WEWeb EcologyWEWeb Ecol.1399-1183Copernicus GmbHGöttingen, Germany10.5194/we-14-79-2014Reproductive ecology of buzz-pollinated Ouratea spectabilis trees
(Ochnaceae) in Brazilian CerradosMontesinosD.danimontesinos@gmail.comOliveiraP.Centre for Functional Ecology, DCV-FCTUC, Universidade de Coimbra,
Calçada Martim de Freitas, 3000-456 – Coimbra, PortugalUniversidade Federal de Uberlândia, Av. Pará 1720, Campus
Umuarama, Caixa postal 593, 38400-902 – Uberlândia, Minas Gerais,
BrazilD. Montesinos (danimontesinos@gmail.com)14January201514179844October201426November20141December2014This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://we.copernicus.org/articles/14/79/2014/we-14-79-2014.htmlThe full text article is available as a PDF file from https://we.copernicus.org/articles/14/79/2014/we-14-79-2014.pdf
Ouratea spectabilis is a ubiquitous tree species in the Brazilian
savannas, or Cerrados, where it plays an important ecological role.
We studied its anthesis phenology, pollination biology, pollen viability, and
pollen tube growth, and executed a set of intra- and interspecific
experimental crosses to determine its mechanisms of incompatibility and
reproductive ecology. The species presents a specialized buzz pollination syndrome and is served by a small array of specialized
pollinator species. It is a mostly self-incompatible species, and although
self-pollination is possible, it strongly reduces fertility, with
reproductive outputs for hand self-pollination similar to those of
interspecific crosses with the co-generic species O. hexasperma.
Incompatibility with another commonly co-occurring co-generic species,
O. floribunda, was complete, with a null fruit set, as occurred
for the autonomous apomixis tests. Our pollen tube growth observations indicate
that incompatibility occurs at the style, and is thus pre-zygotic. All three
Ouratea species presented very high pollen viability. Manual pollen
supplementation did not increase seed sets. Nevertheless, even after excess
manual pollen supplementation, seed-to-ovule ratios were only 30 %. Such
limits are common in stressful environments, and fruit production for most
Cerrado species is reported to be regularly under those levels. The
apparent ubiquity of this fertility limit among Cerrado species
poses interesting ecological questions, such as the role of environmental
stress on reproduction and the potential overproduction of ovules as an
evolutionary strategy to deal with seed predation – questions which deserve
further research in the future.
Introduction
Reproduction is an essential component of fitness and thus is subjected to
strong selective forces. The development of specialized pollination
syndromes, the delimitation of reproductive barriers, and the determination
of the limits to fertility provide essential clues about the relative
importance of the selective forces acting over a species. In this work we
aim to understand the reproductive ecology of an important tree species of
the Brazilian savannas, or Cerrados. “Cerrado” is the local
name given to neotropical savanna areas in central Brazil. A marked dry
season, nutrient-poor soils, and periodic fires have been considered the
determinants of this vegetation type (Oliveira and Marquis, 2002). It is the
second-most important vegetation type in Brazil, after the Amazon rain
forests, and covers approximately one-fifth of Brazilian territory. Its
flora is very rich, comprising more than 800 species of trees, with perhaps
4 times that number of herbs and shrubby species (Oliveira and Gibbs,
2000); however the few studies on reproductive biology of savanna communities
have covered less than 10 % of the species (Oliveira and Gibbs, 2002).
Those savannah areas are characterized by a high beta diversity of plant
species with a turnover of species from area to area (Bridgewaters et al.,
2004).
Several species of Ouratea (Ochnaceae) are ubiquitous across
Brazilian Cerrados (Ratter et al., 2003; Bridgewater et al., 2004).
Nevertheless, few studies have been conducted about species of this genus (but see
Barros-Henriques, 1999), as is the case for many Cerrado plant
species. Phenological and reproductive data are fundamental in order to understand
persistency and distribution of the plants (Rathcke and Lacey, 1985;
Munguía-Rosas et al., 2011), and are particularly important in tropical
environments, where seasonal variations do not limit phenological and
reproductive processes as much as they do in temperate environments (Frankie
et al., 1974; Munguía-Rosas et al., 2011). Significantly,
Ouratea species, including Ouratea spectabilis, present
specialized poricidal anthers which only release pollen when pollinators
vibrate at a determined frequency. This pollination type, also known as
buzz pollination, is a very specialized pollination syndrome
intended to reduce pollen loss to inefficient pollinators and, thus, to
increase pollination specificity and efficiency (Faegri and van der Pijl,
1979; Percival, 1979; DeLuca and Vallejo-Marín, 2013). Typically, in
buzz pollination, pollinators are rewarded by pollen and not by nectar, and
only a small group of specialized visitors are able to vibrate their wings in
the appropriate frequency and then collect the pollen (Buchman and Buchman,
1981; Buchman, 1983). Buzz pollination is present in up to 20 000 plant
species across the world, and has evolved independently many times, occurring
in species from up to 65 families, including the agriculturally important
Solanaceae family (De Luca and Vallejo-Marín, 2013).
