The Western Ghats, one of the Mega Diversity Centre in India is also a biodiversity hotspot. The forests of Kerala are along the southern Western Ghats, which is considered as the most species rich region and endemism in the Western Ghats. The most outstanding feature of the southern Western Ghats is the formation of tropical rain forests along the wind ward side, remarkable for the rich biodiversity. The wet lands and coastal ecosystems are confined to the lowland region of the State.

Champion and Seth (1968) recognized 26 forest types in Kerala of which the major ones are the west coast tropical evergreen, west coast semi-evergreen, southern most mixed deciduous, southern dry mixed deciduous, southern montane wet temperate forests, southern subtropical hill forests, southern montane wet temperate grasslands and littoral forests (mangroves). Certain edaphic types recognized are Bamboo brakes, Cane brakes, Reed brakes, Euphorbiaceous scrub jungles, Laterite thorn forests and Myristica swamp forests. Based on dynamics they recognized secondary forests such as secondary evergreen, secondary moist deciduous, secondary dry deciduous, etc. The important features of the vegetation types are briefly mentioned below.

West-coast tropical evergreen forests

Evergreen forests are characterized by the profusion of species, particularly trees and woody climbers and the canopy is closed. The secondary species and exotic weeds are relatively absent. The lower shrubby layer is composed of mostly seedling and saplings of tree species. The Evergreen forests are found between 400 - 1,200 m. It has been observed that there is difference in the physiognomy and composition of species with altitude. The Evergreen forests below 700 m are occurring in patches and mostly confined to the sides of water courses where the soil is rather deep. Though there is no marked dominance of any group of trees in these forests below 700 m, there is dominance of certain species in the upper stratum. The important associations noted are: (i) Vateria - Calophyllum - Cullenia, (ii) Mesua - Cullenia - Palaquium and (iii) Mesua - Palaquium. In the forests above 1,100 m there is dominance of Lauraceae and Heritiera papilio. Epiphytes and mosses tend to increase with altitude while woody climbers decrease.

a) Species composition below 700 m.

The upper stratum trees are Antiaris toxicaria, Artocarpus heterophyllus, Bischofia javanica, Canarium strictum, Chrysophyllum roxburghii, Cynometra travancorica, Diospyros buxifolia, Dipterocarpus indicus, Drypetes elata, Dysoxylum bedomei, Elaeocarpus tuberculatus, Hopea parviflora, Kingiodendron pinnatum, Mangifera indica, Mastixia arborea subsp. meziana, Ormosia travancorica, Persea macrantha, Syzygium gardneri, Terminalia travancorensis, etc.

The middle stratum trees are Aglaia lawii, A. malabarica, A. perviridis, Agrostistachys borneensis, Alseodaphne semecarpifolia var. parvifolia, Aphanamixis polystachya, Beilschmiedia bourdillonii, Cinnamomum malabathrum, Diospyros spp., Elaeocarpus glandulosus, E. serratus, Garcinia gummi-gutta, G. morella, Gymnacranthera farquhariana, Holigarna arnottiana, Hydnocarpus alpina, Knema attenuata, Lepisanthes tetraphylla, Madhuca neriifolia, Myristica beddomei, M. malabarica, Otonephelium stipulaceum, Reinwardtiodendron anamalaiense, Sageraea dalzellii, Strombosia ceylanica, Syzygium laetum, Vepris bilocularis, Walsura trifolia, etc.

The lower stratum trees are Agrostistachys indica, Atalantia racemosa, Baccaurea courtallensis, Canthium dicoccum, Casearia ovata, Isonandra lanceolata, Leptonychia caudata, Meiogyne ramarowii, Nothopegia racemosa, Orophea erythrocarpa, O. uniflora, Turraea villosa and pole crops of upper and middle stratum trees.

The woody climbers are Acacia concinna, Aganope thyrsiflora, Artabotrys zeylanicus, Bauhinia phoenicea, Combretum latifolium, Derris brevipes, Desmos lawii, Entada rheedei, Mezoneuron cucullatum, Sarcostigma kleinii, Spatholobus purpureus, Strychnos lenticellata, Tetracera akara, Ventilago bombaiensis, etc.

The main undergrowths are Alpinia malaccensis, Barleria courtallica, Blachia denudata, Ixora nigricans, Munronia pinnata, Psychotria spp., Strobilanthes spp., etc.

b) Species composition above 700 m.

