South India – Climate and Geographic factors

The region designated as South India lies between 8 degree -20 degree north of the equator and between 70 degree -85 degree in longitude. The mean temperature is 27 degree C(81 degree F) at sea leavel which it enjoys with Northern Australia , New Guinea , Phillippines , Indonesia , Malaysia , Sri Lanka , Tropical and major part of South Africa , parts of Brazil, Guyana , Columbia ,Venezuela , Panama ,the Honduras and Guatemala . The southernmost regions of the peninsula lie on the northern fringes of the equatorial belt. The peninsula which extends north-south touching Indian Ocean at its southernmost tip , is flanked on the west by the Arabian Sea and on the East by the Bay of Bengal. On the north is the arid expanse of the Deccan plateau beyond which are the Vindhya ranges . Stretching parallel to the Western coastline and bout 40-50 miles inland are the mountain ranges of Sahyadri or the Western Ghats as they are more commonly known. This extends southwards from Bombay to nearly thirty miles short of Cape Comorin. Except for a gap, about 20 km wide somewhere midway between its two extremities , the Western Ghats are one unbroken stretch of mountainous terrain, rising imposingly from the coastal plains and gradually sloping off into the semiarid expanse of land towards east, and throwing up peaks of upto 2300 m at frequent intervals.

Lining the eastern coastline and lying parallel to the Western Ghats , but much less imposing and stopping short at the middle of the peninsula, are the Eastern Ghats.

Our present survey of orchid populations was mostly confined to the coastal state of Kerala and a few nearby areas in Tamil Nadu , which occupy the Southern regions of the peninsula . The combination of the geographical factors discussed above along with the yearly excursions of the sun towards the north and south of the equator , render these southern coastal regions exposed to both South West and North East monsoons . These monsoons drench the land for six moths of the year from June to November ,and provide sporadic showers during the remaining six months .

During the hot summer months of June and July the sun is at its northern zenith. The fierce vertical rays of the sun heat up the entire Central Asiatic region, which sets up a pressure gradient between this region and the comparatively cooler southern parts. As a result , air currents from the equatorial regions below over to the mainland Asia . These air currents , as they come sweeping over the vast expanse of the waters of the Indian Ocean, get saturated with water vapour, which they unload against the cool heights of the Western Ghats. This causes such heavy precipitation over the mountain ranges that this has became one of the wettest spots in the World. As much as 426 cm. of rain has been recorded in some of the southern parts of the Western Ghats. In these regions the greatest amount of precipitation is during the months of June and July , when the sky would be constantly overcast and it would go on raining for days at a stretch. The temperature at this time of the year may vary between 75-80 F at a day time and between 60- 75 degree F during night. The relative humidity is almost always at or near saturation point.

The fury of the south west monsoon is lessened to a considerable degree as it travels north. The costal of Goa , Karnataka and Maharahtra get less of precipitation than the southern regions.

During August and September the force of monsoon abates considerably During its fag end, the sun is once again at the equator ,ready to start its journey southwards. The whole course of events which has set in motion the onset of south-west monsoon , is now reversed . The comparatively cooler air from Central Asia and Himalayas blows to the now hot equatorial and southern regions . This north-east monsoon winds bring showers which the eastern slopes and regions east of the Western Ghats enjoy during the months of October and November . Since north-east monsoon winds travel mostly over land area before they enter South India , the showers these winds bring are far less heavy than those of south-west monsoon. During this season, south-western regions of Western Ghats experience a very predictable weather of bright sunny mornings and cloudy afternoons with thundershowers which may last well into the night.

The moths following north-east monsoon, from December to February ,when the sun is at its southern zenith, are comparatively dry with little or no rain. But during this time , temperature is low for a tropical region and nights are coolest of the year, the temperature dropping down to as low as 60 degree F on most nights.

The months of March , April and May are the hottest of the year, the temperature now soaring to 90-100 F during day-time and never dropping below 80 F at night . There are occasional showers to relieve the monotony of this summer heat.

The heavy rainfall contributed by the two monsoons and spread over six months of the year and the high temperature of the tropics , make the south-western coast of the peninsula , one of the most humid regions of the world. This situation is enhanced to a great degree by the proximity to the sea throughout its entire length . The tropical rain forests of Western Ghats are veriable orchid gardens and the great majority of orchid species reported from South India occur at elevations of 700 m and above in these mountain ranges.

The plants of western costal region include most of Kerala State, parts of Tamil Nadu, Karnataka , Goa and Maharashtra . Here the rainfall is high and there is a dense population of orchids. For a person motoring along the highways , particularly in Kerala ,the wayside avenue trees bearing huge masses of orchids would be a very common sight. But this population of the plains , even though large in number , comprises of only five or six of the more hardly species ,which are adapted to withstand the high temperature without damage to the foliage .

