Tropical rainforest nutrient limitation: integrating individual tree and ecosystem perspectives

Abstract

Lowland tropical rainforests play a significant role in the carbon-climate system through their ability to disproportionately sequester atmospheric carbon compared to other terrestrial ecosystems. In order to predict and manage the dynamic response of these forests to perturbations ranging from land use activities to climate change, we need to better understand the coupling between nutrient and carbon cycling in these ecosystems. Thus far, research on nutrient limitation of tropical rainforest growth has found mixed evidence of whether tree growth is nutrient limited, and if so, by which nutrients (e.g. nitrogen, phosphorus, and potassium). However, most studies have focused on ecosystem-scale responses without incorporating much of the biological complexity of these ecosystems.

In this dissertation, I address key aspects of biological complexity—tree diversity and competition—that may influence tropical rainforest nutrient limitation. Using an in situ fertilization and light gradient experiment, I found functional group- and species-specific nutrient and light co-limitation of sapling stem growth in a Costa Rican tropical rainforest. To further examine the interactions between tree diversity, light competition, and nutrient limitation, I designed a forest simulation model that determines the competitively optimal leaf lifespan of a tree during gap succession. Paradoxically, I found that trees with leaf lifespans that are sub- optimally short for their carbon productivity and nutrient use efficiency are optimal competitors, and that this sub-optimality increases as soil nutrient levels decrease. The optimally competitive leaf lifespan also decreases ecosystem-scale nutrient use efficiency, indicating that adaptation to light competition at the individual tree scale may exacerbate tropical rainforest nutrient limitation. Finally, I examined empirical evidence for leaf trait responses to nutrients and light in the Costa Rican sapling fertilization experiment and found functional group- and species-specific responses, although connections with the stem responses were complex. The combined empirical and theoretical findings of this dissertation indicate that individual tree-scale properties and processes are essential to a mechanistic understanding of ecosystem-scale nutrient limitation in lowland tropical rainforests.