The upper stratum trees are Artocarpus heterophyllus, Calophyllum polyanthum, Chrysophyllum roxburghii, Cullenia exarillata, Dysoxylum malabaricum, Elaeocarpus tuberculatus, Holigarna ferruginea, Mesua ferrea, Paracroton pendulus subsp. Zeylanicus, Palaquium ellipticum, Persea macrantha, Poeciloneuron indicum, Prunus zeylanica, Toona ciliata, Vateria indica, etc.

The middle stratum trees include Actinodaphne malabarica, Agrostistachys borneensis, Beilschmiedia wightii, Bhesa indica, Dimorphocalyx lawianus, Diospyros paniculata, D. sylvatica, Drypetes malabarica, Melicope lunu-ankenda, Glochidion ellipticum, Gordonia obtusa, Hydnocarpus macrocarpus, Litsea bourdillonii, L. floribunda, L. oleoides, Mallotus tetracoccus, Meliosma pinnata, M. simplicifolia, Trichilia connaroides, Turpinia malabarica, etc.

Aglaia tomentosa, Aporusa acuminata, Ardisia pauciflora, Casearia ovata, Dendrocnide sinuata, Gomphandra coriacea, Lepisanthes erecta, Ligustrum robustum, subsp. walkeri, Meiogyne pannosa, Oreocnide integrifolia, Orophea uniflora, Psychotria anamallayana, Sarcococca coriacea, Symplocos cochinchinensis, subsp. laurina, Symplocos macrophylla, spp. rosea, Syzygium munroi, etc. are common in the lower stratum.

The common climbers are Aganosma cymosa, Calamus gamblei, C. pseudotenuis, C. vattayila, Carissa spinarum, Embelia ribes, Kunstleria keralense, Millettia rubiginosa, Pseudaidia speciosa, Rourea minor, Sabia malabarica, Strychnos lenticellata, Tetrastigma sulcatum, Toddalia asiatica etc.

The undergrowth is mainly composed of Amomum cannicarpum, A. muricatum, Aralia malabarica, Elettaria cardamomum, Lasianthus rostratus, Lepianthes umbellata, Munronia pinnata, Mycetia acuminata, Pandanus thwaitesii, Pavetta calophylla, Psychotria flavida, P. nigra, P. nudiflora, Rhynchotechum permolle, Tarenna monosperma, etc.

In the forests above 1,200 m the trees are predominantly of Lauraceae. The other trees are Calophyllum austroindicum, Heritiera papilio, Mesua ferrea, etc.

West-coast semi-evergreen forests

This forest type is found below 700 m. It is intermediate between Evergreen and Moist deciduous types where several species which are common in Evergreen forests and some of the trees in the moist deciduous forests are also seen. In this type also a three layer stratification of the trees are met with.

The upper stratum is composed mainly of Aglaia barberi, Antiaris toxicaria, Artocarpus hirsutus, A. gomezianus subsp. zeylanicus, Bischofia javanica, Bombax ceiba, Carallia brachiata, Chukrasia tabularis, Dimocarpus longan, Diospyros buxifolia, D. crumenata, Drypetes confertiflora, Dysoxylum beddomei, Hopea parviflora, Mimusops elengi, Polyalthia fragrans, Prunus ceylanica, Pterospermum reticulatum, P. rubiginosum, Sageraea dalzellii, Otonephelium stipulaceum, Terminalia bellirica, Tetrameles nudiflora, Toona ciliata, Vitex altissima, etc.

The common trees in the middle stratum are Aglaia barberi, Aglaia lawii, Aporusa cardiosperma, Diospyros bourdillonii, D. assimilis, D. montana, D. paniculata, Flacourtia montana, Ficus callosa, F. nervosa, Harpullia arborea, Holigarna grahamii, Holoptelea integrifolia, Hydnocarpus alpina, H. pentandra, Margaritaria indica, Nothopegia colebrookeana, Olea dioica, Oroxylum indicum, Pajanelia longifolia, Schleichera oleosa, etc.

The lower stratum trees are Acronychia pedunculata, Agrostistachys indica, Antidesma montanum, Callicarpa tomentosa, Drypetes oblongifolia, Hunteria zeylanica, Ixora brachiata, Mallotus philippensis, Meiogyne ramarowii, Memecylon spp., Sapindus trifoliatus, Xanthophyllum arnottianum, etc.

The woody climbers include Acacia concinna, A. pennata, Ancistrocladus heyneanus, Anodendron paniculatum, Bauhinia scandens var. anguina, Calamus hookerianus, C. thwaitesii, Derris brevipes, Entada rheedei, Mezoneuron cucullatum, Chilocarpus denudatus, Erycibe paniculata, Olax imbricata, Salacia fruticosa, Sarcostigma kleinii, Strychnos colubrina, S. minor, etc.

