College of Agriculture & Natural Resources Department of Forestry

MSU assistant professor Scott Stark and research associate Marielle Smith have each spent more than a decade studying the rainforest ecosystem in Brazil. A $1.12 million dollar National Science Foundation grant ($839,000 to MSU) will now allow them to expand their research into the topic of tropical forest drought.

Most research on how drought impacts the Amazon has focused on upland "terra firme" regions of forest, which have deep water tables and regularly experience droughts. But, up until this point,  the MSU researchers have not had a chance to study what Stark labels, “the other side of drought.” Approximately one-third of Amazon forests have shallow water tables—effectively too much water in the soil for ideal tree growth conditions. 

“Trees don’t want to be under water, they need oxygen to get to their roots. When the soil is waterlogged, the trees can’t take in nutrients. It’s the other end of water stress,” Stark said.

Studies have been conducted on the impact of drought in upland areas, but how drought will affect the waterlogged areas is unknown. Stark, and the research team think in these waterlogged areas some aspects of drought could be a positive thing. 

“There is a totally different sensitivity to different levels of drought in these waterlogged forests,” Stark said. “It’s possible that moderate drought will make these forests grow more by creating more favorable root conditions. On the other hand, if these forests really dry out in severe drought, it’s not clear if the trees there that typically have shallow root systems can survive.”

While there may be positive impacts from certain levels of drought, there are still fire risks involved in areas that are ordinarily under water. The extreme effects of seasonally waterlogged soils include flooding of water levels up to 10 meters. When these forests dry out, fire poses a big risk, particularly because these forest’s shallow root mats are vulnerable to destruction, offering new connections for this research teams’ prior work on fire in the Amazon.

The research will be gathered through a combination of methods employed at both upland and waterlogged sites. Ground-based lidar systems will be used to gather information on how the quantity and distribution of leaves changes seasonally within the different forests. Two new tower systems will be installed and a third repaired, housing cameras that capture spectral images of the top of the forest canopy, providing information on how photosynthesis changes through the season and in response to changing soil water.  

The seasonal component is important when gathering this information. Typically, the water table is higher during the wet season and lower in the dry season. By monitoring the area for three years, researchers hope to capture differences between the dry seasons. This will help them understand the impacts of drying in areas with abundant soil water, and what effects this may have on carbon storage.

In addition to ground-based and forest-top levels, satellite data will be used to scale the results up to the level of the Amazon basin, a collaborative component with the University of Arizona. Satellite data will also give researchers a longer timescale to analyze drought cycles, allowing the comparison of data going back approximately 20 years.

“This can be used to analyze impacts of excessive rainfall; the higher highs and lower lows,” Stark said.

A unique aspect of the $1.12M grant is that it continues and strengthens collaboration with Brazilian researchers. Core collaborators Professors Flavia Costa of National Institute of Amazonian Research (INPA, in Manaus) and Juliana Schietti of INPA and the Federal University of the Amazônas state (UFAM) have been pioneers in this area of the Amazon. 

They have championed work on waterlogged soil forest, spending the last 10 years setting up an immense forest plot network spanning almost 400 miles along the BR319 road that runs southwest from Manaus offering unique access to forests of this type.

Without the collaboration of Costa and Schietti, the science objectives and practical challenges would be nearly impossible to meet, with the latter including the construction of the two new canopy photosynthesis observation towers at sites on the BR319 and the refurbishment of another near the city of Manaus. 

Thanks to their research legacy at these sites, and that of scientific networks conducting ecosystem forest research throughout the Amazon like the LBA (Large-scale Biosphere-Atmosphere Experiment in Amazonia), this study will incorporate forest surveys, environmental monitoring and measurements of forest impact on the atmosphere. These data are critical for understanding the contribution of the unique responses of waterlogged forests to climate change and drought. 

This project will mark the first time there has been a network of canopy towers in waterlogged forests in the central Amazon, providing a new opportunity to link knowledge from field studies to satellite data. 

The purpose of this approach is developing an understanding that sweeps across the Amazon basin’s spectrum of soil water conditions to understand the vulnerability and the resilience of the Amazon to drying climate conditions to better confront the challenge of preserving its critical function of cooling the atmosphere.

“Hydrology is changing in many different ways. It’s not fair to only study upland forests,” Stark said.“We know these forests suffer severe water shortages when there is a drought and respond in negative ways. In waterlogged forests there is the possibility of some additional resilience to drought, but that resilience could be limited too; it’s critical we come to understand this waterlogged (portion) of the Amazon. If waterlogged areas do respond positively to drought, they may provide a margin of resiliency for Amazon forests in the face of a drying climate to help reduce global warming.”