Tropical forests cover roughly 17 percent of the Earth’s land surface and hold more than half of all terrestrial plant and animal species, making them the most biologically rich ecosystems on the planet. They are not one uniform biome but a family of forest types shaped by rainfall, seasonality, and deep evolutionary history stretching back over 100 million years. Understanding how they work, why they harbor such extraordinary diversity, and what threatens them requires looking well beyond the postcard image of a dense, dripping jungle.
Not All Tropical Forests Look the Same
The stereotypical tropical forest is a towering rainforest with a closed, evergreen canopy rising above 25 meters. These forests typically form where annual rainfall exceeds about 2,000 millimeters and there is little or no dry season. But that describes only part of the tropics. Where rainfall is distinctly seasonal, with four to nine months of almost no rain, large stretches of savanna and dry forest take over. Tropical dry forests share the seasonal climate of savannas but develop closed canopies and tend to be deciduous, shedding their leaves during the dry months. In drier conditions still, dry forests become shorter and succulents become more prominent in the vegetation.1Current Biology. Tropical savannas and dry forests These distinctions matter because the ecological rules, fire regimes, and vulnerability to climate change differ sharply among these forest types.
Why Tropical Forests Are So Species-Rich
One of the oldest puzzles in ecology is why tropical forests support thousands of tree species in a single landscape when temperate forests manage with a few dozen. A leading explanation, often called the Janzen-Connell hypothesis, proposes that natural enemies like fungi, bacteria, and insect herbivores keep any one species from becoming dominant. Seedlings that land near their parent tree, or in dense patches of the same species, face heavy attack from specialized pests and pathogens. This creates a built-in advantage for rare species and a penalty for being common.
A large meta-analysis of experimental evidence confirmed both predictions: seedlings survive at lower rates near conspecific adults and in high-density patches of the same species, and natural enemies are frequently the cause.2PubMed Central. Testing predictions of the Janzen-Connell hypothesis: a meta-analysis of experimental evidence for distance- and density-dependent seed and seedling survival Modeling work has taken this further, showing that distance-responsive enemies can maintain forest diversity almost indefinitely by imposing a cost on commonness and a benefit to rarity. The mechanism acts like a thermostat for species richness, preventing competitive exclusion without requiring that every species have a unique ecological niche.3PubMed Central. Tropical forests can maintain hyperdiversity because of enemies
But enemy-mediated coexistence does not fully explain why some tropical regions are far richer than others. Neotropical forests in Central and South America and Indomalayan forests in Southeast Asia harbor more species than African tropical forests, even at comparable latitudes and rainfall levels. Recent work tracing 110 million years of evolutionary history suggests these differences trace back to geological and climatic events rather than present-day conditions. Mountain uplift in the Neotropics and Indomalayan region spurred bursts of speciation, while sustained aridification across Africa over the Cenozoic drove higher extinction rates and depressed speciation.4PubMed Central. Earth history events shaped the evolution of uneven biodiversity across tropical moist forests In other words, today’s biodiversity map is a record of deep time, not just current climate.
Life Inside the Canopy
The canopy of a tropical forest does far more than block sunlight. It creates an entirely different climate underneath. In a tropical seasonal rainforest, researchers found that humidity near the forest floor averaged around 97 percent, compared with roughly 87 percent in the upper canopy. Irradiance and wind speed drop dramatically as you descend, and air temperature moderates along the way.5iForest – Biogeosciences and Forestry. Quantifying the vertical microclimate profile within a tropical seasonal rainforest, based on both ground- and canopy-referenced approaches This buffering effect has practical consequences for the organisms living below. Understory plants, amphibians, and insects depend on stable humidity and cooler temperatures. Even soil organisms benefit from the thermal stability the canopy provides.
Research across tropical landscapes shows that canopy height is the key variable dampening microclimate extremes, especially on wind-exposed slopes. But the buffering effect tends to plateau once canopy height exceeds about 20 meters, which means that even selectively logged secondary forests can still provide meaningful thermal shelter.6PubMed. Canopy structure and topography jointly constrain the microclimate of human-modified tropical landscapes A global analysis of forest microclimates found that forests act as thermal insulators broadly: they cool the understory when ambient temperatures are hot and warm it when ambient temperatures are cold. The magnitude of that offset grows as conditions become more extreme, and it exceeds the warming signal of the past century.7Nature Ecology & Evolution. Global buffering of temperatures under forest canopies That last point is striking: the temperature difference between the understory and an open field during a heat wave is larger than the total warming the planet has experienced since the industrial revolution. Keeping the canopy intact is, in a real sense, a form of local climate control.
