Research
The land phosphorus cycle
It is increasingly recognized that nitrogen and phosphorus may significantly constrain the land carbon sink. However, the incorporation of these nutrient cycles into Earth system models is uncommon, especially for phosphorus, and current examples often lack a dynamic ecological framework for resolving the interactions between carbon, nitrogen, and phosphorus. As a result, this double deficiency is hindering our ability to understand and predict the land carbon sink, particularly in tropical forests, which grow on phosphorus poor soils. I am collaborating with Lars Hedin and scientists at NOAA GFDL to design a dynamical plant-soil phosphorus cycle for the GFDL Earth system model.
Currently, we are using a simple, data driven model that integrates the biological dynamics of forest succession after disturbance with ecosystem nutrient cycling to determine time periods when phosphorus and/or nitrogen limit plant growth. We are using this model to examine the role that different plant strategies can play in exacerbating or alleviating phosphorus and nitrogen limitation. Additionally, we will incorporate differences in landscape scale disturbance dynamics across biomes to understand the consequences of disturbance cycles and secondary forest impermanence for nutrient limitation of the carbon sink. The lessons from this simple model will highlight crucial ecological mechanisms to include in our phosphorus cycle for the GFDL land model.
Costa Rica sapling fertilization experiment
Tropical forests currently play a large role in mitigating anthropogenic carbon dioxide from the atmosphere but it is unclear if their future ability to sequester carbon may be limited by soil nutrients. To date, the small number of fertilization experiments in these forests have shown incongruous results of whether they experience nutrient limitation of growth. I conducted a 2.5-year fertilization study in a Costa Rican lowland rainforest to examine how light availability, functional group identity, and species identity can impact nutrient limitation of tropical forest saplings. I found differences in nutrient limitation of growth among species and among functional groups, as well as varying interactions with light. This work stresses the importance of biodiversity and ecological dynamics like light competition for understanding the tropical carbon sink.
Find out more at: Chou, C.B., Hedin, L.O., and Pacala, S.W. 2018. Functional groups, species, and light interact with nutrient limitation during tropical rainforest sapling bottleneck. Journal of Ecology, 106:157-167. https://doi.org/10.1111/1365-2745.12823. pdf
Leaf functional traits
Recent work has demonstrated that functional traits may be an effective way to represent biodiversity in studies of plant function. In particular, leaf functional traits representing resource acquisition strategies are one of the primary dimensions along which plants vary, with tradeoffs in traits between fast growing, resource acquisitive species and slow growing, resource conservative species. Using my Costa Rica sapling fertilization experiment, I examined how leaf functional traits themselves may be plastic to nutrient and light availability, as well as how these traits can impact whole tree nutrient limitation in variable light environments. I found plasticity in some leaf traits but lack thereof in others, with no clear differences in response among the different functional groups of trees we examined. In terms of how functional traits may influence overall plant performance, I found counterintuitive patterns of stem growth nutrient limitation but predictable patterns of leaf productivity nutrient limitation among the different functional groups.
Competition and nutrient limitation
Empirical evidence of the connection between plant diversity and variation in nutrient limitation emphasizes the need to mechanistically understand how ecological competition, and plant strategies in response to this competition, can impact individual plant and ecosystem nutrient limitation. To address this need, I designed a forest simulation model that determines the competitively optimal leaf lifespan of a tree during gap succession. I found the paradoxical result that trees with leaf lifespans that are sub-optimally short for their carbon productivity and nutrient use efficiency are optimal competitors because they can concentrate leaves at the top of their crowns. Due to asymmetric competition for light, leaves at the top of the canopy capture most of the light and also shade all leaves lower in the canopy. This leads to a race to concentrate leaves at progressively higher heights, even if it reduces productivity and nutrient use efficiency of individuals. The optimal competitors have short leaf lifespans because short-lived leaves allow a tree to internally redirect its limited nutrient supply towards new leaves at the top of its crown via resorption from senesced leaves. This means that plant competition may strongly impact both the amount of carbon a forest can store as well as forest regrowth rates because competitively successful traits can have negative effects on nutrient use efficiency and carbon production.
