Báo cáo khoa học: "Physical damage on tropical tree saplings: quantification and consequences for competition through height growth in a neotropical rain forest of French Guiana"

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Original article Physical damage on tropical tree saplings: quantification and consequences for competition through height growth in a neotropical rain forest of French Guiana Rankin-de Mérona b Gilles Koestel a Judy M. 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 / breakage / tropical rain forest stem 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:meronaj@kourou.cirad
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.) / mortalité / / forêt tro- croissance / jeune arbre / compétition mécanique casse 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 regeneration in tropical rain forests between pioneer species and shade toler- tree 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]. Physical damage is 1) In what way does the ecological the mechanical breakage of a stem by an temperment of a species influence the animal (due to tramping, scraping, push- frequency of damage to saplings? ing, biting or boring for example) or by 2) For a given individual of a given material falling from a higher stratum of species, is physical damage automatically the vegetation. Whereas seedlings are detrimental in comparison to another more likely to be completely crushed, individual of the same species with no saplings most frequently suffer from breakage, especially under conditions of breakage or stem deformations resulting active competition? in significant modification of future 3) In which terms does physical dam- growth. This damage may either lead to age affect sapling growth, in height or increased mortality or, in the case of sur- diameter, and in what proportion? Is there vival, to changes in growth trajectories. direct indirect consequence of stem We can surmise that individuals reaching a or these parameters? the young tree stage and emerging from breakage on the understory may already have a long We undertook a study of the effects of and eventful past history. damage by breakage on the growth and survival of saplings of three tropical rain If we take into account the notion of species in the study of physical damage forest tree species in native and silvicul- turally treated forest in order to answer on saplings, we can evaluate how it influ- ences mortality, growth and competition these questions.
The experimental site is composed of 12 2. MATERIALS AND METHODS of 9 ha (each surrounded by a 25 m wide plots buffer zone) distributed over three replicated blocks consisting of four silvicultural treat- 2.1. Study site ments, including a control [27]. The treat- ments, applied once in October 1986 to May This was carried out at the Paracou study 1987 for logging and December 1987 to Forest Research Site of Silvolab, a Tropical January 1988 for poison girdling, are: co-ordination unit set up by French research control (mean basal treatment 0: institutes (current members are CIRAD-Forêt, area - /ha); 2 m 32 (ba) Inra, ENGREF, ORSTOM, ONF and MNHN) = for forest ecosystem studies in French Guiana. selective logging above treatment 1: - Located in the dense upland rain forest of DBH (mean remaining ba after 50 cm north-eastern South America (2-6° N, 24 m /ha; 2 logging = 51°30’53°30W) (figure 1), the climate is con- selective logging plus thin- treatment 2: - sidered as equatorial, characterised by peren- after logging 19 ning by poison girdling (ba = nial high (80-90 %) humidity, low tempera- /ha); 2 m ture variation centred around 26 °C and a treatment 3: selective logging plus thin- rarity of violent winds [15]. Mean annual rain - ning by poison girdling and fuel wood extrac- fall for the last 10 years is 4 976 +/- SD tion (ba after logging 16 m/ha). 2 243 mm (CIRAD-Forêt à Kourou, unpub- = 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. of breakage, survival, height Frequency and diameter growth rates for damaged and The forest structure and composition are undamaged stems were analysed at both the generally similar to other upland rain forest community level and the specific level. sites in South America, all the while possess- ing a Guianian character by virtue of a small At the community level, the frequency of number of relatively abundant tree species in breakage on saplings regardless of species a nonetheless species-rich forest. The three was estimated on randomly oriented transects most representative families of trees 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 (DBH) in the Paracou forest are the transects were censused on three parcels (con- Lecythidaceae (18 % of the individuals), the trol, first level and second level treatments; Caesalpinaceae (13 %) and the with 32 transects per parcel). Measurements Chrysobalanaceae (12 %) [11]. The principal were made on individuals of more than 1.5 m tree species are the Eschweilera spp. in height and less than 10 cm DBH. Diameter (Lecythidaceae), the Licania spp. (Chryso- was measured by classes of 1 cm intervals balanaceae), Eperua falcata, mouamba, with a notched gauge (figure 3). Frequencies prouacensis (this study), bouchi Bocoa of damage were assessed using Clark and mango, Iryanthera spp., bakouman, Eperua Clarks method based on comparison of main grandiflora, Symphonia globulifera, moni, stem diameters above and below breakage Vouacapoua americana, Pradosia cochlearia points as indicated by major discontinuity or (this study), Qualea rosea Carapa procera, scarring [9]. From this a discontinuity ratio patawa, and Dicorynia guianensis [24]. From can be established as: a structural point of view there are on the * discontinuity ratio 100 (diameter above = average 618 stems/ha, with a mean basal area scar/diameter below scar). of 31 m [10]. Approximately 60 % of the /ha 2 Clark and Clark [9] fixed the lower limit of stems are 20 cm in DBH or less [24]. While discontinuity at 25 %. This means that a stem emergents may reach 45 m in height, the gen- with a discontinuity less than 25 % was con- eral level of the canopy is around 40 m (B. sidered as undamaged. But, as they recognise Ferry, pers. com.).
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 cause of the damage. liminary investigation of different critical lev- els for breakage acceptance (10, 15, 20 and The species level effects of damage were 25 %), decided to lower the level of accep- we studied for three tree species of contrasting tance to 20 %. However, results at the com- ecological temperaments. The individuals munity level were not different for the two 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
2.3. Species studied [26] for details of the inventory protocol and 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 tree species which attains 30 m in height and the observations at these two levels were con- is shade tolerant as an adult [4, 11]. Seed dis- ducted in the same zones. All the saplings persal is endozoochorous (spider monkeys) (n = 80 for each species) were more than and synzoochorous (bats, such as Artiibeus) 1.5 m in height and less than 5 cm DBH, the [31]. upper limit being determined by the difficul- 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 [31]. Like Boco, the saplings of Kimboto are up to 7 m. Measurements of DBH for all the relatively shade tolerant [19] yet growth can saplings and the diameter above and below be stimulated by increased light following for- scar for damaged saplings considered as dam- est canopy opening [25]. aged were made twice with a millimetric 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 bank [21]. It is a light-demanding species the diameter and height growth.
from germination until death and grows 3. RESULTS rapidly [18]. It quickly colonises gaps and open sites [6], especially on disturbed soils [28] such as logging trails. For these reasons, 3.1. Frequency of damage in 1994 Goupia glabra usually appears on favourable sites in dense groups of relatively numerous The overall frequency of damage individuals among other pioneer species [17]. observed on the transects, all species con- Mature trees emergent, with maximum are a sidered, is 40.6 % (n 3 681). Damage height surpassing 40 m [12, 20] and a lifespan = of more than 100 years. frequency is essentially equal for the con- trol and treatment level 1 (41.9 and Bocoa prouacensis, Pradosia cochlearia 41.5 % for n 968 and1 421, respec- and Goupia glabra are among the 14 more = common species (for all diameter classes) at tively) and only slightly less for treatment Paracou in the control parcels [3, 10]. The level 2 (38.5 %, n 1 292). = number of Goupia glabra saplings increased For saplings ≤ 4 cm DBH, damage fre- steadily on treated parcels after logging and quency is 43.9 % (n 852) on control = silvicultural interventions [3, 19] while Bocoa plots in our study (table I) as compared prouaceusis remained unchanged or decreased to 19.5 % (n 794) in Clark and Clark’s [19, 25]. = 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 % discon- tinuity threshold or theirs at 25 %. As mentioned these already by authors, frequency of damage and diame- ter class are not independent. At Paracou, find that the ratio of broken to intact we varies by more than chance among stems the four diameter classes under all condi- tions (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 signif-
