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Tropical Biology |
Unité mixte de recherche CIRAD-CNRS-EPHE-INRA-Université Montpellier 2 botanique et bioinformatique de l'architecture des plantes (AMAP), TA40/PS2, 34398 Montpellier cedex 5, France
Received for publication August 30, 2001. Accepted for publication January 29, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: branching • Cecropia obtusa • Cecropiaceae • flowering • growth • synchronicity • phenology
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), ranging from Mexico to northern Argentina and the West Indies (Wheeler, 1942
). The species are pioneer trees that colonize cleared areas and open areas with high light levels (Whitmore, 1989
; Alvarez-Buylla and Martinez-Ramos, 1992
). As the trees are associated with the initial stages of forest succession, determining their age can help to pinpoint the date of the disturbance that prompted their establishment. To this end, it is necessary to understand the dynamics of their development and architecture over time.
In temperate tree species, meristem functioning is synchronized by an enforced rest period in the winter, and each year a shoot is emitted on all the axes of the tree. Observing morphological or macro-anatomical markers (cataphyll scars, reduction in internode length, growth rings, reduction in pith diameter, etc.; Hallé, Oldeman, and Tomlinson, 1978
; Heuret et al., 2000
) often serves to deduce annual growth halt retrospectively. This temporal reference can thus be used to estimate the age of an axis a posteriori and to compare the properties of the annual shoots established in a given year on the axes of a single tree or of several trees. In many tropical plants, the absence of temporal morphological markers makes the retrospective analysis of development difficult, but studies of the regularity of growth, branching, and flowering over time and the analysis of the synchronicity in time of these processes on distinct axes may help the observer to deduce past morphological events.
In Cecropia peltata L. and Cecropia insignis Liebm., flowering has been shown to be annual and to cover a period of several months (Frankie, Baker, and Opler, 1974
; Fleming and Williams, 1990
; Milton, 1991
). In a study conducted on C. obtusifolia Bertol. and C. peltata in Costa Rica, Davis (1970)
showed that internode length fluctuates according to an annual pattern that is correlated to rainfall amount.
Cecropia spp. have a very simple architecture and conform to Rauh's architectural model (Hallé, Oldeman, and Tomlinson, 1978
). Their axes are made of a succession of nodes and internodes whose length and associated lateral productions remain visible and measurable over years. Our aim was to retrospectively study these morphological structures on an individual and stand level in Cecropia obtusa Trécul in order to analyze the regularity of growth, branching, and flowering processes over time with the objective of finding parameters that help reconstruct the developmental dynamic in this species. For this purpose we analyzed the morphological events associated with the successive nodes of an axis (inflorescence or inflorescence scar, branch, underlying internodal length) for all axes of an individual crown and several trees of a same regeneration after forest cutting along a roadside.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The axes are orthotropic, with lateral flowering and rhythmic branching (Fig. 2). Phyllotaxy is alternate and spiral with a fraction of 3/7 (154.3° between two longitudinally adjacent leaves). The leaves are stipulate, entire in young trees, but subsequently become palmatilobate (six to eight lobes). It is a dioecious species with lateral inflorescences. There are three lateral buds in the axil of each leaf (Fig. 3a). The central bud is vegetative and may potentially give rise to a branch. The buds on each side correspond to two proximal axillary buds of the central bud and may give rise to inflorescences, which are thus arranged in pairs, each comprising a set of spikes. The immature spikes are protected by a bract that acts as a sheath, which opens on anthesis (Fig. 3b). Female inflorescences have 4 spikes (Fig. 3c) and male inflorescences have 12 to 15 spikes (Fig. 3d) (Trécul, 1847
).
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), every possible combination of branch and inflorescence scars may be seen (aborted branch and aborted inflorescences, aborted branch and fully developed inflorescences, etc.; Fig. 4).
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The following data were recorded at each node: the type of axillary productions associated with the central bud, with the following coding of the three possible outcomes: (0) no branch, (1) aborted branch, and (2) developed branch (pruned branches were included in this category); the type of axillary productions associated with the lateral buds [(0) no inflorescence, (1) aborted inflorescences and (2) developed inflorescences]; and the length of the underlying internode.
The nodes of all the axes were described on ten trees, while only nodes on the main axis (A1) were measured on the other 20. Due to the frequent development of stilt roots, which made observation difficult towards the foot of the tree, the number of internodes separating the cotyledons of the first node measured is uncertain.
The topology, i.e., the relative positions of the different botanical units described (nodes and axes), was coded using the AMAPmod software (Godin et al., 1997
; Godin, Costes, and Caraglio, 1997
; Godin, Guédon, and Costes, 1999
), enabling the precise location of each structure in space during analysis.
