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The Prevelance of Buttresses in Costa Rica
Buttressing of the roots of trees is very common in the tropical rainforests of the world and especially in Costa Rica. Many theories have been formed to explain the presence of these abnormally large structures that are species specific. Some of the theories involve gas exchange or nutrient uptake, but the most accepted theory is for the support of massive trunks and anchorage to the ground that is needed in high stress environments. It was this latter theory that my group chose to explore upon during our course in Costa Rica.
Buttresses are formed on the roots of trees from extreme epinastic growth on the upper side of the surface lateral roots. The trees that usually produce buttresses experience high amounts of stress as a result of either topography, asymmetric crowns, exposure to wind and strong epiphytic growth or a combination of these. Buttressing is commonly more prominent on the windward side and/or uphill side of the trunk. This is thought to happen because it provides the most support where the most stress is placed upon the tree (ecospace.newport.ac.uk).
There are many similarities that can be found between the different species that produce buttresses. An example of one of these is that they are most prominent on trees that are part of the canopy or emergent trees. The larger buttressed trees tend to grow in similar environments that have: weak silty soils, shallow waterlogged soils, or a shallow humus layer overlying rock or subsoil. Therefore it can be assumed that the extent of buttressing is correlated to the height of the tree, soil texture, and depth. These examples support the theory that they are formed in response to the aforementioned extreme stress, which creates the need for added anchorage (Banks 1704).
Buttressed roots also posses an extensive system of sinker roots not seen on non-buttressed trees. It is thought that these sinker roots provide a large amount of the support needed for the tree and are made possible by the formation of the buttresses; these roots penetrate the subsoil to provide the stability that the superficial buttresses are incapable of. The buttresses then protect the sinker roots from snapping by providing a firm brace to the trunk (Banks 1704).
When first approaching this subject my group had many ideas of how we might be able to research the different purposes, locations and diversity of buttressing using a variety of variables. One ofour original ideas involved comparing the different species based on size of the buttresses with the diameter and height of the trunk. Another idea was to compare species and abundance on the slopes with the lowlands. We also thought of making a comparison between the different species and the size of the buttressing, using the length and number of buttresses, while also comparing the topography and abundance. There was also some consideration given towards the idea of using all of these variables to make a comparison between the different topographical areas and species that we would be studying.
After some consideration all of these ideas were rejected on the basis that we would not be capable of identifying every species that we would encounter which possessed buttresses. We realized that identifying down to the genus would even be an extremely difficult task, so we could not use species or genus classifications in our research. Then we also concluded that we would not be able to make a comparison of the size of the buttresses because that is directly related to the characteristics of the species and/or family.
When making these realizations about the direction we were taking our research we also realized other limitations that we would have to work within. In order to make any measurements of the buttresses we would be required to walk off the designated paths which could be very dangerous and typically not allowed. We would also have to reach into crevices and around areas of the trees that could be very dangerous since they provide the perfect homes for many unfriendly animals. Another limitation would be our sample area, which could only include area along the trail systems. There was also the limitation of time, considering that we would only be in Costa Rica for two weeks and each location for a matter of days.
After taking all of these limitations into consideration we decided that the best approach would have to include only one easily accessible variable. Then we considered the many variables that were earlier discussed. Out of theses we discussed which might give us the most information considering the simplicity of each individually. Our final decision was that the best approach would be to make a comparison of the abundance of buttressing in the different topographical regions that we would be visiting.
After much discussion our hypothesis was established: Buttresses will be more prevalent in the Pacific and Atlantic lowland rainforest than in the cloud forests of Monteverde. We made this prediction based on the knowledge that Monteverde has the highest average wind speed in all of Costa Rica and knowing that Corcovado has a higher average rainfall per year in comparison with the other two regions. This hypothesis then lead to the next area of discussion, which was the methods to use that would best test our hypothesis. Shortly it was decided that we would run transects, using a 100 meter long measuring tape, along the trails making counts of the buttressed and non-buttressed individuals within one meter to both sides of the trail.
Our first area of research was the forest surrounding the resort Almonds and Corals. In this area we were very limited by time and access to the forests, unfortunately resulting in a transect of only 100 meters. In our other locations, Monteverde Cloud Forest Preserve and Corcovado National Park, we were better prepared to make use of our time and able to run transects of 200 meters. A measurement was made in the Mangroves, but not used in comparison with the other sites. It was not possible to run a transect longer than 30 meters through the Mangroves and the zonation and lack of diversity provided skewed results.
After gathering our data it was found that Almonds and Corals had eight of twenty-one trees that were forming buttresses compared out of the total trees in the transect, Monteverde had eleven of fifty-six, and Corcovado had forty-three out of ninety-two. This means that in Corcovado 40% of the trees have buttresses, around Almonds and Corals 33% and Monteverde has 10%. It was found that in the Mangroves, along the Sierpe River, there were five out the seven trees in the transect that had buttresses which means that it had 71% with buttresses, but this data can not be compared with the other locations.
Corcovado National Park, which is along the Pacific Ocean, and Almonds and Corals resort, along the Caribbean Sea or Atlantic Ocean, had significant higher proportions of the forest with buttressed trees compared with Monteverde Cloud Forest Preserve. There are many factors that if possible would have been very beneficial to our research. Having a larger sample area and equal sized sample areas would have provided a more accurate count of the abundance of buttresses. It would have also been beneficial to our data if we could have included the trees that were more than a meter to the sides of the trail where we were running our transect. Taking data within these limitations there are many possibilities for our results to be skewed. We were only measuring within disturbed areas and the sample areas are extremely small in comparison to the size of the entire region, knowing this our data may not be a valid representation of the abundance of buttresses in our three regions used for sampling. There may also be error in that our measurements for Monteverde were taken on the west side of the mountain and the highest amount of wind is on the east side, which may have a higher number of buttressing.
After comparing our data it was decided that we would accept our hypothesis that buttresses are more prevalent in the Pacific and Atlantic lowland rainforests than in the Cloud forests of Monteverde. This conclusion was reached after finding that the Corcovado had the highest percentage of trees with buttresses, Almonds and Corals was slightly less and Monteverde had a significantly smaller percentage. It is realized that there are many factors that would have given more validity to our research and that the acceptance of our hypothesis is based on a very limited and small set of data. If time, money and resources were at endless supply it would be possible to test our hypothesis more thoroughly.
Banks, J.R., M.J. Crook, A.R. Ennos. The function of buttress roots: a comparative study of the anchorage systems of buttressed and non-buttressed tropical trees. Journal of Experimental Botany, vol.48, no. 314. 1997. 1703-1716.
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