Sholas are patches of stunted evergreen tropical and subtropical moist broadleaf forest found in valleys amid rolling grassland in the higher montane regions of South India. These patches of shola forest are found mainly in the valleys and are usually separated from one another by undulating montane grassland. The shola and grassland together form the shola-grassland complex or shola-grassland mosaic. The word 'shola' is probably derived from the Tamil language word cÕlai (சோலை) meaning grove.[1], or possibly from the Sanskrit 'gulma' for a thicket of trees, which occurs in prakrit as 'golla'[2]. The shola-forest and grassland complex has been described as a climatic climax vegetation with forest regeneration and expansion restricted by climatic conditions such as frost or soil characteristics while others have suggested that it may have anthropogenic origins in the burning and removal of forests by early herders and shifting agriculturists.[3]
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Shola forests are found in the higher altitude hill regions of the Nilgiris, Palnis, the Western Ghats and associated ranges in the states of Karnataka, Kerala and Tamil Nadu. Although generally said to occur above 2000 meters above sea level, shola forests can be found at 1600 meters elevation in many hill ranges (e.g. Biligiriranga Hills).
The origin of the shola forest and grassland complex has been the subject of scientific debate. Some early researchers suggested that the floristic composition represents a stable final state or climax vegetation. This stability being maintained by climatic conditions such as frost which allow the grass to grow but kill off any forest seedlings.[4] Others have suggested that the grassland may have been created and maintained by early pastoralists and point out that fire has a major role in the maintenance of the grassland.[5] There is evidence for both and several features of the forest trees and the grasslands have been considered and debated. Pollen analysis from bogs in the Nilgiris suggest that the complex of grassland and forest existed 35,000 years ago, long before human impact began.[6] Long term studies on the dynamic processes of vegetation change continue.[7][8][9]
Due to their isolation, altitude and evergreen character, shola forests are home to many threatened and endemic species. Some of the species found here have close relatives only in the distant evergreen forests of Northeast India and Southeast Asia. Some others are found nowhere else in the world.
The Western Ghats are one of the globally recognized biodiversity hotspots. Among the many larger animals inhabiting a shola-grassland mosaic are tigers and leopards, elephants and gaur. The endangered Nilgiri Tahr (an Asian goat-antelope) is endemic to the shola-grassland, and its range is now restricted to a 400-km stretch of shola-grassland mosaic, from the Nilgiri Hills to the Agasthyamalai Hills.[10] Laughingthrushes, Nilgiri Woodpigeons, Shortwings, and some of the endemic flycatchers (Black-and-Orange and Nilgiri Verditer) are some of the 300+ species of birds that inhabit this area. The area shows high endemicity and is rivalled only by the forests in northeast India; 35 percent of the plants, 42 percent of the fishes, 48 percent of the reptiles, and 75 percent of the amphibians that live in these rain forests are endemic species.[11]
At least 25 types of trees are present in the major sholas of the Nilgiri Hills.[12] The dominant trees in this type of forest are Michelia niligarica, Bischofia javanica (Bishop wood), Calophyllum tomentosa, Cedrela toona (Indian Mahogany), Eugenia (myrtle) spp., Ficus glomerata (Atti or Cluster Fig Tree or Gular Fig tree) and Mallotus spp.Shola forests have an upper storey of small trees, generally Pygeum gardneri, Schefflera racemosa, Linociera ramiflora, Syzygium spp., Rhododendron nilgiricum, Mahonia nepalensis, Elaeocarpus recurvatus, Ilex denticulata, Michelia nilagirica, Actinodaphne bourdellonii, and Litsea wightiana. Below the upper story is a low understory and a dense shrub layer. There is a thick concentration of mosses growing on the ubderstory and many ferns in the sunlit narrow transition to grassland.
Shola forests are interspersed with montane grasslands, characterized by frost- and fire-resistant grass species like Chrysopogon zeylanicus, Cymbopogon flexuosus, Arundinella ciliata, Arundinella mesophylla, Arundinella tuberculata, Themeda tremula, and Sehima nervosum.
Invasive Introduced species are a serious threat to this high altitude ecosystem. Some, like Acacia mearnsii and Eucalyptus globulus are the consequence of commercial plantation and afforestation drives. Other threatening invasives include Lantana camara and Ageratina adenophora.[13]
Periodic fires have been considered to help maintain the grassland, however excessive burning has led to a shrinkage of forest patches and the growth of invasive species.[14]
The shola biome have a high water retention capacity and exists as the precious source of the water for the high altitude organisms are the origin of many streams and rivers in the Western Ghats.[15]