The axis are described node by node from the tip (hence, the ranks of the nodes are counted from the tip) (Fig. 5a). Assuming that the nodes of the axes compared were emitted at the same rate, we can consider that nodes with the same rank were emitted at the same time (rank 1 corresponded to the time of the study). If the nodes of the axes compared were emitted at different rates, this agreement between topology and chronology disappears more or less rapidly as the rank increases. In the case of the "internode length" variable, the A2 of a given tree were compared with A1 as per the following principle: the nodes of A1 were counted from the point of insertion of the first pruned or living branch (which determined the base of the crown) to the top of the tree. The rank on an A2 began at (rank of the bearing node on A1) +1 (Fig. 5b).
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The "branching" variable, with the possible outcome: bud (0), aborted branch (1), and developed branch (2), can be considered as an ordinal variable since the three possible values can be ordered in a meaningful way (corresponding to the code order). This reasoning transposes to the "flowering" variable with its possible (ordered) values: no inflorescence (0), aborted inflorescence (1), and developed inflorescence (2).
An important tool for exploring sequences built from quantitative variables is provided by a series of quantities called sample autocorrelation coefficients, which measure the correlation between observations at different distances apart. When the relevant variable is ordinal, the ordinary or Pearson correlation coefficients can be replaced by the rank correlation coefficients due to Spearman, i.e., the ordinary correlation coefficient between the ranked values. Hence, while the Pearson correlation coefficient looks for a linear relation between two variables, the Spearman rank correlation coefficient looks for a monotonic relation (for more details, see Guédon et al., 2001
).
The correlation properties of multivariate sequences can be investigated by means of sample cross-correlation functions, which are a direct generalization of sample autocorrelation functions. The cross-correlation function measures the correlation between Xt and Xt+k as a function of the lag k, while the autocorrelation function measures the correlation between Xt and Xt+k as a function of the lag k. The sample autocorrelation function is an even function of the lag and hence needs to be plotted for k = 0, 1, 2, ..., while the sample cross-correlation function is not an even function of the lag and hence needs to be plotted for k = 0, ±1, ±2, ...
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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As regards the internodal lengths of the 30 trunks measured, autocorrelation coefficients failed to reveal any significant regularity. Moreover, we did not detect any relation between variations in internodal length and branching or flowering events.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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On a stand level, the rate of emission of new nodes (apparent plastochron) and flowering periods of Cecropia obtusa have never been studied for more than a year or at short, regular intervals, as far as we know. On the basis of an analysis, over 5 wk, on 20 axes of trees whose stage of development was not stated, Lauri (1988)
estimated the plastochron at between 7 and 12 d. Belin-Depoux et al. (1997)
, based on records of the number of nodes established over a 9-mo period, estimated the figure at around 10 d. If one assumes that growth is continuous in this species, the emission of a new node every 10 d or so corresponds to the establishment of around 36 nodes in the course of a year, which coincides with the period observed in the pattern of expression of branching and flowering (Figs. 10a–b). Twice-weekly measurements taken for 2 yr in Costa Rica on Cecropia peltata (Frankie, Baker, and Opler, 1974
; Fleming and Williams, 1990
) showed that flowering was annual and spread from April to August during the dry season. The annual nature of flowering was also demonstrated on Cecropia insignis by 2 yr of growth monitoring on Barro Colorado Island, Panama (Milton, 1991
). As far as we know, no other work refers to flowering patterns with periodicities of more than or less than a year in species of the genus Cecropia. The annual periodicity therefore seems to be appropriate for describing the flowering and branching pattern observed in Cecropia obtusa. The stability of the number of nodes between two zones with branches or flowers, underlined by the significant autocorrelation coefficients over 100 nodes or so (expression of three periods), suggests that the plastochron is highly stable not only for the different axes of a given tree, but also for the different trees in a stand.
It is thus possible to determine the age of tree crowns precisely, simply by counting the number of tiers of branches. In view of these results, it seems to us that the number of nodes and the number of branch tiers should be taken into account to the same extent as branch length or total number of branches when studying Cecropia obtusa growth. On certain individuals (five out of 30 in this study), a branch may be missing from the main axis, or only the scars of aborted buds may be visible. Observing these scars and counting the number of nodes between two successive tiers (from 57 to 63 nodes for the five individuals in question, i.e., more or less double the identified period of 35 nodes between two normally branches developed tiers) proved that a tier was missing, corresponding to a year. This phenomenon was also often observed on branches of the first tier.
