds or pinnae which are shed during dry weather. The fronds of Asplenium phyllitidis, Loxogramme involuta, Microsorium punctatum and Pyrrosia porosa are thick and fleshy. They serve as water storage organs.

Araiostegia pulchra, Asplenium normale, A. phyllitidis, Elaphoglossum beddomei, E. nilgiricum, Grammitis attenuata, Huperzia squarrosa, Loxogramme involuta, Microsorium membranaceum, Leptochilus bahupunctika and Oleandra musifolia are rare and endangered epiphytic ferns found in the area. Among them, Asplenium phyllitidis, Elaphoglossum beddomei, E. nilgiricum, Huperzia squarrosa, Oleandra musifolia are facing serious threat of endangerment and Microsorium membranaceum threat of extinction.

Orchids

About 50 percent of the total epiphytes present in Silent Valley are orchids (Sathish Kumar and Manilal, 1992). The two species of Sirhookera are well distributed and they are seen almost throughout Silent Valley. Sispara and Parathod have the maximum number of epiphytes. The genus Dendrobium is common in this forest and D. herbaceum is the most common species. D. aqueum prefers edge trees for their attachment and D. heyneanum and D. panduratum prefer mostly densely shaded forests. Among the species of Bulbophyllum, B. tremulum is the most common species. Taxa like Coelogyne odoratissima, Eria tiagii and Seidenfadeniella rosea prefer evergreen sholas and are located only at Sispara Ghats. Dendrobium herbaceum, Trias stocksii and species of Aerides, Gastrochilus, Luisia, Sirhookera and Yanda prefer a lower altitude range of 700-900 metre. The leafless orchid Chiloschista lunifera occurs at 800 metre at dam site. Eria reticosa and E. dalzellii are localised in distribution. Eria exi/is grows on lower surfaces of tree trunks and is found in large colonies. Trichoglottis tenera prefers evergreen shola forests of 1000 metre and above.

Other Angiosperms

Among the other epiphytic angiosperms, Peperomia tetraphylla (Piperaceae) is the most common and is located in all the areas. Melasiomataceae has two epiphytic species viz. Medinilla beddomei and M. malabarica. Impatiens has two epiphytic species - I. jerdoniae and a new species I. sivarajanii collected from Sispara slopes. It was observed that a common terrestrial balsam, I. denisonii, showed a tendency to grow on tree trunks at higher elevations. Other epiphytic taxa from Silent Valley are - Aeschynanthus perrottetii (Gesneriaceae), Belosynapsis vivipara (Commelinaceae), Fagraea zeylanica (Loganiaceae), Hoya pauciflora, H. wightii (Asclepiadaceae), Polyscias acuminata (Araliaceae), Remusatia vivipara (Araceae), Utricularia striatula and Utricularia sp. (Lentibulariaceae).

Lower groups

Mosses are abundant in evergreen sholas. Barbella determessi, B. pendula, Fissidens schmidii, Hypnum cupressiforme, Papilaria crocea, Pleoropus nilghirense, Pterobryopsis orientalis, Rhegmatodon orthostegium. Trachypus humi/is and Sematophyllum subhumile have been collected from the shola. They are found growing mostly at an elevation of 1800 metre and above.

Silent Valley has a rich Lichen flora. Almost every tree at Sispara is covered by macrolichens. About 60 species of macrolichens and 10 species of microlichens have been identified. Microlichens are commonm at lower altitudes. Most of the macro lichens collected from Walakkad and Sispara Ghats are found to be either new records to Kerala or new records to South India. Taxa like Heterodermia pellucida and Parmelaria thomsonii are found to be new records to South India and Heterodermia comosa, H. dentritica, H. diademata, H. leucomela, H. leucomela subsp. boryi, H. isidiophora, H. speciosa, Leptogium chloromelum, L. isidiosellum" Parmelia cirrhata, P. nepalensis, P. vartakii, P. wallichiana, Ramalina inflata, Sticta weigelli, Usnea subflorida, U. rigidula are new records to Kerala. Microlichen species like Buellia confusa and Clathroporina duplicascens are common in the evergreen forests of Silent Valley.

DISCUSSION

Approximately 10 percent of all the vascular plant species are epiphytes (Benzing, 1989). The occurrence of epiphytes is influenced by many elements such as forest type, host relationship, humidity, altitude etc. The family Orchidaceae has a greater number of epiphytes than any other family of 'plants; 70 percent of them are epiphytic (Gentry and Dodson, 1986). In Silent Valley, Orchidaceae is well represented compared to other vascular' epiphytes. The humidity of the area plays an important role in the association of epiphytes. Riparian tree species host many orchids and ferns. Trias stocksii is associated with species of Bulbophyllum, Chiloschista lunifera, Vanda etc. Lichen taxa like Parmelia grow extensively on the barks of trees facing river sides. Walakkad and Sispara Ghats have very good representation of epiphytic species. The epiphytic diversity changes in composition on an altitudinal gradient. Their density increases with increase in altitude. Shola forests of Sispara Ghats (2000 m) represent a larger number of epiphytes. Density of epiphytes is also greatest at intermediate altitudes (+ 1000 m ). Areas like Cheriawalakkad and Poovanchola have a good humber of epiphytes.

In humid areas, most of the trees are covered with mosses and lichens. Epiphytes grow well on these moss clad trees. In Silent Valley, upper regions like Sispara, Walakkad and lower regions like Punnamala are very humid. Species like Papilaria crocea form a festooq like appearance on most of the trees. Trees like Bischofia javanica, Calophyllum polyanthum, Phyllanthus emblica and species of Glochidion and Syzygium provide suitable habitats for epiphytic orchids and ferns in Silent Valley. Among microphyllous ferns, taxa like Selaginella involvens and Huperzia sp. are true epiphytes. Many of the orchids prefer ecotone trees like Phyllanthus emblica and species of Glochidion as their hosts. Some of the epiphytic ferns and orchids are associat_d with ants in their root system.

Acknowledgement
The authors are thankful to the Chief Conservator of Forests (Wildlife), Kerala Forest Department for financial support and the Director, Kerala Forest Research Institute for facilities and encouragement