The Silent Valley Reserve Forest is a single block that lies between 11 0 00' and 110 15' North latitude and 760 IS' and 76° 35' East longitude. The whole extent of 89.52 square kilometres was notified as Reserve Forest by the Government of Madras in 1914. This forest is bound on the east by the Attappady Reserve Forest, on the south the vested forests of Mannarkkad, on the west the reserved and vested forests of Nilambur forest division and on the north by the Nilgiri hills of Tamil Nadu. This forest is situated on a plateau of 1000 metre elevation. Due to steep faults on all sides, accessibility of this forest is restricted and this has contributed to the area remaining more or less pristine. The elevation ranges from 700 to 2383 metre. The highest peak is Anginda situated on the northeastern corner at an elevation of 2383 metre. The main river system is the Kunthipuzha and its tributaries. Till it emerges out of Silent Valley, the river and its tributaries flow entirely through the lush green forest and therefore at the point of emergence the water is unpolluted and crystal clear. There are no tribal or non tribal settlements inside the reserved forests of Silent Valley.

The rock formation in this area is of the Archaean age comprising of gneissuss and granites. Soil is formed as a result of the weathering of the ancient crystalline and metamorphic rocks. The soil is generally kiolinitic in nature, acidic in reaction, highly porous and friable.

The climate is tropical with summer rains constituting the bulk of the precipitation. There is slight variation in the rainfall depending upon the elevation. The elevated hills on the western side of the Silent Valley Reserve receive an average of 5045 millimetre of rain as recorded at Arthala (KSEB, 1978) at 1200 metre elevation followed by Neelikkal receiving 4364 millimetre of rain at 1005 metre elevation. At higher elevations near Walakkad and above the rainfall even reaches up to 6500 millimetre.

Out of the total annual precipitation of 4364 millimetre at Neelikkal in Silent Valley, the southwest monsoons which last from June to September account for 79.5 percent, the northeast monsoons (retreating monsoons) from October to December 12.1 percent, the premonsoon thunder showers from April to May 7.6 percent and the dry season from January to March receives 0.8 percent.

The average temperature is 20.2° C in Silent Valley, the maximum being 23.6°C and minimum 17.800C felt in January. In the higher reaches the temperature goes down in the cold month even reaching 0° C. The relative humidity remains more or less above 80 percent from June to February. The minimum is recorded during March-May.
The forests exhibit considerable variation in the floristic composition, physiognomy, life forms etc. primarily owing to climatic, edaphic and altitudinal variations.

The following are the forest types found in the Silent Valley Reserve Forest.

A brief description of each type of forest is given below.

WEST COAST TROPICAL EVERGREEN FOREST

Westcoast tropical evergreen forest is the climax vegetation and is characterised by its luxuriance consisting of at least three tiers, the highest tier attaining a height of 40-45 metre. Very often trees are buttressed at their base and boles are clear, cylindrical, unbranched at least upto two-third of their height and are generally with umbrella shaped crowns at the top. The middle stratum is more or less candle shaped and the lower characteristically conical. Trees with heavy infestation of orchids, aroids, mosses, ferns and lichens are common. Species with cauliflory are found. The multiplicity of species coexisting in a complex biological equilibrium gives this type of forest the highest status viz., 'climatic climax.'

This type is encountered between 600 and 1100 metre elevation. Under favourable conditions like shelter and moisture accumulation it extends upto an elevation of 1200 metre.

Floristics

Physiognomically three storeys can be recognised in this type of forests. The dominant upper storey which is usually of about 40-45 metre in height consists chiefly of Artocarpus heterophyllus, Bischofia javanica, Calophyllum elatum, Canarium strictum, Cullenia exarillata, Drypetes elata, Dysoxylum malabaricum, Elaeocarpus tuberculatus, Holigarna arnottiana, H. grahamii, Mesua ferrea, Palaquium ellipticum, Persea macrantha, Poeciloneuron indicum, Polyalthia coffeoides and so on.