Goupia glabra; analysis chi- icant differences exist between this and 3.873; df 1; P < 0.05 for the frequencies observed in the first and = square = Pradosia cochlearia). The median value second treatment levels for this size class was used in this comparison, being more (df 2; chi-square 5.8; P > 0.05), nor = = representative of location due to distribu- for any of the other size classes. tion skewness. We also examined the initial overall frequency of stem damage for the three species chosen for the growth and sur- 3.3. Rates of growth in height vivorship studies. Differences in fre- and diameter quency of damage among species are 2, chi- highly significant (df = Growth rate between March of 1994 17.3; P < 0.001) because fre- = square and March 1996 were measured for the quency of damage is very high for Bocoa saplings that did not die, suffer disease prouacensis (64 %; n 72) and Pradosia = and/or physical damage during the 78) and nearly cochlearia (59.5 %; n = period, as far as we were able to discern. double that observed for Goupia glabra For this reason, the sample size for this (34 %; 85). n = part of the study was reduced as com- pared to the initial sample of tagged seedlings. True values of mean diameter 3.2. Two year mortality rates growth are difficult to establish due to difficulty in accurately repeating mea- Mortality between 1994 and 1996 due surements on an irregular stem where the physical damage varied widely to site of measurement can not be perma- between the three species studied. nently marked without risk of damage to Mortality was high for Goupia glabra (21 the plant, and due to the great difference of 85 individuals) but less than half of in overall diameter and actual diameter this is apparently due to physical damage: growth over the observation period (table nine saplings broken among the 21 which III). Moreover, regarding stem shrinkage died over the period. Six of the seven [16], for a short observation period such Pradosia cochlearia that died (of 78 ini- 2 years, true gains may be masked if tially) had their stems broken. Only one as the second measure is made when the Bocoa prouacensis of 72 tagged stems stem is significantly less hydrated or died and that individual had its stem bro- under greater hydric stress than at the ken in 1994, and thus does not figure in first measure. In this study, this problem the following analysis. was minimised by taking the measure- The comparison of heights and diame- ments during the same seasonal period. in 1994 between stems dying during ters we can observe highly Nevertheless, the subsequent 2 year period and those differences among species significant surviving (table II) shows certain signifi- (table III) in diameter growth rates cant differences by species, with those (Kruskall-Wallis test; df = 2; chi- dying have smaller initial heights and 10.5; P < 0.01) and height diameters (for initial diameters: median = square growth rates (Kruskall-Wallis test; df = 2; 1-way analysis chi-square 7.228; = chi-square 11.9; P < 0.01) with Goupia df 1; P < 0.01for Goupia glabra; anal- = = glabra greatly outpacing Pradosia ysis chi-square 5.745; df 1; P < 0.05 = = cochlearia for both parameters and for Pradosia cochlearia; for initial height: median 1-way analysis chi- Pradosia cochlearia outpacing Bocoa 4.789; df 1; P < 0.05 for prouacensis. = = square
No correlation is height (table VI). There is no significant difference observed between the variables men- between damaged and undamaged stems tioned above for Bocoa prouacensis of the same species (table IV) in diameter (damaged or undamaged), the damaged growth (median 1-way analysis chi- Goupia glabra and the undamaged 0.072; df = 1; P 0.79 for = square = Pradosia cochlearia. There is a signifi- 0.282; Goupia glabra; chi-square = cant but not strong correlation between 0.59 for Pradosia cochlearia, df 1; P = = these variables for damaged Pradosia 0.24 for 1.38; df 1; P chi-square = = = cochlearia. Bocoa prouacensis). Sapling height in 1994 can be parti- Spearmans correlation between height tioned into the following components: growth rate over 2 years and the initial height (1996) height (1994) + height = 1994 DBH is high and significant for the growth (1994-1996) undamaged Goupia glabra (table V). The The comparison of a linear regression same kind of correlation is observed model between stem height in 1994 and between height growth rate and initial
