Variation in bark thickness in a tropical forest community of Western Ghats in India

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1 Functional Ecology 8, ORIGINAL ARTICLE OA EN Variation in bark thickness in a tropical forest community of Western Ghats in India V. HEGDE,* M. D. S. CHANDRAN* and M. GADGIL *Dr A.V. Baliga College of Arts and Science, Kumta, 8343, Uttara Kannada, Karnataka, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur Campus, Bangalore 66, and Centre for Ecological Sciences, Indian Institute of Science, Bangalore 6, India Summary. We examined the relation between bark thickness and girth in a large sample of trees from evergreen and semi-evergreen rain forest.. There is a significant tendency for bark thickness to increase with tree girth. Removing this tendency, we find a significant trend for species from more disturbed habitats to possess thicker bark. 3. Species from more disturbed habitats also have a greater propensity for secreting gums and resins. 4. Nine of the species occurring in more than one habitat type and with a sample size of at least individuals show a tendency for possession of a thicker bark by individuals in the more disturbed habitats.. We conclude that bark thickness and occurrence of gums and resins are physiognomic structural attributes of value in characterizing tree communities created by different levels of disturbance. Key-words: Disturbance, evergreen forests, gums, resins Functional Ecology (8), British Ecological Society Introduction The bark of a tree serves a protective function, insulating against extremes of temperature, fire, desiccating winds and against herbivory and microbial infections (Romberger, Hejnowicz & Hill ). It has been noted that barks of tropical rain-forest trees are thinner and smoother than those of species in drier habitats (Richards ; Ashton 6; Whitmore 84). Jacobs (88) observed that monsoon-forest trees with a thick bark or a bark rich in moisture, such as Eucalyptus spp., have a better chance of withstanding fires. A study in Gabon showed that the barks of woody species in disturbed forests were more frequently associated with toxic chemicals than the woody species characteristic in less disturbed forests (Levin 76). These observations suggest that thickness of bark and occurrence of secretions like resins and gums might be correlated with the habitat of a tree species. These attributes could therefore be useful in arriving at a physiognomic structural classification of vegetation (Webb 68). We wanted to identify such physiognomic structural attributes to characterize the different components of the vegetation mosaic created by varying degrees of human impacts on the hill chain of Western Ghats in southern India. This calls for a community level quantitative study of bark thickness and occurrence of secretions, such as gums and resins. We have found no published investigation of this, although considerable information on mean bark thickness and secretions is scattered in the literature (Talbot ; Troup ; Council for Scientific & Industrial Research 6 66). We present here results of a systematic community level study which confirms such bark attributes to be useful indicators of the status of vegetation in the evergreen forest tracts of the Western Ghats. Materials and methods THE SETTING Our study area, the Uttara Kannada district (3 ' 3 'N; 74 ' 7 'E) is located at the centre of Western Ghats hill chain running parallel to the west coast of India. The annual rainfall of the district varies from mm along the west coast to mm along the crest of hills, declining to mm to the east. The hills, plateaus and valleys of the district have an extensive forest cover comprising the wet evergreen and semi-evergreen types. These natural vegetation types have been degraded to varying degrees by human influences over the last several centuries. The human influences, which have 33

2 34 V. Hegde et al. thus transformed the vegetation into a patchwork of different types, include shifting cultivation, fires, grazing by cattle, collection of leaf litter, lopping of green leaves for manure and extraction of timber and fuelwood. Such degradation is often accompanied by incursion of deciduous elements. This vegetation has been surveyed and mapped on a : scale by Pascal (8, 86). According to his classification, our study sites, located in the coastal hills and valleys of Kumta taluk (= county) harbour the following types of vegetation: () low elevation evergreen and semievergreen climax forests dominated by Persea macrantha, Diosyros spp. and Holigarna spp.; () low elevation disturbed evergreen and semi-evergreen forests; (3) secondary moist deciduous forests; (4) thickets; () tree savannas. The disturbances in type (), i.e. low elevation disturbed evergreen and semievergreen forests, usually involve selective harvesting of timber resulting in canopy