PHYTOSOCIOLOGICAL OBSERVATIONS ON ECONOMICALLY IMPORTANT PLANTS IN A DRY DECIDUOUS FOREST OF MARUTHAMALAI HILLS, COIMBATORE, TAMIL NADU

The present investigation was carried out in a dry deciduous forest of Maruthamalai hills to know the changes in species composition according to altitude and ecology of economically important plants. A total number of 128 plant species were identified and 112 of them are recognized as economically important. Based on importance value index, the species like Acacia torta, Chloris barbata, Eragrostis viscosa, Erythroxylon monogynum Pterolobium indicum and Zizyphus oenoplia are ecologically well established plants in the study forest. On the other hand the spices such as Polygala Jacobi, Portulaca guadrifida, Ruellia patula, Sida rhomboidea, Waltheria indica, Calotropis gigantean, Solanum torvum, Acacia leucophloea, Acacia nilotica, Acacia trotitis, Agave Americana, Bambusa arundinacea, Cassia fistula, Chloroxylon swietenia, Peltophorum pterocarpum, Pithecellobium dulce, Pongamia pinnata, Prosopis juliflora, Samanea saman, Thespesia populnea, Canavalia mollis, Leptadenia reticulata, Rivea hypocrateriformis etc., are considered as ecologically weaker species in the community. Hence priorities must be given to these species so as to protect the genetic stock and species as well.


INTRODUCTION
Maruthamalai, the shrine of lord muruga, is situated in the Western Ghats of Coimbatore District, Tamil Nadu. It is also called as Karumalai, Maruthuvamalai and Marundhumalai. In the past 3 yugas of the age of the world, it is well known for its herbal wealth and for the history of Pambatti Siddhar, one of the 18 Siddhars who established the temple at a height of 1175 m above msl. According to Champion and Seth (1968) the vegetation of the Maruthamalai hills comes under the dry deciduous forest. Ramachandran and Nair (1981) documented nearly 66 medicinal plant species in this area. However since last few decades the floristic wealth of Maruthamalai hills is depleted at an alarming rate due to the influence of heavy biotic pressure. In this juncture, the present ecological investigation was aimed to determine the ecological position, the level of establishment and the fitness to the habitat for all component species.

Study area
forms the western boundary to Coimbatore district. The hill area raises up to 1699 m high, forms scrub jungle up to 700 m with dry rocky soil from the foot hill and evergreen vegetation with grasslands above 700 m height. The trees in this region are small with stunted growth.

Phytosociological analysis
Phytosociological studies were carried out during the dry month of March, 2011 in a dry deciduous forest of Maruthamalai hills to obtain the quantitative characters such as frequency, density, basal cover and their relative values and importance value index. A one ha plot was established in each of three study plots and it was divided into 20 x 20 m workable units (quadrat). The species and their individuals' occurring in each quadrat were recorded. The basal areas at the point of emergence were measured for all the species. The quantitative characters of the constituent species were calculated as per the following formulae of Cottam and Curtis, (1956).

Number of quadrats in which the species present
The present study was carried out in a dry deciduous forest of Maruthamalai hills, which is Importance value index (IVI) is the sum of Since most of the stems are cylindrical, the basal area was calculated by using the formulae: Basal area = π(r) 2 Where, π = 3.14 and r is the radius of the stem at the point of emergence.
Number of occurence of the species environmental variables generally determine the degree of distribution of many plant species (Belsky, 1988 hirta, Zizyphus oenoplia etc., were showed high density during the time of sampling. Tansley (2003) stated that in the slopes of mountains where the X 100 Number of individuals of all species quantities of the relative frequency, relative density and relative dominance expressed per 300.

RESULTS AND DISCUSSION
The vegetation of each study plot (1ha) sorted out into four compartments viz., herbs, shrubs, trees and climbers. In all the three studied plots, a total number of 128 species has been enlisted. Off which a high number of 119 species was recorded in the study plot II followed by 117 species in the study plot I and 88 species in the study plot III. Out of 128 species available in three studied plots, 112 species are recognized as medicinally and economically important. The utilization value of the studied plots in a dry deciduous forest of Maruthamalai hills was found to be higher because of the presence of large number of plant species (87.50% of the total flora) as economically important. Paulsamy (2005) also identified a great percentage of economically important species in the floristic list of Nilgiri sholas, adjacent mountain range to the present study area.
The distribution of some of the economically important plants like, Acacia torta, Erythroxylon monogynum, Fluggea leucopyrus and Zizyphus oenoplia was even in all three studied plots. It may be explained that the factors like suitability of microhabitat, dispersal mechanism of seeds, germination efficiency, degree of survivability of seedlings and many other intrinsic characters are playing major role for their successful distribution. Many species in three studied plots like Calotropis gigantean, Solanum torvum, Acacia nilotica, Acacia torta, Delonix regia, Eucalyptus globules, Peltophorum pterocarpum, Samanea saman, Tectona grandis etc., have showed restricted distribution. The external factors like topography, soil conditions and biotic disturbances and some intrinsic factors like dispersal mechanism, seed longevity, dormancy period and germination efficiency are some of the subtropical and temperate vegetations are available, many local climates are existing which result the variation in the population size of many plant species in the communities. On the other hand, many species like Calotropis gigantea, Solanum torvum, Acacia trotitis, Agave americana, Cassia fistula, Eucalyptus globulus, Pithecellobium dulce, Pongamia pinnata, Samanea saman, Tectona grandis, Terminalia arjuna, Thespesia populnea, Leptadenia reticulata etc., were present with low densities in all studied plots may also be due to their poor reproductive potential with less seed output and weaker competitive ability (Chandrasekaran and Swamy, 1995).
Similarly, a high number of economically important plants occupied higher basal area in three studied plots. This may be due to the presence of suitable climate and soil conditions for the growth of such economically important plants in Maruthamalai hills. In addition, the shade provided by the trees also enhancing the growth of these species which naturally being a shade tolerance. Padmavathy (2005) reported in a similar fashion that the forest understories of Nilgiri contained more number of economically important plants with greater density and basal area.
In all the three studied plots of Maruthamalai hills, the ecological picture of economically important plants is highly notable. Among the 112 species of economically important plants, many species like Acacia torta, Chromolaena odorata, Euphorbia hirta, Erythroxylon monogynum, Fluggea leucopyrus, Mollugo pentaphylla, Pterolobium indicum, Tarenna asiatica, Zizyphus oenoplia etc., were determined as well established species on basis of their higher IVI values in comparison to other species. Suitability of habitat, dispersal mechanism of seeds, seed output, reproductive efficiency, longer viability, less demand, rapid regeneration and development of adaptive features according to seasons are accounted to be the reasons for their success in the environmental of present study area (Ramakrishnan, 1991;Paulsamy, 2005).
Total basal area of the species Relative dominance = X 100 Total basal area of all species