In this study we aim to describe O. spectabilis anthesis phenology and
pollination ecology, and to experimentally study the intra- and
interspecific reproductive barriers and mechanisms of incompatibility by
controlled intraspecific crosses, self-pollination, and apomixis in O. spectabilis and also through interspecific crosses with the close
co-generic species O. hexasperma and O. floribunda. This
information is essential to understand the auto-ecology of the species and to
assess the ecological importance of buzz pollination in Cerrados.
Materials and methodsStudy species
Ouratea spectabilis (Mart.) Engl. or folha-de-serra;
O. hexasperma (St. Hill.) Benth. or vassoura de bruxa; and
O. floribunda Engl. or batiputá are three closely
related shrubs or trees from the Ochnaceae family. They frequently co-occur
in Brazilian Cerrado habitats. Trees are deciduous and
typically reach up to 4–5 m. All three species flower from August to
September, and fruits ripen from October to November. Flowers are
hermaphrodite, yellow, and without nectar. Extra-floral nectaries are known to
be critical in a number of interactions with different species of ants and
O. spectabilis (Byk and Del-Claro, 2010). However, pollinators are
apparently rewarded exclusively in the form of pollen, which is produced by
poricidal dehiscent anthers.
Study site
Fieldwork was carried out between July and September 2001 in the Cerrado
reserve (640 ha) of the Clube de Caça e Pesca Itororó,
Uberlândia, State of Minas Gerais, Brazil (18∘59′ S,
48∘18′ W). The vegetation is a cerrado sensu stricto,
consisting of a dense scrub of shrubs and trees with a fair amount of
herbaceous plants (Oliveira and Marquis, 2002). A rainy and hot season occurs
from September to April, and a dry and cold season from May to August
(Oliveira and Marquis 2002). We selected six distant (> 10 m)
individuals of O. spectabilis which were individually marked. Within
each tree, more than 100 flowers (Ntotal=634 flowers) were
bagged before the anthesis to exclude pollinators, and were subsequently used
in the hand pollination experiments and to estimate fruit set from natural
pollination.
Flower phenology and pollen viability
Flowers from each marked tree were marked in different stages of the
anthesis. Characteristics of each stage and time to change from one stage to
the next were recorded. Pollen viability was estimated from three stamens
from three flowers of three different individuals of each of the three
Ouratea species. Although only O. spectabilis was tested as
a pollen receiver, the quantification of pollen viability allowed us to discard
this factor as limiting for pollination success. For each stamen, the
viability of 500 pollen grains was assessed by using the acetic carmine
procedure, which allows distinguishing stainable cytoplasm of putatively
viable pollen grains from empty non-viable grains (Radford et al., 1974).
Generalized linear models with quasi-binomial distribution of errors were fit
to the pollen germination data with R 3.1.2 (Ihaka and Gentleman, 1996) in
order to test for differences in pollen viability among the three studied
species. Plant species was used as a fixed factor, and each individual sample,
from each of three different trees for each species, as a replicate. We
considered each germinated pollen grain as a positive count within the total
number of grains observed (500 per sample) by using the command
cbind in glm.
Pollination ecology
Insects visiting flowers at the marked trees were observed at the peak of the
flowering season (August 2001), on two different sunny days, for a total of
21 h. Number of flowers visited and time spent in each flower were
recorded. Individuals from each visitor species were collected and
taxonomically determined to the genus level. General linear models were
performed in SPSS 19.0 (IBM, 2010) in order to find differences among average
time per flower spent by each particular pollinator species.
Pollination treatments
Eight different pollination treatments were applied to a total of 1077
flowers from six different individual trees in order to study the reproductive
system of O. spectabilis. Pollen exclusion bags of nylon mesh were
set on a total of 634 flowers before the anthesis to prevent insect
pollination; we also left untouched a large number of flowers from each
individual as controls for natural pollination. In order to avoid
self-pollination, some of the flowers were emasculated before stigmas were
receptive. Pollen was extracted from the poricidal anthers by vibrating
stamens with a diapason and collecting expulsed pollen in a microscope plate.
