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Forest landscapes of the southern western Ghats, India

 | 
B.R. Ramesh
, 
Rajan Gurukkal

Part II. Analysing The Landscape: The Anamalai Region (Nelliyampathi Hills)

3. Forest Types and Status: Ecological Characteristics and Impact of Disturbance

B.R. Ramesh, P.V. Karunakaran, Pierre Couteron, G. Muthu Sankar et Danny Lo Seen

Texte intégral

1Vegetation as a template of a landscape is the visible expression of the prevailing environment. Within a landscape, heterogeneity in vegetation depends on local variability in bioclimate, soil and topography. However, interferences from anthropogenic activities further modify vegetation into various stages of degradations and transformations. Thus the characterization and classification of vegetation based on its dynamics and succession along the natural gradient and the disturbance regime are fundamentally important for developing any management plan at landscape level.

2To understand the effect of the natural gradient, especially of bioclimate on vegetation, the concept of “climatic climax” has been adopted. According to Clements (1916, 1936), a climax is the mature, stable and optimal state of a phytocoenosis, representing the final outcome of a progressive series of successive stages of vegetation. Under a given climate, all the successions converge towards a unique climax, which is the “climatic climax”. Pascal (1986) further extended the definition of the ‘climax’ to include sylvigenetic dynamics, structure and floristic composition of a forest mosaic (Aubroville, 1938) when it is in equilibrium with the environmental conditions. Further, based on the dynamic relationship of formations he classified the vegetation into three main groups (Figure 3.1):

  • Climax formations in equilibrium with the prevailing condition of environment when biotic interference is minimum;
  • Formations potentially linked to the climax after moderate exploitation, however, may return to climax if they are protected;
  • Highly degraded formations with different physiognomy due to repeated logging and other biotic pressures like grazing and fire. These formations may no longer return to a climax under natural condition

Figure 3.1 Stages of successions derived from a climax forest

(adapted from Pascal, 1986)

Based on these concepts and principles of ‘dynamic relationships’ and ‘succession’, vegetation types of the study area are defined using phenology, floristic composition and physiognomy and are classified along the gradients of bioclimate as well as degradation stages.

Materials and methods

Criteria and terms used for classification

Phenology

3Forests are separated into 3 categories based on the relative percentage of evergreen and deciduous species:

  1. Evergreen forests comprise pure patches of evergreen species, although a few deciduous ones may be found in the openings or margins.
  2. Deciduous forests, where the majority of species shed their leaves during the dry season.
  3. Semi-evergreen forests represent a mixture of deciduous and evergreen species in various proportions.

Physiognomy

4The physiognomy is external appearance of the vegetation that includes biological forms, structure and density. The physiognomy varies according to the nature and intensity of disturbance (Figure 3.1). In the land cover map, the physiognomy is mainly classified based on density of stand and canopy.

5The term forest is strictly reserved for formations dominated by a tree stratum. A forest is considered dense when the canopy cover exceeds 70 %; moderately disturbed between 50 and 70 %; highly disturbed between 30 and 50 %.

  • For woodland, the canopy cover is generally varying between 50 to 70 % with paucity in under storey trees. When the undergrowth is dominated by a carpet of tall grasses it is called a savanna woodland and when bamboo is dominated as undergrowth it is called a bamboo woodland.
  • Tree savanna to grassland: these formations are characterized by a continuous grass cover with a few scattered trees (tree savanna) or devoid of any woody species.
  • Thickets refer to a continuous or discontinuous shrubby layer either along the edges of the forest and grassland or as regenerations in the clearings of forests and plantations.

Floristic types

6Generally the term ‘primary forests’ is widely used to refer to ‘climax forests’. The primary forests and their potentially linked formations, with the same physiognomy or phenolgy are further divided into ‘floristic types’ based on the main differences in their floristic composition in relation to gradient in rainfall and temperature. Floristic types are named after certain tree species, which are selected by their abundance or their characterizing value to indicate that particular bioclimatic zone. For example in evergreen forests, Dipterocarpus indicus, which is not observed above 700 m, is one of the key species selected to characterize the low elevation wet evergreen forests. Similarly Cullenia exarillata, generally found above 700 m is considered as an indicator species of the medium elevation type.

Geographical Information System and satellite image analysis

7The land cover and land use map of the study area is prepared using a Geographical Information System (GIS) derived from following information sources.