Poor Soils, Rich Forests
One of the great paradoxes of tropical forests is that many sit on some of the most nutrient-depleted soils on Earth. Millions of years of heavy rainfall have leached away base cations, leaving behind soils rich in aluminum and iron oxides that lock up phosphorus in forms plants struggle to access. In Southeast Asian forests, this process creates intensely acidified soils where exchangeable aluminum becomes toxic to most plant species, yet dipterocarp trees thrive. Their secret lies partly in partnerships with ectomycorrhizal fungi, which mine phosphorus from the soil and make it available for critical life events like mast fruiting. In the Neotropics and Africa, legume trees that fix atmospheric nitrogen help ease nitrogen limitations, though excess nitrogen fixation can itself accelerate soil acidification.8Ecological Research. Plant–soil interactions maintain biodiversity and functions of tropical forest ecosystems
These fungal partnerships vary in their chemistry. Ectomycorrhizal trees respond well to inorganic phosphorus but can also break down complex organic forms like phytic acid, giving them an edge in soils where phosphorus exists mainly in hard-to-reach organic compounds. Arbuscular mycorrhizal trees, which are far more common globally, are less effective at scavenging organic phosphorus but still benefit from simple organic forms.9PubMed. Partitioning of soil phosphorus among arbuscular and ectomycorrhizal trees in tropical and subtropical forests The upshot is that the productivity of tropical forests depends less on the raw fertility of the soil than on these intricate biological recycling networks. Disrupt the fungi and the leaf litter layer, and the forest’s nutrient engine stalls.
Woody Vines and Their Outsized Influence
Lianas, the thick woody vines that drape through tropical canopies, are not mere decoration. They compete intensely with trees for light, water, and soil nutrients. In a controlled experiment, tree sap velocity jumped about 60 percent soon after lianas were removed, and tree diameter growth rose roughly 25 percent within a year.10PubMed. Liana competition with tropical trees varies seasonally but not with tree species identity That competitive drag carries real consequences for the carbon budget of tropical forests. Modeling across a landscape of regenerating forests estimated that lianas reduced tree biomass growth by about 22 percent, and the effect intensified over time, rising from 19 percent after five years to a projected 47 percent after six decades.11PubMed. Liana effects on biomass dynamics strengthen during secondary forest succession
There is also evidence that lianas do not just reduce the total amount of carbon a forest stores but shift its composition. Heavy liana infestation tends to favor tree species with lower-density wood, which store less carbon per unit of volume.12Biogeosciences. Liana infestation impacts tree growth in a lowland tropical moist forest With warming temperatures and longer dry seasons predicted for many tropical regions, lianas may gain a competitive edge over trees because they tend to photosynthesize more actively during dry periods. If liana abundance continues to climb, as observations in several Neotropical forests suggest, the capacity of tropical forests to absorb atmospheric carbon could weaken.
Drought, Fire, and the Tipping Point
Tropical moist forests are not adapted to burn. When fire enters these systems, the consequences are abrupt. In a long-term experiment in the Amazon simulating annual and triennial burn regimes, researchers observed enormous spikes in tree mortality during a severe drought year: increases of over 200 percent and over 400 percent, respectively, compared to non-drought years. The cascading effects included sharp declines in canopy cover and live biomass, along with invasion by flammable grasses at the forest edges, creating a feedback loop that makes future fires more likely.13PubMed Central. Abrupt increases in Amazonian tree mortality due to drought-fire interactions A similar dynamic played out in Ghana, where severe drought in 2016 triggered extensive fires concentrated almost entirely in degraded forest reserves, underscoring how prior degradation sets the stage for fire during drought.14Forest Ecology and Management. Forest degradation promotes fire during drought in moist tropical forests of Ghana
Even without fire, drought alone kills trees, and recent research has clarified the mechanism. For years, scientists debated whether drought-killed trees died of carbon starvation (running out of stored sugars) or hydraulic failure (their water-conducting tissue breaking down). The world’s longest-running tropical drought experiment, spanning over a decade in the Brazilian Amazon, found that hydraulic failure is the trigger. Droughted trees showed no depletion of stored sugars compared to non-droughted neighbors, and growth rates did not decline before death. Instead, the water transport system from soil to leaf collapsed. Large trees were at far greater risk because their taller stature means water must travel further and under greater tension.15Nature. Death from drought in tropical forests is triggered by hydraulics not carbon starvation
The interaction between heat and drought makes things worse. During the 2015-2016 El Niño, researchers tracking canopy trees in Amazonia found that the safety margins of leaf water potential and xylem function narrowed sharply during warm periods. The drop in water transport was driven by a combination of air-drying and water-pipe collapse inside the wood. The stress correlated most tightly with atmospheric dryness rather than soil moisture alone, suggesting that hotter droughts are disproportionately dangerous even when rainfall does not change dramatically.16PubMed Central. Dry and hot: the hydraulic consequences of a climate change–type drought for Amazonian trees
