IAWA Bulletin n.s., Vol. 11 (4), 1990: 319-336 COMPARATIVE WOOD ANATOMY OF AFROMONTANE AND BUSHVELD SPECIES FROM SWAZILAND, SOUTHERN AFRICA by J. A. B. Prior 1 and P. E. Gasson 2 1 Department of Biology, Imperial College of Science, Technology & Medicine, London SW7 2BB, U.K. and 2Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3DS, U.K. Summary The habit, specific gravity and wood anat­ of the archaeological research, uses all the omy of 43 Afromontane and 50 Bushveld well preserved, qualitative anatomical charac­ species from Swaziland are compared, using ters apparent in the charred modem samples qualitative features from SEM photographs in an anatomical comparison between the of charred samples. Woods with solitary ves­ two selected assemblages of trees and shrubs sels, scalariform perforation plates and fibres growing in areas of contrasting floristic com­ with distinctly bordered pits are more com­ position. Some of the woods are described in mon in the Afromontane species, whereas Kromhout (1975), others are of little com­ homocellular rays and prismatic crystals of mercial importance and have not previously calcium oxalate are more common in woods been investigated. Few ecological trends in from the Bushveld. wood anatomical features have previously Key words: Swaziland, Afromontane, Bush­ been published for southern Africa. veld, archaeological charcoal, SEM, eco­ The site of Sibebe Hill in northwest Swazi­ logical anatomy. land (26° 15' S, 31° 10' E) (Price Williams 1981), lies at an altitude of 1400 m, amidst a Introduction dramatic series of granite domes in the Afro­ Swaziland, one of the smallest African montane forest belt (White 1978). At least countries, lies in the subtropics between lati­ 1500 mm ofrain falls mainly during the sum­ tudes 25° and 27° S. Although its total area is mer months, when the mean temperature is only 17,565 Ian 2, large altitudinal differences 16.5 ° C. The high rainfall promotes the for­ from west to east result in a wide environ­ mation of a black, acidic hill peat typical of mental diversity. Ancient charcoal fragments the tropical, north-eastern mountain sourveld spanning the last 10,000 years were excavat­ (Acocks 1975). Moderate to severe frosts oc­ ed from two sites in the high, wet northwest cur during most winters, producing variable and low, dry northeast of the country, during periods of physiological drought. The lichen a multidisciplinary investigation of the covered trees and shrubs of the area cluster in detailed pattern of climatic change during the sheltered areas between the domes. They are Holocene period (Prior & Price Williams predominantly evergreen and include a high 1985). The fragments were identified by percentage of specific endemics. comparison with charred branchwood col­ Siphiso rock shelter is situated in north­ lected from 93 modern trees and shrubs of east Swaziland (26° 18' S, 31° 58' S), at an common occurrence around the two sites. altitude of 320 m. Vegetation in the area, de­ The modern taxa chosen, which represent scribed by Acocks (1975) as semiarid low­ over 60% of the woody flora of each area, veld, includes tropical bush and savanna spe­ were biassed in favour of those species cies known as Bushveld. An annual rainfall known to be carefully selected for burning varying between 500 and 700 mm and mean and for a range of utilitarian purposes today, summer temperatures in excess of 20° C lead as in the past. The present study, a byproduct to levels of evaporation which greatly exceed Downloaded from Brill.com10/05/2021 12:37:45PM via free access 320 IAWA Bulletin n.s., Vol. 11 (4), 1990 precipitation. Upper soil levels become saline in diameter represent mature wood where and calcrete modules, composed of calcium further changes in cambial initial length and carbonate, are deposited at varying soil changes in most other cambial products are depths. Some woody species typically occur negligible. 1-2 cm3 cubes cut from each wood on the shallow, raw mineral soils or lithosols sample were embedded in washed silver sand mantling the slopes; others, particularly mi­ in crucibles and charred by heating in a muf­ crophyllous legumes, characterise the black, fle furnace at 450 0 C for 30 minutes. They alkaline, hydromorphic vertisols of the bot­ were cooled in the sand to minimise ash for­ tomlands (Young 1976). On both soil types, mation, then scored and manually fractured in a high proportion of species are deciduous three planes of section. Fractured pieces were and many show xeromorphic characters. mounted on aluminium stubs with Durofix, All the charred branch wood was examin­ desiccated for at least 24 h and coated with ed by scanning electron microscopy. The use 40 nm of gold in an atmosphere of argon, of charred material inevitably presents prob­ using a Polaron E5000 diode sputter coater. lems in the interpretation of certain wood ana­ Stubs were examined in