Geographical Distribution and Ecology of Orchids

Orchids have a very wide range of distribution. They are found to occur in all parts of the world except, perhaps, in the Antarctica. Polunin (1959 as quoted by Stearn 1960) in his Circumpolar Arctic Flora has reported 7 species to occur in the Arctics. Orchidaceae is also the most successful of all plant families , as evidenced by the largest number of existing species-25,000 to 35,000 species in 600-800 genera-reported among all the families of flowering plants, Compositae with over 12,000 sepcies occupying the second place.

According to the mode of distribution , orchids fall under two categories –temperate and tropical. The former are found in the temperate regions of the Northern hemisphere such as, Europe, Northern Asia and North America. The climatic conditions in these regions are characterized by sharp seasonal variations and minimum of warmth and rainfall. The orchid species here are invariably terrestrial and with the exception of Diandrae are tuberous , constituting the most primitive of all forms with either sectile or granular pollinia .

The tropical population of orchids , on the other hand, comprises of species which are both terrestrial and epiphytic , the latter vastly greater in number than the former. The more advanced tribes with waxy pollinia exist side by side with the primitive forms. The tropical belt extends upto 23 ½ degree North and South of the Equator ,encompassing about half of the earth’s surface. But nearly three quarters of this area are occupied by water. Even within the remaining land mass, which, according to Veitch (1887-1894) amounts to about 1/3 of the total land mass on earth, there are extensive tracts which have been rendered unfit for orchid growth due to certain geographic and climatic factors. Such areas exist in all continents. The Arabian desert, the plains of Iran, the Sind desert of North Western India and the Deccan Plateau of Peninsular India are such tracts occupying large chunks of land in the continent of Asia . The major portion of Australia is desert land . In Africa , there is the great Sahara desert on the north and the Kalahari desert in the South. In the Americas, the campos of Brazil , the pampas of Argentina , savannas of Venezuela and Guyana and the desert plains of northern Mexico are regions far too arid to support orchid growth (Veitch 1884-1892).The remaining tropical land surface which is congenial for proper growth of orchids comes to only about ¼ of the total land mass. Here we find the three major orchid-growing regions of the world:

1. The tropical American zone which lies along two distinct tracts –the South Eastern parts of Brazil and the North Western region from Bolivia to Western Venezuela, Central America and Southern Mexico. This is the land of the flamboyant Cattleyas, Oncidiums and their allies . The Amazon basin which lies in between these two tracts, in comparatively poor in orchids.

2. Tropical Africa where the majestic Angraecoids with their footlong spurs flourish.

3. The Indo-Burmese and South East Asiatic region , which is one continuous tract of lush orchid growth, from the Himalayas through Assam, Burma, Thailand, Malaysia and Indonesia to Philippines and New Guinea. This is the home of the finest Dendrobiums , Cymbidiums, Vandas and Phalaenopsis .

The tropical climate is characterized by a high amount of rainfall , reaching to more than hundred inches annually, contributed to the South West and North East Monsoons. Though the heaviest showers are confined to certain months of the year , there is no prolonged dry season, since even these comparatively dry months have a mean rainfall of 3-4 inches. The temperature is high, ranging between 70-90 F at sea level. The heavy rainfall and high temperature provide the atmosphere with a relative humidity which is almost always near saturation point.

This belt of warm , humid tropics is the home of epiphytic orchids, where they thrive , breed and interbreed and evolve at such an amazing rate that all the other families of flowering plants are left far behind on the scale of evolution .

In the equatorial region,i. e., 7-12 degree North and South of the Equator, the climate is equable throughout the year, with no great variations in temperature . The rainfall is distributed uniformly throughout the year, although there are two yearly peaks consequent upon the sun’s journey north and south of the Equator. The monopodial forms of orchids, characterized by their terminal growing point, reach their zenith of development under these climatic conditions. They grow round the year, and since there is an abundant supply of warmth and moisture at all times of the year, they do not go in for any prolonged resting period. Growth , as a rule, starts around 60 degree F, reaches an optimum at about 90Degree F and then starts to decline , coming to a halt around 110 degree F . Since temperatures in the equatorial belt is usually maintained at 70 degree -90 degree F , plants continue growth almost throughout the year. The various Vandas, Angraecums, Stauropsis etc. come under this category.

As the distance from the equator increases , most of the rainfall gets confined to one particular season , the rest of the year getting only sporadic showers. The epiphytic orchids which are more dependent on rainfall than those species which are rooted in soil, develop storage organs which serve as protection against possible scarcity of atmospheric moisture. They store water in modified parts of stems called pseudobulbs, as do most of the deciduous sympodial forms like Dendrobium, Bulbophyllum , Cattleya, Laelia etc. Where they are evergreen , leaves are converted into storage organs as do the terete-leaved Vandas, Diplocentrums, Luisias and Oberonias . The plants also develop a characteristic of completing their growth during the rainy season and taking a prolonged rest during the unfavourable dry months.

With the increase of distance from the equator , the amount of rainfall and warmth and resultant humidity also decrease and the occurrence of epiphytic orchids gradually diminishes . Eventually , in the temperate regions, where there is a prolonged dry season, the epiphytic species are completely replaced by terrestrial forms with underground tubers, which are adapted to survive by hibernating during the unfavourable conditions.