  1. Toposheets (scale1: 50,000): Eight toposheets covering the study area were scanned and georeferenced in the GIS. They also served as geographic reference system to which all other spatial data are converted. Relevant information such as settlements, plantations and Reserved Forest boundaries were also digitised.
  2. GPS points: GPS receivers were used to identify the coordinates of samplings or ‘ground truth’ points during field surveys. This spatially referenced information was included in the GIS and used further to interpret satellite images.
  3. Field inventory: Intensive field surveys enabled us to identify species characteristic to the forest type as indicated through their frequency of occurrence and pattern of distribution. Further the surveys helped us in making out the principal changes in the stand structure and floristic composition in relation to altitude and degrees of disturbance. To interpret the satellite image, 139 ‘ground truth’ points in the study area were established. Around each point (nearly 50m radius), the extent of common species and density of trees were estimated. In addition, information on vegetation from 145 sampling plots (0.1ha, for studying vegetation structure) and 82 transects (for studying distribution and abundance of fauna) has been also used.
  4. Satellite images: Satellite scenes, (24 m pixel resolution) acquired during the dry (IRS-P6 LISS3, March 2004) and after the wet (IRS-P6 LISS3, December 2003) seasons have been used in the study. The satellite images were georeferenced in the GIS and served mainly as the basis for vegetation mapping.
  5. Vegetation mapping: A detailed (1: 50,000 scale) vegetation map of the study area has been prepared through supervised classification of satellite images of two-seasons. First, the four image layers (1: green, 2: red, 3 near infrared and 4: shortwave infrared) of each of the two cases of coverage, were joined into a single image with 8 layers. The supervised classification procedure was carried out on that resulting image by taking training areas according to the ground knowledge and GPS control points. The image was then classified following similarities of spectral characteristics with those of the spectral signatures of the training areas. The most distinct and easily separated classes thus obtained are: evergreen forests, deciduous forests, forest plantations, cash crops, grasslands and water bodies. These relatively broad classes were further refined by carrying out supervised classification on one class at a time to segregate dense forests, disturbed to degraded formations, reeds, bamboos, teak plantations, commercial plantations and swamps (vayals). Further, using ancillary information on rainfall isohyets and temperature gradients and using distribution pattern of certain species that are indicators of these climatic zones, the evergreen and deciduous forests were subdivided into primary and secondary, medium and low elevation forests.

Land cover and land use map

8Based on the above concepts, materials and methods, a land cover and land use map of the study area that include LU13 and 16 has been prepared (since the map is prepared at A0 size, it is separately given in a CD at the back cover of this book). Although the map is prepared at 1: 75,000 scale, the information in the map is at the resolution of 1: 50,000 scale. In the map, the land cover generally refers to natural vegetation and their degradation stages and the land use corresponds to human made formations like forest and commercial plantations, settlements, home gardens etc. The drainage and roads networks (taken from topo maps) and, forest administrative boundaries (Division and Ranges) are superimposed on the map. The extent of each land cover and land use class is given in Table 3.1.

  • 1 DDS: Dipterocarpus indicus – Dipterocarpus bourdilloni – Strombosia ceylanica type
  • 2 CMP: Cullenia exarillata – Mesua ferrea – Palaquium ellipticum type
  • 3 LTD: Lagerstroemia microcarpa – Tectona grandis – Dillenia pentagyna type

Table 3.1 Natural1 vegetation and2 other land use3 types

Table 3.1 Natural1 vegetation and2 other land use3 types

Forest types in relation to bioclimate

9In the study area, 4 types of primary forests and their respective degradation stages have been recognised. These types are closely correlated with the temperature and rainfall regimes (Table 3.2). Wet evergreen forests are mostly found in the zone with more than 2000 mm rainfall; where as dry fringe evergreen and primary moist deciduous types are found in the Parambikulam basin where the rainfall varies between 1400 and 2000 mm. The low (<700 m) and medium elevation (>700-1500 m) wet evergreen forests are clearly distinguished by a decrease in the minimum temperature with increasing altitude. The temperature of low elevation forests generally corresponds to >23ºC mean temperature during the coldest month (t), whereas medium elevation forests are mostly found in the zone where ‘t’ varies between 16 and 23ºC. The length of dry months varies from 2 to 4 months in the areas of wet evergreen forests and 4 to 6 months in the areas of dry fringe forests and primary moist deciduous forests.

Table 3.2 Primary forests in relation to rainfall, temperature and length of dry season

Primary forest types Rainfall (mm) Mean temperature of the coldest month ‘t’ (° C) Length of dry season
Low elevation wet evergreen forests >2000 >23 2-4
Medium elevation wet evergreen forests >2000 16 - 23 2-4
Dry fringe evergreen forests 1500 -
2000
16 - 23 4-6
Primary moist deciduous forests 1500-2000 >23 4-6

Primary evergreen forests and degradation stages

Primary evergreen forests (dense to disturbed)

10The dense forests of low and medium elevation when they are least disturbed, are tall with 30 to 40 m high upper storey and the emergent often reaching some times over 50 m. The trunks are tall, straight and smooth with a high branching. The base of the bole is often enlarged into buttresses, some times terminating into surface-roots. Stilt roots are common when species belongs to Myristica and Poeciloneuron. These forests are also characterized by the abundance of lianas, epiphytes and cauliflorus trees (Baccaurea). The herbaceous stratum is nearly absent. The vertical profile of dense forest generally shows 4 structural ensembles (sensu Halle et al, 1978; Pascal, 1988). The upper ensemble represents the canopy and includes trees ranging between 24 and 40 m high. The lower level of ensembles- II, III and IV-occupies 16-24, 8-16 and <8 m respectively.

