Plant Diversity in a Tropical Rainforest

My training as a molecular biologist meant that both my classes and research focused on a kind of biology visible at best under a microscope, and often not at all by the naked eye. This background made the first few days of the tropical field course eye-opening with regards to the breadth of readily visible ecological diversity presented in the rainforest. Hiking through Barro-Colorado Island (BCI) gave me a much greater appreciation for the diversity of plant life and the competition that such a densely-occupied area presents. And while the landscapes from the forest floor on BCI and the crane near Fort Sherman are remarkably different, they are analogous in demonstrating the multitude of varieties that flourish in the tropical rainforest.

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The approach to BCI, as seen from the boat. Photo by Karthik Yarlagadda.

Limited resources are always a source of competition, and canopy trees fight to occlude their smaller neighbors, sometimes even growing beyond what they themselves may require to crowd out competitors from other species. An introduction to the Janzen-Connell hypothesis provided another glimpse into the difficulties plants face, as growth near conspecifics, where presumably the local environment is suitable for growth of that species, is problematized by the increased likelihood of pathogens that can feed on the conspecific species. The workaround is to distribute seeds farther from the parent species, encouraging plants to utilize various forms of seed dispersal. One plant has an interesting take on these problems – the aptly named “suicide tree”, of the genus Tachigali. As a canopy tree, it grows to great heights, competing with the tallest of other species, prior to flowering once and dying, dispersing a large number of seeds, and eventually leaving behind a gap in the canopy.

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The view from the canopy crane near Fort Sherman. Photo by Karthik Yarlagadda.

I also had the pleasure of working on a small project while on BCI. With limitations on time and resources, I worked with two other students and a STRI post-doc to examine the viability and density of Zanthoxylum ekmannii seeds by distance from the parent tree and depth in the soil. As a dioecious species, Z. ekmannii has both male and female trees, but we were lucky enough to find two female trees to sample from (though regrettably, one had recently fallen over in a storm). Conclusions were sparse due to sample size, but we did observe a sharp decline in seed density past 5m away from the maternal parent, as well as a decline with depth. While seed viability seemed unchanged by distance or depth, the sample size from seeds collected past 10m does not provide adequate power to suggest these results are meaningful. Nevertheless, the project was an educational experience in attempting to decide on a question and hypothesis testable in a short timeframe with limited resources.

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