The lower shrubby layer is composed of Barleria courtallica, Dichapetalum gelonioides, Dracaena terniflora, Gomphandra tetrandra, Glycosmis spp., Justicia santapaui, Leea indica, Murraya paniculata, Strobilanthes spp., etc.

Southern Moist mixed deciduous forests

This forest type is seen below 600 m. During wet season, because of the thick foliage, the canopy looks similar to that of semi-evergreen forests and is therefore scarcely distinguishable. However, during dry season the moist deciduous forests reveal their true identity as the trees shed leaves. The leafless period varies from a few weeks up to 5 months depending on the species. Among the trees, Bombax insigne, Hymenodictyon obovatum and Lagerstroemia microcarpa have a leafless period up to five months. Terminalia paniculata and Dalbergia sissoides have a leafless period of less than 2 weeks. In Moist deciduous forests also a three tier stratifications for trees.

The trees in the upper stratum are Albizia lebbeck, A. odoratissima, A. procera, Alstonia scholaris, Bombax ceiba, B. insigne, Dalbergia sissoides, Dillenia pentagyna, Gmelina arborea, Grewia tiliifolia, Haldina cordifolia, Hymenodictyon orixense, Lagerstroemia microcarpa, Lannea coromandelica, Melia dubia, Pterocarpus marsupium, Radermachera xylocarpa, Stereospermum colais, Tectona grandis, Terminalia bellirica, T. elliptica, T. paniculata, Tetrameles nudiflora and Xylia xylocarpa.

The middle stratum is composed mainly of, Bauhinia malabarica, B. racemosa, Bridelia retusa, Cassia fistula, Careya arborea, Cleistanthus collinus, Dalbergia lanceolaria, Ficus callosa, F. exasperata, F. racemosa, Garuga pinnata, Hymenodictyon obovatum, Macaranga peltata, Miliusa tomentosa, Olea dioica, Phyllanthus emblica, Sapindus trifoliatus, Schleichera oleosa, Spondias pinnata, Sterculia guttata, S. villosa, Streblus asper, Strychnos nux-vomica, Trema orientalis, Zanthoxylum rhetsa, etc.

At the lower stratum, Casearia tomentosa, Cipadessa baccifera, Cochlospermum religiosum, Grewia glabra, G. nervosa, Helicteres isora, Holarrhena pubescens, Naringi crenulata, Securinega leucopyrus, Tabernaemontana alternifolia, Tamilnadia uliginosa, Wrightia tinctoria, etc. are common.

The climbers include Acacia pennata, A. torta, Bauhinia scandens var. anguina, Bridelia scandens, Calycopteris floribunda, Cissus heyneana, C. latifolia, Dalbergia volubilis, Olax imbricata, Spatholobus roxburghii, Ziziphus rugosa, etc.

The undergrowth is composed of Abutilon persicum, Baliospermum montanum, Barleria prattensis, Chromolaena odorata, Canthium angustifolium, Dendrolobium triangulare, Flemingia strobilifera, Lantana camara var. aculeata, Phyllodium pulchellum, Polhillides velutinum, Pseudarthria viscida, Rotheca serrata, etc.

Southern montane wet temperate forests

This type is characterized by the short-boled and highly branched trees. The branches are densely clothed with moss and epiphytes. There is no stratification for trees. Leaves of trees in general are small. Climbers are few.

The characteristic species are Actinodaphne bourdillonii, Cinnamomum sulphuratum, Daphniphyllum neilgherrense, Elaeocarpus munroii, Elaeocarpus recurvatus, Euonymus indica, Eurya japonica, Fagraea ceylanica, Gordonia obtusa, Ilex gardneriana, Ligustrum robustum subsp. walkeri, Maesa indica, Mahonia leschenaultii, Michelia champaca, Microptropis ramiflora, Pittosporum neelgherrense, Neolitsea scrobiculata, Rapanea thwaitesii, Rhododendron arboreum, Symplocos cochinchinensis subsp. laurina, Ternstroemia gymnanthera, Turpinia nepalensis, etc.

The common epiphytes are Aerides ringens, Bulbophyllum fischeri, Coelogyne nervosa, Dendrobium nanum, Diplocentrum congestum, Eria reticosa, Impatiens parasitica, Liparis elliptica, Medinilla malabarica, Oberonia santapaui, O. verticillata, Peperomia tetraphylla, Trichoglottis tenera, etc.