11Within this general framework of dense forests, in spite of having dense canopy, certain primary forests show variations in stand structure due to different kinds of anthropogenic pressures like fire, grazing etc.

Low elevation type (<700 m)

12The low elevation type is typical of ‘Dipterocarp’ forests, potentially found in LU13 and in the highly dissected portion of LU16. According to Forest Map of South India (Ramesh et al, 2002), these ‘Dipterocarp’ forests in the study area are referred to as the Dipterocarpus indicus - D. bourdilloni – Strombosia ceylanica type. Like elsewhere in the Western Ghats, the ‘Dipterocarp’ forests were selectively logged for softwood species like Dipterocarpus indicus, D. bourdilloni and Vateria indica that are generally canopy or emergent trees. Consequently, D. bourdilloni that is having a lower limit of altitudinal range (<500 m) has become very rare, with only a few individuals reported from Pooyamkutti valley. Dipterocarpus indicus, although recorded sporadically, the occurrence of big trees are less. The Vateria indica, relatively resilient compared to other two ‘Dipterocarps, ’ is commonly found even in disturbed forests of low elevation and also shows its extension into medium elevation forests.

13Other species that are characteristic at canopy and subcanopy level include Otonephelium stipulaceum, Chrysophyllum lanceolatum, Semecarpus auriculata, Poeciloneuron indicum, Strombosia ceylanica, Cryptocarya bourdillonii, Drypetes elata, Plalaquium ellipticum, Hopea malabarica, Myristica dactyloides, Diospyros bourdilloni, Diospyros ghatensis and Drypetes elata. At understorey level (ensembles 3 and 4), Hydnocarpus alpina, Hydnocarpus pentandra, Knema attenuata, Taraktogenos macrocarpa, Hopea ponga, Diospyros paniculata, Baccaurea courtallensis etc., are common. In disturbed to highly disturbed forests, species with wider ecological amplitude viz Polyalthia fragrans, Pterygota alata, Artocarpus gomezianus, Antiaris toxicaria and Bombax ceiba dominate the canopy. Atuna travancorica and Terminalia travancorensis, endemic to southern Western Ghats, are found characteristically in the low elevation forests between Chalakudi and Pooyamkutti valleys.

Medium elevation type (700-1500 m)

14Medium elevation forests are structurally very similar to low elevation ones, especially at the lower limits they are tall (canopy 30-40 m) with four structural ensembles. Towards the upper limit of high hills, the forests are stunted with two to three strata (canopy <18 m). In the study area, these medium elevation forests are mostly confined to partially dissected surfaces on plateaus and upper limits of the highly dissected hills of LU16. They are more compact and relatively less disturbed compared to the low elevation type. However, in Nelliyampathi and in the northeastern corner of the Sholayar plateau, considerable areas of these forests have cardamom as undergrowth under the thick canopy of original forest.

15Floristically medium elevation forests differ from low elevation types for the disappearance of species like Dipterocarpus indicus, Dipterocarpus bourdillonii, Kingiodendron pinnatum, etc. and for the appearance of Cullenia exarillata, Ardisia pauciflora, Drypetes venusta etc. At this elevation range, the relative abundance of certain species like Strombosia ceylanica, Vateria indica, Diospyros bourdilloni etc. have also become less.

16Floristically, the medium elevation forests in the study area are referred to Cullenia exarillata – Mesua ferrea – Palaquium ellipticum type (Ramesh et al, 2002). All these three are canopy species. Although Cullenia is exclusive to medium elevation type, other two species are found in low elevation too. However, they are relatively more frequent at medium elevation. Apart from these typical species, other common canopy tree or emergent species are Diospyros sylvatica, Drypetes elata, Cinnamomum keralaense, Syzygium gardneri, Dimocarpus longan, Semecarpus travancorica, Holigarna nigra, Aglaia jainii, and Litsea oleoides. In the second and third ensembles Agrostistachys borneensis, Symphilia mallotiformis, Tricalysia apiocarpa, Myristica dactyloides, Homalium travancoricum, Diospyros paniculata, Drypetes venusta, Litsea bourdillonii, Litsea keralana, and Aglaia exstipulata are frequently encountered. In the fourth ensemble Paracroton pendulus, Orophea erythrocarpa, Tabernaemontana gamblei, Psychotria nigra, and Acronychia pedunculata are common.

