IAWA Bulletin n.s., Vol. 10 (2),1989: 123-132 APPEARANCE AND PERIODICITY OF GROWTH RINGS IN SOME TROPICAL WOODS by Pierre Detienne Centre Technique Forestier Tropical, 45bis avenue de la Belle Gabrielle, 94736 Nogent-sur-Marne Cedex, France Summary Trees belonging to 30 tropical African and thus calculating their growth rate; detecting South American hardwood species were variations in growth rate during the tree life wounded annually. This made it possible, or accurately dating phenomena that left after the trees had been felled, to mark pre­ traces in the wood, e.g. insect attack or slight cisey the annual growth rings and determine fire damage. their boundaries. These boundaries are al­ ways formed during the longest dry season Materials and Methods and clearly express the rhythm of cambial Experiments were carried out on 2 to 10 activity. The appearance and nature of these trees selected from 30 species belonging to growth rings vary according to genera rather 26 genera of 16 families, in Guinean-Congo­ than to types of climate. lese Africa and French Guiana. The species Key words: Annual growth rings, tropical are listed in Table 1. African and American hardwoods, wood The cambial activity was recorded every structure. two weeks with band dendrometers at a pre­ cision of 0.2 mm over periods of 4 to 7 years Introduction depending on species or observation stations. For a long time tropical trees have been The accurate detection of growth rings described as fast and continuously growing was accomplished thanks to scars left in the plants for their entire life span. Consequent­ wood by wounds inflicted on living trees. ly, the occurrence of annual growth rings in These wounds, 0.5 cm wide and 4 or 5 cm their wood was not acknowledged, although high, were made in the bark, up to the cam­ several authors had hinted at their existence, bium which was locally destroyed, but the yet without being able to demonstrate their wood was kept devoid of lesions. Wounding annual character (Chowdhury 1939, 1940; was done once a year, during the longest dry Alvim 1964), except in Swietenia mahagoni season, either between December and March Jack, Azadirachta indica Juss. and in Tectona in Guinean -Congolese Africa, or between grandis L.f. (Coster 1927). However, some July and September in French Guiana, authors succeeded in explaining periodicity in always at the same height in the trunk wood formation linking physiology and anat­ (Detienne & Barbier 1988). omy (Fahn 1958; de Fay 1986). Mter the trees had been annually wounded From 1965, studies have been carried out several times, they were felled and cross cut at the Centre Technique Forestier Tropical in­ at wound height. Mter drying, the end grain to the possible annual periodicity of diameter was sanded with extremely fine carborundum growth of tropical trees, and into the struc­ paper (400 grains per cm2). The scars left by tural characters by which this periodicity is the wounds were still quite visible and some­ marked in the wood. The knowledge of the times underlined by local formation of dis­ appearance and the annual periodicity of these coloured wood in species with well-defined variations can be applied to several problem heartwood (Figs. 1-5). Dating each scar was areas, e. g. determining the age of trees and performed easily, the oldest one being the Downloaded from Brill.com10/01/2021 08:31:48AM via free access 124 IAWA Bulletin n.s., Vol. 10 (2),1989 most distant from the bark, the others short dry season, interruptions in cambial ac­ following from right to left or left to right tivity starting a few months before the dry according to the direction chosen to inflict the season or, but less frequently - and only in wounds. trees with very slow growth - temporary The growth rings are visible to the naked shifts in the normal rhythm. The possibility eye on trees with considerable growth during that these abnormalities were caused by phe­ the experiment. However, a magnifying lens nological events (e.g., massive flowering or (x 6 or x 12, or even x 25) is generally fruiting, top damaged by insect attack) cannot needed to determine the growth rings of most be excluded, but this has not been studied in species accurately (Figs. 1-5). detail. Rhythm of cambial activity Appearance and nature of growth rings The trees studied always showed a rhyth­ The wood formed before the period of mical circumference increment. Our band complete standstill, or during the long period dendrometer measurements showed that the of reduced growth, generally shows a thin growth in trunk diameter generally starts layer of fibres with slightly thicker walls around the beginning of the rainy season and and/or with a more flattened outline in cross goes on, on a regular