Original article Physical damage on tropical tree saplings: quantification and consequences for competition through height growth in a neotropical rain forest of French Guiana Gilles Koestela Judy M. Rankin-de Méronab Station de recherches forestières, Inra, Centre Régional de Guyane, BP 709, 97387 Kourou cedex, France (Received 29 July 1996; revised 2 April 1997; accepted 25 May 1998) Abstract - This paper deals with the quantification and the effects of physical damage on tree regeneration dynamics in the tropical rain forest. We define physical damage as breakage resul- ting in a greater than 20 % reduction in stem diameter and its associated effects. A study of phy- sical damage at the community level was made in March 1994 in primary forest and forest distur- bed by silvicultural treatments at the Paracou research site in French Guiana. The frequency of damage varies with diameter class and the degree of forest disturbance due to the silvicultural treatments, ranging from 14.9 % for saplings greater than 6 cm DBH in undisturbed forest to over 50 % for smaller saplings in disturbed forest. Study at the specific level was made at the same site on saplings of three tree species with contrasting ecological temperaments, Bocoa prouacen- sis, Pradosia cochlearia and Goupia glabra, from March 1994 to March 1996. Damage frequen- cies varied from 34 % for saplings of the pioneer species Goupia glabra to 64 and 60 %, respecti- vely, for the more shade tolerant species Bocoa prouacensis and Pradosia cochlearia. Physical damage does not directly influence height or diameter growth rates nor mortality within a species except for Pradosia cochlearia. However, diameter growth rates irrespective of damage are significantly different between species. Under certain circumstances, stem breakage may be an influential factor affecting the long term survival of pioneer species saplings because it modifies their social status. (© Inra/Elsevier, Paris.) growth / saplings / competition / mortality / stem breakage / tropical rain forest Résumé - Casse mécanique sur des jeunes arbres tropicaux : quantification et consé- quences sur la compétition par la croissance en hauteur dans une forêt néotropicale humide de Guyane française. Ce travail portant sur la quantification et les effets de la casse mécanique sur la dynamique de la régénération des arbres en forêt dense humide tropicale, a été envisagé à *Correspondence and reprints E-mail: [email protected] deux niveaux de perception sur le dispositif de recherche de Paracou, en Guyane française. Une étude au niveau peuplement sans prise en compte des espèces a été réalisée en mars 1994 en forêt primaire et en forêt perturbée par des traitements sylvicoles. Le pourcentage de casse varie selon les classes de diamètre et l’importance de la perturbation due aux traitements, depuis 14,9 % pour les jeunes arbres de DBH supérieur à 6 cm en forêt naturelle, à plus de 50 % pour ceux de petits diamètres dans des sites perturbées. L’étude au niveau spécifique a été réalisée sur trois espèces aux tempéraments écologiques contrastés, Bocoa prouacensis, Pradosia cochlearia et Goupia glabra, entre mars 1994 et mars 1996. Les jeunes arbres de l’espèce pionnière Goupia glabra sont moins fréquemment endommagées (34 %) que celles des espèces plus tolérantes d’ombre, Bocoa prouacensis (64 %) et Pradosia cochlearia (60 %). La casse mécanique n’influence pas directement les taux de croissance des tiges en hauteur ou en diamètre pour une espèce, sauf pour Pradosia cochlearia. Cependant, la croissance en diamètre, sans prendre en compte des dégâts mécaniques, est significativement dif- férentes entre espèces. La casse mécanique, dans certaines circonstances, peut avoir une influence importante sur la survie à long terme des jeunes arbres d’espèces pionnières, car elle modifie leur statut social. (© Inra/Elsevier, Paris.) croissance / jeune arbre / compétition / mortalité / casse mécanique / forêt tro- picale humide / 1. INTRODUCTION dynamics. This approach is interesting in light of the nuances revealed by some Studies on population dynamics and studies contrasting the differences tree regeneration in tropical rain forests between pioneer species and shade toler- have shown the importance of physical ant ones [1, 9]. We ask the following damage on seedling and sapling mortality questions. [1, 7-9, 14, 29, 30]. is Physical damage 1) In what way does the ecological the mechanical of a stem an breakage by temperment of a species influence the animal to (due tramping, scraping, push- frequency of damage to saplings? ing, biting or boring for example) or by 2) For a individual of a material falling from a higher stratum of given given is the vegetation. Whereas seedlings are species, physical damage automatically detrimental in to another more likely to be completely crushed, comparison individual of the same with no saplings most frequently suffer from species under conditions of breakage or stem deformations resulting breakage, especially