Dry fringe evergreen forests

17The evergreen forests along the Palghat Gap, especially towards Tamil Nadu are unique in floristic composition. One such patch of evergreen forest, which falls in the Parambikulam Basin known as Karian shola (Topslip). This patch of evergreen forest, amidst primary deciduous forests is spread over between 700 and 1100 m elevation. Although situated in the less rainfall zone (1800 mm), the stand structure is very close to that of medium elevation wet evergreen forests, probably due to an edaphic factor.

18The absence of Cullenia exarillata and Palaquium ellipticum stands out significantly in the floristic composition. The common canopy and subcanopy species includes: Syzygium gardneri, Polyalthia fragrans, Artocarpus gomezianus, Fahrenheitia zeylanica, Aglaia barberi, and Diospyros sylvatica. Mesua ferrea is rarely present. Among the lower storey species Drypetes oblongifolia and Dimorphocalyx lawianus are very common. Other species frequently found in this layer include Excoecaria crenulata, Litsea stocksii, Orophea erythrocarpa, Miliusa sp., Scolopia crenulata, and Canthium dicoccum. Dry evergreen forest species like Alseodaphne semicarpifolia and Diospyros ovalifolia have also been recorded.

Secondary or degraded stages

19The formation resulting from the degradation of the primary dense or moderately disturbed forests is grouped under secondary or degraded stages. The dynamic relationships of these successive stages are already shown in the Figure 3.1.

Highly disturbed forest to secondary evergreen and semi evergreen forest

20The degradation of primary dense forests into highly disturbed or secondary forests is reflected in the following changes in stand structure and floristic composition. These changes are applicable to all low and medium elevation primary evergreen forests as well as dry fringe forests.

  • Disappearance of emergent trees;
  • Upper strata relatively low and not forming continuous cover;
  • Dense undergrowth due to the proliferation of invasive species and by the saplings of secondary species;
  • Disappearance of least tolerant sciaphylous species, which are often characteristic primary species (Dipterocarpus, Cullenia etc.).
  • Multiplication of evergreen species with wider ecological amplitude or helliophyllus species- as seen in the highly disturbed or secondary forests. Such species include: Alstonia scholaris, Polyalthia fragrans, Persea macrantha, Hopea malabarica, Holigarna grahamii, Syzygium cumini, Olea dioica, Epiprinus mallotiformis etc. at upper strata; Actinodaphne malabarica, Antidesma menasu, Aporosa lindleyana, Archidendron monodelphum, Clerodendron viscosum, Euodia lunuankenda, Leea indica, Macaranga peltata, Mallotus phillipensis, Ixora brachiata, Xanthophyllum flavescens etc. at lower levels.
  • In case of semi-evergreen forests the height of the stand hardly exceeds 20 m. The proportion of deciduous and evergreen species varies between 40 to 60 %. Upper strata are generally dominated by deciduous species like Albizia odoratissima, Bombax ceiba, Dalbergia latifolia, Holoptelea integrifolia, Lagerstroemia microcarpa, Sterculia guttata, Dillenia pentagyna, Tabernaemontana heyneana, Pterospermum spp., Terminalia bellerica, T. paniculata, Sterospermum chelonoides, Trewia nudiflora etc. The lower stratas have more or less same species like in the secondary evergreen forests.

Secondary moist deciduous forests (dense forests, woodland to bamboo woodland)

21Based on the moisture regime, moist deciduous forests are divided into primary and secondary moist deciduous types. The primary moist deciduous forests, an intermediary between wet evergreen and dry deciduous, generally occupy the zone of 1500 to 2000 mm rainfall. The secondary moist deciduous forests occur within the potential area of wet evergreen formations, where the rainfall is more than 2000 mm. Although the stand structure and floristic composition are almost similar in both the types, the relative dominance of certain species varies. In the study area the secondary moist deciduous formations (dense forest to woodland or savanna woodland) are predominantly spread over in LU13 and highly dissected part of LU16, especially around the Idmala Ar valley.

22The height and the quality of stand mainly depend on the depth and fertility of soil as well as on the intensity of anthropogenic pressures including fire. The height can vary between 15 and 30 m. Generally, dense forests have two strata, the upper and lower. The upper one has individuals, ranging from 12 to 30 m with a compact canopy, whereas the lower stratum, less dense and has individuals between 6 and 12 m high. Undergrowth represents shrubs or treelets and is lower than 6 m. In the study area, except a few good patches, the moist deciduous forests in the form of woodland cover a large area. The paucity of understorey trees, either due to illicit cutting or frequent burning is a characteristic feature of woodlands. The undergrowth is either covered by a thick carpet of weeds or tall grasses.