How Forests Bounce Back
Despite these vulnerabilities, tropical forests are remarkably resilient when given the chance to recover. A global synthesis of recovery data found that after 20 years of regrowth, secondary tropical forests had regained about 78 percent of old-growth values across a range of attributes. Soil properties recovered fastest, within about a decade. Plant functioning recovered within about two and a half decades, while structural features and species diversity took roughly three to six decades. Full recovery of biomass and species composition, however, requires more than a century.17PubMed. Multidimensional tropical forest recovery
The speed of recovery depends heavily on what the forest is recovering from. In southwestern Costa Rica, forests regrowing after clear-cuts accumulated biomass faster in the early years, reaching about 52 percent of old-growth levels after two decades. But species richness lagged behind, reaching only about 31 percent of old-growth richness in the same period. Forests recovering from pastures showed the reverse pattern: species numbers climbed faster, but biomass accumulated more slowly.18Forest Ecology and Management. Recovery of aboveground biomass, species richness and composition in tropical secondary forests in SW Costa Rica This means there is no single recovery trajectory; the type of prior land use leaves a lasting signature on how the forest reassembles itself.
A common assumption is that tree planting always accelerates restoration. That is not always the case. A meta-analysis of 133 studies found that natural regeneration actually outperformed active planting for all three biodiversity groups examined (plants, birds, and invertebrates) and for five measures of vegetation structure. Restoration success was 34 to 56 percent higher for biodiversity and 19 to 56 percent higher for vegetation structure in naturally regenerating forests compared to planted ones.19Science Advances. Ecological restoration success is higher for natural regeneration than for active restoration in tropical forests That does not mean planting is useless. In heavily degraded areas with no seed sources or where soil conditions are poor, planting may be the only viable option. And for soil recovery specifically, both strategies perform comparably. After 18 years, planted and naturally regenerating forests in a Brazilian study showed similar levels of soil carbon and nitrogen cycling, though the microbial communities in both still differed from those of old secondary forest.20Restoration Ecology. Natural regeneration or tree planting in a tropical forest‐to‐pasture damaged area: which is more efficacious for soil ecosystem recovery?
Indigenous Lands as Forest Strongholds
Some of the best-preserved tropical forests sit on Indigenous Peoples’ lands, and the data backs this up across continents. A pan-tropical analysis comparing more than three million square kilometers of Indigenous lands to matched non-protected areas found that deforestation and degradation rates from 2010 to 2018 were lower on Indigenous lands across the tropics. In Africa, Indigenous lands avoided more deforestation than formal protected areas did.21Nature Sustainability. Reduced deforestation and degradation in Indigenous Lands pan-tropically Losses of intact forest landscapes have also been considerably lower on Indigenous lands than on other lands overall.22Frontiers in Ecology and the Environment. Importance of Indigenous Peoples’ lands for the conservation of Intact Forest Landscapes
The picture gets more nuanced when you look at forest quality rather than just deforestation rates. Across the tropics, areas that are both Indigenous lands and formally protected (sometimes called Protected Indigenous Areas) had the highest average forest integrity scores, while non-protected areas had the lowest. In Africa, all governance types with some form of recognition outperformed unprotected lands by three to five percent on forest integrity metrics. In the Americas and Asia, results were more mixed: formal protected areas and Protected Indigenous Areas outperformed non-protected lands, but Indigenous lands without formal protection sometimes showed slightly lower forest integrity than the surrounding landscape. Researchers note that this could partly reflect the fact that many formally recognized Indigenous territories are located far from deforestation pressures, making it hard to separate the governance effect from the geography effect.23Current Biology. Protected-Indigenous areas have the highest forest integrity across the tropics
What Tropical Forests Release Into the Air
Tropical forests do not just absorb carbon dioxide; they emit a vast and chemically active cocktail of volatile organic compounds that shapes atmospheric chemistry at regional and global scales. In measurements over Amazonian forest, isoprene and monoterpenes together accounted for roughly 75 percent of the total reactivity of the air with hydroxyl radicals, the atmosphere’s primary cleaning agent.24Journal of Geophysical Research: Atmospheres. The tropical forest and fire emissions experiment: Emission, chemistry, and transport of biogenic volatile organic compounds in the lower atmosphere over Amazonia These compounds are not pollutants in the conventional sense. They influence cloud formation, ozone chemistry, and even the oxidative capacity of the atmosphere. When forests burn or are cleared, the shift from biogenic emissions to combustion-related pollutants alters atmospheric chemistry in ways that ripple far beyond the tropics.
