The importance of large arborescent palms on the dynamics of seasonally-dry Amazonian forests
A conceptual model that represents the importance of large arborescent palms in the
dynamics of seasonally dry Amazonian forests was conceived to guide this thesis.
Specifically, the model is aimed at guiding the investigation of the role of large arborescent
palms on forest regeneration and succession. Aspects of the model were examined with an
ecosystem experiment that took advantage of a palm-rich secondary forest patch
(=palmeiral) close to the Pinkaiti research station, south-eastern Amazon, immersed in a
matrix of notably well protected seasonally dry forests, preserved for the purpose of
scientific investigation and preservation. We focused on the palms Attalea maripa and
Astrocaryurn aculeatum, the first naturally abundant and the second rare at the Pinkaiti,
attempting to establish comparisons between the two species, whenever possible.
The Pinkaiti, like other seasonally dry forests, have great heterogeneity in forest
structure and composition, associated with biotic characteristics of the most important tree
species, natural disturbance and history of land-use. The palm grove, moderately dominated
by the arborescent palm Attalea maripa (Aubl. ) Mart., presented high tree species diversity
and was floristically similar to undisturbed forests at the study site.
Despite being superficially similar, the palms A. maripa and A. aculeatum are
distinct in the structure and development of their stem. The samples of A. maripa stems
were more homogeneous in density than those of A. aculeatum, both from the internal to
the external, and from the lower to the upper parts of the stem. Field estimates of stem
growth rates revealed that A. maripa growth is also more constant through development in
height. Short A. aculeatum palms had faster growth rates than A. maripa, as they get taller,
stem growth rates are reduced and approach A. maripa rates.
The study of A. maripa distribution in relation to forest disturbance suggests that,
both natural and anthropic disturbance affects favorably their occurrence. However, due to
the long time scale of palm development in relation to ordinary scientific research, the
study of the importance of forest disturbance to the development of arborescent palms must
consider the historical perspective and explicit variations of palms distribution across the
The seeds of A. aculeatum are substantially larger that those of A. maripa and carry
more reserves for the embryo. The study of the seed rain of both species, based on the
seedling distribution patterns in relation to reproductive adults, contradicts general patterns
among Angiosperms as, at the scale considered, the larger seeds of A. aculeatum were more
homogeneously dispersed than the seeds of A. maripa. The result can be explained by the
seed dispersal services paid by scatterhoarding caviomorph rodents (agoutis, Dasyprocta
aguti). The rodents apparently prefer the more nutrient rich seeds of A. aculeatum, being
more efficient dispersers of this palm, as compared with their behaviour towards A. maripa,
which "pays" smaller rewards for the seed dispersal service.
Hypotheses for the maintenance of tropical forest tree diversity were tested with a
system that includes the palm Attalea maripa, the beetle Pachymerus cardo and vertebrate
predators of large seeds in a palm patch and its adjacent forest in Southeastern Amazon.
Seed predation was affected by the degree of host-specificity of the animals involved. Seed
survivorship depended positively (when the predator was a specialist) or negatively (when
the predator was a generalist) on the distance from the palm-patch but was unrelated to
distance from individual fruiting palms.
The results presented in the data chapters of this thesis suggest that the proposed
model is appropriate and that its study is rewarding for understand the importance of large
arborescent palms on the dynamics of seasonally dry Amazonian forests. The results
highlight their role as tools for environmental restoration in a likely future of larger scale
deforestation in the Amazon.
We also investigated and compared the determinants of palm diversity at different
spatial scales: along a moisture gradient in a seasonally dry Amazonian forest and using a
climatic model comprising the entire Brazilian territory. Climatic hazards seem to strongly
determine the distribution of palms, with the relative importance of moisture being highly
consistent across scales.