Physical Damage on Tropical Tree Saplings

Physical damage on tropical tree saplings: quantification and consequences for competition through height growth in a neotropical rain forest of French Guiana Gilles Koestel, Judy M. Rankin-de Mérona

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Gilles Koestel, Judy M. Rankin-de Mérona. Physical damage on tropical tree saplings: quantification and consequences for competition through height growth in a neotropical rain forest of French Guiana. Annales des sciences forestières, INRA/EDP Sciences, 1998, 55 (6), pp.727-742. hal-00883232

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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 resulting in a greater than 20 % reduction in stem diameter and its associated effects. A study of physical damage at the community level was made in March 1994 in primary forest and forest disturbed 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 prouacensis, Pradosia cochlearia and Goupia glabra, from March 1994 to March 1996. Damage frequencies 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 tropicale 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 treatments, including a control [27]. The treatments, 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. After a pre- depends on the species themselves and not the liminary investigation of different critical lev- cause of the damage. els for 20 and breakage acceptance (10, 15, The species level effects of damage were 25 %), we decided to lower the level of accep- studied for three tree species of contrasting tance to 20 %. However, results at the com- ecological temperaments. The individuals level were the two munity not different for studied were tagged saplings located on circu- limits. Measures above and below the scar lar plots (radius 3.72 m) located throughout were made with a caliper every time disconti- the 12 plots of 9 ha mentioned above, and nuity was doubtful. It is difficult to distinguish forming part of the Inra (Institut national de la vertebrate damage from limbfall or treefall recherche agronomique) natural tree regenera- effects and we assume that the greatest contri- tion study (see [27] for the site description and [26] for details of the inventory protocol and 2.3. Species studied data base). The positions of the plots corre- spond to the origins of the community level Bocoa prouacensis Aubl. (Caesalpi- transects on the 40 by 40 m square grid, thus niaceae) is a relatively common mid-canopy the observations at these two levels were con- tree species which attains 30 m in height and ducted in the same zones. All the saplings is shade tolerant as an adult [4, 11]. Seed dis- (n = 80 for each species) were more than persal is endozoochorous (spider monkeys) 1.5 m in height and less than 5 cm DBH, the and synzoochorous (bats, such as Artiibeus) upper limit being determined by the difficul- [31]. ties in accurately measuring heights of more Pradosia cochlearia (Lecomte) Pennington than 7 m, the maximum length of our cali- (Sapotaceae) is an emergent tree with an adult brated telescopic pole. Height measurements height of about 40-45 m. Its large typically were thus made to the centimetre for heights sapotaceous berries are dispersed by monkeys up to 7 m. Measurements of DBH for all the [31]. Like Boco, the saplings of Kimboto are saplings and the diameter above and below relatively shade tolerant [19] yet growth can scar for damaged saplings considered as dam- be stimulated by increased light following for- aged were made twice with a millimetric est canopy opening [25]. caliper, each diameter being the mean of two Goupia glabra Aubl. (Celastraceae) is a perpendicular diameters. All measurements long-lived pioneer tree species [6] with very were made once in March 1994 and were small fruits of about 1 cm and few seeds [2, 3, repeated in March 1966 in order to evaluate 13] present throughout the rain forest soil seed the diameter and height growth. bank [21]. It is a light-demanding species from germination until death and grows 3. RESULTS rapidly [18]. It quickly colonises gaps and open sites [6], especially on disturbed soils such as trails. For these [28] logging reasons, 3.1. Frequency of damage in 1994 Goupia glabra usually appears on favourable sites in dense groups of relatively numerous The overall of individuals among other pioneer species [17]. frequency damage observed on the con- Mature trees are emergent, with a maximum transects, all species height surpassing 40 m [12, 20] and a lifespan sidered, is 40.6 % (n = 3 681). Damage of more than 100 years. frequency is essentially equal for the con- Bocoa prouacensis, Pradosia cochlearia trol and treatment level 1 (41.9 and and Goupia glabra are among the 14 more 41.5 % for n = 968 and 1 421, respec- common species (for all diameter classes) at tively) and only slightly less for treatment Paracou in the control The parcels [3, 10]. level 2 (38.5 %, n = 1 292). number of Goupia glabra saplings increased For 4 cm DBH, fre- steadily on treated parcels after logging and saplings ≤ damage silvicultural interventions [3, 19] while Bocoa quency is 43.9 % (n = 852) on control prouaceusis remained unchanged or decreased plots in our study (table I) as compared [19, 25]. to 19.5 % (n = 794) in Clark and Clark’s study [9] limited to nine canopy tree species at the La Selva Biological Station, Costa Rica. The difference for saplings > 4-10 cm DBH on control plots is somewhat less, with a damage rate of 27.6 % (n = 116) in our study compared to 22.8 % (n = 281) in Clark and Clark’s study [9]. These figures remain virtually identical whether using our 20 % discontinuity threshold or theirs at 25 %. As already mentioned by these authors, frequency of damage and diameter class are not independent. At Paracou, we find that the ratio of broken to intact stems varies by more than chance among the four diameter classes under all conditions (table I: for df = 3, in the control, chi-square = 16.5, P < 0.001; in level 1, chi-square = 14.7, P = 0.01; in level 2, chi-square = 24.7, P = 0.001). In the control plots there is a greater occur- rence of damage in the two lower size classes while damage is greatest for the intermediate sizes in the treatment plots (table I). For saplings < 2 cm DBH, the frequency of damage is significantly higher in the control over the treated plots (df = 2, chi-square = 12.3, P < 0.001). The least damage (14.9 %) is encountered for the largest saplings > 6-10 cm DBH in undisturbed forest. However, no significant differences exist between this and Goupia glabra; analysis chi- the frequencies observed in the first and square = 3.873; df = 1; P < 0.05 for second treatment levels for this size class Pradosia cochlearia). The median value (df = 2; chi-square = 5.8; P > 0.05), nor was used in this comparison, being more for any of the other size classes. representative of location due to distribu- We also examined the initial overall tion skewness. frequency of stem damage for the three species chosen for the growth and sur- vivorship studies. Differences in fre- 3.3. Rates of growth in height and diameter quency of damage among species are highly significant (df = 2, chi- Growth rate between March of 1994 square = 17.3; P < 0.001) because fre- and March 1996 were measured for the quency of damage is very high for Bocoa that did not suffer disease prouacensis (64 %; n = 72) and Pradosia saplings die, and/or the cochlearia (59.5 %; n = 78) and nearly physical damage during double that observed for Goupia glabra period, as far as we were able to discern. For this the size for this (34 %; n = 85). reason, sample part of the study was reduced as compared to the initial sample of tagged of mean 3.2. Two year mortality rates seedlings. True values diameter growth are difficult to establish due to Mortality between 1994 and 1996 due difficulty in accurately repeating mea- to physical damage varied widely surements on an irregular stem where the between the three species studied. site of measurement can not be perma- Mortality was high for Goupia glabra (21 nently marked without risk of damage to of 85 individuals) but less than half of the plant, and due to the great difference this is apparently due to physical damage: in overall diameter and actual diameter nine saplings broken among the 21 which growth over the observation period (table died over the period. Six of the seven III). Moreover, regarding stem shrinkage Pradosia cochlearia that died (of 78 ini- [16], for a short observation period such tially) had their stems broken. Only one as 2 years, true gains may be masked if Bocoa prouacensis of 72 tagged stems the second measure is made when the died and that individual had its stem bro- stem is significantly less hydrated or ken in 1994, and thus does not figure in under greater hydric stress than at the the following analysis. first measure. In this study, this problem The comparison of heights and diame- was minimised by taking the measurements the same seasonal ters in 1994 between stems dying during during period. the subsequent 2 year period and those Nevertheless, we can observe highly surviving (table II) shows certain signifi- significant differences among species cant differences by species, with those (table III) in diameter growth rates dying have smaller initial heights and (Kruskall-Wallis test; df = 2; chi- diameters (for initial diameters: median square = 10.5; P < 0.01) and height 1-way analysis chi-square = 7.228; growth rates (Kruskall-Wallis test; df = 2; df = 1; P < 0.01 for Goupia glabra; anal- chi-square = 11.9; P < 0.01) with Goupia ysis chi-square = 5.745; df = 1; P < 0.05 glabra greatly outpacing Pradosia for Pradosia cochlearia; for initial cochlearia for both parameters and height: median 1-way analysis chi- Pradosia cochlearia outpacing Bocoa square = 4.789; df = 1; P < 0.05 for prouacensis.

