IAWA Journal, Vol. 14 (2),1993: 173-185 BARK STRUCTURE AND PREFERENTIAL BARK UTILISATION BY THE AFRICAN ELEPHANT by J ohn W. Malan I and Abraham E. van Wyk 2 1 Mammal Research Institute, Department of Zoology, 2H.G.W.J. Schweickerdt Herbarium, Department of Botany, University of Pretoria, Pretoria, 0002 Republic of South Africa Swnmary Bark fracture properties are thought to In any particular area, elephants show a influence the debarking of selected trees by preference for stripping and ingesting the the African elephant. This hypo thesis was bark of certain tree species; bark of Acacia tested for large riverine tree species in the tortilis and A. nigrescens are especially fa­ Northern Tuli Game Reserve, Botswana. An voured. Elephants loosen the bark with their index of bark breakage strength and pliability tusks and then strip it off. Even if only apart of secondary phloem tissue was compiled for of the bark is stripped off, debarked trees are 11 common riverine species, and the bark susceptible to further damage by fire, insect anatomy of these species was investigated to borers and fungi, and often succumb to their determine relative fibrosity. The majority of injuries (Thomson 1975). species preferred by elephants have strong Little is known about the effects of bark and pliable barks, associated with a high pro­ structure and strength on the extent of debark­ portion of fibres. However, not all preferred ing. In fact, the possible influence of plant species have these characteristics, which in­ fracture properties on animal behaviour is dicates that factors other than bark fracture rarely considered (Vincent 1990). Thomson properties affect species preference. Bark (1975) suggested that the ease with which structure influences the way pieces of bark bark is stripped from a tree may account, in are stripped from a tree trunk during debark­ part, for the differences in species preference ing. It is hoped that this paper will stimulate shown by elephants. Laws et al. (1975) re­ further studies on the effects of bark structure marked that the mechanical properties of bark, on the preferential feeding behaviour of the which either facilitate or hinder debarking, African elephant. may be involved in determining the incidence Key words: African elephant, bark anatomy, of debarking of a particular tree species. debarking, gelatinous fibres, mechanical The present study was initiated following properties, sclereids. a suggestion that the breakage strength and pliability of bark play an important role in the Introduction preference shown by elephants for certain The devastating effects that large numbers tree species in southern Africa (McKenzie, of elephants have on their habitat are evident pers. comm.). In this paper we explore the in many parts of eastern and southern Africa relations hip between the intensity of bark (Anderson & Walker 1974). The relatively damage caused by elephants and aspects of large size and high mean age of survival en­ bark anatomy. We test the hypothesis that able this species to have a major, often long­ fibrous barks tend to be tough, pliable, and term, impact on the ecosystem (Watson & hence easily stripped from extensive areas of Bell 1968), particularly in areas where the stern, whereas barks in which the sclerenchy­ movements of elephant herds are restricted. ma is completely lacking or is comprised of Elephants are second only to man in their ca­ mainly sclereids, are brittle and therefore less pacity for altering their environment (Skinner likely to be removed in relatively large pieces. & Smithers 1990, and references therein). This study forms part of a comprehensive Downloaded from Brill.com09/24/2021 10:28:38PM via free access 174 IAWA Journal, Vol.14 (2), 1993 project on the association between African located along the main rivers within the Re­ e1ephants and the large riverine trees of the serve and inc1ude the Majale, Pitsani, Njaswe, Northern Tuli Game Reserve in Botswana. Matabole and Jwala rivers. Two hundred large trees, both living and dead, were sampled at each site. Only large trees were included in Materials 8lld Methods the sampie as these constitute the most im­ portant structural component of the riverine Studyarea forests. Large trees were defined as trees The Northern Tuli Game Reserve (NTGR) with a trunk circumference exceeding 1 m is situated in the eastern corner of Botswana, when measured just above the basal swelling. between 21° 55' Sand 22° 15' S, and 28° For each living tree sampled, the species 55' E and 29° 15' E, where Botswana, South and extent of bark damage inflicted by ele­ Africa and Zimbabwe meet. It is bounded phants were recorded. Bark damage was de­ in the north by the Tuli Circ1e, in the south fined as the width of bark removed at the by the Limpopo and Motloutse rivers, in the height of greatest width damage, expressed east by the Shashe River, and in the west by as a percentage of the stern circumference at the Tuli Block backline, a veterinary control that height: fence. The Reserve, which covers an area of Percentage bark removed (%BR) = 100' Ai / C approximately 60,000 ha, is neither gazetted nor proc1aimed as a private game reserve, but where: relies solelyon a shared interest in conserva­ Ai = the width of a debarked area at tree height x tion between the landowners of several pri­ C circumference of tree at height x vately owned farms. From aerial censuses x = the height at which the greatest area of bark conducted in 1986, 1987 and 1988 (Le Roux has been removed. 