gaps along with incidental damage to neighbouring trees and ground vegetation, as well as disturbance attendant on formation of logging roads and skidding trails. Type (3), i.e. secondary moist deciduous forests, are the result of wholesale vegetational changes involving slashing and burning of the original evergreen semi-evergreen types. In the course of time, if disturbances of this type do not repeat, type (3) gets converted into semievergreen through recruitment of more evergreen elements. Thickets and tree savannas occur closer to human settlements. The thickets are constantly being hacked for biomass, arresting their progressive succession. The tree savannas, comprising isolated trees or clumps of trees in a matrix of grass, are the result of periodic fires along with extensive harvests of woody biomass (Pascal 86; Chandran 3). METHODOLOGY Barks were sampled of a total of 36 individuals of tree species in three different localities in each of these five types of vegetation. In any given locality, samples were taken of three to five individuals of each species with a girth or circumference of cm at cm from ground level (girth at breast height cm). This led to the sampling of three to 3 individuals with an average of 6 per species. The sampling involved measurement of girth and removal of a piece of bark of a few cm in width and length at a height of cm above the ground, avoiding warts, thorns or other protuberances. The thickness of this bark was then measured with Vernier calipers to the nearest mm. Table. Sample size, preferred habitat, mean bark thickness for different girth at breast height (gbh) classes, significance of change in bark thickness in relation to girth and habitat, and occurrence of defensive chemicals for 77 tree species arranged in order of increasing preference for more disturbed habitats Mean bark thickness (in mm) for gbh classes Sample Preferred > Species name size habitats cm cm cm cm cm G D C Calophyllum polyanthum a * Chrysophyllum lanceolatum 3 a * 8 4 Dimocarpus longan 7 a * 7 Diospyros candolleana 8 a * Diospyros paniculata 3 a * 4 Ficus nervosa 7 a 6 6 Garcinia gummi-gutta a * Garcinia morella 4 a * 7 Holigarna grahamii 4 a * R Reinwardtiodendron anamallayanum 8 a * 3 6 Lophopetalum wightianum 3 a 6 4 Myristica dactyloides 3 a * Strombosia ceylanica 8 a * Syzygium gardneri 3 a * 6 + Polyalthia fragrans a,b * Diospyros oocarpa 3 a,b * Actinodaphne malabarica a,b * 6 6 Knema attenuata 3 a,b * Bischofia javanica a,b Mimusops elengi 6 a,b * G,T Myristica malabarica 8 a,b * Artocarpus hirsuta 7 a,b * Cinnamomum spp. a,b *

3 Table. Continued. Mean bark thickness (in mm) for gbh classes Sample Preferred > Species name size habitats cm cm cm cm cm G D C Diospyros buxifolia 4 b Ficus callosa 3 b 7 7 Persea macrantha 3 b G Pterospermum acerifolium 6 b * 4 3 Terminalia crenulata 3 b 8 3 T Pterospermum reticulatum 8 b * Lagerstroemia speciosa 4 b * R,T Garcinia spicata a,c * T Macaranga peltata a,b,c * T Olea dioica 3 a,b,c * Lagerstroemia microcarpa 7 a,b,c * T Aglaia elaeagnoidea 3 a,b,c * Hopea ponga a,b,e * Cissampelos pareira a,b,e * Artocarpus gomezianus a,c,e T Flacourtia montana 4 b,c * Sapindus emarginatus b,c Ficus virens 4 b,c * Terminalia paniculata 7 b,c * T Vitex altissima b,c * F Diospyros montana b,c * Syzygium cumini a,e * R,T Mangifera indica a,b,c,d,e * G,R,T Xantolis tomentosa 4 a,b,c,d * Ixora brachiata a,b,c,e * Aporusa lindleyana a,b,c,e * Syzygium laurifolia a,d,e * Alstonia scholaris 4 b,d * R Carallia brachiata b,e * P Hydnocarpus pentandra b,e * Dillenia pentagyna c * T Alseodaphne semecarpifolia 3 c *4 3 Tabernaemontana heyneana c * R Microcos paniculata c * Murraya koenigii 3 c * 4 6 Grewia tiliifolia 7 c * Zanthoxylum rhetsa 3 c * Holigarna arnottiana 4 b,c,d,e * R Lannea coromandelica b,c,d,e * R,T Strychnos nux-vomica 33 b,c,d,e * A,P Stereopspermum colais b,c,d * Schleichera oleosa 7 b,c,e * T Mammea suriga c,d * Careya arborea c,e * Catunaregam spinosa c,e * Phyllanthus emblica 6 c,e * T Memecylon umbellatum 7 c,e * 3 6 Bombax ceiba 4 c,e * 3 8 Buchanania lanzan 7 c,d,e * G,T Acacia chundra 6 d * 8 T Ficus arnottiana 6 d * 7 3 Sapium insigne 3 e * P Artocarpus heterophyllus 3 e 8 8 G Ficus benghalensis 3 d,e *6 a, low elevation evergreen and semi-evergreen; b, low elevation disturbed evergreen and semi-evergreen; c, secondary moist deciduous forest; d, thicket; e, tree savanna; * girth cannot be measured in case of two species of strangler fig; G, increase in bark thickness with girth increment. For species distributed in different habitats: D, increase in bark thickness in more disturbed habitats: + significant at % level; ++ significant at % level; +++ not significant at % level; C, presence of defensive chemicals: gum (G), resin (R), tannin (T), flavenoids (F), alkaloids (A), poisons (P): from the literature (Council for Scientific & Industrial Research 6 66).

