Overview: Understanding what determines the current distribution of biomes is fundamental for projecting how vegetation will respond to future climate and disturbance regimes. Tropical savanna-forest boundaries mark the transition between the two most extensive tropical biomes, yet the factors that determine the location, structure, and dynamics of these boundaries are poorly understood. Our understanding has been hindered by strong positive feedbacks and other non-linear processes that cause complex dynamics and hysteresis (i.e. the dependency of system not only on its current environment but also on its past environment). These legacy effects confound our ability to observe vegetation responses to environmental change.
The proposed research will combine field data and modeling to test for and quantify sources of hysteresis in savanna-forest dynamics. The aims of the research are (1) quantify key processes that underlie switches between savanna and forest states, (2) use this information to refine and parameterize the CLM(ED-SPITFIRE) model for simulating savanna-forest dynamics, and (3) perform simulations to understand environmental controls on the distribution of tropical savanna and forest, with emphasis on causes of hysteresis. This model represents the coupling of a demography-based ecosystem model (ED), a mechanistic model of fire occurrence (SPITFIRE), and a model of land surface processes (CLM) that can optionally be coupled to a general circulation model (CESM).
Much of the data needed to refine, parameterize, and validate the model are available from previous NSF-funded research. Gaps in this information will be resolved with a fire experiment at a natural savanna-forest boundary in Brazil. In the experiment, flammability trials will be used to understand thresholds that mediate vegetation-fire feedbacks. Measurements of biomass production and turnover will quantify the relative roles of primary productivity and mean residence time in governing biome shifts. Monitoring of tree dynamics will provide a more complete understanding of the distinct roles of savanna and forest tree species in these shifts.
Intellectual merit: The proposed research aims to understand the sources of hysteresis that confound projections of future vegetation dynamics in the seasonal tropics. This work will make it possible to systematically address uncertainties in modeling savanna-forest dynamics that result from climate feedbacks, fire feedbacks, plant demographic thresholds, and shifts in tree functional types. Furthermore, the field experiment will refine our understanding of ecological and biophysical thresholds that mark the transition between savanna and forest. The research team is uniquely qualified for this work because of the resources brought to the problem, including a state-of-the-art fire-vegetation model that can be coupled to a general circulation model, a growing database of comparative trait data of savanna and forest tree species, and a rare opportunity to establish a fire experiment at a savanna-forest boundary in Brazil.
Broader impacts: Savannas and evergreen forests are the most important tropical vegetation types in terms of area, biodiversity, total carbon stocks, and use by humans. These biomes once covered 82% of the tropical land area and are home to approximately 40% of the global human population and perhaps 50% of all terrestrial species.
Despite an undeniable natural role of fire in tropical savannas, fire suppression is a standard policy in protected areas of savanna in Brazil. Many reserves, particularly those that are small and isolated, are undergoing succession to forest, and causing decline of species adapted to open environments. To promote the acceptance of the natural role of fire, we will prepare an article for the Brazilian popular science magazine Ciencia Hoje and a white paper to be distributed to managers of Brazilian savanna reserves and state ministers of environment. These documents will review the evidence for the natural role of fire and the consequences of fire suppression. As the project generates results, we will invite journalists to visit our study sites to inform the public of the natural role of fire in Brazilian savannas.
We will provide four US undergraduate students with international research experiences that will contribute directly to the research objectives of this proposal. In doing so, we will stimulate interest and expertise in tropical ecology among US students while fostering interactions that we hope will grow into long-term collaborations.