The second storey which is about 15 to 30 metre high is comprised of Actinodaphne hookeri, Aglaia elaeagnoidea, Baccaurea courtallensis, Cinnamomum malabatrum, Dimocarpus longan, Elaeocarpus serratus, Garcinia morella, Gomphandra polymorpha, Litsea wightiana, Meliosma pinnata, Myristica dactyloides, Oreocnide integrifolia and others.

The third storey which is less than 15 metre in height consists of small trees like Agrostistachys meeboldii, Euonymus angulatus, Jambosa munronii, J. laeta, Memecylon sisparense, Turpinia malabarica, Xanthophyllum flavescens and shrubs like Dendrocnide sinuata, Lasianthus sp., Moghania sp., Psychotria sp., Sarcococca brevifolia, Solanum surattense, Strobilanthes sp. and Thottea siliquosa.

Monocots are few and their presence is localised. Important among them are Arenga wightii, Calamus gamblei, C. thwaitesii, Ochlandra travancorica, Oxytenanthera sp., Pandanus furcatus, Pinanga dicksonii and so on.

The ground flora is. sparse and is mainly represented by Amomum sp., Arisaema leschenaultii, Centella asiatica, Elettaria cardamomum, Neurocalyx calycinus, Ophlorrhiza brunonis, Sarcandra chloranthoides and species of Begonia and Elatostemma.

Woody lianas are represented by Caesalpinia bonduc, Entada pursaetha and Gnetum ula while straggling ones with or without thorns are represented by Derris sp., Dioscorea sp., Luvunga sarmentosa, Paramignya beddomei, Senecio sp., Smilax zeylanica, Thunbergia mysorens is, Tragia involucrata etc.

Epiphytes are represented by Fagraea zeylanica, Roya pauciflora, various species of orchids, aroids and pteridophytes. Common parasites are Elytranthe sp., Loranthus sp. and Vis cum capitellatum. Occasionally root parasites like Aeginetia pedunculata and Balanophora fungosa are seen.

Occurrence of vegetational subtypes

The evergreen forests occupy different ecological niches depending on local factors like elevation, slope and soil conditions and there is characteristic preponderance of one or more tree species at specific locations. While preparing the working plan Aiyar (1935) identified six types of species associations in the Silent Valley forests.
i. Cullenia - Palaquium association
ii. Palaquium - Mesua association
iii. Poeciloneuron - Palaquium association iv. Mesua – Calophyllum
association
v. Ochlandra - Calophyllum association vi. Ochlandra - Poeciloneuron association

Aiyar was of the opinion that single species dominance can never be encountered in wet evergreen forests. He supported his arguments by citing the example of the Amazon Valley. Although Muhammed (1959) agreed with this while revising the working plan, Chand Basha (1977) was of a different opinion. Based on enumeration data, he stressed that the so called associations were nothing but preponderance of a few tree species in specific ecological niches. This may be due to edaphic, altitudinal or other such conditions. This view was further supported by Pascal (1984) who considered the dominance of a few species in the area as 'facies of vegetation'.

Presence of Cullenia, Mesua and Palaquium as the most commonly occurring tree species in the evergreen forests having minor variations in their relative composition can be ascribed to the status of vegetational subtypes. Accordingly, based on five percentage enumeration; the following subtypes have been recognised. The subtypes are identified taking into consideration the relative higher incidence of one or two species.

i. Palaquium - Cullenia subtype

Palaquium - Cullenia subtype is the commonest one found to occur at lower elevations. This type is found in areas with deep and well drained loamy soil at moderate elevations from 750 to 1100 metre.

