Phytogeography: Introduction, continuous and discontinuous distribution, Phytogeography of India, Vegetation regions of India

Phytogeography:- It is the branch of biogeography that is concerned with the geographic distribution of plant species and their influence on the earth's surface. 

Introduction:-

> The term is derived from two words ‘phyto’ meaning plant and ‘geography’ meaning distribution. 

> It covers all aspects of plant distribution including the individual species and the factors that regulate the composition of communities and flora.

> Phytogeography encompasses various fields such as environment, flora (taxa), vegetation (plant community) and origin. 

> Phytogeography is often divided into two main branches: 

i. Ecological phytogeography:- It deals with the role of biotic and abiotic interactions on plant distribution.

ii. Historical phytogeography:- It is related to historical reconstruction of the origin, dispersal, and extinction of taxa. 

> Alexander von Humboldt is called as the "father of phytogeography". 

> This branch of science helps in understanding the patterns of adaptation of species to the environment. This is done by studying the specific traits of a population. These patterns are termed as ecogeographical rules. 

Distribution of vegetation:-

a. Continuous distribution

b. Discontinuous distribution

a. Continuous distribution:- Some taxa are uniformly distributed in the area and give rise to continuous distribution. The area of a taxon or of a vegetation feature is never really continuous. It is of four main types:

i. Cosmopolitan:- These taxa are distributed all over the globe. They occur in all six widely inhabited continents, Eg.- many weeds and lower groups of cryptogams.

ii. Circumpolar:- These taxa are distributed around the North or South Pole Eg.- Saxifraga oppositaefolia, Eutrema edwardsii.

iii. Circumboreal:- These taxa are distributed around the boreal zone Eg.- Ribes (Gooseberries) and Danthonia (poultry grass).

iv. Pantropic:- These taxa occurring practically throughout the tropics and sub-tropics or at least widespread in the tropical region of Asia, Africa and America, Eg.- Palm family (Arecaceae).

b. Discontinuous distribution:- Most taxa may have isolated patches of areas widely separated apart. These constitute discontinuous distribution pattern. The plants are separated by wider gaps which cannot be overcome or crossed by the propagules. The main causes of discontinuity are environmental (due to particular topographic, climatic, edaphic or biotic characteristics of tracts separating the areas). Following are the familiar discontinuous types:

i. Arctic – Alpine:- Taxa are distributed in arctic region and in mountainous system of temperate zone. Eg.- Salix herbacea, Saxifraga oppositaefoilia.

ii. North Atlantic:- Taxa are distributed in North America and Europe and also some times locally in Asia. Eg.- Lycopodium inundatum and Spiranthes sp.

iii. North Pacific:- Taxa are distributed chiefly in North America and Eastern Asia, Eg.- Torreya (Torrey pine) Symplocarpus foetida.

iv. North—South American:- Taxa are distributed in North and South America but lacking continuity in between, Eg.- Sarraceniaceae (pitcher-plant family)

v. Europe – Asian:- Taxa are distributed in Europe and Asia but lacking continuity between. Eg.- Leontice altaica, Cimicifuga foetida.

vi. Mediterranean:- Taxa are distributed at European and African shore of Mediterranean sea. Eg.- Platanus

vii. Tropical:- Taxa are distributed in two or more separate tropical regions. Eg.- Buddleia

viii. South pacific:- Taxa are distributed at least in South America and New Zealand. Eg.- Jovellana

ix. South Atlantic:- Taxa are distributed at least in South America and Africa. Eg.- Asclepias

x. Antarctic:- Taxa are distributed on Arctic mainland and parts of South America, New Zealand and Austral islands. Eg.- Nothofagus

xi. Gondwana type:- Taxa are distributed in more than one continent which tends to embrace Africa, Madagascar, India and Australia. Eg.- Some sedges.

Phytogeography of India:- The Indian sub-continent is characterised with a variety of climate types and the flora of the country is also correspondingly of different types in its different parts. A phytogeographical region is defined as an area of uniform climatic conditions and having a distinctly recognisable type of vegetation. 

