The pollination and reproductive biology of a cerrado woody community in Brazil
Abstract
The pollination biology and reproduction of a woody plant
community of cerrado, the neotropical savanna vegetation in Brazil, was
studied. A delimited area of relatively dense cerrado which included 59
woody species was studied from July 1988 to September 1989 in the
Ecological Reserve area of Brasilia Botanic Garden. Aspects of the
phenology, floral biology, breeding systems and fruit-set patterns were
studied for the whole community or for a subsample of the species in the
area. Some species, group of species or aspects of the reproductive biology
of these plants were further analyzed in separate case history studies.
The species in the community showed varied phenological
behaviour with vegetative growth and reproduction distributed throughout
the year. It seems that the phenology of adult individuals is, to a certain
extent, independent of the markedly seasonal wet and dry climate of the
area. Only the seedling establishment seems to be confined to a specific
period at the beginning of the rains. Wind and animal dispersal
mechanisms are partially constrained showing two different peaks but
flowering phenology seems to be much more flexible and even for
congeneric species flowering periods may differ markedly. The
phenological strategy of each species seems to be a more or less
independent assemblage of the possible options in each phenophase.
The plants could be grouped in seven pollination guilds based
on their main pollinators. These guilds were basically similar to the ones
described for other tropical communities. Unspecialized flowers
pollinated by small insects, including flies, wasps and small bees formed
the most Common group. The more conspicuous and specialized flowers
pollinated by medium to large, long-tongued bees formed the second most
important group. Other characteristically tropical pollination systems
such as bat and beetle pollination were also present. Hummingbird
pollination was rare and wind and butterfly pollinated flowers were
absent. The fauna of visitors, their foraging habits and possibly their
seasonal segregation is similar to that described for Costa Rican seasonal
forests, particularly in the case of the large bee fauna. No pollination
system seems to be seasonally limited although a peak of large bee
pollinated flowers, for example, could be identified at the onset of the
rains. Breeding systems could be established for a sample of 22
species with different pollination systems, the majority (86%) of which
presented self-incompatibility mechanisms. This sample and the
frequency of dioecious species (15% of the total) permitted an estimate of
84% as the frequency of obligatory outcrossing species in the area. Such
frequency is similar to those obtained for other lowland tropical forest
communities and much higher than the estimates for altitudinal cloud
forests in the tropics. Apomixis was present in two species in the
community and is possibly present in another two, which indicate that,
although rare, this may be an important phenomenon amongst the
cerrado woody species. The site of the incompatibility reaction was studied
for most of the self-incompatible species and a majority presented "lateacting
self-incompatibility", with self-pollen tubes reaching the ovary or
even penetrating the ovule. More "classical" self-pollen tube arrest in the
style was also observed for some groups.
Fruit-set was usually low but the data should be regarded
with caution since they varied both spatially and temporally.
Nevertheless, some differences between pollination system groups and
relationships with breeding system are suggested by the results.
The case history studies involved a self-incompatible shrub
species, Yellozia squamata (Velloziaceae), which presented a distinct
flowering phenology; a bat-pollinated tree, Hymenaea stigonocarpa (Leg.
Caesalpinioideae), which presented "late acting self-incompatibility";
another legume tree, Sclerolobium paniculatum, which present two
varieties segregated ecologically but which were not clearly isolated
reproductively; contrasting breeding systems, self-incompatibility and
apomixis, in two cerrado Eriotheca spp. (Bombacaceae); six species of
Vochysia (Vochysiaceae) which occur in different vegetation
physiognomies in the Ecological Reserve of the Brasilia Botanic Garden
but have very similar floral biology and outbreeding system; a study on
some small "settling" moth pollinated species in the study area which
have similar outbreeding systems as their large hawkmoth pollinated
counterpart; and a general study on the occurrence of dioecy which is less
important in cerrado areas than in the contiguous forest habitats.
The breeding features emerging from this study support the
idea of the cerrado vegetation as stable communities where biomass
output is possibly limited by availability of nutrients and restrict
establishment conditions, but not restricted by seasonality or disturbance
in terms of opportunity and predictability for the reproductive process of
the woody plants. If outbreeding systems are indicative of environmental
stability, then cerrado conditions seems to be comparable to those in the
lowland tropical forest.
Type
Thesis, PhD Doctor of Philosophy
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