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Ecology |
Universidad Central de Venezuela, Facultad Ciencias, Centro Botánica Tropical, Apartado 48312, Caracas 1041-A, Venezuela
Received for publication June 15, 2001. Accepted for publication October 11, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: flowering phenology • fruiting phenology • habitat • life-form • mature fruit phenology • vegetation unit • Venezuela
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Bawa, 1983
; Rathcke, 1983
; Petanidou et al., 1995
) and abiotic (Frankie, Baker, and Opler, 1974
; Lieberman, 1982
; Borchert, 1983
; Bell and Stephens, 1984
; Zimmerman, Roubik, and Ackerman, 1989
; Friedel et al., 1993
; Johnson, 1993
; Petanidou et al., 1995
) factors, and a combination or interaction of both kinds of factors related to seed dispersal (Wheelwright, 1985
; Seres and Ramírez, 1993
; Oliveira, 1998
) and seed germination (Burtt, 1970
).
In general, differences in flowering phenology among plant species illustrate an overlooked mechanism for maintenance of high species diversity in tropical plant communities (Gentry, 1974
). Differences in phenological strategies have been found between herbaceous and woody savanna species (Monasterio and Sarmiento, 1976
; Sarmiento and Monasterio, 1983
; Seghieri, Floret, and Pontanier, 1995
), between overstory and understory species (Shukla and Ramakrishnan, 1982
), and between canopy and subcanopy (Newstrom et al., 1994
). In the seasonal neotropics, most herbs and shrubs flower in the rainy season, but twice as many tree species flower in the dry season than in the rainy season (Rathcke and Lacey, 1985
). Herbs flower and fruit in the wet season because of moisture limitation (Monasterio and Sarmiento, 1976
; Sarmiento and Monasterio, 1983
; Seres and Ramírez, 1993
). The pattern of seasonal development, the phenological niche, in tropical plants must be seen in the context of life-forms, vegetation structure, habitat, or successional seres. Grubb (1977)
suggests that coexistence in species-rich communities is only possible due to differentiation of the regeneration niche, of which flowering phenology forms a part. The importance of habitat specialization in determining how plant species avoid interference has been previously reported. Many related species are separated by habitat rather than by flowering time (Hurlbert, 1970
; Stiles, 1975
; Bullock and Solís-Magallanes, 1990
). However, nonoverlapping staggered sequences in reproductive phenology can be a biological reality. One of the proximate elements, which explain how phenological patterns operate physically (Newstrom et al., 1994
), may be associated with the life-form strategies.
The aim of this study was to evaluate the reproductive phenology of species distributed across a successional and edaphic gradient on the Venezuelan Central Plain, as previously described by San José and Fariñas (1983, 1991)
, and to analyze whether plant life-forms and vegetation heterogeneity in the same locality have effects on flowering and fruiting patterns. Specifically, I examined the hypothesis that the reproductive phenology patterns in the Venezuelan Central Plains are affected by life-form frequency and habitats.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Aristeguieta, 1966
) at the Estación Biológica de los Llanos, Sociedad Venezolana de Ciencias Naturales, located approximately 12 km Southwest of Calabozo, Estado Guárico, Venezuela (8°56' N, 67°25' W). The 250-ha site includes the following four habitats at least, reflecting the interaction of soil types and past anthropogenic disturbance: (1) forest vegetation or groves, locally called "matas" (Blydenstein, 1962
; Aristeguieta, 1966
), which are composed of discontinuous patches with several tree stems and an understory layer of herbs and shrubs (see San José and Fariñas, 1983
) (larger clusters of trees also interrupt the grassland); (2) ecotone vegetation or forest border, which is transitional vegetation between forest and savanna; (3) savanna vegetation consisting of scattered tree stems and a continuous grass stratum dominated by Trachypogon and Axonopus spp.; and (4) secondary vegetation, consisting of vegetation dominated by pioneer species on disturbed areas that formerly supported forest or savanna but which are disturbed by fire and grazing periodically (outside protected area). Disturbed areas, outside the protected areas, are burned and pastured very frequently. The study area has been protected from fire and cattle grazing since 1961. As a result of protection, the savanna of Estación Biológica de los Llanos is gradually changing from an open to a closed bush savanna with a taller grass layer (San José and Fariñas, 1983, 1991
). From these last two studies, it could be concluded that savanna vegetation exist as a number of sequential successional stages from disturbed areas to forest condition and that the soil properties and periodical fires control the rate of vegetation change. According to Santamaría and Bonazzi (1963, 1964)
, the main operational factors in the savanna are the presence of an excessively dry soil and an indurated ironstone horizon close to the surface.