There is no significant difference height (table VI). No correlation is between damaged and undamaged stems observed between the variables men- of the same species (table IV) in diameter tioned above for Bocoa prouacensis growth (median 1-way analysis chi- (damaged or undamaged), the damaged and the square = 0.072; df = 1; P = 0.79 for Goupia glabra undamaged Pradosia cochlearia. There is a Goupia glabra; chi-square = 0.282; significant but not correlation between df = 1; P = 0.59 for Pradosia cochlearia, strong these variables for Pradosia chi-square = 1.38; df = 1; P = 0.24 for damaged cochlearia. Bocoa prouacensis). in 1994 can be Spearmans correlation between height Sapling height parti- tioned into the following components: growth rate over 2 years and the initial 1994 DBH is high and significant for the height (1996) = height (1994) + height undamaged Goupia glabra (table V). The growth (1994-1996) same kind of correlation is observed The comparison of a linear regression between height growth rate and initial model between stem height in 1994 and stem height in 1996 for undamaged stems consider this phenomena not just simply with the line y = x is one way to evaluate as frequency at a given time, but also as a the relationship between initial height and function of time spent in the understory. subsequent height growth (figure It reveals both the importance and the 4a-b-c). For Goupia glabra, the regres- effect of physical damage in the history sion coefficient is significantly > 1 (com- of a young tree. of the coefficient value parison regression The low rate of damage in the higher to 1; one-tailed test: n = 39; df = 37; diameter classes is consistent with the a = 0.025; t = P < 5.793; 0.001). greater mechanical resistance of larger Difference is not for the significant stems and the fact that they occupy pro- Bocoa = df = prouacensis (n 25; 23; gressively higher levels within the forest t = 1.027) nor for Pradosia cochlearia and are thus subject to less material = df = t = Thus for a (n 30; 28; 1.892). falling from overhead. species such as Goupia glabra, stem breakage acts to reduce subsequent The importance of canopy overhead concurs with the results from the tran- growth as well as current height, among other effects. This alters the saplings sects: saplings < 2 cm DBH are more immediate and future social status in the damaged on the control parcel because stand and its ability to compete. this kind of stem is very fragile and exposed to falling debris. Furthermore, on treated parcels, the increase in light 4. DISCUSSION reaching the soil leads to a very important recruitment of pioneer species. Such The frequencies of damage observed stems grew fast in open sites with con- are relatively high, due in part to the deci- comitantly fewer limbfalls. Both charac- sion criterion we used. Our method con- teristics are presented by Clark and Clark siders as damaged stems that have been [9] and Aide [1] as the best way for broken a long time ago. Such stem trau- avoiding physical damage by limbfalls. mas have partially healed with time and Thus the competitive sorting processes may not be taken into account with a potentially at work on saplings (damage, more conservative criterion [9]. This way disease, competition for light and nutri- of tackling physical damage leads us to ents) intervene in a different sequence and certainly with different degrees of cal damage as a function of time spent in influence in native versus disturbed tropi- the understory [1], they must be more cal rain forest. broken than the younger ones. The higher mortality of the smaller Frequency of damage of the 6-10 cm stems may lead us to underestimate the DBH class is less in the control. This can importance of physical damage in pri- probably be explained by the greater mary forest for the first diameter class. resistance of these individuals in the con- This would explain why the second DBH trol parcels as compared to the treated class presents higher frequencies of dam- parcels in the early years after logging, age in each treatment. The other explana- where slower growth rates characteristic tion for the higher frequencies in this of a lower light environment favour the diameter class is the fact that these formation of denser woody structures and saplings are older. If we consider physi- thus greater resistance to breakage. The differences in damage frequency not share the same age nor the same his- between pioneer species, such as Goupia tory. and shade tolerant ones can thus glabra, Our results on stem breakage as a fac- also be due to differences in the micro- tor contributing to mortality of saplings habitat and rate height growth [1]. are not unanimous regarding species. Frequency of damage is less for pioneer Bocoa prouacensis has the highest rate of species such as Cecropia spp. or breakage among the three species studied Simaropuba amara Aubl. [9]. Overall, here but only one individual died Goupia glabra was found here to grow between 1994 and 1996, showing that much faster than