- Tropical forest plant roots have not received as much research attention as aboveground vegetation. This knowledge gap affects our understanding of how rainforests adapt to change, including their ability to capture and store atmospheric carbon.
- An emerging field of research is looking at how root systems respond to global change. New evidence dramatically underlines the outsized importance of tropical forests in the global carbon cycle. Tropical forests represent one of the world’s largest carbon sinks, largely thanks to plant roots, which add carbon to soils.
- Despite the challenge of studying tiny roots hidden underground, researchers are uncovering important insights. Some tropical forests send roots deeper into the soil under dry conditions, possibly seeking moisture, which may aid in drought tolerance. Others seem unable to do this, making them more vulnerable to climate change.
- Recent plant root studies are confirming the immense stress tropical rainforests are under, with conditions changing faster than roots belowground can adapt. Knowing more precisely which forests can, and can’t, tolerate escalating climate change and other stressors could better inform management and conservation decisions.
The destruction of tropical rainforests — seen in shocking images of huge wildfires, vast clear-cuts and dying drought-stressed trees — is unfolding annually before the world’s eyes. But some serious changes in growing forests are occurring unseen, below the surface.
As aboveground changes in those forests intensify, those effects are also filtering underground, and researchers are sounding the alarm due to potentially adverse changes to tropical forest roots.
Roots are key to how rainforest trees and soils capture and store atmospheric carbon dioxide, especially as climate change progresses, says Daniela F. Cusack, associate professor in the Department of Ecosystem Science and Sustainability at Colorado State University.
The hidden world of plant roots, and the impacts of human-caused stressors on them, is not yet well understood, but important clues are emerging.
An overlooked science
While underground plant growth is more difficult to study than what’s happening aboveground, roots are no less important than forest floor to canopy processes, explains Cusack, who is also a research associate at the Smithsonian Tropical Research Institute.
Scientists began looking at the complex tropical forest underworld about 40 years ago. Today, they know tropical tree species’ roots grow in a mat on the forest floor to collect nutrients from composting vegetative matter. Some trees have shallow root systems that run for more than 100 meters (325 feet) to steady the tall trees, especially since strong winds often rock the canopy. Others have evolved large, thin extensions of the trunk that begin some 6 m (20 ft) above the ground to buttress them. Other trees, like palms, have stilt roots for support.
Roots are key to critical ecological processes. Plants remove carbon from the air via photosynthesis, depositing it in the soil through their roots, which exist in symbiosis with a fungus called mycorrhiza. These fungi provide the roots with mineral nutrients and water as well as protection against pathogens. In return, the roots offer the fungi carbon as an energy source. This important relationship helps maintain the stability of the entire tropical forest ecosystem.
A recent line of research is zooming out from individual plant species or particular roots to look at how entire root systems within forests respond to global change, and especially how those changes are impacting carbon storage belowground. Cusack, an ecosystem ecologist, is focused on these emergent patterns.
“We look at the changes as a whole, instead of trying to mine out a singular root,” she says. The hope is that the revelations provided by data demonstrating root system responses to change will help climate scientists more accurately predict future scenarios of carbon cycling and help forest managers make better decisions regarding tropical forest conservation and restoration.
Much of this groundbreaking work is being spearheaded by TropiRoot, an international group of root researchers. Led by Cusack, this global team published one of the first comprehensive review papers on tropical root strategies in 2021. They followed up in 2024 with an overview of how tropical root dynamics should be represented in ecosystem models to improve climate change predictions. This work synthesizes what researchers know about tropical root characteristics and functions and includes new data from Costa Rica, Panama, Puerto Rico and Singapore.
Carbon banks at risk
TropiRoot’s findings underline the enormous importance of tropical forests, and their roots, in the global carbon cycle. The 30% of global soil carbon that tropical forests store — transported primarily through plant roots — form one of the world’s largest terrestrial carbon stocks. Cusack explains that live roots act like conduits, moving carbon from aboveground vegetation into the soil, where it remains sequestered.
The TropiRoot team postulates that while these immense tropical carbon banks currently help stave off some of the severe effects of climate change, these stores are at growing risk as tropical forests experience global warming, shifting rainfall patterns, forest degradation and deforestation and depletion of soil nutrients related to elevated levels of atmospheric CO2.
Studies are now investigating the nature of that risk, but even with technological advances, roots are still hard to study, especially on a large scale. The immense richness and diversity of tropical forest vegetation — possibly encompassing 40,000-53,000 tree species alone — is also reflected underground in vastly diverse and complex root systems, which are in turn hidden from researchers’ prying eyes by exceedingly dense tropical overgrowth.
Compared with temperate ecosystems, tropical forests have more varied and unique combinations of root characteristics and strategies, both at species and community scales. An additional challenge to this research: Tropical humid forests have lots of “fine” roots, a classification that typically includes roots measuring less than 2 millimeters (0.1 inches) in diameter.
The slow, slow study of roots
Studying all of this across the tropical world is a gargantuan task — a slow, slow process, necessitating an exorbitant number of human work hours. Take, for example, the Ph.D. work of TropiRoot member Amanda L. Cordeiro, a postdoctoral scholar at the University of Minnesota. She studied the effects of chronic drying on root dynamics and characteristics across four different tropical forests in Panama.