Deforestation and Disease Outbreaks
The consequences of tropical forest loss extend to human health in ways that are only recently being quantified. A global analysis spanning 1990 to 2016 found that increases in outbreaks of zoonotic and vector-borne diseases are linked with deforestation, predominantly in tropical countries. The same study identified a separate association between outbreaks of vector-borne diseases and expansion of oil palm plantations.25PubMed Central. Outbreaks of Vector-Borne and Zoonotic Diseases Are Associated With Changes in Forest Cover and Oil Palm Expansion at Global Scale The mechanisms are not hard to imagine: clearing forest pushes wildlife into closer contact with people and livestock, creates standing-water habitats that favor mosquito breeding, and shifts microclimates toward warmer, sunnier conditions that some disease vectors prefer. This link between deforestation and disease risk is one of the strongest arguments for treating tropical forest conservation as a public health issue, not just an ecological one.
Listening to the Forest to Measure Its Health
One of the practical challenges of tropical forest science is monitoring. These ecosystems are vast, remote, and densely layered, making traditional surveys expensive and slow. Passive acoustic monitoring is emerging as a complement to satellite imagery and on-the-ground fieldwork. By recording the soundscape continuously over weeks or months and analyzing it with machine learning, researchers can track biodiversity recovery across decades. In studies of regenerating forests, both standard acoustic indices and deep-learning models trained on the recordings performed well at explaining variation in species richness, and not just for animals that vocalize. The acoustic measures also correlated with the abundance of largely silent groups like nocturnal insects, suggesting that soundscapes capture a broader signal of ecosystem health than you might expect.26Nature Communications. Soundscapes and deep learning enable tracking biodiversity recovery in tropical forests
Acoustic monitoring is even reaching below the ground. Researchers have demonstrated that recording soil sounds, the clicks, buzzes, and movements of invertebrates in the litter and topsoil, can serve as an indicator of soil biodiversity in tropical forests.27Ecological Indicators. Listening to tropical forest soils The approach is cheap relative to laboratory sorting of soil samples and can run continuously without disturbing the site. It is still early days for calibrating what different soil soundscapes mean, but the potential for long-term, large-scale monitoring is clear.
Peatland Forests and Their Outsized Carbon Stores
Not all tropical forests grow on mineral soil. Across Southeast Asia and parts of the Amazon basin, forests sit atop waterlogged peat deposits that can be meters deep. These peatland forests store enormous amounts of carbon both above and below ground. In Amazonia, peat-forming wetland forests supply fruit, fiber, and timber to local communities while maintaining high belowground carbon stocks, with tree species composition and peat formation governed by local hydrology.28Hydrological Processes. The presence of peat and variation in tree species composition are under different hydrological controls in Amazonian wetland forests In Indonesia, where peatland forests have been extensively logged and drained for plantations, the difference between intact and degraded stands is dramatic. Conservation-zone forests in Riau province stored roughly twice the aboveground biomass and carbon of rehabilitation-zone forests at the same growth stage, reflecting how heavily management history shapes these systems.29E3S Web of Conferences. Aboveground Biomass and Carbon Stock of Trees and Poles in Tropical Peatland Forest Ecosystems: A Case Study from Riau, Indonesia When peatlands are drained, the exposed peat oxidizes and burns, releasing centuries of stored carbon in a matter of weeks. The fires that swept Indonesian peatlands in 2015 briefly made the country one of the world’s largest greenhouse gas emitters. Keeping peat wet is, by a wide margin, the most efficient way to protect these carbon stocks.