According to D. Chattarjee (1962), India can be divided into nine phytogeographical regions:

1. Western Himalayas

2. Eastern Himalayas

3. Indus plain

4. Gangetic plain

5. Central India

6. Deccan

7. Western coasts of Malabar

8. Assam

9. Bay Islands of Andaman and Nicobar

1. Western Himalayas:-  It extends from Kumaon to Kashmir and has annual rainfall up to 200 cm. Altitu-dinally there are following three zones of vegetation corresponding to three climatic belts:

a. Submontane zone:- This extends up to 1500 metres altitude and comprises mostly of Siwalik ranges. The forests are tropical and subtropical having trees such as Shorea robusta, Dalbergia sissoo, Cedrela toona, Ficus glomerata, Eugenia jambolano, Acacia catechu, Butea monosperma (dhak), Zizyphus and thorny succulent eu- phorbias on slopes.

b. Temperate zone:- Above submontane zone extend montane tem- perate forests up to 3500 metres altitude. They are dominated by plant species such as Quercus, Acer, Ulmus, Rhododen- dron, Betula (birch), Salix (cane), Populus, Cornus, Prunus, Fraxinus, Pinus, Cedrus, Picea and Taxus.

c. Alpine zone:- This zone ex- tends from 3500 to 4500 metres alti- tudes (snow line) and is characterised with alpine forest vegeta- tion with scrub and meadows. Most common tree species are Abies, Betula, Juniperus and Rhododendrons. The herbs which occur near the snow line include species such as Primula, Potentilla, Polygonum, Geranium, Saxifraga, Aster, etc.

2. Eastern Himalayas:- It includes regions of Sikkim and NEFA and is characterised by more rainfall, less snow and higher temperature. This is also divided into the following three zones altitudinally:

a. Tropical zone:- Up to 1800 metres altitudes, this zone has tropical semi-evergreen or moist deciduous forests. These forests comprise the plants such as Shorea robusta, Acacia catechu, Delbergia sissoo, Terminalia, Albizzia, Cedrela, Dendrocalamus (bamboo), etc.

b. Temperate zone:- This zone extends between 1800 metres to 3800 metres altitudes and has typical montane temperate forests which are dominated by oaks such as Michelia, Quercus, Pyrus, Symplocos, Eugenia, etc., at lower levels and by conifers as Juniperus, Cryptomeria, Abies, Pinus, Larix and Tsuga and also Salix, Rhododendron and Arundinaria (bamboo) at higher cooler levels.

c. Alpine zone:- Beyond the temperate zone extends alpine zone up to 5000 metres altitudes. It has alpine vegetation including Juniperus and Rhododendron with its other typical flora.

3. Indus plains:- This zone includes the arid and semiarid regions of Punjab, Rajasthan, Kutch, part of Gujarat and Delhi. The rainfall is less than 70 cm. The vegetation is tropical thorn forest in semi-arid region and is typical desert in the arid region. The plants of this zone are primarily xerophytic. The common plant species of this zone are Acacia nelotica, Prosopis sp., Salvadora, Tecomella, Capparis, Tamarix, Zizyphus, Calotropis, Panicum, Saccharum, Cenchrus, Euphorbia, etc.

4. Gangetic plains:- This region extends over Uttar Pradesh, Bihar, Bengal and part of Orissa and is characterised by moderate amount of rainfall and most fertile (i.e., alluvial) soils. Vegetation of this zone is chiefly of tropical moist and deciduous and dry deciduous forest type. The common plants of this zone are Dalbergia sissoo (shesham), Acacia nelotica (babul), Saccharum munja, Butea monosperma, Madhuca indica (mahua), Terminalia arjuna (arjuna), Buchanania lanzan (chiraunji), Diospyros melanoxylon (tendu) , Cordi myxa (lisora), Acacia catechu (khair), Azadirachta indica (neem), Ficus bengalensis (bergad), Ficus religiosa (pipal), Mangifera indica (mango), and weeds and grasses such as Xanthium, Cassia, Argemone, Amaranthus, etc. In Gangetic delta (South Bengal) mangrove vegetation is common.

5. Central India:- It comprises Madhya Pradesh, parts of Orissa and Gujarat. The rainfall is 150–200 cm and its vegetation is thorny, mixed deciduous and teak type. The chief plants of this region are Tectona grandis, Madhuca, Diospyros, Butea, Dalbergia, Terminalia, Carissa, Zizyphus, Acacia, Mangifera, etc.