4 36 V. Hegde et al. 8 British Ecological Society, Functional Ecology,, The term bark is often loosely used to describe everything external to the wood. This typically includes the innermost layer of vascular cambium, secondary phloem, phelloderm or secondary cortex, phellogen or cork cambium and phellum or cork as one progresses outwards. Because in practice it is difficult to measure thickness of each individual layer of bark, the thickness as reported here is a gross measure of all tissues external to wood. The occurrence of exudations like gums or resins, as shown by the liquid exuding from the localized zones of the cut bark as well as older injuries, was also recorded for these trees. This information was supplemented by that from the literature; the nature of the exudation was also confirmed by referring to the available literature (Troup ; Council for Scientific & Industrial Research 6 66). Results The primary objective is to assess the value of bark thickness and occurrence of gums and resins as indicators of the habitat of the plant community. The 77 tree species studied were arranged along the gradient of increasing disturbance, from climax and disturbed evergreen and semi-evergreen forests to secondary moist deciduous forest, to thickets and tree savannas (Pascal 88). Forty-five of the 77 tree species studied occur in more than one kind of habitat; for example, Mangifera indica (Mango) is present in all five. The trees exhibit a wide range of variation in girth in these habitats, from the lower threshold of cm to over cm. The higher girth classes are more frequent in the less disturbed habitats. This may introduce a bias because bark thickness is known to increase with girth in species such as Sequoia sempervirens ( Berger 73). Our own data confirm this: in 4 out of 38 species where sample size exceeded individuals the thickness of bark showed an increase with girth at % level of significance (Z-test, Sokal & Rohlf 87). In order to remove this bias, Table gives data on mean bark thickness for five different girth classes for the 77 species studied. The sample size is insufficient for the two highest girth classes but for the three lower girth classes, namely, and cm, there is a significant tendency for bark thickness to increase in species from the more disturbed habitats at % level of significance (Spearman s rank correlation, as well as Z-test, Sokal & Rohlf 87). There is also a greater tendency for the species from more disturbed habitats to possess gums and resins (column C, Table ). Apart from this interspecific, presumably genetically fixed response, there is also a tendency for nine of the species with a sample size of at least trees that occur in more than one kind of habitat to exhibit thicker bark in more disturbed habitats (column D, Table ). This significance is at % level for seven and % level for two species on the Wilcoxon signed rank test (Sokal & Rohlf 87). This suggests that species may be selected for a phenotypically plastic response towards developing a thicker bark in the more disturbed habitats. Discussion Figure summarizes the entire data set on bark thickness for the five distinct habitats. Evidently, even without correcting for girth, there is a clear tendency for greater prevalence of thicker bark along the gradient of increasing disturbance from climax evergreen through thicket vegetation. The distribution of mean bark thickness for tree-savanna habitat is bimodal with many species exhibiting relatively thin bark in the range of 4 mm. This may be related to twofold influences in the origin of tree savannas: fire and overharvests of wood. Several thin barked evergreen species, such as Hopea ponga, Sageraea dalzelli, Ixora brachiata, Aporosa lindleyana and Holigarna arnottiana, regenerate in parts of tree savannas protected from fire. These co-occur with fire-hardy and thicker-barked species like Syzygium cumini, Lannea coromandelica, Careya arborea and Bombax ceiba, resulting in the bimodal distribution. This evolutionary and plastic response of bark thickness and defensive compounds is presumably generated by species with thicker barks and defensive compounds