23Floristically they are similar to primary moist deciduous, though some deciduous species like Dillenia pentagyna, Albizia odoratissima, Terminalia bellirica, Tabernaemontana heyneana, Strychnos nux-vomica, Alstonia scholaris, are relatively more common than in the primary forests. Xylia xylocarpa is found abundant as in the primary moist deciduous. Tectona grandis is often found mixed with other species in dense formations. In the dense forests, the under-storey is dominated by evergreen species like Ixora brachiata, Olea dioica, Persea macrantha, Dimocarpus longan, Flacourtia montana, Macranga peltata etc.

Primary moist deciduous forests and degradation stages

24The primary moist deciduous forests in the study area come in the Parambikulam basin where the rainfall is around 1500 mm. The denser part of the primary forest rarely covers large continuous areas and it often forms a mosaic with the open forests. The dense forests generally have 2 strata. The first stratum represents trees of 20 to 30 m heights with nearly 70 % canopy cover. The trees are generally low-branched, though some species (like Terminalia, Haldina, Tectona, Lagerstroemia) have individuals with straight boles. The second stratum is formed of scattered trees between 6 and 12 m high. The under storey, which includes shrubs and under shrubs is generally less than 6 m high. The bushy and herbaceous strata are not well developed in the dense forest, though they become more apparent with an increase in the openings of the stand. In the Paramikulam basin, woodland with bamboos cover a large area compared to dense forests.

25Floristically primary moist deciduous forests in the study area belong to Lagerstroemia microcarpa – Tectona grandis – Dillenia pentagyna type (Ramesh et al, 2002). Dillenia pentagyna and Tabernaemontana heyneana are the characteristic species of this type. Lagerstroemia microcarpa and Tectona grandis, together with other species such as Anogeissus latifolia, Grewia tiliaefolia, Dalbergia paniculata, Pterocarpus marsupium, Terminalia paniculata, Terminalia crenulata, Haldina cordifolia, Schleichera oleosa are common. Like in secondary deciduous, here also Xylia xylocarpa is very common, often found as gregarious patches. In the lower stratum, Careya arborea, Cassia fistula, Emblica officinalis, Radermachera xylocarpa are frequent. In some moist locations where the rainfall is between 1800 and 2000 mm, Glycosmis pentaphylla is found abundant as undergrowth.

Other degraded and natural formations

Bamboo

26The presence of bamboo (Bambusa bambos) formations is one of the dominant features of vegetation in the study area. They are more or less spread in patches all along deciduous formations (primary and secondary) either as pure patches or as bamboo woodlands especially on skeletal soil or steep slopes.

Reeds

27The reeds cover 11 % (207 km2) of the study area and are predominant around Idamalayar and Pooyamkutti valleys in LU16, where the rainfall is more than 3500 mm. Generally the reed grows in clusters of clumps. Each clump consists of a number of culms that attain a height varying from 4 to 12 m. The upper part of reeds is fire prone and the rhizomes are fire resistant. During the fire, the culms act as a fuel load, which burn the forest around and creat openings. In the openings thus created, with the availability of moisture in the soil during rainy season, reeds regenerate and proliferate gregariously through rhizome. Consequently reeds occupy the potential area of evergreen forests in a very high rainfall zone as pure patches or as undergrowths in a disturbed forest or woodlands.

28Two species of reeds are recorded in the study area: Ochlandra travancorica and O. scriptoria. Of these, the former is most common and widely spread. It is also exploited in large quantities for the pulp industry. Whenever reeds are found in the woodland, the tree species are mostly heliophyllus viz Vernonia arborea, Macaranga peltata, Mallotus tetracoccus, Tetrameles nudiflora, Sterculia guttata, Xylia xylocarpa, Hydncarpus pentandra etc. When they are as undergrowth in evergreen forests, at low elevation, they are found under the canopy of Hopea parviflora, Calophyllum polyanthum, Dipetrocarpus indicus, Vateria indica, Polyalthia fragrans, Bischofia javanica, Acrocarpus fraxinifolius etc. At medium elevation they found beneath the trees of Cullenia exarillata, Canarium strictum etc.

Swamps (vayal)

29Vayals are marshy lands dominated with sedges and grasses and occupy the flat bottom of the valleys. This ecosystem, though less in extent, forms an important constituent in the landscape on account of its biological diversity and animal utilization. The vayals are mainly found in the PAs such as Parambikulam and Thattekkad and non-PAs such as Malayattoor and Vazhachal. Some of the grass genera common to this ecosystem are Axonopus spp, Paspalum spp, Eragrostis spp. The common sedges found are Lipocarpa argentea, Fuirena umbellata, Fimbristylis tetragona, F. accuminata, F. junciformis, F. dephylla, Cyprus cuspidatus, C. difformis, C. tegetum and Rynchospora corymbosa. In Parambikulam WLS, the presence of coarse and unpalatable sedge, R. corymbosa and the invasion of bamboo along the fringes are posing severe management problem.