The climate is markedly seasonal, with a rainy season from May to November and a dry season from December to April (Sarmiento and Monasterio, 1968
; Walter and Medina, 1971
). The annual precipitation varies between 800 and 1839 mm, and the average annual temperature is 27°C for 25 yr of records (from 1971 to 1995). Seasonal patterns of precipitation and temperature are shown in Walter and Medina (1971)
.
Plant species and habitat preferences
Plant species were categorized as tree, shrub, liana, annual herb (short-lived species were recorded during phenological observations), perennial herb, and epiphyte; the last group included hemiparasitic and nonparasitic species. Plant species frequency was recorded in a total area of 4000 m2 in the reserve of the Estación Biológica de los Llanos (1000 m2 in each habitat). Ten 100-m2 quadrats were positioned at random in each habitat. Each 100-m2 quadrat was divided into four 25-m2 quadrats, and the 25-m2 quadrats were divided into ten 1-m2 plots. Trees were counted in the 100-m2 plots, shrubs and lianas were counted in 25-m2 plots, and herbs were counted in 1-m2 plots. Habitat preference by plant species was established comparing the number of plant species in each pair of habitats with equal expected frequencies (chi square test). Each plant species could be associated with more than one habitat, when the observed frequencies were not statistically different from the equal expected frequencies.
Phenology
A total of 171 plant species belonging to 57 families of angiosperms were phenologically censused. Individuals were selected to encompass as much heterogeneity of soil, plant size, and shade or sun conditions as possible. Plants were tagged and monitored at 1-mo intervals. The censuses included 5–10 reproductively mature individuals of each species. Individuals that died, mainly annual plant species, were replaced during the reproductive period in the same area.
Flowering and fruiting (including ripe and unripe fruits) were recorded during a 3-yr period (from 1983 to 1986). Flowering was considered to be the occurrence of open flowers. The occurrence of unripe fruit was considered to be between flower disappearance and ripe fruit. Fruit was considered to be ripe when fully developed green fruits displayed a change of color and/or texture between successive observations. In those plant species without apparent change in ripe fruit, the full development of seeds was examined. An individual plant could possess more than one phenological state at a given time, depending upon the synchronization of its reproductive activity.
Data recorded during the 3-yr period was pooled for flowering, unripe fruit, and ripe fruit respectively, and then the monthly presence of each phenological phase was established for each plant species. The percentage of species recorded monthly over all species, as well as in each life-form, and in each habitat was plotted. The relationship of life-form, habitat, and time, and their interaction effects were established using log-linear analysis of frequency (Statistica, 1994
). Log-linear analysis provides a way of looking at cross-tabulation tables. This method allows testing different factors that are used in the cross-tabulation and their interactions for statistical significance.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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2 = 155.48, P = 0.0001). The forest was dominated by trees and in a lesser proportion by shrubs and lianas, while the most frequent life-forms for the ecotone were lianas, shrubs, and perennial herbs and to a lesser degree annual herbs and trees. The savanna was markedly dominated by perennial and annual herbs, with only a few species of other life-forms present. The disturbed area was mainly dominated by annual herbs, and in a lesser proportion by perennial herbs (Table 1).
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Habitats and ripe fruits phenology
Mature fruit patterns showed a minimum in May for all habitats. An increase between September and December occurred for the ecotone and the savanna (Fig. 1C). The marginal association and interaction effects between habitat and time were statistically significant; mature fruit production was affected by habitats (Table 2). There was not a pronounced seasonality in ripe fruit abundance for the forest; two minor peaks of activity seem to occur during the year, the first during the dry season (February–April), and the second, a less conspicuous peak, during the wet season, from July to October (Fig. 1C). The disturbed area showed the most seasonal activity in the proportion of mature fruit, with a maximum between September and December.
Life-forms and flowering phenology
Flowering patterns were similar for many life-forms irrespective of habitat, except for epiphytes in the forest (Fig. 2A–F). The significant association and interaction effects between life-form and time indicated that flowering phenology was differentially affected by life-form (Table 2). A large proportion of herbaceous (annual and perennial) and liana species flowered during the wet season, whereas only a small fraction flowered during the dry season (Fig. 2C–E). The proportion of flowering annual species decreased drastically during the second half of the dry season, in which only two species remained alive until the next wet season; however, some perennial herbs continued flowering during this period (Fig. 2D–E). Flowering activity in perennial herbs commenced earlier than in annual herbaceous species. Trees, shrubs, and epiphytes showed a peak of flowering activity during the dry season; after this season, flowering continued to decrease until October for trees, until December for shrubs, and until July for epiphytes (Fig. 2A, B, and F).
