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A Comparison of Species Richness in
Buttressed and Non-Buttressed Trees at La Selva Biological Reserve
Jeannie Okray, Miriam Jimenez, Jack Green and
Patti Laird
Introduction
Buttress formation is a physiognomic structure commonly associated with
many species of emergent trees in primary forest of the tropics. Buttresses
have been analyzed using engineer models and are believed to provide
support for trees. Buttress height is
well correlated with trunk diameter (Holdridge,et al. 1971), but not all trees
found in the tropical wetlands are buttressed. One physiological explanation
for buttresses is that flooding causes low soil oxygen and inhibits the
downward translocation of food causing food and auxins to accumulate at the
tree base thus stimulating the localized growth (Kramer and Kozlowski,
1960). There is a strong negative
correlation between buttressing and bark thickness. Thicker bark tends to be a characteristic of temperate zones and
tropical montane areas due to thermal properties. It seems that when temperatures decrease as in mid montane and
upper montane wet forests within the tropics the decreased temperature seems to
select for lower surface area growth and thicker bark and deeper rooting. Increased risk of fire and drought also may
select for the decrease of buttressing with the increase of latitude (Smith,
1972). Buttressed trees are among the largest in the La Selva Rainforest. They provide a microclimate for a variety of
species. These big trees are being cut
for commercial purposes. Their destruction
may result in the loss of a component microclimate of the rainforest. In turn, this may result in the reduction of
species richness.
Problem
Do the buttressed trees have a higher species richness of epiphytic tracheophytes
and arthropods than non-buttressed trees? Secondly, is there a difference
between the species richness of trees in altered areas as compared with those
in the primary forest?
Hypothesis
We hypothesize that buttressed trees have greater species richness of
epiphytic tracheophytes and arthropods than non-buttressed trees, and that
buttressed trees in altered areas will lack the species richness of the primary
forest.
Methods and Materials
Due to safety concerns and time
constraints, we selected a random sample of six different species of forest
trees which were small to medium sized -15 to 20 inches (38cm. to 50cm.) in
diameter at breast height. Three species of buttressed trees and three species
that lacked buttresses were observed in the Holdrige Arboretum (the altered
area) and a coresponding set of buttressed and non-buttressed trees from the
primary forest at La Selva Biological Station in Costa Rica. We collected data on the trees from the ground
up to a height of five feet. We counted the number of morphospecies of
arthropods and epiphytic traecheophytes present on the trunk and on the
buttresses. If scientific names were
known, they were recorded along with the numbers of morpho-species that were
found on each tree. If tree taxonomy
was not known, we recorded the texture and color of the bark and leaf patterns
to help us to verify that six different tree species were studied in the
primary forest. For safety reasons, the
species richness assessment technique we used for arthropods was to scrape the
tree trunks and buttresses gently with stiff paper and part the foilage with an
umbrella tip in order to count the arthropod species. No ocular devices were
used to examine the surface area. A flashlight was needed in the primary forest
where light was scarce. Once the arthropod morphospecies were counted we then
counted the number of tracheophyte morphospecies growing on the same area.
Data
Conclusion
With the analysis of our data
we found a slightly lower number of arthropods and traecheophytes in
non-buttressed trees in both the primary and arboretum sites. This supports our hypothesis that buttressed
trees have greater species richness than non-buttressed trees. This may be due to a greater surface area
(due to the bifurcation of the trunk) of the buttressed trees and additional
micro-habit formation.
Comparing species richness of the altereded area (Arboretum) with the
primary forest had mixed results. Arthropod diversity was greater in the
primary forest than the altered area, while diversity was less for the
epiphytic tracheophytes. Further study
might determine if the lack of leaf litter in the arboretum is the cause for
fewer species of arthropods. Regarding tracheophyes, it is assumed the presence
of more light in the Arboretum was responsible for the greater diversity in
this habitat type. Further investigations would be needed to draw conclusions.
Limitations
It is widely assumed that older trees with larger buttresses would show
higher species diversity, as they provide a much greater surface area and
possilbly more microclimates. The trees we selected were relatively young with
small buttresses for safety and time restraints, so the mature, emergents were
omitted.
Another limitation was the number of trees within each species
assessed. A broader based study would provide more reliable data.
Further, different individuals observed at the two sites, thus
introducing possible variability in technique and skill.
Classroom
Applications
Our investigation can easily be applied to other learning
environments. By researching the lower
portions (from ground level up to eye level) of trees of varying age and
species, a comparison of epiphytes and arthropods is possible. Since the main epiphytes in temperate
forests are mosses and lichens, these should be included. Also, the different aspects (north, south,
east, west) and other abiotic factors should be noted for these may be
significant in the higher latitudes.
By researching plants based on their ability to provide habitat
for diversity of insects and plants, students develop an understanding of the environmental impact which results when
humans alter the population of species in an ecosystem. These assessments may provide data
supporting the exponential impact of altering ecosystems.
This activity can be expanded to the evaluation of the impact of
invasive species on plant and animal diversity. By examining arthropods and
selected plant types, students could investigate the organism diversity
supported by invasive species in comparison with endemic species which are
being replaced by the invasive species and extrapolate potential impacts for
biodiversity in these ecosystems.
Following presentation of their findings, students may discuss and
debate the economic, environmental, and political implications related to their
investigation. Environmental action
should be encouraged. This may range
from writing to legislators to becoming involved in local or regional
environmental groups. Students could
present their findings from the field study to appropriate groups and community
leaders to make them more aware of the significance of primary forests and
other ecosystems in maintaining biodiversity.
Reference
Henwood, K. (1973). A structural model of forces in buttressed
tropical rain forest trees. Biotropical
5, 83-93.
Holdridge, L.R., Grenke,
W.C., Hatheway, W.H., Liang, T. and Tosi, J.A., Jr. (1971). Forest Environments in Tropical Life
Zones: A Pilot Study. Pergamm
Press.
Saur, E., Bonheme, I.,
Nygren, P., and Impert, D. (1998).
Nodulation of Pterocarpus officinals in the swamp forest of Guedeloupe
(Lesser Antilles). Journal of
Tropical Ecology, 14(c), 761-770.
Smith, A.P. (1972).
Buttressing of tropical trees: a descriptive model and new
hypotheses. The American Naturalist,
106 (947), 32-46.
Smith, A.P. (1979). Buttressing of tropical trees in relation to
bark thickness in Doninica, B.S.I. Biotropical II, 159-160.
Richards, P.W. (1952).
The Tropical Rainforest: An Ecology Study. Cambridge: Cambridge University Press.
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