Individual- and ecosystem-level consequences of competition on tropical forest nutrient cycling
Due to their high levels of net primary productivity, tropical rainforests are highly competitive environments, with light becoming a precious resource as it is intercepted by many layers of leaves from the top of the canopy to the forest floor. Thus, competition for light and the corresponding need for potentially rapid height growth play a strong role in shaping successful tree traits in these ecosystems. In addition to structuring population level dynamics, there is a potential for feedbacks of this competition on nutrient cycling in these forests, which can affect their role as a carbon sink. We built a forest gap simulation model to explore the nature of and consequences of this competition on nutrient limitation of individual trees and the ecosystem as a whole. In addition, we compared our results to data from a nutrient fertilization experiment in Costa Rica.
Using our model, we were able to find the evolutionarily stable strategy for leaf lifespan during the rapid height growth phases of these forests such as gap succession or secondary succession. The winning trees grew most rapidly in height compared to their competitors, and also needed to grow leaves at a faster rate to match their height growth. This created a high nutrient demand for the winning individuals since leaves are nitrogen rich. Shorter leaf lifespans allowed these trees to re-translocate the limiting nutrient of nitrogen to the top of their crown for a competitive advantage in intercepting light. We will compare the theoretical results with field measurements of leaf production and leaf lifespan amongst tree species with varying degrees of stem growth rates and nutrient limitation. At the ecosystem level, the rapid height growth during successional competition can also elevate ecosystem level nutrient demand. This work explains a mechanism of nutrient limitation in tropical rainforests.