In Silent Valley, this subtype is found to occupy the southern portion below Chembotti stream (east of Thondakkulam) on the west and Valiaparathod on the east of Kunthipuzha except the higher reaches of Kummathanthod and Valiaparathod where these species in the community are replaced by Mesua and Calophyllum.

ii. Palaquium - Mesua subtype

The dominant species in this subtype are Palaquium ellipticum and Mesua ferrea. In Silent Valley this subtype is found in the Thondakkulam plateau on the west where the incidence of Palaquium is more than that of Mesua. In areas north of Valiaparathod and in compartment six lying on the west of Kunthipuzha, the incidence of Mesua is more than that of Palaquium. In the upper reaches of Madiramaramthod, the incidence of Mesua is found to dwindle and that of Calophyllum to increase. As the elevation increases further Mesua disappears. North of Periamallathod, a higher incidence of Palaquium is seen though this species loses stature at higher elevations.

iii. Mesua - Calophyllum subtype

This subtype is found to occur between elevations from 1150 and 1525 metre. Beyond 1375 metre Mesua tends to disappear though Calophyllum struggles in a more stunted form upto 1525 metre where the wet evergreen forest merges with the subtropical evergreen forests. The under storey is predominantly a thick growth of Strobilanthes belonging to several species growing to even six to seven metre in height and preventing natural regeneration of dominant species.

In Silent Valley, this subtype is encountered on the slopes of Valiamullumala and on the eastern side on the slopes of hills forming the catchment of Kummathanthod, Valiaparathod and Erramalathod.

iv. Calophyllum - Syzygium subtype

This subtype is met with in the elevations above 1400 metre and goes up to 1525 metre. It occupies the upper reaches of Madiramaramthod and Karingathod in the eastern side of Silent Valley. The chief species are Calophyllum elatum and Syzygium sp. Mesuaferrea is almost absent. The quality of Calophyllum and Palaquium is very poor due to stuntedness. Trees of higher girth classes (above 240 cm ) are almost absent. Regeneration is very poor. The incidence per hectare is also poor.

v. Ochlandra - Poeciloneuron subtype

In this subtype, the chief species is Poeciloneuron indicum which shows exceptional tolerance to the marshy conditions brought about by heavy rainfall, impeded drainage due to physiographic conditions and clayey soil. The Poeciloneuron trees stand as standards over thick growth of Ochlandra which form thickets spreading continuously along the sides of Kunthipuzha in the northern half below Walakkad at elevation ranging between 950 and 1250 metre. This species thrives in water logged conditions due to the presence of aerial roots. Poeciloneuron exhibits remarkable capacity to regenerate under these conditions leading to local gregariousness. The other species found in very low proportions are Calophyllum elatum, Hopea glabra, Syzygium sp., Bischofia javanica with Agrostistachys meeboldii in the under storey among the reeds.

vi. Ochlandra - Calophyllum subtype

This is purely an edaphic subtype found occurring only in marshy
localities where small streams stagnate most of the year. The ground is covered by a series of pits and mounds and Calophyllum is seen to occupy mostly the mounds which offer a reasonable degree of drainage for the regeneration and survival of this species. Calophyllum with associates like Bischofia javanica, Hopea glabra and Syzygium sp. mostly stand as standards over thick growth of Ochlandra which grows in thickets causing prevention of free flow of water. This subtype is encountered only in the western portion of Silent Valley restricted to very small areas south of Kathisundamala and northern portions of Thondakkulam plateau. The undergrowth chiefly comprises of Agrostistachys indica and A. meeboldii.

It is seen that at least three subtypes viz., Palaquium - Cullenia, Ochlandra - Poeciloneuron and Ochlandra - Calophyllum are governed by edaphic factors with drainage playing a major role. The Mesua Calophyllum subtype is controlled by elevation while the areas with less rainfall show preponderance of Mesua.

The tropical wet evergreen forests are well known for their floristic richness and diversity. According to Foggie (1960), the humid tropical forest with very few exceptions is a most complex mixture with hundreds of trees growing intimately together, of which the commercially important ones constitute only a fraction. In terms of number of individuals Silent Valley is quite rich as it contains 383 individuals of over 10 centimetre girth at breast height in an area of a quarter hectare. Species diversity wise also the wet evergreen forests of Silent Valley are rich.

