Next Generation Ecosystem Experiments—Tropics: Tropical forest responses to a changing climate

Nov 24, 2015

Because tropical forests recycle more CO2 than any other biome, they play a major role in Earth’s carbon cycle, which has implications for climate change. And while they have been studied extensively, there are still critical gaps in our knowledge about them, gaps that contribute to uncertainties in Earth system models and predictions of future climates. Most critically, as carbon levels in the atmosphere increase, we need to know whether the tropical forests will respond by cycling more carbon, or at the very least continue to be a carbon sink through the twenty-first century.

The $100M, 10-year, US Department of Energy–funded Next Generation Ecosystem Experiments–Tropics (NGEE-Tropics) was initiated to answer critical questions related to gaps in our knowledge of tropical forests and determine whether they will continue to function as carbon sinks as temperatures rise. The project identifies gaps in current models, and experiments and fieldwork are then designed to provide the missing data. The results are used to develop more precise models, which will then influence the direction of future research. Project participants hope to develop the most representative model of the way tropical forests interact with the environment that we have ever had, with detailed information from bedrock to the tree canopy—a pretty ambitious project that involves perhaps not a cast of thousands, but certainly hundreds; spans continents and hemispheres; and includes researchers from five national labs, the US Forest Service and other government agencies, the Smithsonian Tropical Research Institute, and numerous international collaborators such as Brazil’s National Institute of Amazonian Research (the Instituto Nacional de Pesquisas da Amazônia or INPA).


New beginnings

Phase 1 of the project, which runs from March 2015 to March 2018, is envisioned as an exploratory phase, during which current models, knowledge, and gaps will be assessed; a data synthesis–management framework developed; and pilot fieldwork started. Results from Phase 1 will guide model development and additional pantropical fieldwork when the project goes global in Phases 2 and 3.

Three main sites have been selected for Phase 1 fieldwork: Manaus, Brazil, where project researchers will collaborate with INPA; El Yunque National Forest in Puerto Rico; and several sites across Panama, where project researchers from the Smithsonian Tropical Research Institute will join the team. Work at Manaus will focus on how tropical forests respond to daily and seasonal moisture changes, including drought. In Puerto Rico, the emphasis will primarily be belowground biogeochemistry and how soil fertility impacts CO2 cycling. In Panama, the focus will be on species-level characteristics and how they enable some species to survive in hostile conditions. Work at all three sites has been jumpstarted by the need to get equipment in the field to record this year’s El Niño (see separate article).


What lies beneath

Climate Change Science Institute’s (CCSI’s) Rich Norby, who is on the project’s executive committee, leads the Nutrient Biogeochemistry research area. A key task for that research, led by CCSI’s Xiaojuan Yang, involves the phosphorus nutrient cycle in the tropics and efforts to improve the “phosphorus model.” While the main point of all NGEE-Tropics research is the global carbon cycle and carbon balances, the phosphorus cycle is one of the critical gaps in knowledge that NGEE-Tropics is designed to address. Yang’s research suggests that if there is not enough phosphorus, plants can’t be as productive as they might otherwise be; and therefore, they don’t draw down as much CO2 from the atmosphere. Melanie Mayes, who is also working on this aspect of the project, points out that you get very different results when you include phosphorus limitations in models. “The tropics,” she says, “are so big, so important to global climate, and have such massive amounts of biomass, that if you include phosphorus limitations, the models do a much better job of predicting atmospheric CO2 concentrations, which helps reduce uncertainty.”

Mayes, a hydrologist by training and predilection, is interested in linking the hydrologic information from the project with the soil biogeochemistry findings. She says moisture is the key to everything. “If it’s really, really wet, you have fluctuating redox reactions that cause soil minerals to dissolve, liberating phosphorus . . . . We know this happens in Puerto Rico, but we don’t know a lot about the other sites.” In later phases of the project she hopes to take what she learns in Puerto Rico to other sites. Echoing these sentiments, CCSI’s Jeff Warren says that the “interactions between plant and soil span surface energy, hydrology, and nutrient biochemistry, and it’s exciting to look at all the critical links between all these critical components.”

Figure Caption: Graphical representation of factors controlling nutrient availability and relationship of nutrient availabilityto forest productivity, plant allocation and turnover, and post-disturbance recovery rates.


Norby, an enthusiastic supporter of the “model–experiment–model” concept, believes the phosphorus module is one of the best examples of embracing the concept. “We’ll see where we are in a year,” he says philosophically, “but I really like the way the models are guiding our work in the field. We’ll have to see how well the data lead to improvements in the models . . . I have high hopes they will.” 


Water is the key to everything

How do trees in the tropics respond to drought?

How does drought response vary across species?

How can we better model plant-soil hydrological interactions?

What are the implications for climate modeling?