1989), it is evident that the density of ele­ phants within the NTGR is approximately Choice 0/ species one per square kilometre. The vegetation of the area is broadly c1assified into Colopho­ An objective of this study was to distin­ spermum mopane woodland and scrub wood­ guish, on the basis of bark damage, preferred land (White 1983). woody plants from non-preferred ones. A pre­ ferred species is defined as one that is utilised more frequently than indicated by its avail­ Choice 0/ trees ability in the environment (Petrides 1975). A stratified sampie of the riverine tree com­ For this purpose preference ratios were cal­ munities within the NTGR, comprising 16 culated using the following equations, adapt­ sites, was taken in May 1991. The sites were ed from Petrides (1975): Percentage utilisation (U i) Preference ratio (PR) Percentage availability (Ai) where: Number of debarked trees of species i in all sites sampled Ui = 100 • Number of debarked trees of all species within all sites sampled Number of available trees of species i within all sites sampled Ai = 100 • Number of available trees of all species within all sites sampled Downloaded from Brill.com09/24/2021 10:28:38PM via free access Ma1an & Van Wyk - Bark utilisation by the African elephant 175 For each tree, the time of debarking prior to Bark anatomy sampling (weeks, months, and/or years) was Fresh bark sampies, collected as described determined from the appearance of the expos­ above, were fixed in FAA (Johansen 1940). ed wood and/or stripped bark and/or signs Unembedded fixed material was softened of regenerative bark growth. From observa­ with steam and transverse and radial sections tions made on the response of trees from cut at 15 - 20 ~m on a sliding microtome. which we have removed bark (to test bark Sections were stained with safranin 0 and fast strength), the criteria were found to be a re­ green (Johansen 1940), and mounted in en­ liab1e measure of estimating the relative time tellan. The presence of lignin was determined of debarking. Preference ratios were calcu­ using phloroglucinol (Jensen 1962). Draw­ lated based on1y on bark damage that had ings of transverse sections were made using a occurred weeks and/or months prior to the projection light microscope. Unless other­ time of sampling (= recent damage) and only wise indicated, descriptive bark anatomy ter­ for species for which ten or more trees were minology follows Trockenbrodt (1990). sampled. Bark strength Results The fracture properties of bark were tested Preference ratios for 11 NTGR riverine tree species. For each The number of large riverine trees sam­ species, bark from six randornly chosen trees, pled (both accessible and inaccessible to located along the Maja1e and Jwala rivers, elephants), and their percentage occurrence was collected in October 1991 (the end of the within the riverine tree communities of the dry season). The bark was removed from NTGR, are listed in Table 1. Based on bark each tree at breast height, and only from trees damage inflicted by elephants, preference with a trunk circumference of between 100 ratios calculated for species with ten or more and 200 cm when measured just above the trees sampled are given in Table 2. The me­ basal swelling. Bark sampled from the vari­ dian percentage bark removed from all trees ous species differed in thickness. For com­ of each species sampled (Table 3) was cal­ parative purposes, a standard 1 x 2 x 100 mm culated, and resuIts are plotted in Figure 1. axial strip of tissue, removed from the cen­ tral part of the living secondary phloem was Bark structure used in the following tests (n = 6 for each The predominant type of sclerenchyma in species). the secondary phloem of each investigated To measure breakage strength (stress at species is recorded in Tab1e 3. Sclerenchyma which the sampIe breaks), ahomemade wood­ arrangement, as seen in transverse section, is en clamp was attached to each end of a bark illustrated in Figures 2-13. Because the sec­ strip, leaving a 10 mm gap between each ondary sclereids that are associated with the clamp. The clamps were then pulled apart dilation zone are variable in occurrence and with a mini hand-wrench.