Southern montane wet grasslands

This type is confined to the hill-tops, and the vegetation is dominated by grasses.

The common grasses are Apocopis courtallica, Arundinella leptochloa, A. mesophylla, Chrysopogon asper, C. hackelii, Cymbopogon flexuosus, Eulalia trispicata, Dimeria ornithopoda, Ischaemum indicum, Jansenella griffithiana, Sacciolepis indica, Themeda triandra, Zenkeria elegans, etc.

The grasslands also support several herbaceous and a few subshrubby species. The subshrubby species are Exacum anamallayanaum, Hedyotis articularis, Hypericum mysorense, Impatiens hensloviana, Lobelia nicotianaefolia, Osbeckia aspera, O. leschenaultiana, etc.

The herbaceous species are Aeginetia pedunculata, Anaphalis lawii, Andrographis neesiana, Arisaema tortuosum, Cajanus heynei, C. lineata, Cyanotis spp., Dichrocephala integrifolia, Drosera peltata, Dumasia villosa, Emilia scabra, Eriocaulon spp., Crotalaria fysonii, Habenaria spp., Hedyotis swertioides, Heracleum candolleanum, Gynura nitida, Lobelia heyneana, Juncus prismatocarpus, Juncus inflexus, Pedicularis perrottetii, Leucas hirta, Malaxis acuminata, Neanotis decipiens, N. indica, N. tubulosa, Peucedanum anamallayense, Pecteilis gigantea, Phyllocephalum courtallense, Plectranthus nilgherricus, Pogostemon rotundatus, Teucrium tomentosum, etc.

Southern tropical thorn forest (scrub jungle)

This forest type is restricted to the rain shadow regions in Idukki and Palakkad districts. The species are mostly xerophytic with short bole and low branching. The canopy is wide open. Therefore the canopy level differentiation is not very distinct. The hardwood trees, thorny shrubs and climbers are characteristic features of the forest type. The undergrowth is furnished with some herbaceous forms during monsoons and remains exposed for the rest of the time.

The major species representing the forest type are Acacia spp., Euphorbia spp., Capparis spp., Opuntia spp., Ziziphus spp., Grewia spp., Cordia spp., Albizia amara, Atalantia monophylla, Pleiospermium alatum, Prosopis juliflora, Dichrostachys cinerea, Diospyros cordifolia, Pisonia aculeata, Carissa carandas, Strychnos potatorum, Ceropegia juncea, Pergularia daemia, Barleria spp. Caralluma spp., Helixanthera spp., etc.

Southern dry mixed deciduous forest (dry deciduous forest)

The forest type is characterised predominantly by hardwood deciduous tree species. The canopy is open with poor undergrowth. Bamboos are barely represented. The canopy level is vague in this type also. The lower storey consists of mostly shrubs and small trees.

The most characteristic species present invariably in the forest type are Albizia amara, A. ferruginea, A. planifrons, Alphonsea lutea, Anogeissus latifolia, Boswellia serrata, Capparis grandis, Chloroxylon swietenia, Hardwickia binata, Santalum album, Manilkara hexandra, Commiphora caudata, C. pubescens, Diospyros ovalifolia, D. cordifolia, Premna tomentosa, Cassia fistula, Ficus mollis, Givotia moluccana, Gtrocarpus asiaticus, Limonia acidissima, Sterculia urens, Sapindus emarginatus, Canthium coromandelicum, Tarenna asiatica, Dodonaea viscosa var. angustifolia, Garuga floribunda, Shorea roxburghii, etc.

Southern hill-top tropical evergreen forest

This forest type is confined to places above 1,500 m elevation. There is no stratification of tree canopy into different tiers. The trees are not very large and often highly branched. Epiphytes are common on branches of trees.

The trees are Aglaia bourdillonii, Agasthyamalaya pauciflora, Poeciloneuron indicum, Meliosma pinnata subsp. barbulata, Vernonia travancorica, Gordonia obtusa, Garcinia travancorica, G. imberti, G. rubro-echinata, Actinodaphne campanulata, A. bourdillonii, Syzygium cumini, Syzygium densiflorum, Syzygium rubicundum, Isonandra candolleana, Neolitsea scrobiculata, N. fischeri, Mastixia arborea, Turpinia nepalensis, Ardisia rhomboidea, Bhesa indica, Elaeocarpus venustus, Eugenia discifera, etc.

The lower shrubby layer is composed of Octotropis travancorica, Ardisia blatteri, Symplocos wynadense, Goniothalamus wightii, Polyscias acuminata, Strobilanthes asper, S. lurida, S. tristis, Euonymus paniculatus, etc.