Questions like these inform the work of CCSI’s Warren and the other scientists working on the NGEE-Tropics Drought and Hydrologic Heterogeneity research objectives. In Warren’s words, they want to “develop a mechanistic model of H2O fluxes and resistances through the soil to the roots, through the stems to the leaves, and to the atmosphere.” As part of the fieldwork for this task, they will apply resistances to H2O uptake and thresholds to investigate cavitation of the soil-root system, which happens when the xylem is damaged (cavitated) due to stressors such as drought.

This highlights one of the many challenges of this project. The tropics are a hugely diverse area; at the Manaus site alone, there are 300–400 species that could potentially be studied. “Realistically, we can’t go out and measure 300 or more trees,” Warren says. So then it becomes an exercise in determining how to survey/measure a representative number of plants, and which ones, in the context of their relation to drought.

Hydrological research is also being conducted in Panama and Puerto Rico, where currently CCSI staff members and other NGEE-Tropics personnel are installing sap flowmeters and dendrometers to assess drought stress and correlate it with tree growth. The sites in Panama reflect another of the challenges: power. Some NGEE-Tropics field sites don’t have access to energy sources. So then project logistics have to encompass decisions on alternate energy sources—battery devices and chargers, for example, or solar; siting the alternate energy sources, including proximity to vegetation; and, in the case of foreign countries, import requirements and other restrictions. 

Measurements of soil moisture, tree growth, and sap flow are essentially “ground-level” measurements; however, measurements of water potential in the leaves at the top of the forest canopy are also necessary, and this is a challenge in the rainforest. To resolve this challenge, sensors are being placed in towers adjacent to the trees where possible. In other cases (e.g., Panama), cranes are being used. And where neither of these alternatives is available, in some cases researchers may have to resort to climbing trees to obtain samples. Predawn leaf-water potential, closely correlated with plant water status (stress) and transpiration rates, is another critical measurement, according to Warren, and where towers and cranes aren’t available, obtaining samples in the dark can be particularly challenging.


Collaboration and leverage

One of the distinctive hallmarks of NGEE-Tropics is the level of collaboration and use of data from prior or simultaneous research projects to leverage NGEE-Tropics goals. Norby, Mayes, and Warren are all enthusiastic about the levels of cooperation, collaboration, and sharing associated with the project. The levels of collaboration they are seeing, in the words of one, will allow them to do “so much more.”

For example, early on, Mayes became aware of a massive data set based on soil samples collected at 30 different locations in Puerto Rico under a previous project funded by the National Science Foundation. Mayes contacted Dr. Erika Marin-Spiotta of the University of Wisconsin, Madison, the principal investigator (PI) on that project, and as a result will be using some of those soil samples in her work on phosphorus. Thanks to Marin-Spiotta, Mayes says we already know a lot about these soils—pH and carbon and nitrogen content, for example. “This much information is really powerful,” she says. “With all this information, we can build an equation to predict how much phosphorus is going to adhere to the soil/soil minerals and use that in our models.”

Mayes is also collaborating in Puerto Rico with Dr. Whendee Silver of the University of California, Berkeley, to obtain data on soil-gas fluxes using new state-of-the-art Picarro soil-gas flux measurement systems. By combining forces, they can maximize the results from their efforts, generating a huge amount of information (in Mayes’s case, a 43% increase in data collected). “Once you have this many [21] Picarro chambers,” she says, “you can do things you just couldn’t do with only a few.”

Warren and Norby likewise anticipate collaborating with Molly Cavaleri, the PI on another DOE project in Puerto Rico (TRACE or the Tropical Responses to Altered Climate Experiment), to boost the potential of both projects.

Warren points out that one of the sites in Panama, Barra Colorado Island—an island in the middle of Lake Gatun, literally in the middle of the Panama Canal—has a research center operated by the Smithsonian Tropical Research Institute, one of the NGEE-Tropics participants. The island and its environs have been heavily studied for nearly 100 years, and that’s information that will definitely be leveraged for the project. Norby says he also expects to leverage photosynthesis-phosphorus data from a previous Oak Ridge National Laboratory–funded project in Panama. He says the project was very much geared toward deepening understanding of tropical research conditions, and the data generated is providing a good baseline for the NGEE-Tropics phosphorus work.

So while a lot of original research is being conducted for NGEE-Tropics, people aren’t forgetting that there’s a lot of excellent existing data and new ways of combining and looking at the data sets to improve models and guide NGEE-Tropics. “It’s really about building partnerships,” says Mayes, “and that’s a good model for all research.”

NGEE-Tropics is funded by the US Department of Energy Office of Science and is led by Lawrence Berkeley National Laboratory. For more information on the project, go to the website at For more information on ORNL CCSI contributions, contact Rich Norby.

by VJ Ewing