a Philips SEM tomical characters such as the thickness of 500 and photographed at magnifications lignified walls and the number of crystals between 50 x and 6400 x. At least fifteen to present. Great care was taken to distinguish twenty photographs were taken of each charring artefacts from genuine anatomical wood. characters of taxonomic importance (Prior & For each set of photographs numbered Alvin 1983). Charring results in differential characters were recorded, according to the shrinkage of the tissues of the wood. Al­ list of microscopic features (I AW A Commit­ though this invalidates any quantitative ana­ tee 1989). Maximum and minimum ray widths tomical measurements, the large amount of were noted for most specimens and are in­ qualitative information remaining in the char­ cluded in the Appendix. The specific gravity coal enabled each wood to be clearly identi­ of most of the woods was determined either fied, using the list of microscopic features for by measuring the ratio of the oven-dry weight hardwoods recently published (IA WA Com­ to the weight of water displaced by the green mittee 1989). Only one species, the softwood volume or by reference to Kromhout (1975) Podocarpus latifolius, an important compo­ or Van Wyk (1984). The habit of each spe­ nent of the vegetation occurring amongst the cies was determined either by field observa­ granite domes of Sibebe Hill, could not be tion or by reference to Palmer (1977) or Van characterised in this way. Every wood is de­ Wyk (1984). The distinction between trees scribed in coded form in the Appendix. A and shrubs is not always clear in relation to comparison is made of the vessel, fibre, axial the species of southern Africa. In this paper, parenchyma and ray characters which pre­ plants with a woody stem of 3 m or more in dominate in the two distinct tropical montane height are called trees, though these are fre­ and tropical lowland areas. quently multi-stemmed, particularly under conditions of climatic stress, which induce Materials and Methods natural coppicing. The arrangement of the Wood samples from 43 Afromontane spe­ families and the nomenclature of species used cies and 50 Bushveld species were collected at in the Appendix follows Coates Pal grave the beginning of several dry seasons. Vege­ (1983). tative voucher specimens of each were col­ lected in duplicate for authentication at the Results Royal Botanic Gardens, Kew and at the Bo­ The habit and specific gravity of Afromon­ tanical Research Institute, Pretoria. Branch­ tane and B ushveld species are compared in wood samples were between 4 and 6 em in the table. Anatomical features of particular diameter. Wood of this size was selected ecological significance, such as the compara­ since it equates best with archaeological tive number of species with exclusively soli­ samples. Baas (1973), in his detailed study tary vessels, scalariform perforation plates, of !lex, concludes that wood samples of 5 cm fibres with distinctly bordered pits, predom- Downloaded from Brill.com10/05/2021 12:37:45PM via free access Prior & Gasson - Wood anatomy of Afromontane and Bushveld species 321 Table showing comparative ecological trends in Afromontane and Bushveld species. Qualitative character Afromontane species Bushveld species (%) (%) Tree 58 68 Shrub 42 32 Evergreen 65 32 Deciduous 26 60 Tardily deciduous 9 8 Growth ring boundaries distinct 7 14 Basic specific gravity 0.40-0.75 21 16 Basic specific gravity> 0.75 35 56 Unknown 44 28 Vessels exclusively solitary 39 14 Perforations scalariform 19 2 Intervessel pits scalariform 12 0 Intervessel pits opposite 14 0 Intervessel pits alternate 93 100 Vestured pits 26 36 Vessel-ray pits + distinct borders 81 80 Vessel-ray pits + reduced/simple borders 21 22 Vessels + helical thickenings 5 0 Fibres with distinctly bordered pits 46 20 Septate fibres present 39 26 Fibres very thin-walled 21 14 Fibres thin- to thick-walled 91 96 Fibres very thick-walled 14 22 Parenchyma very rare 24 20 Parenchyma predominantly apotracheal 72 64 Parenchyma vasicentric to aliform and confluent 16 36 Parenchyma banded 16 30 Rays homocellular 12 28 Very narrow rays (1-3 cells wide) 84 92 Ray height> 1 mm 14 8 Crystals present 33 58 Silica bodies present 5 4 Downloaded from Brill.com10/05/2021 12:37:45PM via free access 322 IAWA Bulletin n.s., Vol. 11 (4), 1990 inantly apotracheal and paratracheal axial commonly arranged in radial multiples of parenchyma or homocellular rays are also four or more are three times more frequent in listed. Features which are of rare occurrence the cooler, Afromontane area (see Appendix). or of less ecological significance are only In this area too, 39% of all species have 90% shown in the Appendix. These include the or more of solitary vessels. The comparable storying of one or more elements, vessel figure for the semiarid Bushveld sample is grouping other than solitary and the occur­ only 14%.