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Life-forms and ripe fruit phenology
Once more, mature fruit patterns were similar for each life-form irrespective of the habitat, except for epiphytes in the forest (Fig. 2M–R). The interaction effect indicates a differential response of ripe fruit pattern to life-form (Table 2). The proportion of species with mature fruit peaked in the period of maximum rainfall for shrubs, in the mid-rainy season for perennial herbs, and at the end of the rainy season for annual herbs (Fig. 2N, P, and Q). The largest proportion of tree and liana species with ripe fruits occurred during the late dry season (Fig. 2M and O). For the sample studied, epiphytes with mature fruits were seasonal during the year, with an increase during the wet season (Fig. 2R).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Opler, Gordon, and Frankie, 1980
; Lieberman, 1982
; Shukla and Ramakrishnan, 1982
; Murali and Sukumar, 1994
; Machado, Barros, and Sampaio, 1997
; Oliveira, 1998
). This phenological pattern at the community level is frequently related to climatic factors, life-form, and the optimal time for reproductive success. Flowering and fruiting patterns are biased to a large extent by the frequencies of life-forms in the Venezuelan Central Plain. General patterns of flowering and fruiting during the rainy season in ecotone, savanna, and disturbed area is influenced by the dominance of herbaceous plants in terms of species number studied in the Venezuelan Central Plain. Differences among habitats may be associated with the relationship between life-form and habitat.
Community patterns of growth and reproduction depend on the composition of species and individuals included in the sample (Hopkins and Graham, 1989
); therefore, reproductive phenology includes the sum of variation given by the vegetation heterogeneity in the area studied. The major differences occurred between the most contrasting habitats in the Venezuelan Central Plain: the forest and the disturbed area. The flowering pattern of early seral stages differed strikingly from that of nearby mature communities. Most mature forest species flowered during the mid-dry to early wet season, while the pioneer seral stages exhibited a sharp flowering peak during the wet-dry season interface in a lowland forest ecosystem (Opler, Baker, and Gordon, 1980a
). Ecotone and savanna exhibited less pronounced flowering peaks during the rainy season compared with the disturbed area, while there was not a clear flowering peak for forest. In this sense, ecotone and savanna phenology may be considered in their reproductive phenology as stages between the disturbed area and forest, and therefore, the ecotone and savanna may be considered as previous seres to the forest vegetation in absence of disturbance (e.g., fire and cattle). Under protection of fire and cattle, the study area change into a denser arboreal community, even if the ironstone outcrop is faulted or when indurated ironstone is dept (San José and Fariñas, 1983
).
This idea is supported by the following results: (1) the significant association between habitats and life-forms, where the number of herbs and the number of trees decreased and increased from disturbed area to forest, respectively, (2) the patterns of flowering, unripe fruit, and mature fruit tend to increase in the seasonality of the proportion of plant species during the year from the forest to disturbed area, and (3) flowering peaked during the rainy season for herbaceous species, compared with a dry season peak for trees and shrubs. Under the same climatic regime, the proportion of each life-form occurring in each habitat will determine the degree of difference between habitats in flowering and fruiting phenology. These trends could reduce the competition for common resources, such as pollinators or seed dispersers, among plant species having the same life-form and at the same geographical area.
Trees, treelets, and shrubs tended to flower in the dry season, at the end of the dry season, or during the early wet season (Burger, 1974
; Frankie, Baker, and Opler, 1974
; Croat, 1975
; Monasterio and Sarmiento, 1976
; Opler, Gordon, and Frankie, 1980
; Foster, 1982
; Sarmiento and Monasterio, 1983
; Heideman, 1989
; Lampe et al., 1992
; Sun et al., 1996
; Williams et al., 1999
). The peaks of flowering activity of trees, shrubs, and epiphytes were concentrated at the end of the rainy season, the dry season, and the end of the dry season in the Venezuelan Central Plain. Synchronization of flowering with a particular season by many species appears to be under the control of prevailing climatic conditions (Frankie, Baker, and Opler, 1974
; Sarmiento and Monasterio, 1983
). Flowering during drought periods in tropical rain forests suggests a predominant role of seasonal changes in water status as determinants of flowering (Reich and Borchert, 1984
). Moisture-related factors may play the most important role in controlling flowering in tropical trees, shrubs, and epiphytes. Therefore, flowering strategies of trees, shrubs, and epiphytes converged in peaks during the dry season in the forest. Alternatively, the absence of strong water stress in the forest area would allow more flexibility of flowering phenology and its concentration during the dry season; long roots of trees and shrubs may reach the water table.