Single species dominance has not been encountered and the 33 tree species are spread over in 17 families with Lauraceae standing first followed by Euphorbiaceae. Families with more than one species are Clusiaceae, Urticaceae, Myristicaceae, Sapotaceae, Meliaceae and Myrtaceae. Studies conducted by Pascal (1984) in the adjacent Attappady evergreen forests in an area of 2000 square kilometres showed 25 species belonging to 16 families dominating with Lauraceae, Euphorbiaceae and Clusiaceae:

Percentage distribution of individuals occupying different tiers in the study plot showed 38.4 percent belonging to the first storey, 28.9 percent to the second storey and 32.7 percent to the third storey. Vegetation structure shows that the girth classes in the range of 181-210 centimetre are comparatively less represented.

SOUTHERN SUBTROPICAL BROAD LEAVED HILL FOREST

This type of forest, although floristically rich, is commercially very poor. It is inferior to the wet evergreen forests of lower elevations. Although species like Bischofia javanica, Calophyllum elatum are found here, they are nanic and crooked with spreading crowns. The height of the trees seldom exceeds 20 metre and trunks and branches are heavily festooned with mosses, lichens, aroids and ferns. At an elevation from 1200 to 1300 metre a transition between this type and the wet evergreens may be found. These forests are encountered from 1300 to 1800 metre elevation.

Floristics

Principal species are Calophyllum elatum, Cinnamomum sulphuratum, Elaeocarpus munronii, Dimocarpus longan, Garcinia sp., Gordonia obtusa, Memecylon sp., Mesua ferrea, Syzygium sp. and members of Lauraceae. A good number of them show affinity towards temperate flora.

SOUTHERN MONTANE WET TEMPERATE FOREST
(Temperate Hill Forest)

This type of forest is always found in cliffs and sheltered folds above 1900 metre. Like the former type, wind and altitude make these forests very stunted and they are interspersed with rolling grassy downs. Trees seldom attain a height of over 10 metre. In all the upper reaches of Silent Valley above 1900 metre this type is encountered.

Floristics

The principal arborescent species in this forest are Elaeocarpus munronii, Gordonia obtusa, Meliosma pinnata, Schefflera racemosa, Symplocos sp. with profuse Lauraceous and Myrtaceous members. Interspersed with these forests are open grassy patches.

GRASSLAND

Two types of grasslands are encountered in the Silent Valley.

i. Low level grassland below 1500 metre.

They are distributed within the wet evergreen forests of Silent Valley. The main floristics are Allophylus serratus, Breynia vitis-idaea, Canthium dicoccum, Careya arborea, Dalbergia latifolia, Emblica officina lis, Gordonia obtusa, Maesa perrottetiana, Phoenix humilis, Rubus ellipticus. Wendlandia thyrsoidea, Ziziphus rugosa and herbs like Aeginetia, Commelina, Curculigo, Leucas, Merremia, Oldenlandia, Oxalis, Polygala and Tridax. Grasses like Cymbopogon, Heteropogon and Themeda are common. Most of the tree species are pyroresistant.

ii. High level grassland

These are found in the upper reaches of Silent Valley above 1500 metre. Grasses are stunted and carpet like. Though rainfall is high, heavy run off is observed due to steep slopes and grasses are exposed to strong wind. These gras.slands are often interspersed with Montane Temperate Forests and Subtropical Hill Forests.

Important species that inhabit this area are Gaultheria fragrantissima,Rhododendron arboreum ssp. nilagiricum and grasses like Arundinella fuscata, Bothriochloa pertusa, Heteropogon contortus and others.

The forests of Silent Valley are to be considered an 'ecologist's paradise' as different successional stages leading to the formation of climax type viz., the wet evergreen forests in the tropics can be witnessed clearly. Different ecological niches are the store houses of incredible faunistic and floristic diversity.