Davis (1970)
described variations in internodal length in Cecropia peltata and Cecropia obtusifolia in Costa Rica, which depended on the variations in annual rainfall. The internodes established during the dry season were shorter than those established during the rainy season. Under these conditions, observing the alternation of short and long internodes on the main axis would be a way of estimating the age of individuals. Although we observed variations in internodal length, we did not observe any such marked patterns in our study. This can be attributed to the less marked difference between the dry and rainy seasons at our site than in Costa Rica and to the occasional occurrence of a short, more or less severe dry season in March. If we assume that the variations in internodal length observed on the various axes of a given tree are directly linked to the climatic conditions, the slight lag observed between the variations in internodal length on A1 and A2 suggests that the A2 produce slightly more nodes in the early stages of their growth. Internodal length, which is directly affected by environmental factors such as rainfall (Davis, 1970
), seems to be less stable than the rhythm of new node emission.
In the genus Cecropia, the height of the first branch is always highly stable for a given species (Oldeman, 1974
; Alvarez-Buylla and Martinez-Ramos, 1992
; Sposito and Santos, 2001a
, b
). In our study, the first branch was at a height of around 7 m, which corresponds to around 120 nodes (with a degree of uncertainty due to the difficulty of observing the lowest nodes). In view of the stability of the number of nodes produced each year (around 35 nodes) and of the number of nodes below the first branches in the crown, the age at which the tree begins to branch would apparently be between 4 and 5 yr. It is highly likely that the plastochron changes during the establishment phase, in line with a "establishment effect" found in most plants obtained from seed (Barthélémy, Caraglio, and Costes, 1997
). Moreover, tree development may be modified by environmental conditions. Barthélémy (1988)
observed Cecropia obtusa growing on bare soils that were both eroded and deprived of humus, which remained single-stemmed all their life and did not exceed a height of a few meters. For trees growing in nonoptimum conditions, the point at which the tree produces its first tier of branches may be linked more to a stage of development than to a chronological age. Knowledge of the stage at which Cecropia obtusa individuals produce branches and of the variability of that age would enable an easy assessment of the age of a regenerated stand, based simply on counting the number of branch tiers on the individuals in the stand. As Cecropia spp. are associated with the initial phases of vegetation sequences (Whitmore, 1989
; Alvarez-Buylla and Martinez-Ramos, 1992
), this would enable an estimation of the age of gaps and a better understanding of the development over time of other pioneer species associated with recolonization.
The annual pattern seen for growth, branching, and flowering would also be worth studying on other species of the genus Cecropia, particularly those that live longer, so as to see how less favorable environmental conditions (physical interference, competition for light, etc.) affect tree architecture. It is likely that the plastochron diminishes with trees age as a result of a drift phenomenon (Barthélémy, Caraglio, and Costes, 1997
).
This study, which was solely based on a posteriori observations, revealed that growth, flowering, and branching were both temporally and topologically organized. Combining this method with growth monitoring could help to detect markers in other species that would provide a clearer understanding of the phenology of those species and of their growth dynamics over time.
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FOOTNOTES |
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2 Current address: Ecole Nationale du Génie Rural, des Eaux et Forêts, 14 Rue Girardet, 54000 Nancy, France
3 Author for reprint requests (patrick.heuret{at}cirad.fr
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Barthélémy D. 1988 Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique. Ph.D dissertation, Université Sciences et Techniques du Languedoc Montpellier II, France
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Belin-Depoux M. P.-J. Solano C. Lubrano J.-R. Robin P. Chouteau M.-C. Touzet 1997 La fonction myrmécophile de Cecropia obtusa Trecul (Cecropiaceae) en Guyane française. Acta Botanica Gallica 144: 289-313[ISI]
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Guédon Y. D. Barthélémy Y. Caraglio E. Costes 2001 Pattern analysis in branching and axillary flowering sequences. Journal of Theoritical Biology 212: 481-520
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Sposito T. C. F. A. M. Santos 2001a Architectural patterns of eight Cecropia (Cecropiaceae) species of Brazil. Flora 196: 215-226[ISI]
Sposito T. C. F. A. M. Santos 2001b Scaling of stem and crown in eight Cecropia (Cecropiaceae) species of Brazil. American Journal of Botany 88: 939-949
Trécul M.-A. 1847 Sur les Artocarpées. Tribus 1—Conocephaleae. Cecropia Linn. Annales des Sciences Naturelles—Botanique 3: 78-85
Wheeler W. M. 1942 Studies on neotropical ant-plants and their ants. Bulletin of the Museum of the Comparative Zoology Harvard 90: 1-162
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