The lush green canopy might look unchanged from above, but beneath the soil of Panama’s tropical forests, roots are on the move. Facing longer, harsher dry spells, trees are pushing their roots deeper underground, using what researchers call a “rescue strategy” to hang on to water and stay alive.
Panama’s forests are quietly rewiring themselves
Tropical forests hold more than half of all life on land and are among the planet’s biggest carbon stores. A huge share of that carbon sits out of sight, locked in vast networks of roots and the soil around them.
Yet those forests are under mounting pressure. Rising temperatures and shifting rainfall patterns are piling stress onto ecosystems that, until now, have relied on fairly reliable wet seasons. Central America, including Panama, is already seeing more frequent and intense droughts linked to climate change and El Niño events.
To understand how forests might cope, researchers launched a long-running field project known as the Panama Rainforest Changes with Experimental Drying, or PARCHED. Their latest findings show that trees do respond, and fast – but there are limits to what this emergency response can achieve.
Trees in experimental drought plots shifted growth away from shallow, surface roots and into longer, finer roots reaching deeper, wetter layers of soil.
Inside the PARCHED drought experiment
The PARCHED team set up 32 experimental plots across four different tropical forest sites in Panama. Each site has its own character: different tree communities, soils, nutrient levels and rainfall patterns. That diversity gave scientists a rare chance to see how multiple forest types respond to the same stress.
How you fake a drought in a rainforest
To mimic chronic drying, researchers installed clear plastic roof panels high above the forest floor. These structures, which resemble partial greenhouse roofs, intercepted roughly half to two-thirds of the rain before it reached the ground below.
They paired this with deep trenches around each plot, lined with thick plastic sheets. That barrier stopped roots from sneaking in water from neighboring, non-droughted areas. Inside those fenced-off patches, the only water available was what fell through the reduced rainfall or was already in the soil.
- Rainfall reduced by around 50–70% using roof panels
- Plots isolated with plastic-lined trenches to block lateral water flow
- Four distinct forest types used, each with unique soils and species
- Experiment has been running for several years, tracking gradual drying
Three different ways to watch roots react
Roots are notoriously hard to study because they work out of sight. The PARCHED team used three complementary approaches to see what was happening underground over five years:
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| Method | What it measured |
|---|---|
| Soil cores | Root biomass and distribution down to about 20 cm below the surface, collected several times a year |
| Root traps | New root growth into mesh-filled columns, checked every three months |
| Underground cameras | Fine-scale changes in root length and density via cameras inserted into acrylic tubes 1.2 m deep |
By combining these techniques, the scientists could see not just how many roots there were, but where they were growing and how their behavior changed as the soil slowly dried out.
Deeper roots, fewer surface lifelines
Across all four forests, the pattern was strikingly consistent. As conditions dried, the quantity of fine roots close to the surface dropped. Those shallow roots usually play a vital role in grabbing both water and nutrients during and shortly after rainfall.
Chronic drying triggered a clear trade-off: trees reduced shallow fine roots and invested more in deeper, moisture-hunting roots.
With less water in the surface layers, those shallow roots became less useful and more vulnerable to dieback. In response, trees started growing more fine roots at depth, where remaining moisture persisted for longer into the dry season.
That deeper rooting helps maintain what ecologists call “hydraulics” – the internal water transport system that keeps leaves hydrated and photosynthesis running. Without that, trees risk wilting, shedding leaves or dying outright during extended droughts.
A rescue strategy, not a cure
Researchers describe this shift as a rescue strategy because it keeps trees functioning, but it does not fully compensate for losses. The total amount of root biomass, and therefore carbon stored in roots, still declined under chronic drying.
In other words, the trees are staying alive, but they are doing so with a leaner root system and less below-ground carbon. That matters because tropical forests play such a large role in soaking up human-produced carbon dioxide.
Deeper rooting helps survival, yet does not restore the lost carbon or biomass in the upper soil layers.
Fungi step in to help stressed roots
The drought experiment also revealed a quiet ally: fungi living around the roots. Many tropical trees form close partnerships with arbuscular mycorrhizal fungi. These fungi wrap around root tips and extend filament-like strands through the soil, effectively enlarging the plant’s reach.
Under chronic drying, the reduced number of surface roots showed a stronger association with these fungi. With fewer shallow roots remaining, those that do persist seem to attract more fungal partners. The fungi, in turn, improve access to both water and scarce nutrients held in dry soil.
This kind of symbiosis can make a meaningful difference during stress. It helps trees wring the last drops of moisture and minerals out of the topsoil, even as they send new roots deeper down in search of a more stable supply.
Can these forests adapt fast enough?
Not all tropical forests are equally prepared for more severe or frequent drought. Some tree species in naturally dry regions have had millennia to evolve drought-tolerant traits, such as thick bark, dense wood or inherently deeper roots.
Other forests, especially those on typically wet, nutrient-poor soils, are more vulnerable. There, trees have not historically needed heavy-duty drought strategies. Sudden shifts in climate risk outpacing their ability to adjust.
Scientists worry that rapid climate change could push some tropical species beyond their tolerance, leading to local declines or disappearances.
If sensitive species fail to keep up, forest composition is likely to change. Drought-tolerant trees and shrubs may expand, while water-loving species fade. That shift would affect not just the forest’s carbon balance, but also the wildlife that depends on particular trees for food and habitat.
Why these root changes matter for carbon and climate
When roots die off, the carbon they contained can be released back into the atmosphere as microbes break them down. Deeper roots might slow that process, because carbon buried further underground tends to be more stable and decomposes more slowly.
The balance between lost shallow roots and new deep roots will shape how much carbon these forests can continue to store. If root systems continue to thin overall, tropical forests could become weaker carbon sinks over time.
Scientists involved in the PARCHED project now want to know how persistent the deeper-rooting response will be. If droughts become more intense or more frequent, trees may hit physiological limits. Constant stress can reduce growth, seed production and the ability to recover after damage from storms or pests.
Key terms that help make sense of the study
A few pieces of jargon used in this kind of research are worth clarifying, because they reveal what is at stake:
- Fine roots: The thinnest, most active roots, usually less than 2 millimetres in diameter, that absorb most water and nutrients.
- Chronic drying: Long-term reduction in water availability, as opposed to a single short, sharp drought.
- Hydraulics: The tree’s internal water transport system, moving water from roots to leaves through xylem tissues.
- Carbon storage: Carbon held in wood, leaves, roots and soil, which keeps it out of the atmosphere.
Understanding these concepts helps explain why scientists pay such close attention not just to tree trunks and leaves, but also to what is happening underground.
What this means for future forests
The PARCHED experiment hints at both resilience and risk. On one hand, trees are not passive victims. They adjust root architecture, forge stronger fungal partnerships and tap deeper water reserves when surface conditions dry out.
On the other, those adjustments come with trade-offs: less surface-root biomass, potentially reduced growth and uncertain long-term impacts on carbon storage. If droughts grow harsher than those simulated roofs can create, some species may run out of options.
For conservation planners and policymakers, these findings suggest that protecting a variety of forest types could help spread risk. Forests already used to seasonal dryness might become increasingly important refuges for drought-tolerant species. Meanwhile, wetter forests may need extra attention, including limits on logging or fragmentation, to reduce added stress from human activity.
The Panama results also highlight why long-running experiments matter. A five-year project only captures a small slice of a tree’s life, yet it already shows significant changes underground. Over coming decades, continued monitoring will reveal whether deeper roots are a temporary adjustment or part of a more profound reshaping of tropical forests under a warming climate.