Northwestern and Southwestern Ghats montane rainforests

• Category: Forest Biomes.
• Geographic Location: Asia.
• Summary: These montane rainforests in western India support very high biodiversity that is under threat from habitat fragmentation.

India's main extent of tropical montane rainforests are found on a continuous chain of mountains along the west coast of the country, a region known as the Western Ghats. The mosaic of vegetation here arises from interactions between gradients of rainfall, latitude, and elevation. This biome is part of the Western Ghats and Sri Lanka biodiversity hot spot, and is home to many endemic (found nowhere else on Earth) species.

The Southern Ghats have higher peaks, above 6,560 feet (2,000 meters); these high mountains have a distinct forest structure referred to as the Shola forests. These Shola are patches of relatively low forested areas interspersed with grasslands. The Northern Ghats do not have high mountains, but have flora and fauna that are also distinct to this region. Given the pressure on the resources of this geographic region due mainly to a growing human population, this ecosystem needs to be protected in order to ensure that its unique flora and fauna are conserved.

Climate change is of particular concern to this region, with the Western Ghats having been identified as one of several areas in Asia most likely to be impacted by global warming. Some studies suggest significant forest dieback will occur, as a result of higher average temperatures, disrupted timing of rainfall, and altered seasonal storm patterns, which combined would be devastating to many of the habitat niches here.

The Western Ghats

The Western Ghats are a continuous chain of mountains found along the southwestern coast of India. The 994-mile (1,600-kilometer) mountain chain runs north-south, approximately 20 to 30 miles (30 to 50 kilometers) parallel to the west coast. The Western Ghats chain is interrupted by a 22-mile (35-kilometer) stretch of flat terrain, the Palghat Gap. The northwestern Ghats include the states of Gujarat, Maharashtra, and Goa; those in the southwest include the states of Karnataka and Kerala.

The Western Ghats have their origin in volcanic eruptions that occurred during the prehistoric movement of the formation of continental Gondwanaland and its journey toward Asia 150 to 100 million years ago. The Western Ghats receive heavy rainfall primarily in two seasons: the southwest monsoon that spans from June to September, and the northeast monsoon that spans from October to November. Almost 80 percent of the rainfall occurs during the southwest monsoon.

Rainfall is heaviest on the western slopes, and the intensity of rainfall decreases toward the north. A similar pattern exists from west to east. The leeward, or the east-facing, slopes are rainshadow areas and receive less than 10 percent of the rainfall; a corresponding area on the windward or the west-facing slope is inundated with the lion's share of precipitation. The southernmost Ghats also have a shorter dry season, typically two months less than the northern Ghats.

Southwestern Montane Rainforests

The southern Western Ghats have numerous peaks taller than 6,560 feet (2,000 meters). These areas feature distinct vegetation types along their elevational gradients. They receive most of their rainfall during the monsoons, but due to the cloud-scraping function of the ridge lines, they also receive substantial rainfall during other months, and hence have a shorter dry period. The tallest mountain peak in this region is the Anaimudi at 8,840 feet (2,695 meters). The mountains at this elevation have the Shola formation of wooded forest and grasslands. These forested stands here feature a dense understory with stunted trees. Shola forests are found along the hill slopes and valleys. Since this unique vegetation occurs only in isolated patches at the higher elevations, they are also referred to as sky islands, and serve as hot spots of endemic flora and fauna.

The transition between the forest patches and the grasslands is abrupt. The trees generally have a spread-out canopy and are covered with mosses, ferns, and epiphytes; they are particularly rich in orchids. These forests have high levels of humidity even during the dry seasons. The grasslands, however, get extremely dry during the summer, becoming highly flammable, and resulting in seasonal man-made fires and clearings. In some areas, these grasslands and the montane forests have been cleared to grow plantation crops such as tea.

At lower elevations, the southwestern Ghats have evergreen forests which are structurally complex and provide niches for a diverse community of fauna. These forests are generally old growth and the canopy height ranges between 131 to 262 feet (40 to 80 meters). Many of the tree species in these forests are endemic to the southern Ghats.

At elevations in the range of 1,640–2,950 feet (500–900 meters), moist deciduous forests with bamboo thickets are seen on the leeward side of the mountains. Many of these stands continue to thrive. Low-elevation evergreen forests and marsh communities, however, have largely been decimated. Still, some small, scattered patches of water-logged Myristica swamps persist. Since the midelevation forests are almost contiguous in the southern Ghats, they serve as corridors for dispersal of large mammals, Asian elephants, Indian gaur, lion-tailed macaques, sloth bears (Ursus ursinus), Bengal tigers, Nilgiri tahrs (Hemitragus hylocrius), an ibex, and a type of monkey called Nilgiri langurs (Trachypithecus johnii).

Northwestern Montane Rainforests

Drier than the southern areas, the northern montane rainforests are also more affected and adapted to sharper seasonal contrast in precipitation. The average annual rainfall here tends to be about 98 inches (2,500 millimeters); most comes during the monsoon, while the dry season tends to extend for a full four months. This climate favors wet-evergreen and deciduous forest types.

Higher elevations in these northern Ghats areas harbor subtropical broadleaf forests; a few stands are old growth. Canopy height ranges from 130 to as high as 260 feet (40 to 80 meters).

The levels of endemism across taxonomic groups of both trees and animals decline toward the north generally in the Ghats. The northern Ghats also have a longer history of more intensive human use and exploitation, although there are some remnant forests where large-girth trees can still be seen. Portions of these forests have in the past been cleared for raising extensive plantations of teak (Tectona grandis).

The Khas Plateau in the northern Ghats has a unique geomorphology and vegetation. These flat hill tops are found at about 3,280 feet (1,000 meters) in elevation. A characteristically thin layer of topsoil here supports scant woody vegetation. The rocky outcrops on the hilltops support herbaceous vegetation, dominated by annuals that flower only in a narrow window of time during the monsoon season. These mountains are of volcanic origin and have rich deposits of iron and manganese, helping provide nutrients.

Biodiversity and Conservation

The Western Ghats of India are considered a global, mega-biodiversity hot spot. An astonishing three-fourths of all amphibians identified in India may be found in this area, as well as approximately half of India's reptiles. This region also accounts for more than 27 percent of all the flowering plants in India. The Western Ghats were designated a World Heritage Site in 2012.

There are 325 globally threatened species found in the Western Ghats, of which 51 are critically endangered. Apart from the high species richness and endemism, the region is also known to harbor evolutionarily distinct species surviving as relic populations in its evergreen forests. Examples of such species are found among trees, mollusks, fish, amphibians, reptiles, and mammals. Only 9 percent of the total area of the Western Ghats is protected, however, and new species of flora and fauna of great evolutionary significance are periodically still being discovered. This further enhances the conservation importance of this biome.

Habitat fragmentation is one of the most pressing issues that need to be addressed to ensure the safety of this unique ecosystem. Fragmentation usually occurs either by a change in the human land-use practice, which often entails commoditization of one or more local species of plant or animal, or from outright industrialization or urbanization of an area. Separately, during the dry season, when the grasslands and the moist deciduous forests are flammable, human-induced fires cause extensive damage to natural vegetation, sometimes undermining the ability of a particular habitat to recover. The Western Ghats essentially are undergoing predictable degradation and habitat fragmentation in order to support a large and growing population. The region is also vulnerable to climate change, as its natural infrastructure is tuned to a moisture and temperature regime of long standing.

Bibliography

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Myers, N. “Threatened Biotas: ‘Hot-Spots' in Tropical Forests.” Environmentalist 8, no. 1 (1988).

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