As mentioned earlier , the area east of Western Ghats receives only a nominal amount of rainfall and is mostly a tropical savannah towards south and an arid semidesert towards north. Both these regions have only a sparse orchid population because of the low degree of humidity and the high temperature . Amidst this expanse of arid plains , the Eastern Ghats, provide a sanctuary of sorts to orchids. But compared to Western Ghats , the amout of rainfall here is low and orchids of Eastern Ghats are mostly succulents or semi-succulents , adapted for existence in moisture-deficent areas. As a result of the efforts of Wight , Hooker , Fischer , Blatter & McCann Santapau and Kapadia ,nearly 240 species of orchids have been reported from South India . In later years stray reports of a few more species have also been added to this . Altogether there are under 250 species in about 70 genera reported from this region . Nearly 60 % of the species are epiphytic the remaining 40 % constituting the terrestrial flora. Habenaria and its allies form half of the terrestrial population , the other half being constituted by the various Neottieae , Malaxis, Liparis , Eulophis etc. Among the epiphytic forms there is a preponderance of sympodial forms.

No.of genera which are monotypic -37

Genera with number of species 2 –Calanthe, Sirhookera, Phaius , Gastrochilus , Acampe, Diplocentrum, Vanilla ,Zeuxine, Disperis , Porpax, etc.

Genera with number of species 3- Cymidium

“ 4-Nervilia, Vanda
“ 5- Luisia , Malaxis
“ 6- Saccolabium ,Aerides , Coelogyne
“ 9- Eulophia ,
“ 10 – Oberonia , Liparis , and above - Bulbophyllum , Dnedrobium ,Eria, Habenaria , Peirstlylus .

The largest genus among South Indian Orchids is Habenaria and among epiphytic species Dendrobium.

The orchid population of Western Ghats becomes especially interesting when we take into consideration its apparent isolation . The main orchid growing regions nearest to south India are separated from it by either vast expanses of water or by equally vast expanses of arid land mass. Tropical Africa and Madagascar on the West, the Indo-Burmese and South East Asiatic region on the North and East are effectively separated from South Indian orchid growing regions. The island of Sri Lanka ,which lies nearest to South India and has almost the same climatic conditions is , nevertheless, separated from it by the Gulf of Mannar.

Depending upon the influence of these neighbouring orchid growing regions , the orchid flora of South India may be divided into the following sections:

1. The endemic element
2. The Ceylonese element
3. The Himalayan and South East Asiatic element..

1. The endemic element

Of the 240 species , 102 are endemic to South India .But rather surprisingly , none of the 64 genera are endemic. Two genera, Cottonia and Sirhookera are exclusive to South India and Sri Lanka. All species of Bulbophyllum , and Eria and more than half of the larger genera like Dendrobium and Habenaria are endemic .

2.The Ceylonese element
Hooker reported 180 species of orchids from Ceylon (Now Sri Lanka). As in the case of the South Indian species, the orchid flora of Sri Lanka also maintains a considerable amount of individuality , as is evidenced by the 104 species , which are endemic to the island. There are only 37 species which South India and Sri Lanka share together . It is surprising that these two geographically juxtaposed regions, separated by a narrow stretch of water ,should maintain such a high degree of individuality .

2. The Himalayan and South East Asiatic element

The greatest of outside influences on orchid flora of South India appears to be that of Himalayan region . About 68 of the species found here are native of the cool Himalayas . Twently –three of these extend also to Burma and 15 to Malaysia and possibly to Thailand also.

Besides these individual influences , there are some 30 species which are found to occur in all three of the above regions.

Though Africa is as much of a neighbour to South India as Malaysia and Thailand are ,yet the African influence on South Indian orchid population is only at the generic and never at the specific level. Flora of both places are typically of the Old World with such genera as Habenaria , Disperis ,Satyrium, Bulbophyllum , Eulophia ,Acampe, Vanilla, Nervilia , Liparis , oberonia etc. But no species are reported from both places so far, even though there is very close resemblance between certain species which occur in East Africa and Sounth India ,eg. Calanthe volkensii and Calanthe masuca , Eulophia schimpeiana and Eulophia epidendraea etc.

Occurrence of the same species in different parts of the same continent is usually explained by long range dispersal . Species like Dendrobium crepidatum and thrixspermum arachnites, which are of common occurrence in Himalayan and South East Asiatic regions , but are of comparatively rare and localized distribution in South India , may have come to occupy the latter terrain through human agency , or better still , through the agency of migrating birds. This may very well be the case with the thirty odd species which south India shares with Sri Lanka.

We have to seek explanation elsewhere for the similarities between South Indian and African flora . As pointed out earlier ,the relationships are at the generic rather than at the specific level. This naturally points to common ancestors in the distant past and subsequent divergent evolution in Asiatic and African continents. Such a situation can best be explained by the theory of continental disjunction as propounded by wegener during the early part of this century . According to this theory , earth’s land surface was one solid mass before Pleistocene; it split up into continents which slowly drifted away to occupy their present positions later on . The main evidence in favour of this theory is the shape of the present continents ,which if put together ,will fit like the pieces of a jig-saw puzzle. In this united condition the present peninsular India may have remained appressed to the South East African Coast on one side and the South East Asiatic countries on the other . During this time they might have possessed many plants in common. After disjunction , due to climatic changes which might have happened in the case of each continent ,divergent evolution might have taken place giving rise to the different yet related population that we see now.