Grassland

30In the landscape units, grasslands are found scatted all over the area. Prominent patches are seen in Parambikulam WLS (Karimalagopuram, Vengoli, Pandaravari), Nemmara (Meenampara and Padagiri hill tops), Vazhachal (Charapapadam), Malayattoor (Soolimudi, Manjakallan, Vagiriyan), etc. The total extent of grasslands that could be identified in the study area is 51 km2. All these grasslands, lying generally below 1500 m, are situated on top of the hills and associated with shrubby vegetations. Grasslands are ecologically important as an ecosystem for the endangered Nilgiri tahr. The grass species commonly present are Andropogon lividus, Arundinella purpurea, Agrostis peninsularis, Chrysopogon zeylanicus, Eulalia phaeothrix, Sehima nervosum, Heteropogon contortus, Pollinia sp. Themeda sp., Ischaemum indicum and Tripogon bromoides. The woody species are represented by Wendlandia thyrsoidea, Glochidion sp. Terminalia chebula, Emblica officinalis, Careya arborea, Bridelia crenulata. In some places a dwarf palm, Phoenix is found in patches. The grasslands found in the lower reaches (valleys and valley bottom) are with tall grasses, and reach height up to 1.5 m. The grasslands both in the valleys and hilltops are equally infested with weeds and burn frequently. As a result, unpalatable Cymbopogon flexuosus and Pteridium (a fern) have become dominant features.

Trees savannas / thickets / clearings / fallows / rocky outcrops

31In the study area there are other naturally open or degraded formations. As their signature (at the given resolution) on the satellite image looks similar to grasslands, it is difficult to distinguish them from grasslands. These formations include:

32Tree savanna (usually towards the summit of the hills) as a mid zone between grasslands and forest edges. They generally compose of tall grasses like Themeda, Cymbopogon etc. along with sparse tree species like Glochidion malabaricum, Bridelia sp., Syzygium cumini, Pterocarpus marsupium, Terminalia alata, Careya arborea etc. These trees are mostly in stunted format (<8 m).

33Clearings, thickets, fallows (open areas corresponding to either the areas of forest after burning or extracted or failed areas of plantation): Often these patches are in the form of thickets composed of Lantana, Ipomea, and Eupatorium. Thickets are also found in the areas where the grazing is predominant. Fallows represent cultivation areas abandoned by adivasi-s.

Land use categories

Forest plantations

34Forest plantations largely represent teak. In the study area, teak plantations are mostly found in LU13 and LU16- along the valleys of the Chalakudi and Parambikulam basin. The teak found in Parmbikulam looks more robust where the moderate rainfall is conducive for such growth.

35In the map, two types of teak areas have been distinguished viz pure teak and mixed teak. In mixed ones, the teak trees generally found along with other naturally growing deciduous species like Terminalia spp. Lagerstroemia microcarpa, Grewia tiliaefolia, Sapindus tetraphylla, Tabernaemontana heyneana, Diospyros montana etc. There are two giant natural teak trees in the study area, ‘Kannimara Teak’ in the Parambikulam WLS and at Kappayam area in the Malayattoor division. Small areas of Eucalyptus plantations are found in Chalakudi valley. However, it seems to be the failed plantation.

Commercial plantation

36The main commercial plantations that are lying within the study area include coffee, cardamom, tea and pepper especially in the Nelliyampathi Hills and in Malakkappara of Vazhachal division. The rubber plantations are mostly found along the periphery of LU13 and the fringe areas. Extensive oil palm plantations are found along lower reaches of Chalakudi River.

37The total extent of the commercial plantation visible on the satellite images is about 135 km2. However, there are considerable areas of cardamom cultivation under the dense canopy of evergreen forests at Poopara (Parambikulam WLS), Variyam (Malayattoor division) and Nelliayampathi Hills that cannot be detected on satellite images.

Settlements / home gardens

38Home gardens and (or) settlements constitute one of the important land covers in the study area. These two together constitute an area of 44 km2 in the LUs. The main crops found in the home gardens are coconut, areca nut, cocoa, pepper, rubber, plantains, cashew and fruit trees such as jack tree, custard apple, guava and mango.

Overall status of forests and fragmentation of evergreen forests

39In the study area, the combined influence of heavy to moderate monsoon rains and the effect of relief and aspects, have naturally brought out the multiplicity in vegetation types (Figure 3.2) and associated flora (Box 3.1). The intense and diverse anthropogenic pressures have further affected part of these natural vegetations into various stages of degradation in terms of stand structure and floristic composition (see chapter 4).