Mangroves or tidal swamp forests

Mangroves are mostly confined to the estuaries and banks of backwaters where the influence of tidal waves is pronounced. Kerala had a substantial area under mangroves. However, due to alternate land use, this unique vegetation is now occupies only 17 km2 area (Chand Bash, 1991). The trees in the mangroves are Avicennia officinalis, Avicennia marina, Cerbera manghas, Rhizophora apiculata, Rhizophora mucronata, Ceriops tagal, Bruguiera cylindrica, Bruguiera gymnorrhiza, Barringtonia racemosa, Dolichandrone spathacea, Sonneratia alba, Sonneratia caseolaris, Excoecaria agallocha, Kandelia kandal, etc. Climbers commonly associated are Caesalpinia crista, Cayratia trifolia, Derris trifoliata, Dalbergia candenatensis, Ipomoea violacea etc. The commonly found shrubs are Acanthus ilicifolius, Aegiceras corniculatum, Lumnitzera racemosa, Premna serratifolia, Clerodendrum inerme etc.

Wetlands and Aquatic Vegetation

These vegetation types are confined to lowlands and midlands. In the ponds, lakes, lagoons and marshy areas where plant like Nymphaea nouchali, Nymphaea pubescens, Nelumbo nucifera, Aeschenomene aspera, Aponogeton natans, Ceratophyllum demersum, Hydrilla verticillata, Limnophylla aquatic, Ludwigia perennis, Myriophyllum oliganthum, Nymphoides indica, N. hydrophylla, Ottelia alsinoides, Sphaeranthus africanus, Utricularia spp, thrive. While species like Najas graminea, Polypleurum spp., Zeylanidium olivaceum, etc. are attached to submerged rocks in the rivers. Semi-aquatic species such as Hydrolea zeylanica, Monochoria vaginalis, Hydrophyla auriculata, Baccopa monnieri, Lindernia spp., Eriocaulon spp., Ammania spp. Rotala spp. etc. are found in marshy areas and banks of ponds and lake shores.

Coastal vegetation

Kerala has a seacoast of 590 km long. The vegetation along the coast in dominated by species like Scaveola sericea, Pandanus spp. Canavalia maritima, Cyperus arenarius, Gisekia pharnaceoides, Glinus oppositifolius, Hydrophylax maritima, Hedyotis diffusa, Ipomoea pes-caprae, Launaea sarmentosa, Polycarpaea corymbosa, Spinifex littoreus, Wollastonia biflora, Alternanthera pungens, Alternanthera sessilis, Spermacoce hispida, Boerhavia diffusa, Leucas aspera, Chloris barbata, etc.

Agricultural and horticultural crops

Kerala is a land of intensive cultivation. Several exotic species have been introduced as cash crops. They are raised in pure crops or as mixture along with indigenous species. Coconut, Arecanut and Paddy are the most extensively raised indigenous species. A variety of herbaceous as well as shrubby species are often grown as under crop in coconut and Arecanut; whereas paddy is raised as a pure crop. Tea, coffee and rubber are the extensively raised exotic cash crops in the State. Tapioca, cashew, Coco and tobacco are also cultivated to a large to medium scale. Several exotic trees were introduced to meet the pulp and paper industry, mostly Eucalyptus grandis, E. tereticornis, E. camaldulensis, Acacia auriculiformis, A. mangium, A. mearnsii, A. dealbata, Paraserianthes falcataria, Grevillea robusta, Swietenia macrophylla, etc. Exotic ornamental and avenues trees are Spathodea campanulata, Kleinhovia hospita, Kigelia africana, Cananga odorata, Polyalthia longifolia, Jacaranda mimosifolia, Melaleuca citrina, Peltophorum pterocarpum, Delonix regia, Plumeria rubra, Cassia siamea, C. nodosa, Couropita guianensis, etc. The important horticultural plants are Achras sapota, Psidium guajava, Averrhoa bilimbi, A. carambola, Phyllanthus acidus, Syzygium aqueum, S. jambos, S. malaccensis, Diospyros mangostana, etc. A few species which were introduced as garden plants have become naturalised and attain the status of weed. The most widespread and obnoxious weeds are Chromolaena odorata, Ageratina adenophora, Lantana camara, Mikania micrantha, Eichhornia crassipes, Parthenium hysterophorus and Mimosa diplotricha. These aggressive weeds, fast establishing in the aquatic as well as terrestrial ecosystems are serious threat to the indigenous flora. It is estimated that about 11 per cent flora of the State is composed of exotic species.