surviving or reproducing better in more disturbed habitats. Some of these are species with very broad habitat tolerance, for example, M. indica and S. cumini. Some species, such as Terminalia paniculata and Tabernaemontana heyneana, may have existed originally in naturally open habitats within climax forest, such as on steep escarpments. Most of the thicker barked species favouring more disturbed habitats have, however, probably originated in the drier regions to the east of the evergreen-forest tract on the Deccan plateau and slowly dispersed into the evergreen forests with human activities beginning with fire and shifting cultivation at least years ago (Gadgil & Chandran 88). Examples of such species include Xantolis tomentosa, Dillenia pentagyna, Grewia tiliifolia, Lannea coromandelica and B. ceiba. Bark thickness and occurrence of gums and resins are then clearly attributes that may be used to characterize vegetation at different stages of disturbance, at least so far as the evergreen-forest vegetation of the Western Ghats is concerned. Acknowledgements This study was supported by a summer student fellowship to V.H. from the Jawaharlal Nehru Centre for Advanced Scientific Research and by a grant from the Ministry of Environment and Forests, Government of

5 37 Bark thickness in a tropical forest community Percentage individuals (a) (b) (c) 4 3 (d) Percentage individuals (e) Percentage individuals Bark thickness classes Fig.. Distribution of individuals in different bark-thickness classes in (a) climax evergreen and semi-evergreen, (b) disturbed evergreen and semi-evergreen, (c) secondary moist deciduous forests, (d) thickets and (e) tree savannas. Bark thickness classes: (up to 3 mm), (4 6 mm), 3 (7 mm), 4 ( mm), (3 mm), 6 (6 8 mm), 7 ( mm), 8 ( mm), (6 8 mm), (> mm). 8 British Ecological Society, Functional Ecology,, India to the Centre for Ecological Sciences. We are grateful to the Forest Department of the Government of Karnataka for help in undertaking this field word. References Ashton, P.S. (6) Speciation among tropical forest trees: some deductions in the light of recent evidence. Biological Journal of Linnaean Society, 6. Berger, G.A. (73) Development and Shedding of Bark. Shedding of Plant Parts (ed. T. T. Kozlwski), pp. 36. Academic Press, New York. Chandran, M.D.S. (3) Vegetational changes in the evergreen forest belt of Uttara Kannada district of Karnataka State. PhD thesis, Karnatak University, Dharwad. Council for Scientific and Industrial Research (6 66) The Wealth of India, Raw Materials. CSIR, New Delhi. Gadgil, M. & Chandran, M.D.S. (88) On the history of Uttara Kannada forest. Changing Tropical Forests (eds J. Dargavel,

6 38 V. Hegde et al. K. Dixon & N. Semple), pp Centre for Resource and Environmental Studies, Canberra. Jacobs, M. (88) The Tropical Rain Forest. A First Encounter. Springer-Verlag, Berlin. Levin, D.A. (76) Alkaloid bearing plants: an ecogeographic perspective. American Naturalist, Pascal, J.P. (8) Forest Map of South India, Shimoga. Karnataka Forest Department, Bangalore and French Institute, Pondicherry. Pascal, J.P. (86) Explanatory Booklet on the Forest Map of South India. French Institute, Pondicherry. Pascal, J.P. (88) Wet Evergreen Forests of the Western Ghats of India: Ecology, Structure, Floristic Composition and Succession. French Institute, Pondicherry. Richards, P.W. () The Tropical Rain Forest: An Ecological Study. Cambridge University Press, Cambridge. Romberger, J.A., Hejnowicz, Z. & Hill, J.F. () Plant Structure: Function and Development. Springer-Verlag, Berlin. Sokal, R.R. & Rohlf, F.J. (87) Introduction to Biostatistics, nd edn. W.H Freeman & Co, New York. Talbot, W.A. () Forest Flora of Bombay Presidency and Sindh, vols &. Government Photoincographic Press, Poona. Troup, R.S. () The Silviculture of Indian Trees. Clarendon Press, Oxford. Webb, L.J. (68) Biological aspects of forest management. Proceedings of Ecological Society of Australia 3,. Whitmore, T.C. (84) Tropical Rain Forests of the Far East, nd edn. Clarendon Press, Oxford. Received October 6; revised 8 April 7; accepted April 7 8 British Ecological Society, Functional Ecology,, 33 38