Flower stigmas were rubbed to plates in order to pollinate them. Flowers were
bagged again after the treatment until fruits were formed and counted.
Pollination treatments were (a) controls: composed by a large number of
flowers marked and simply left without any treatment; (b) intraspecific
cross-pollination (not emasculated): flowers were pollinated with pollen from
other individuals in the population of the same species; (c) intraspecific
cross-pollination (emasculated): flowers had the same treatment as b but
were manually emasculated before anthers were ripe (this group served as a
control of the effect of the emasculation treatment); (d) spontaneous
self-pollination: flowers were left inside pollen exclusion bags without
further treatment; (e) hand self-pollination: flowers were pollinated with
pollen from a different flower from the same tree; (f and g) interspecific
pollination: flowers were emasculated before stigmas were receptive, and were
later pollinated with pollen either from O. hexasperma or O. floribunda from the same location; and (h) apomixis: flowers were
emasculated and kept inside pollen exclusion bags to test for autonomous
apomixis.
Number of O. spectabilis flowers visited by each pollinator species and
average time per visit (mean ± SD). Different letters indicate
statistically significant differences (p≤ 0.05) for flower visitation times.
PollinatorNumber of flowersAverage time perspeciesvisitedflower (seconds)Centris sp. 12452.88 ± 0.75aCentris sp. 21233.19 ± 0.80bApismellifera404.02 ± 2.20c
Total number of O. spectabilis flowers used on each treatment, and number
of fruits (and percentage) and carpels observed as a result of each
treatment.
TreatmentN of flowersN of fruits (%)N of carpelsControl443196 (44.2)438Intraspecific (not emascul.)7135 (49.3)105Intraspecific (emasculated)265 (19.2)16Self-spontaneous28825 (8.7)57Self-manual663 (4.5)8Interspecific O. hexasperma281 (3.6)4Interspecific O. floribunda250 (0)0Apomixis1300 (0)0
Observed number of flowers with pollen and/or pollination
tubes ending in that point of the feminine structures for each pollination
treatment.
O. spectabilisStigmaIn the styleEnd of the styleTotalSelf-pollination1135Intraspecific72918Inter-sp. O. hexasperma4307Interspecific O. floribunda1304O. hexaspermaStigmaIn the styleEnd of the styleTotalSelf-pollination0134Intraspecific1146Interspecific O. spectabilis4004Interspecific O. floribunda2114
When the fruits developed, the number of mature carpels from each treated
flower was counted in order to calculate seed sets obtained for each
treatment. This rate was calculated as the number of mature carpels observed,
divided by the number of flowers treated, multiplied by 5, which was the
number of carpels usually present in O. spectabilis fruits.
Generalized linear models with quasi-binomial distribution of errors were fit
to the data with R 3.1.2 in order to test for differences among pollination
treatments. Pollination treatment was used as a fixed factor, and each
individual tree as a replicate. We considered each observed seed (carpel) as
a positive count within the maximum number of ovules available in the marked
flowers (five ovules per flower) by using the command cbind in
glm. Unfortunately, we did not record how many carpels per flower
within each tree were observed.
Finally, we calculated an index of self-incompatibility (ISI; sensu
Bullock, 1985) in order to define the breeding system. This index ranges from
0 to 1 and is the ratio between self- and cross-pollination fertility, where
0.25 has been considered the upper limit for self-incompatible species. The
reproductive efficacy index (REI; sensu Ruiz and Arroyo, 1978) is the ratio
between seed set from open pollinated and cross-pollinated flowers; it also
ranges from 0 to 1 and is used to estimate the relative efficacy of natural
pollination.
Pollen tube growth
Pollen germination and growth were studied using fluorescence microscopy with
aniline blue stain (adapted from Martin, 1959). Four to 16 flowers per
treatment were marked before the anthesis and bagged to exclude pollinators.
Half of the flowers were collected 24 h after either hand self- or
cross-pollinations, and the other half of the flowers were collected 48 h
after hand pollination treatments. Treatments were the same as described for the
estimation of fruit sets.
ResultsFlower phenology
We successfully characterized six different stages of the flower development
of O. spectabilis: Immature flowers (1) were characterized by small
green buds, with calix equal in size to corolla – this stage lasted 7
days; (2) Calix remained similar in size with the corolla, but petal color
starts to change to yellow – this stage lasted from 3 to 11 days;
(3) corolla was already bigger than chalice and buds turned yellow, and stamens
started to open from this point on – this stage will last only 1 day;
(4) anthesis began when flowers started to open and was completely developed
with stigmas receptive – this stage lasted 2 days; (5) petals fell, but
stamens remain in the flower for 2 more days; and (6) no petals, sepals,
or stamens left, and fruit development started.