consider this just simply stem height in 1996 for undamaged stems phenomena not with the line y x is one way to evaluate but also as a frequency at a given time, as = the relationship between initial height and function of time spent in the understory. It reveals both the importance and the subsequent height growth (figure 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. parison of the regression coefficient value The low rate of damage in the higher 1; one-tailed test: n 39; df 37; to = = diameter classes is consistent with the 5.793; P < 0.001). 0.025; t a = = greater mechanical resistance of larger Difference is not significant for the stems and the fact that they occupy pro- Bocoa prouacensis (n = 25; df 23; = gressively higher levels within the forest t = 1.027) nor for Pradosia cochlearia and are thus subject to less material (n = 30; df 28; t 1.892). Thus for a = = falling from overhead. species such as Goupia glabra, stem The of canopy overhead importance breakage acts to reduce subsequent with the results from the tran- concurs growth as well as current height, among sects: saplings < 2 cm DBH are more other effects. This alters the saplings damaged on the control parcel because immediate and future social status in the this kind of stem is very fragile and stand and its ability to compete. exposed to falling debris. Furthermore, on treated parcels, the increase in light reaching the soil leads to a very important 4. DISCUSSION recruitment of pioneer species. Such stems grew fast in open sites with con- frequencies of damage observed The comitantly fewer limbfalls. Both charac- relatively high, due in part to the deci- are teristics are presented by Clark and Clark sion criterion we used. Our method con- [9] and Aide [1] as the best way for siders as damaged stems that have been avoiding physical damage by limbfalls. broken a long time ago. Such stem trau- Thus the competitive sorting processes mas have partially healed with time and potentially at work on saplings (damage, may not be taken into account with a disease, competition for light and nutri- more conservative criterion [9]. This way ents) intervene in a different sequence of tackling physical damage leads us to
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 of the smaller Frequency of damage of the 6-10 cm higher mortality lead us to underestimate the DBH class is less in the control. This can stems may 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 share the the his- not same nor same age between pioneer species, such as Goupia tory. glabra, and shade tolerant ones can thus Our results fac- stem breakage on as a also be due to differences in the micro- to mortality of contributing saplings tor habitat and height growth rate [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
(86 %) among the dead ones. In the by modifying this social status of the stems acts of Goupia glabra, while fewer stems individual. When a stem is damaged, the case 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 damaged stem declines and ultimately The differences in ecological tempera- dies. This scenario is valid only in the described in the methods can be ment case of growth under direct competition illustrated in the specific case of stem in groups on a small area. We observed breakage. Goupia glabra is a pioneer damaged Goupia glabra which had a species whose waiting stage is the seed, high growth rate due to the open unen- while Bocoa prouacensis and Pradosia cumbered nature of the site. According to cochlearia are primary forest species and Brokaw [5], on the basis of numerous their waiting stage is the seedling or experimental studies " (...) canopy open- young plant. Goupia glabra and Bocoa ing occasions accelerate seedling and prouacensis are not affected by physical sapling growth in perhaps all middle- and damage in their mortality and growth upper-story tree species". Bocoa proua- while Pradosia cochlearia seems to be censis, because it is adapted to shade con- affected directly in its survival but not in ditions, is able to persist in low light its growth. Except for the Pradosia environments and still react to the arrival cochlearia, these results do not imply a of better light conditions as shown by its direct effect of physical damage on com- low mortality rates. For this species, petition between individuals of the same competition for height growth is not vital or different species. Of course, the break- during the sapling stage and physical ing of a stem is effectively a height damage, even if it implies more time reduction and implies more time before species in the understory, has no effect on the individual will reach the canopy stra- sapling survival. Pradosia cochlearia is tum, especially for a slow-growing an opportunist like Bocoa prouacensis species, but in what way does it affect and a sapling exposed to good light con- competition and its outcome? ditions 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 mortality and growth rate in our sam- 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 environ- ment, 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 It reveals how ecological tem- dynamics. 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
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