When drought tightens its grip on tropical forests, the biggest changes are not always in the canopy. In a Panama experiment launched in 2018, researchers found that persistent drying cut fine root production in the top 4 inches of soil by about half, even as many trees pushed new growth deeper underground. The bottom line is a mix of resilience and risk, forests can adjust to drying to a large extent, but the adjustments may still drain carbon from soils over time.
It is an unsettling question for a warming planet. What happens when the “rainy season” does not show up the way it used to, and the forest floor stays dusty longer than normal? Scientists say part of the answer is hidden below our feet, in roots that quietly decide how much water and carbon a forest can bank for the future.
A drought test built for the real world
The study, titled “Drying suppresses fine root production to 1 m depths and alters root traits in four distinct tropical forests,” followed belowground changes across four lowland forests on the Isthmus of Panama. The sites range from about 92.5 to 134.6 inches of rain per year (2,350 to 3,420 millimeters), with an average temperature near 79 degrees Fahrenheit (26 degrees Celsius).
To simulate long-lasting drying without clearing land, the team built partial “throughfall exclusion” setups over 33 by 33 foot plots (10 by 10 meters), paired with controls. Clear roofing panels initially covered about half the ground area, then expanded to around 70 percent in 2021, while plastic-lined trenches about 20 inches deep (50 centimeters) helped limit sideways water flow.
The shallow root slowdown was dramatic
Fine roots are the forest’s frontline plumbing, the tiny strands that take up water and nutrients. Across all four forests, drying reduced fine root biomass productivity in the top 4 inches (0 to 10 centimeters) by about 51 percent, dropping from roughly 10.1 milligrams per kilogram of dry soil per day (about 4.6 milligrams per pound) in controls to about 5.0 milligrams per kilogram (about 2.3 milligrams per pound) under drying.
Just below that, in the 4 to 8 inch layer (10 to 20 centimeters), productivity fell by about 35 percent. Live fine root biomass “stocks” also shrank by about 21 percent in both layers, and turnover slowed by about 42 percent in the top layer, meaning the system was replacing itself less often. That matters because fine roots are one of the main ways forests move fresh carbon into soil.
Deep roots offered an escape route, with limits
When the surface soil became hostile, underground cameras captured a shift in strategy. Using minirhizotron imaging recorded every three weeks from 2019 to 2022, the team analyzed 237,360 images of roots growing down to about 3.3 feet (1 meter) without digging the plots up.
The patterns were not uniform, but the signal was clear in many places. In deeper soil layers below about 31.5 inches (more than 80 centimeters), drying increased root productivity across forests, and the strongest increase showed up around 39 to 43 inches (100 to 110 centimeters), except in the wettest, most nutrient-poor forest. As CSU ecologist Daniela Cusack put it, “some trees are able to send roots deeper in search of moisture,” though “the increase in deeper roots is not enough biomass to offset carbon losses” from surface roots.
Fungi stepped in as quiet allies
Roots did not face drought alone. The study found higher colonization by arbuscular mycorrhizal fungi in dried plots, rising about 6.7 percent in the top 4 inches and about 14.6 percent in the 4 to 8 inch layer during the wet season.
These fungi form tight partnerships with plant roots and send threadlike filaments into soil pores that roots cannot easily reach. In practical terms, it is biological outsourcing, a tree spends some of its carbon to “hire” fungi, and the fungi can help deliver water and nutrients back to the plant.
Why the results matter for carbon storage
Tropical forests are home to some of the largest carbon stores on Earth, so small shifts in drought response can add up. The results underline that drought is not just a canopy story but also a soil carbon budget story, because fine roots are constantly dying, regrowing, and feeding carbon into the ground.
If long-term drying keeps suppressing shallow root growth, less fresh carbon may enter surface soils in the long run. Shifting some root activity deeper could move carbon inputs into layers with different chemistry and decomposition rates, so the net climate effect may vary by forest and by soil type. And that is the tricky part, the clock on drought risk is moving faster than our belowground data.
What to keep in mind as droughts spread
The headline finding is flexibility, but the fine print is limits. A deeper-root “rescue” does not erase the carbon losses tied to surface root decline, and not every forest in the experiment responded the same way.
That matters for climate forecasting and restoration planning, because it is hard to protect what you do not measure. The study was published on New Phytologist.
