When hit by drought, Panama’s tropical forests have a “relief strategy” that sends roots deep underground to adapt to water shortages, a new study has found. But scientists warn that this may not be enough to save people from the ravages of climate change.
Tropical forests are home to more than half of the world’s terrestrial biodiversity and store large amounts of the planet’s carbon. Much of this carbon is held in underground roots. However, climate change is increasing temperatures in these forests and is expected to lead to extreme droughts.
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The scientists built transparent roof structures over the plots to prevent 50% to 70% of the rainfall from reaching the forest floor. The structure “looks like a partial greenhouse roof,” study co-author Daniella Cusack, an ecoecologist at Colorado State University, told Live Science. She has been leading the PARCHED experiment since 2015. The researchers also dug trenches around the plots and lined them with thick plastic to prevent roots from accessing water from outside the plots.
The researchers used three methods to find out what was happening to tree roots.
They sampled soil cores four times a year for five years. The core extended approximately 8 inches (20 centimeters) below the surface. The researchers also used root traps, which are mesh posts filled with soil. Every three months, they checked how many roots were growing on these pillars.
The third method was to use a small camera to observe how the roots grow. When the PARCHED experiment was set up, the researchers buried an acrylic tube about 4 feet (1.2 meters) into the ground. These tubes have gaps at regular intervals through which cameras peer into the soil.
All four forests, although different from each other, responded similarly to the slowly drying environment.
Chronic dryness significantly reduced the amount of surface fine roots, reducing available water and nutrients, but trees had a number of strategies to survive chronic drought.
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“The trees were probably compensating for the depletion of surface roots by sending thin roots deep into the soil to acquire water,” Cusack said.
“Root growth alone is not enough to compensate for the loss of carbon and biomass,” she says. This is more of a “rescue strategy for trees to maintain their hydraulic and physiological functions.”
At the same time, surface roots were more likely to be colonized by arbuscular mycorrhizal fungi. This type of fungus forms a symbiotic relationship with plants, increasing the availability of water and nutrients.
The remaining surface roots appear to attract more of these fungi, giving them better access to nutrients, Cusack said.
Daniela Jaffar, who was not involved in the study and studies roots in tropical forests at Oak Ridge National Laboratory in the US, welcomed the study but said more research was needed to understand how roots behave in other tropical forests.
“Some species have long adapted to drier environments, and these adaptations typically evolve over long periods of time,” she told Live Science. “The new challenge is that tropical forests, especially in areas that are not used to such dry conditions, are undergoing major changes and may not have enough time to adapt.”
Species that are less able to adapt to more extreme droughts may decline or disappear from the ecosystem, she said.
Cusack warned that root adaptation is not a bulwark against climate change. “Our five-year study is pretty short in terms of the lifespan of a tropical forest,” she says. “We don’t know how long forests can maintain these adaptations.”
Lead author Amanda Cordeiro, a researcher at the University of Minnesota and a doctoral candidate at Colorado State University during the study, told Live Science that the next step is to assess the long-term effects of the root changes and how they affect the entire ecosystem in terms of carbon storage and plant fitness. “For example, it is currently unknown whether increased production of deeper roots will enable tropical forests to withstand ongoing chronic desiccation over several years,” she said.
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