Pollen viability
Pollen viability estimates were high for all three species, averaging
99.97 ± 0.03 % of viable pollen grains for all three species, which
did not differ in their pollen viability rates (t=-0.772, df=8,
p=0.469, t value derived from the division of the estimates by the
standard error). This indicates that our subsequent fertility results for the
different pollination treatments were not limited by pollen viability.
Pollination ecology
Ouratea spectabilis experienced a high number of flower visits,
although trees were visited only by three bee pollinator species. Those bees
visited a total of 408 flowers during the 21 h of observations. Only the
two Centris species vibrated their wings to liberate pollen.
Apismellifera bees did not vibrate the flowers, and their
activity was limited to collect pollen spilled by previous visitors, spending
the largest time per flower among the three observed species. Significant
differences were found in the average time spent per flower for each species
(F1,2= 23.6, p < 0.001). Centris sp. 1 spent
significantly less time per flower than the other two visitors (post hoc
p= 0.001 and p= 0.007) and was by far the most common pollinator, while
Centris sp. 2 presented marginally significant pollination
visitation rate differences than Apis mellifera (p= 0.068;
Table 1). Centris pollinators stand hung on the stamens and vibrated
their wings in order to liberate pollen from the poricidal anthers
(buzz pollination or sonication). Pollen was deposited on the
ventral side of the insects' thorax, abdomen, and legs. Contact of the
pollinator body with the stigmas of flowers visited subsequently resulted in
pollination. Each fruit bore up to five (O. spectabilis, O. floribunda) or six seeds (O. hexasperma), from five or six carpels,
so seed set can be easily determined.
Pollination treatments
Pollination success varied greatly between treatments in O. spectabilis. Percentage of fertility, as the proportion of mature carpels
over the total carpels pollinated, is shown in Table 2. Statistical analysis
showed significant differences between several pollination treatments.
Controls and non-emasculated intraspecific crosses resulted in high but
significantly different fertility rates (t=-2.153; df= 39; p= 0.039),
while emasculated intraspecific crosses presented a slightly lower and also
statistically significant reduction in fertility (t=-4.180; df= 39;
p≤ 0.001). However, the main difference was observed between these three
successful kinds of crosses and the rest of the treatments, which resulted in
very low fertilities (t=-0.008; df= 39; p= 0.994; Table 3 and Fig. 1).
Emasculation treatments did have a somewhat negative effect on fruit sets,
but this group still presented high fertility values. In the low fertility
group two treatments, the apomixes and interspecific crosses with O. floribunda, gave a rotund zero, indicating both that O. hexasperma
does not present apomixis and that the species is extremely incompatible with
O. floribunda. Interspecific crosses with O. hexasperma
resulted in very low seed sets, but not as low as the ones with the other
species, indicating a weakest reproductive barrier with this species.
Index of self-incompatibility, calculated as the division of the average
self-pollination value (spontaneous and manual) and the average
cross-pollination value (emasculated and not emasculated), was 0.15,
indicating that O. spectabilis is clearly a self-incompatible
species.
Reproductive efficacy index, resulting from the division of the controls' value
by the average cross-pollination value, was 0.94, indicating an extremely
high efficacy for the natural pollination mechanism.
Pollen tube growth
Differentiation of pollen tubes on microscope preparations was difficult
both for O. spectabilis and for O. hexasperma. Callose
fluorescence was mostly weak. For both species, self-pollination treatments
and intraspecific cross-pollinations allowed for the observation of tube
growth down the end of the style. In contrast, interspecific crosses showed
tube growth arrested at the beginning or middle of the style, indicating that
interspecific incompatibility mechanisms are likely pre-zygotic (Table 3).
Discussion
Ouratea spectabilis trees showed strong reproductive barriers to
self-pollination and can be classified as self-incompatible trees,
presenting an index of self-incompatibility (Bullock, 1985) of 0.15,
well below the commonly agreed limit of 0.25 for self-incompatible species.