in significant modification of future active competition? growth. This damage may either lead to 3) In which terms does physical dam- increased mortality or, in the case of sur- age affect sapling growth, in height or vival, to changes in growth trajectories. diameter, and in what proportion? Is there We can surmise that individuals reaching a direct or indirect consequence of stem the young tree stage and emerging from breakage on these parameters? the have a understory may already long We undertook a study of the effects of and eventful past history. damage by breakage on the growth and If we take into account the notion of survival of saplings of three tropical rain species in the study of physical damage forest tree species in native and silvicul- on saplings, we can evaluate how it influ- turally treated forest in order to answer ences mortality, growth and competition these questions. 2. MATERIALS AND METHODS The experimental site is composed of 12 plots of 9 ha (each surrounded by a 25 m wide buffer zone) distributed over three replicated 2.1. Study site blocks consisting of four silvicultural treat- ments, including a control [27]. The treat- ments, applied once in October 1986 to May This study was carried out at the Paracou 1987 for logging and December 1987 to Tropical Forest Research Site of Silvolab, a January 1988 for poison girdling, are: co-ordination unit set up by French research institutes (current members are CIRAD-Forêt, - treatment 0: control (mean basal area Inra, ENGREF, ORSTOM, ONF and MNHN) (ba) = 32 m2/ha); for forest ecosystem studies in French Guiana. - treatment 1: selective logging above Located in the dense rain forest of upland 50 cm DBH (mean remaining ba after north-eastern South America (2-6° N, logging = 24 m2/ha; 51°30’53°30W) 1), the climate is con- (figure - treatment 2: selective logging plus thin- sidered as equatorial, characterised by peren- after = 19 nial low ning by poison girdling (ba logging high (80-90 %) humidity, tempera- m2/ha); ture variation centred around 26 °C and a rarity of violent winds [15]. Mean annual rain - treatment 3: selective logging plus thin- fall for the last 10 years is 4 976 +/- SD ning by poison girdling and fuel wood extrac- 243 mm (CIRAD-Forêt à Kourou, unpub- tion (ba after logging = 16 m2/ha). lished data). Rainfall distribution is unequal over the year (figure 2). The main dry season occurs between August and November, with 2.2. Methods another short dry season during March or April. Average annual temperature is 26 °C. Frequency of breakage, survival, height The forest structure and composition are and diameter growth rates for damaged and generally similar to other upland rain forest undamaged stems were analysed at both the sites in South America, all the while possess- community level and the specific level. ing a Guianian character by virtue of a small At the community level, the frequency of number of abundant tree in relatively species breakage on saplings regardless of species a nonetheless forest. The three species-rich was estimated on randomly oriented transects most families of trees representative attaining 20 m long and 2 m wide, with origins located at least 10 cm in diameter at breast height every 40 m on a square grid. A total of 96 in the Paracou forest are the (DBH) transects were censused on three parcels (con- % of the Lecythidaceae (18 individuals), the trol, first level and second level treatments; and the Caesalpinaceae (13 %) with 32 transects per parcel). Measurements The Chrysobalanaceae (12 %) [11]. principal were made on individuals of more than 1.5 m tree are the Eschweilera species spp. in height and less than 10 cm DBH. Diameter the Licania (Lecythidaceae), spp. (Chryso- was measured by classes of 1 cm intervals mouamba, balanaceae), Eperua falcata, with a notched gauge (figure 3). Frequencies Bocoa (this bouchi prouacensis study), of damage were assessed using Clark and bakouman, mango, Iryanthera spp., Eperua Clarks method based on comparison of main grandiflora, Symphonia globulifera, moni, stem diameters above and below breakage americana, Pradosia cochlearia Vouacapoua points as indicated by major discontinuity or (this Qualea rosea study), Carapa procera, scarring [9]. From this a discontinuity ratio and From patawa, Dicorynia guianensis [24]. can be established as: a structural of view there are on the point * ratio = 100 above average 618 stems/ha, with a mean basal area discontinuity (diameter of 31 m2/ha [10]. Approximately 60 % of the scar/diameter below scar). stems are 20 cm in DBH or less [24]. While Clark and Clark [9] fixed the lower limit of emergents may reach 45 m in height, the gen- discontinuity at 25 %. This means that a stem eral level of the canopy is around 40 m (B. with a discontinuity less than 25 % was con- Ferry, pers. com.). sidered as undamaged. But, as they recognise themselves, this criterion is quite conserva- butions to any observed species differences tive, leading to under-estimates.