basis or not, until section than the others. The phenomenon is the long dry season, i.e. for 7 to 10 or similar to that observed in species of tem­ 11 months. Then the cambial activity either perate climates; however, it is less marked comes to a complete halt, or the growth rate and thus less clearly visible. Seen with a is clearly slowed down. magnifying lens, this thin layer of wood The complete halt lasts for various lengths looks slightly darker. This character is not of time depending on species, specific years, always very obvious, so it can be used only but, above all, on the vigour of the tree which in species with no variations in other tissues, may change with age or depend on external such as softwoods or hardwoods with a very factors such as for instance social position. low tissue proportion of parenchyma. Growth is generally interrupted for 15 days The only experiment on softwoods was up to 3 months, but sometimes for 6 months carried out on two trees of Agathis moorei in moderately vigorous trees, or even over Mast. from New Caledonia. The annual 1 year in weak and suppressed trees. In the growth ring boundary is marked by a thin latter case, on the disk of the felled tree, no dark band composed of fibres with thicker growth ring was shown to have been formed walls, but it may be blurred, or even invisible for such a period of time. In some species, on part of the section. The small number of a small decrease in the circumference was samples does not permit to define the real noted during the inactive period. This phe­ annual growth ring exactly; often individual nomenon can probably be attributed to a dark bands are only present on part of the slight contraction of the bark during the dry disk surface; these could represent false rings season. when they are only present in one sector; al­ Periods of reduced cambial activity during ternatively annual growth rings may be partly the long dry season were principally observ­ (locally) or completely missing. Such a dark ed in fast growth species or vigorous speci­ band formed by flattened fibres with thicker mens. They always last longer and/or their walls is also the only charactistic of the ring increment reduction is greater than in other boundaries of Yayamadou, Virola melinonii periods of slow growth occurring during the A.C. Smith (Myristicaceae). The transition vegetative period, e.g., during the short dry between this band of latewood and the lighter season. coloured earlywood is abrupt or more or Abnormalities occasionally occur in some less gradual depending on the year. It rarely trees during certain years, but these do not occurs in combination with a thin continu­ directly call the annual rhythm into question. ous band of marginal parenchyma. They mainly concern short periods of re­ duced growth during rainy seasons or the (text continued on page 128) Downloaded from Brill.com10/01/2021 08:31:48AM via free access Detienne - Appearance and periodicity of growth rings in tropical woods 125 . ",--- / Figs_ 1-5. Annual scars showing growth ring boundaries. - 1: Chlorophora excelsa. - 2: Triplochiton scleroxylon. - 3: Ajzelia ajricana. - 4: Entandrophragma angolense. - 5: Entandrophragma cylindricum. Downloaded from Brill.com10/01/2021 08:31:48AM via free access 126 IAWA Bulletin n.s., Vol. 10 (2),1989 Figs. 6-9. Annual growth rings. - 6 & 7: Entandrophragma utile, x 1.5 & 7.7 respectively.- 8: Jacaranda copaia, x 14. - 9: Terminalia ivorensis, x 7.7. Downloaded from Brill.com10/01/2021 08:31:48AM via free access Detienne - Appearance and periodicity of growth rings in tropical woods 127 Figs. 10-13. Annual growth rings. - 10: Tectona grandis, x 7.7. -11: Chlorophora excelsa, x 7.7. - 12: Mansonia altissima, x 14. - 13: Pterocarpus soyauxii; the double arrow shows a false limit which is identical to a real annual boundary, x 7.7. Downloaded from Brill.com10/01/2021 08:31:48AM via free access 128 IAWA Bulletin n.s., Vol. 10 (2),1989 No false rings or boundary duplication were but they can be misleading in a superficial observed in this species. The same does not study with the naked eye. hold true for Okoume, Aucoumea klaineana Well-marked boundaries of annual growth Pierre (Burseraceae), where the situation is rings are those delimited by marginal bands slightly different and more complex (Mariaux of parenchyma. These bands are easy to de­ 1970). The wood formed during a vegetative tect in species having mostly paratracheal or year comprises four different layers of fibres: apotracheal parenchyma in short bands. The a light one, then a dark layer composed of boundaries are sometimes not as easily de­ narrower fibres, then a second light layer and tectable when they appear in woods with at last a thin terminal dark layer.