Self-pollination is nonetheless possible, but strongly penalized in terms of
fertility, with self-pollinated flowers presenting seed sets as low as the
ones obtained for interspecific crosses with O. hexasperma. They
are closely related species which seem to present a strong but incomplete
reproductive barrier. Incompatibility with the other co-occurring co-generic
species O. floribunda is complete, and no carpel developed into
fruit after experimental crosses, a level of fertility similar to that of
apomixes treatment, which was also null. Pollen tube growth observations
strongly pointed towards the existence of a pre-zygotic reproductive barrier,
since pollen tube growth was arrested at the style for incompatible
treatments.
Carpel-to-ovule ratio for each of the treatments in
O. spectabilis flowers, calculated as the % of the ratio
between the number of mature carpels developed per total number of ovules
available. Mean ± SE is shown. Different letters indicate statistically
significant differences (p≤0.05).
We did not observe any activity of pollen-eater insects, i.e., those that
“rob” pollen without providing any pollination service. Pollen eaters seem
to be uncommon for the genus, and previous studies (Barros-Henriques, 1999)
noticed only two inflorescences attacked by this kind of insect in the
closely related O. hexasperma. However, that same study found a
70 % pollen viability for O. hexasperma, while our
estimations of pollen viability neared 100 % for the three studied
species. Given the simplicity of the methodology, it is plausible to think
that differences were due to environmental or regional conditions, and that
pollen viability was not a limiting factor in either study, but that it can
potentially experience important regional and temporal variability.
Pollinators continuously visited O. spectabilis trees throughout the day
and among days. As expected for the extremely specialized buzz-pollinated
flowers, the array of pollinators visiting them was small, resulting in a
remarkably high REI of 0.95 (in a 0–1 range;
Ruiz and Arroyo, 1978). Such specialization is consistent with the high beta
diversity characteristic of Cerrados (Bridgewaters et al.,
2004). Although pollen supplementation resulted in a slightly higher
seed set, maximum fertility was still under 30 %, suggesting that some
pollen limitation might be present but that other factors could be playing a
more important role in limiting tree fertility. We hypothesize that water or
nutrient limitation, or even a physiological mechanism anticipating such
limitation in the future, could be playing a role in setting a maximum
investment in reproduction. Similar fertility levels are common for Cerrado
species, and most of the species for which fertility studies have been
performed show seed-to-ovule ratios below 30 % (Barbosa, 1983; Barros,
1992; Oliveira et al., 1992; Oliverira and Gibbs, 1994; Oliveira and Sazima,
1990; Proença et al., 1994). Ecological theory also suggests that ovule
overproduction could be a mechanism to control for pre-dispersal seed
predation when associated with early abortion of preyed ovules (Obeso, 2002;
Montesinos et al., 2010). In any case, this apparent waste of resources in
producing ovules which are unlikely to become seeds poses interesting
ecological and evolutionary questions with broad implications for the
understanding of Cerrado dynamics, and it deserves further attention in
future research.
D. Montesinos executed fieldwork and led the analyses of data and
the writing of the manuscript. P. Oliveira conceived and designed the study and
collaborated on the analyses of data and on the writing of the manuscript.
Acknowledgements
The authors want to thank Christian Westerkamp for his help determining
pollinator species, and to William Zaca for his help with fieldwork. Thanks
go to Asociación Española de Cooperación Internacional
(AECI) for an Intercampus scholarship to D. Montesinos (AECI-2001-115).
Edited by: J. Stadler
ReferencesBarbosa, A. A. A.: Aspectos da ecologia reprodutiva de três espécies
de Qualea (Vochysiaceae) num cerrado de Brasília, DF,
Brasília, UnB, Disertação de Mestrado, 1983.Barros, M. A. G.: Fenologia da floração, estratégias reprodutivas
e polinização de espécies simpãtricas de Byrsonima
Rich (Malpighiaceae), Rev. Brasil. Biol., 52, 343–353, 1992.Barros Henriques, R. P.: Ecologia da polinização de Ouratea hexasperma (St. Hil.) Bail (Ochnaceae) em cerrado no Brasil central,
Brasília, 4, 46–64, 1999.
Bridgewater, S., Ratter, J. A., and Ribeiro, J. F.: Biogeographic patterns,
b-diversity and dominance in the cerrado biome of Brazil, Biodiv.
Conserv., 13, 2295–2318, 2004.
Buchmann, S. L.: Buzz pollination in angiosperms, edited by: Jones, C. E. and Little,
R. J., Handbook of experimental pollination biology, Van Nostrand
Reinhold, New York, 73–113, 1983.Buchmann, S. L. and Buchmann, M. D.: Autoecology of Mourini myrtiloides (Melastomatacea, Memecylae), an oil flower in Panama, Biotropica
13, 7–24, 1981.