Figure 3.2 Simplified vegetation types

Box 3.1 Floral wealth of the study area
The compilation of flora with the help of available literature (Sasidharan & Sivarajan, 1996; Sasidharan, 2002) and fieldworks indicate a record of 1835 species falling into 151 families and 859 genera in the study area. Among the families, 35 are monogeneric and 517 genera are monospecific. The most dominant families are Fabaceae (Faboideae – 117 sps.) Orchidaceae (110 sps.), Poaceae (109 sps.) and Euphorbiaceae (102 sps.). The dominant genera are Ficus (20 sps.), Impatiens (19 sps.), Crotalaria (16 sps.), Litsea (14) and Diospyros (14 sps.).
Among the species recorded, herbaceous flora dominate (799 species) the biological spectrum. There are 459 tree species, of which 335 are evergreen and the remaining are deciduous. Among the evergreen species 243 are primary and 82 secondary in nature.
Endemism and Conservation status
The number of endemic species recorded in the landscape units indicates the biogeographic significance of the area. The present study reported 473 (26 %) species of flowering plants, which are endemic to the Western Ghats. Out of these, 162 are trees, 89 shrubs 165 herbs, 29 climbers, 18 lianas and 8 stragglers. The endemic genera are represented by Anaphyllum, Blepharistemma, Decalepis, Haplothismia, Indobanalia, Indotristicha, Kanjarum, Moullava, Nilgirianthus, Ochreinauclea, Otonephelium, Poeciloneuron, Utleria and Willisia.
The critical conservation status of the recorded species is ascertained based on IUCN criteria and the species are categorized under endangered, vulnerable, threatened, rare, possibly extinct, extinct and species with low risk and data deficient. Among the recorded species 145 are categorized under critical conservation status. Of these, 40 species are endangered, 38 vulnerable, 49 rare and remaining under other categories. Among the rare and endangered species, 85 are endemics. Haplothismia exannulata, thought to be extinct has been recorded from Parambikulam WLS. Similarly Colubrina travancorica, a climber recorded in last century and since then thought to be extinct has also been rediscovered during this study. Some of the endangered species includes: Aglaia malabarica, Aporosa bourdillonii, Ardisia sonchifolia, Atuna travancorica, Cordia octandra, Derris benthamii, Desmos viridiflorus, Dysoxylum beddomei, Dysoxylum malabaricum, Eugenia indica, Hopea erosa, Hopea ponga, Impatiens auricualta, Impatiens wightiana, Kingiodendron pinnatum, Palaquium ravii, Polypleurum filifolium, Psychotria globicephala and Tarenna monosperma.

40The existing natural vegetation in the study area accounts for 72 % (1322 km2) of the total (1837 km2) area. Of this, 22 % are primary evergreen forests and 3 % are relatively dense deciduous forests of both primary and secondary. Degraded formations of the evergreen and deciduous forests together contribute to 47 %. However, when landscape units considered separately, LU16 has 30 % of primary forests and LU13 accounts to only 3 %.

41In general, primary evergreen forests are one of the most productive and highly biological rich ecosystems. In the context of Western Ghats, the low elevation evergreen forest types are unique because they represent the western most domains of the ‘Dipterocarp’ forests of the South and Southeast Asia. These forests are potentially rich in hardwood (Hopea, Calophyllum and Mesua) and softwood species (Dipterocarpus, Vateria, Bombax, Acrocarpus). Like else where in Kerala and other parts of the Western Ghats (Nadkarni et al, 1989), in the study area also, during the colonial times, large numbers of hardwood species were extracted for railway sleepers. Even the tram way was specially constructed to transport timber from Parambikulam to Chalakudi. In the post-colonial period, the low elevation forests were further exploited for soft wood species. According to the annual reports of the Kerala Forest Department, between 1960 and mid 1980s nearly forty thousand trees of Dipterocarpus bordillonii, D. indicus and Vateria indica were “selectively” logged for plywood industries (Ramesh et al, 1996). Forests were also clear felled for raising teak plantations and creation of reservoirs. In the last two decades, with concomitant demand for natural rubber and spurt in agro-economic conditions, rubber plantations, oil palm, pineapple and cashew have increased and replacing the already disturbed or degraded evergreen forests in the foothills of the Ghats (LU13).

42As the bioclimate and the forest type of medium elevation regions are suitable for cash crops, considerable extent (58 km2) of medium elevation forest type has been converted into tea, coffee and cardamom plantations in Nelliyampathi hills and Malakkappara in Sholayar region.

43The process of deforestation of natural forests into various land uses leads to several changes in landscape. Degradation and fragmentation of forests generally precede deforestation. The fragmentation of forests is especially recognized as detrimental to biodiversity conservation. Isolated small forest patches are more prone to disruption of ecological processes and to species extirpation or extinction than large and continuous forest tracks. This is mainly due to increases in "edge effects" corresponding to climatic, biotic or anthropogenic disturbances that originate in surrounding non-forest areas. The impact of edge effects on forest diversity is generally increasing with the length of patch perimeter while declining with patch area. Hence, patch shape indices are often used in addition to patch area to analyze patterns of forest fragmentation (Forman and Godron, 1986).

