Functional traits as competitive strategies driving nutrient dynamics of tropical rainforest trees during gap succession

Background/Question/Methods

Tropical rainforests play a large role in the global carbon cycle, but our ability to predict their dynamics and function still lacks a comprehensive understanding of the relationship between trees, their traits and growth strategies, and soil resources in these ecosystems. While there is evidence that recovering secondary forests are nutrient limited, ecosystem-level studies of nutrient dynamics in mature forests are inconclusive on whether forest growth is nutrient limited and if such limitation interacts with plant traits. Of particular concern is gap phase succession, where dramatic sapling growth driven by intense competition to fill a canopy gap may raise nutrient demand. We are taking a tree-level approach to resolve plant trait-nutrient dynamics in these forests, focusing on how light and nutrients interact with functional traits in these competitive gap phases. We will present a theoretical forest gap model and field data to examine how traits may play an essential role in helping trees win a position in the canopy.

Results/Conclusions

Our model competed trees with varying traits in a simulated canopy gap until one individual was able to out-shade the others, causing them to die of light starvation. In addition to this competition for light, trees were also competing belowground for a shared pool of nitrogen. We found that the winning trees had an elevated nitrogen demand because they had the fastest height growth and needed to constantly grow new leaves for the top of their stem, which had a high cost of nitrogen. We will present results from the model and compare them to results from a field experiment in a Costa Rican lowland tropical rainforest, where growth of saplings with different traits was measured in response to a fertilization treatment across a gradient of light conditions. Differential responses of trees with different traits in both the model and field experiment provide evidence of how traits can affect the nutrient status of tropical trees with ecosystem implications.