LTREB: How does inter-annual variation in rainfall interact with soil fertility and chronic disruption of soil moisture dynamics to alter soil C cycling in tropical forests?

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Many tropical forests are experiencing drying and increased variation in rainfall events. Tropical forests are among the most diverse ecosystems on Earth and hold some of the largest stocks of carbon in plants and soils globally. Thus, they are a “carbon bank,” storing carbon in solid form and reducing potential carbon in the atmosphere as the greenhouse gas carbon dioxide (CO2). Therefore, these ecosystems are important to global societies for biodiversity, nutrient cycling, and climate moderation. This project studies the effects of drying on carbon storage and cycling in tropical forests, using a long-term experiment that reduces rainfall inputs to four distinct tropical forests in Panama. The forests have different background rainfall and soil fertility, which could influence how they respond to drying. The project was established in 2015, and thus is accumulating a long-term dataset. In the initial seven years of data there was large variation in carbon fluxes and cycling from year to year, possibly related to variation in rainfall between different years. This new project, called Panama Rainforest Changes with Experimental Drying (PARCHED), will now continue for an additional 5 – 10 years of measurements. These results will help improve understanding of how tropical forest carbon storage changes over decades in response to climate variability. Undergraduate students will have opportunities to be involved in research, and there will be outreach materials produced in both English and Spanish. Results from the past seven years of the PARCHED experiment have some anomalies relative to what would be predicted. Some model predictions matched experimental results, while some mismatches suggested that important processes in tropical forests were not represented in the model. In particular, the importance of reduced water fluxes into soils, and related reductions in delivery of nutrients into soils, were lacking from the model, which could be important in the context of inter-annual variation in rainfall and soil hydrology. This project works to: a) determine whether observed chronic drying effects on tropical carbon cycling years persist over one to two decades of background rainfall variation; b) allow for detection of changes in long-term soil carbon storage in tropical forests. The project measures tree stem and root growth, soil carbon levels, and changes in carbon losses from soils via CO2 fluxes. Ongoing work aims to improve the model characterization of important processes in rainforests under drying. In conducting these scientific activities, the project trains and support a full-time field technician, involves undergraduate researchers, and will be used as part of the Smithsonian’s science outreach efforts.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.