44To study the fragmentation of the evergreen forests in the landscape units, we extracted from the GIS all the polygons classified as evergreen forests with mention of their status related to disturbance (undisturbed, disturbed and highly disturbed) (Figure 3.3). The aim of the analysis was to determine to what extent patch size/shape attributes differ according to the disturbance status of the patch. Shape was characterized via a classical index:

Figure 3.3 Fragmentation of evergreen forest

45The ratio would reach a minimal value of 1 for a patch having a strictly circular shape and would reach high figures for patches having either elongated shapes or scalloped boundaries (Forman and Godron, 1986).

46Statistical distributions of patch area and patch shape ratio (D) were compared between disturbance classes using the non-parametric Kolmogorov-Smirnov (K-S) test for comparison of cumulated frequency distributions (Sokal and Rohlf, 1995). For this analysis, we considered only GIS polygons (i.e. patches) with an area of above 5 ha. The smaller polygons are not considered, because they may be merely artifacts during vegetation map preparation. To ensure the legibility of Figure 3.4, we did not plot directly the cumulated frequency distributions of patch areas and patch shape index, but rather their respective “log-odds-ratios” (as it is used in logistic regression) called LOR:

47LOR (x) = log [(1 − p (x)) / p (x)], where p (x) is the probability to see a given patch index take a value below x (cumulated frequency).

48Results of the K-S test showed that the patch area distribution of undisturbed evergreen forests was significantly different (P<0.01) from the distribution observed for the two classes of disturbed forest, due to a smaller proportion of patches less than 25 ha in undisturbed conditions (Figure 3.4a). In addition, no disturbed patch had an area above 600 ha, while there were eight and four patches of undisturbed forest exceeding 600 ha and 1,000 ha, respectively. Most of the small patches, as remnants, were generally found in LU13 and disturbed parts of LU16. The difference between patch area distributions of the two classes of disturbed evergreen forests was weaker, however, found slightly significant (P=0.04).

49Further analysis indicates that there was no significant difference (P>0.05) between the frequency distributions of values of the perimeter-area index and thus, no clear relationship appeared in this dataset between disturbance level and patch shape. But it can nevertheless be noted that the index never reach above 2.5 for patches of undisturbed (Figure 3.4b).

Figure 3.4 Comparison of cumulated frequency distributions for undisturbed (dotted line), disturbed (interrupted line) and highly disturbed (solid line) evergreen forests: (a) of patch area and (b) for patch shape index. The log-odds-ratios, LOR (x) is plotted instead of the cumulated frequency p (x) to ensure the legibility of the plot (see text for definitions).

Conclusion

50Escalation of anthropogenic factors, ranging from the increase in population in fringe areas to frequent fires, grazing, extractions of forest products, hydroelectric projects and commercial plantations, has contributed severely to the heterogeneity of the landscape at the cost of otherwise more homogenous vegetation types with the given bioclimatic variability. Among the forest patches, although large fragments are seen in the medium elevation ‘Cullenia type’ of forests, biologically rich low elevation dipterocarp forests are almost shrunk to one third of its potential area. Consequently, several typical endemic species like Dipterocarpus bourdilloni, Pseudoglochidion anamalayanum, Orophea thomsonii, Holigarna ferruginea, Palaquium ravii, Anacolosa densiflora, Syzygium travancoricum, S. beddomei, Ardisia sonchifolia, and Dysoxylum beddomei, which were recorded earlier, have either become very rare or extirpated. In order to control or effectively manage the factors that lead to degradation of forests, a comprehensive map which highlights the dynamic relationship of land cover and land use is required. Such a thematic map provides base line information to generate spatially explicit management zones to develop prudent management system.

Notes

1 DDS: Dipterocarpus indicus – Dipterocarpus bourdilloni – Strombosia ceylanica type

2 CMP: Cullenia exarillata – Mesua ferrea – Palaquium ellipticum type

3 LTD: Lagerstroemia microcarpa – Tectona grandis – Dillenia pentagyna type

Table des illustrations

Légende Figure 3.1 Stages of successions derived from a climax forest
Crédits (adapted from Pascal, 1986)
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Titre Table 3.1 Natural1 vegetation and2 other land use3 types
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Légende Figure 3.2 Simplified vegetation types
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Légende Figure 3.3 Fragmentation of evergreen forest
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Fichier image/jpeg, 46k
Légende Figure 3.4 Comparison of cumulated frequency distributions for undisturbed (dotted line), disturbed (interrupted line) and highly disturbed (solid line) evergreen forests: (a) of patch area and (b) for patch shape index. The log-odds-ratios, LOR (x) is plotted instead of the cumulated frequency p (x) to ensure the legibility of the plot (see text for definitions).
URL http://books.openedition.org/ifp/docannexe/image/6996/img-6.jpg
Fichier image/jpeg, 29k

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