Flowering of liana species may continue throughout the year, with slight increases in the mid-dry and mid-rainy seasons (Putz and Windsor, 1987
; Morellato and Leitão-Filho, 1996
), or during the late dry season for anemochorous species (Ibarra-Manríquez, Sánchez-Garfia, and González-García, 1991
). A large number of lianas, annual herbs, and perennial herbs flowered during the wet season in the Venezuelan Central Plain, which may be associated with the response of these life-forms to rains. Included in liana species are woody and nonwoody species; this last group includes many species with aerial or subterranean organs for reserve and survival (e.g., Dioscorea, Passiflora, Operculina, and Cissus), which may depend on new growth and, therefore, on moisture availability to produce flowers. Morellato and Leitão-Filho (1996)
have recorded two peaks among liana species, including vines, for a semideciduous forest in Brazil. Liana species present a variety of life strategies that make their flowering phenology converge in a peak during the rainy season in the Venezuelan Central Plain.
Herbs and grasses commence flowering early in the rainy season, and a large number flower in the middle of the rainy period or in the later part of the wet season (Burger, 1974
; Croat, 1975
; Monasterio and Sarmiento, 1976
; Sarmiento and Monasterio, 1983
; Seres and Ramírez, 1993
; Seghieri, Floret, and Pontanier, 1995
, but see Ramírez and Brito, 1987
). Flowering activity among herbaceous species seems to be constrained by drought and mostly associated with the disturbed area. The fact that herbs must complete their vegetative growth before flowering and fruiting may explain why the flowering of this group began only 1 or 2 mo after the onset of the rainfall period (Janzen, 1967
). This situation is slightly more evident among annual species than among herbaceous perennial species in the Venezuelan Central Plain. Perennial herbs may resemble annual ones when they flower after vegetative growth has occurred early in the growing season (Rathcke and Lacey, 1985
). Although perennial and annual herbaceous species exhibited a similar yearly flowering pattern, perennial species flowered and reached a peak 1 mo earlier than annual species. It is likely that the presence of roots, stems, rhizomes, and other persistent reserve structures in perennial herbs reduced the time between the first rains and flowering activity.
The peaks of unripe and mature fruits occurred during the mid-wet season and late wet season, respectively, for herbs in the Venezuelan Central Plain. Fruiting phenology in perennial herbaceous species occurs 1 mo earlier than fruiting phenology in annual species. The longer fruiting activity of annual species compared to that of herbaceous perennial species may be related to the following factors: (1) many individuals of annual species grow close to humid sites where the drought sets in later than in other places of the same area and (2) the mature fruit production over long periods has been related to the fact that colonizing weedy herbs commonly have long periods of seed release (Harper, Landragin, and Ludwig, 1955
).
The peaks of mature fruits for lianas and trees overlapped during the dry season in the Venezuelan Central Plain. These results differ from those found by Ibarra-Manríquez, Sánchez-Garfia, and González-García (1991)
, who found that period of fruiting in trees and lianas was clearly separate in time. Fruiting peak in lianas and trees may be biased by convergent adaptation to wind seed dispersal under different patterns of unripe fruit phenology in the Venezuelan Central Plain. Morellato and Leitão-Filho (1996)
found that most of liana and tree species fruiting during the dry season were wind dispersed. In fact, most of the lianas and trees are dispersed by wind in the Venezuelan Central Plain. Epiphytes and shrubs exhibited different unripe fruit phenology that converged in a similar mature fruit phenology, irrespective of the habitat. The fruiting peak for shrubs and epiphytes during the rainy season may be related to the production of animal-dispersed fleshy fruits. Fruiting peak during the major rainy season is correlated with fleshy or animal-dispersed fruits in a tropical savanna (Gottsberger and Silberbauer-Gottsberger, 1983
) and tree species in a tropical montane forest (Sun et al., 1996
).
Finally, functional criteria such as reproductive phenology may only be used for a rough guide to establishing differences among habitats in the same geographical area. Each life-form tended to present a similar reproductive phenology irrespective of the habitat. Differences among phenological patterns in habitats are caused mainly by life-form and promote a wider distribution of reproductive events in habitat. Combinations of life-forms in different habitats could cause a better distribution of reproductive events among plant species along the year through differential flowering and fruiting times. This could offer more continuous resources for a fauna of pollinators and seed dispersal and maintain, at any time, an active seed rain in the area, which could improve establishment opportunities. Therefore, the proportion of life-forms could be considered as a proximate factor for adjusting reproductive events through time at the community level.
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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= ecotone, 
= savanna, 
= disturbed area, and 
= total.