Pradosia cochlearia and this species has an important capacity to Bocoa prouacensis. Moreover, it is well survive after breakage. Frequency of known that this species reiterates easily damage for Pradosia cochlearia is nearly after physical trauma [20]. Thus we can the same as for Bocoa prouacensis but its assert that a Bocoa prouacensis and a influence on mortality seems to be impor- Goupia glabra with the same height do tant regarding the percentage of broken stems (86 %) among the dead ones. In the acts by modifying this social status of the case of Goupia glabra, while fewer stems individual. When a stem is damaged, the were broken and a greater percentage reduction in height entails a change in died over the 2 year observation period, microhabitat: the undamaged stems dom- overall mortality from all causes was also inate the broken one which receives less high, giving the impression of a less light and so grows less rapidly. This resistant and less vigorous species. domination increases with time and the The differences in ecological tempera- damaged stem declines and ultimately is in ment described in the methods can be dies. This scenario valid only the case of under direct illustrated in the specific case of stem growth competition in on a small area. We observed breakage. Goupia glabra is a pioneer groups which had a species whose waiting stage is the seed, damaged Goupia glabra while Bocoa prouacensis and Pradosia high growth rate due to the open unen- cumbered nature of the site. to cochlearia are primary forest species and According Brokaw on the basis of numerous their waiting stage is the seedling or [5], studies young plant. Goupia glabra and Bocoa experimental " (...) canopy open- occasions accelerate and prouacensis are not affected by physical ing seedling damage in their mortality and growth sapling growth in perhaps all middle- and tree Bocoa while Pradosia cochlearia seems to be upper-story species". proua- because it is to shade con- affected directly in its survival but not in censis, adapted its growth. Except for the Pradosia ditions, is able to persist in low light environments and still react to the arrival cochlearia, these results do not imply a of better conditions as shown its direct effect of physical damage on com- light by low rates. For this petition between individuals of the same mortality species, for is not vital or different species. Of course, the break- competition height growth the and ing of a stem is effectively a height during sapling stage physical even if it more time reduction and implies more time before damage, implies in the has no effect on the individual will reach the canopy stra- species understory, survival. Pradosia cochlearia is tum, especially for a slow-growing sapling an like Bocoa species, but in what way does it affect opportunist prouacensis competition and its outcome? and a sapling exposed to good light conditions will surely grow better than Observations on Goupia glabra another in shade conditions, even if it has showed the importance of initial height been broken before; however Pradosia for and rate in our sam- mortality growth seems to be sensitive to the trauma ple. This is due to this species’ heliophile caused by physical damage. temperament: its main stem grows very fast because, in a competitive environment, vertical growth is the only way to 5. CONCLUSION satisfy its needs in light energy. For this reason, competition within and between it This study shows in what way physical and other species is very important [2, damage can interfere at the specific level, 22]. Lacoste [17] showed that selective in regeneration and plant competition logging in favour of Goupia glabra dynamics. It reveals how ecological tem- seedlings and saplings significantly low- peraments will determine a reaction, sig- ers mortality and stimulates growth. He nificant or not, in the face of this phe- underlines the importance of initial social nomenon. It also reveals growth status, determined by its height, of indi- mechanisms that permit the characterisa- viduals for this species. Physical damage tion of behaviours in a stage of tropical tree species dynamics that has rarely been [8] Clark D.B., Clark D.A., The rôle of physical in the of a studied in situ and a damage seedling mortality regime using demographic neotropical rain forest, Oikos 55 (1989) 225-230. with individuals, protocol tagged by [9] Clark D.B., Clark D.A., The impact of physi- establishing the relationship between cal damage on canopy tree regeneration in tropical height or diameter growth rate and the rain forest, J. Ecol. 79 (1991) 447-457. social status of a sapling as expressed by [10] Durrieu de Madron L., Mortalité, chablis et rôle des trouées dans la avant et its initial This should us in sylvigenèse après height. help exploitation sur le dispositif d’étude sylvicole de the definition of functional groups [11] Paracou, Guyane française, thèse, Engref/Cirad, for juvenile stages of tree growth. 1993. 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