Bullock, S. H.: Breeding systems in the flora of a tropical deciduous forest,
Biotropica, 17, 287–301, 1985.
Byk, J. and Del-Claro, K.: Nectar- and pollen-gathering Cephalotes ants
provide no protection against herbivory: a new manipulative experiment to
test ant protective capabilities, Acta Ethol., 13, 33–38, 2010.
De Luca, P. A. and Vallejo-Marín, M.: What's the “buzz” about? The
ecology and evolutionary significance of buzz-pollination, Current Op.
Plant Biol., 16, 429–435, 2013.
Faegri, K. and van der Pijl, L.: The principles of pollination ecology,
Pergamon Press, New York, 244 pp., 1979.
Frankie, G. W., Baker, H. G., and Opler, P. A.: Tropical plant phenology:
applications for studies in community ecology, edited by: Lieth, H., Phenology
and seasonality modelling, Springer-Verlag, Berlin, 287–296, 1974.IBM: SPSS Statistics for Windows, Armonk, NY, IBM Corp, 2010.
Ihaka, R. and Gentleman, R.: a language for data analysis and graphics,
J. Comp. Graph. Stat., 5, 299–314, 1996.
Martin, F. N.: Staining and observing pollen tubes in the style by means of
fluorescence, Stain Technol., 34, 125–128, 1959.
Munguía-Rosas, M. A., Ollerton, J., Parra-Tabla, V., and De-Nova, J. A.:
Meta-analysis of phenotypic selection on flowering phenology suggests that
early flowering plants are favoured, Ecol. Lett., 14, 511–521, 2011.Montesinos, D., Verdú, M., and García-Fayos, P.: Relictual distribution
reaches the top: Elevation constrains fertility and leaf longevity of the
mountain tree Juniperus thurifera, Acta Oecol., 36, 120–125,
2010.
Obeso, J. R.: The costs of reproduction in plants, New Phytol., 155,
321–348, 2002.Oliveira, P. E.: Reproductive biology of two species of Kielmeyera
(Guttiferae) in the cerrados of Central Brazil, J. Tro. Ecol.,
9, 67–79, 1993.
Oliveira, P. E.: Reproductive biology of woody plants in a cerrado community
of Central Brazil. Flora, 195, 311–329, 2000.Oliveira, P. E. and Gibbs, P. E.: Pollination biology and breeding systems of
six Vochysia species (Vochysiaceae) in Central Brazil, J.
Trop. Ecol., 10, 509–522, 1994.
Oliveira, P. and Marquis, R. J.: The cerrados of Brazil : ecology and natural
history of a neotropical savanna, New York, Columbia
University Press, 398 pp., 2002.Oliveira, P. E. and Sazima, M.: Pollination biology of two species of
Kelmeyera (Guttiferae) from Brazilian cerrados vegetation, Plant
Syst. Evol., 172, 35–49, 1990.
Oliveira, P. E. and Gibbs, P. E.: Pollination and reproductive biology in cerrado
plant communities, In: The Cerrados of Brazil. Ecology and natural history of
a tropical Savanna, edited by: Oliveira, P. and Marquis, R. J., 424, New York,
Columbia University Press, 329–349, 2002.Oliveira, P. E., Gibbs, P. E., Barbosa, A. A., and Talavera, S.: Contrasting
breeding systems in two Eriotheca (Bombacaceae) species of the
Brazilian cerrados, Plant Syst. Evol., 179, 207–219, 1992.
Percival, M.: Floral Biology, Pergamon Press, Oxford, 243 pp., 1979.
Proença, C. E. B. and Gibbs, P. E.: Reproductive biology of eight sympatric
Myrtaceae from central Brazil, New Phytology, 126, 343–354, 1994.
Radford, A. E., Dickson, W. C., Massey, J., and Bell, C. R.: Vascular plant
systematics, Harper and Row, New York, 891 pp., 1974.
Rathcke, B. and Lacey, E. P.: Phenological patterns of terrestrial plants,
Ann. Rev. Ecol. Syst., 16, 179–214, 1985.
Ratter, J. A., Bridgewater, S., Ribeiro, J. F.: Analysis of the floristic
composition of the Brazilian cerrado vegetation III: Comparison of the woody
vegetation of 376 areas, Edinburgh J. Bot., 60, 57–109, 2003.
Ruiz, T. Z. and Arroyo, M. T. K.: Plant reproductive ecology of a secondary
deciduous tropical forest, Biotropica, 10, 221–230, 1978.