Cusack, her supervisor and co-author on subsequent papers, notes that the drying conditions they observed in their research did not reflect an extreme drought (like that which might be seen as climate change escalates) but rather an overall reduction of rainfall, like that seen during El Niño-Southern Oscillation (ENSO) events.
The four different forests varied in soil fertility and mean annual precipitation, and Cordeiro investigated the effects of drying on fine roots and coarser roots across different soil depths, from shallow roots down to those growing 1 m (3.3 ft) deep.
She applied a variety of research methods. One entailed inserting 32 transparent acrylic tubes into the soil containing minirhizotron cameras, which photographed the roots surrounding them. Cordeiro analyzed 237,360 images over two years, tracing all of them by hand using specialized software.
This “super time-consuming” exercise took thousands of hours. (She listened to scientific podcasts and Brazilian music to “get in the mood of tracing roots.”) She also analyzed 3,824 soil samples, collecting the roots in each, cleaning and separating them according to diameter (fine or coarse) and determining whether they were alive or dead.
The messages in the roots
Importantly, Cusack and Cordeiro found that with chronic drying, the Panamanian forests suppressed the growth of shallow roots (shallower than 20 cm or almost 8 in). However, deeper down (more than 60 cm or about 23 in), three of the four Panamanian forests increased their root production.
Only the wettest, most fertile forest didn’t increase deep root production. That forest receives 3,400 mm (about 134 in) mean annual precipitation, Cusack explains, and might not have the adaptive capacity to grow deeper roots since dry conditions have been rarer in the past in these wetter forests. “Drier” forests (naturally adapted to longer drier seasons) seemed more able to adapt to less rainfall.
Cusack explains that while shallow rainforest roots are adapted to high moisture, they die during prolonged drought or the dry season (which is becoming longer in many tropical regions due to climate change). Far down in the soil, some forests can compensate. They send more roots down, likely searching out deeper pockets of moisture.
In some instances, Cusack says, this may occur at the forestwide community scale. “The whole forest can send more roots deeper into the soil when there’s drying,” she says, and they can possibly rescue themselves from drought in this way. But, she adds, the wettest forests don’t appear able to apply this rescue strategy, making them more vulnerable to drying.
Other findings noted that the mycorrhizal fungi and root diameter increased with drying, likely to compensate for the decrease in dying roots at the soil surface, altering the plant’s resource acquisition strategy.
The researchers concluded that future drying is likely to change tropical forest-climate feedback via changes in root production and inputs of root biomass to soil carbon storage.
More answers, more questions
Scientists know that tropical rainforests are experiencing immense stresses due to human activities. Cusack emphasizes that tropical forest root systems are also now displaying clear stress responses.
Rainforests are undergoing accelerating extreme changes in drought and moisture conditions that they’re not adapted to, she says, with conditions “changing faster than their evolutionary adaptation.”
The good news, she notes, is that some tropical forests are more resilient, with the key to their resilience largely being their belowground root flexibility. If roots can respond and adapt, then a rainforest will be more likely to survive climate change for longer.
Understanding which rainforests can respond and adapt quickly to drier conditions, and which cannot, could help forest managers to frame their conservation decisions. We don’t know all the answers, Cusack says, but scientists are now amassing data as to which forests could be more resilient and which ones more vulnerable. “We have a starting place.”
Banner image: Amanda L. Cordeiro (left), a postdoctoral scholar at the University of Minnesota, and Daniela F. Cusack, associate professor in the Department of Ecosystem Science and Sustainability at Colorado State University, at one of their research sites to investigate how root systems react to climatic changes. Image courtesy of Daniela F. Cusack.
Citations:
Hijri, M., & Bâ, A. (2018). Editorial: Mycorrhiza in tropical and neotropical ecosystems. Frontiers in Plant Science, 9</em. doi:10.3389/fpls.2018.00308
Cusack, D. F., Addo-Danso, S. D., Agee, E. A., Andersen, K. M., Arnaud, M., Batterman, S. A., … Yaffar, D. (2021). Tradeoffs and synergies in tropical forest root traits and dynamics for nutrient and water acquisition: Field and modeling advances. Frontiers in Forests and Global Change, 4. doi:10.3389/ffgc.2021.704469
Cusack, D. F., Christoffersen, B., Smith-Martin, C. M., Andersen, K. M., Cordeiro, A. L., Fleischer, K., Wright, S. J., Guerrero-Ramírez, N. R., Lugli, L. F., McCulloch, L. A., Sanchez-Julia, M., Batterman, S. A., Dallstream, C., Fortunel, C., Toro, L., Fuchslueger, L., Wong, M. Y., Yaffar, D., Fisher, J. B., Norby, R. J. (2024). Toward a coordinated understanding of hydro-biogeochemical root functions in tropical forests for application in vegetation models. New Phytologist, 242(2), 351-371. https://doi.org/10.1111/nph.19561
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F. Slik, J. W., Arroyo-Rodríguez, V., Aiba, S. I., & Venticenque, E. M. (2015). An estimate of the number of tropical tree species. PNAS>. Retrieved from https://www.pnas.org/doi/full/10.1073/pnas.1423147112
Cordeiro, A. L., Cusack, D. F., Dietterich, L. H., Valdes, E., Gonçalves, N. B., Oliveira, M., Quesada-Ávila, G., & Wright, S. J. (in review). Drying suppresses fine root production to 1m depths and alters root traits in four distinct tropical forests [Manuscript submitted for publication].
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