6. Malabar (west coast):- This region includes western coast of India from Gujarat to Cape Comorin and has heavy rainfall. The forests are tropical evergreen in west, semievergreen towards interior, subtropical or montane temperate evergreen forests in Nilgiris and mangroves near Bombay and Kerala coast.

7. Deccan Plateau:- This region extends all over peninsular India (i.e., Andhra Pradesh, Tamil Nadu and Karnataka) and has rainfall up to 100 cm. Its central hilly plateau has tropical dry deciduous forests of Bowsellia serrata, Tectona grandis and Hardwickia pinnata, while, the low eastern dry Coromandal Coast has tropical dry evergreen forests of Santalum album (chandan), Cedrela toona and plants such as Acacia, Prosopis, Euphorbia, Capparis, Phyllanthus, etc.

8. Assam:- This region is characterised by heavy rainfall (200 to 1000 cm). The vegetation is either dense evergreen forest or sub-tropical. The evergreen forests include trees such as Dipterocarpus macrocarpu, Mesua ferrea, Shorea robusta, Ficus elastica, etc. bamboos as Bambusa pallida, Dendrocalomus hamiltonii, etc., grasses such as Imperata cylindrica, Saccharum sp., Themeda sp., insectivorous plants as Nepenthes sp., and also epiphytes (ferns and orchids). In the northern cooler regions, wet hill forests include plants such as Alnus, Betula, Rhododendron, Magnolia, etc., The hilly tracts also have pine forests of Pinus khasiya and P. insularis.

9. Andmans:- This region possesses a varied type of vegetation: mangroves and beech forest at its coasts and evergreen forests of tall trees in the interior. Important plant species of this island are Rhizophora, Mimusops, Calophyllum, Lagerstroemia, etc.

Vegetation regions of India:- There are 5 major kinds of Natural vegetation in India, that are listed as:

1. Tropical Evergreen Forests

2. Tropical Deciduous Forests 

3. Tropical Thorn Forests and Scrubs

4. Montane Forests

5. Mangrove Forests

1. Tropical Evergreen Forests:- Forests are grown in heavy rainfall areas where the annual rainfall is over 200 cm with a short dry season. These are also known as Tropical Rainforests. These forests are found in the Western Ghats and on the island groups of Lakshadweep, Andaman, and Nicobar, upper parts of Assam and Tamil Nadu coast.

2. Tropical Deciduous Forests:- Tropical Deciduous Forests are one of the most widespread forests of India. These forests are found over the region where the annual rainfall is between 200 cm and 70cm. Trees of These forests types shed their leaves for about six to eight weeks in dry summer. These forests are also known as Monsoon Forests.

3. Tropical Thorn Forests and Scrubs:- These specific tree species provide insight into natural vegetation as well as the environments in which they flourish. It is obvious that these areas receive just 70 cm of yearly rainfall. This is why these plantations have begun to produce thorns in order to conserve water in this arid climate. In nature, the trees are often bushy and thorny. India’s dry environment and natural vegetation reflect the characteristics of these plants. These plants may be found in Rajasthan and Gujarat’s higher reaches, where the environment is arid. The main plant species are Acacias, Palms, Euphorbias, and cacti.

4. Montane Forests:- Montane Forests are the forest that is usually found in a mountainous region where the decrease in temperature with increase in altitude leads to the corresponding change in the natural vegetation. This is a unique sort of vegetation in India, where the species develop larger leaves and are found at elevations of 1000-3000 meters above sea level. Some of the mountainous areas are Jammu Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, Arunachal Pradesh. Chestnuts, oaks, and other broad-leaved evergreen trees may be found here.

5. Mangrove Forests:- Mangrove Forests are the forests that are found in the coastal areas influenced by tides, because of this, they are also known as Tidal Forests. The Ganga, Mahanadi, Godavari, Krishna, and Kaveri deltas, for example, develop along the shore and on the edges of deltas. These woodlands are known as ‘Sundarbans’ in West Bengal. The trees include the hogla, garan, gewa, golpata, and pasur, among others. The most common varieties in these forests are Dense Mangroves and their roots are submerged underwater.