College of Agriculture & Natural Resources Department of Forestry

These findings published by Michigan State University (MSU) Researchers Scott Stark (Assistant Professor in Forestry) and Marielle Smith (Postdoctoral Research Associate in Forestry) and colleagues Flávia Costa (Professor, National Institute for Amazonian Research, Brazil) and Juliana Schietti (Assistant Professor, Federal University of Amazonas) examine how climate change shapes the future of the world’s largest rainforest and the impacts drought has on the forest growing on various soil water and water table conditions— a particularly topical subject in light of the recent UN COP26 climate conference.

This research is the first to be published from a $1.12M US National Science Foundation project to investigate the influence of soil water table depth on Amazon forest responses to drought, led by Stark and Smith and colleagues Costa and Schietti.

Amazon forests are globally important for carbon storage, the climate system, and biological diversity. As such, the importance of studying how these ecosystems respond to climate change, particularly drought, is well recognized. However, research frequently neglects a critical component of the hydrological cycle—groundwater. 

This oversight is surprising considering the wide range of hydrological conditions that exist across the Amazon basin – from forests growing over deep water tables where soil water can become scarce, to seasonally inundated forests where soil water remains plentiful year-round. Failure to assess Amazon forest drought responses across the full gradient of soil water availability limits scientists’ ability to predict the future of these forests.

“New research clearly needs to incorporate soil hydrology and groundwater in the design of monitoring plots and their distribution, and in the modeling of forest functioning and carbon balances. Public policies need to incorporate conservation planning for areas with the greatest potential to maintain biodiversity and ecosystem services such as regulation of CO₂ and water in the future under climate change, and these areas are defined by soil hydrology,” said Costa.

The ability of tree roots to access groundwater may be key to improving our understanding of Amazon forest drought responses. Until now, research has focused almost entirely on forests growing on deep water tables, leaving forests on shallow water tables understudied.

“About half of the Amazon basin is made up of shallow water table forests, while forests on deep water tables comprise only 8%. And yet, the majority of long-term forest monitoring plots are located in deep water table regions,” Smith explained.

The team’s latest research, published in the New Phytologist on January 17, 2022 presents a broad review of the importance of water table depth and summarizes previous studies showing the potential for shallow water table forests to act as hydrological ‘refugia’, or oases—showing resilience to droughts while other forests in the landscape are negatively affected. 

“A refugia is basically a safe place for an ecosystem that allows it to persist amidst a greater inhospitable landscape,” said Stark.

This led the researchers to hypothesize that sites with different soil water table conditions have highly contrasting responses to drought, and particularly that shallow water table forests may actually benefit from moderate drought. 

The team review literature and present findings from an extensive body of work developed over the last decade by Costa and Schietti from a new network of forest plots concentrated in shallow water table regions of the central Amazon. Evidence is presented that during “normal”, or non-drought years, shallow water table forests have higher tree mortality rates and are less productive than deep water table forests. These conditions — when soil water is excess — are actually stressful for trees, as they lead to low oxygen levels at the roots. 

However, during moderate droughts, these forests appear to hit a “sweet spot” in terms of having enough water but not too much. Drying shortens the length of the stressful waterlogged period, effectively increasing the length of the growing season and resulting in lower mortality and enhanced productivity.

Findings of resilience among shallow water table regions challenge the prevailing view of widespread negative effects of climate change on Amazonian forests. Enhanced productivity of shallow water table forests under drier conditions may offset the large carbon losses expected for deep water table areas. 

However, the team’s review also highlights that shallow water table forests may be more vulnerable to severe droughts because trees in these areas evolved in wet conditions and therefore lack adaptations for drought tolerance. 

New research suggests that whether shallow water table forests will act as a carbon sink—absorbing CO₂ and offsetting emissions from deep water table forests, or as a carbon source—releasing CO₂ to the atmosphere, will depend upon drought intensity.  

Understanding the factors that promote resilience versus susceptibility of Amazon forests to climate change is critical to setting conservation priorities and understanding the likelihood of reaching a “tipping point”, beyond which these globally important forests may collapse.

“In a nutshell, if half of the Amazon responds totally differently to climate change droughts, possibly growing more and changing in a way that does not lead to widespread tree death and release of carbon into the atmosphere, then that changes our whole perspective on the resilience and future of this region,” said Stark.

But while shallow water table regions could be an important carbon sink, absorbing carbon even as the Amazon becomes drier, Schietti urged against focusing conservation efforts on these areas alone. 

“Shallow water table forests should not be treated as a ready-made solution to a climate where droughts are more frequent. These forests should be prioritized for conservation because of their role as a carbon sink during moderate droughts, but conservation of the Amazon needs to be thought of as a single, continental block that connects the Atlantic Ocean to the Andes and is responsible for the production of rains that supply much of South America. Without the large block of forests on both shallow and deep water tables, the flow of rains that starts in the Atlantic will be interrupted, putting the water supply for people and agriculture at risk, as well as the long term survival of the Amazon” said Schietti.