Puerto Rico holds a remarkable variety of tree-covered landscapes packed into a territory smaller than Connecticut. From coastal mangrove swamps to high-elevation cloud forests where trees rarely exceed the height of a two-story building, the island hosts wet, dry, and montane forest types that have been shaped by centuries of human land use and a relentless cycle of Atlantic hurricanes. What makes Puerto Rico’s trees especially interesting to ecologists is how dramatically the island’s forest cover has shifted over the past century, rebounding from near-total deforestation to a patchwork of recovering native and introduced forests that are still evolving today.
How Puerto Rico Lost and Regained Its Forests
By the mid-twentieth century, Puerto Rico had been stripped of most of its original forest. Centuries of sugarcane, coffee, and cattle farming had reduced tree cover to a fraction of what existed before European colonization. Then something unexpected happened: the island’s economy industrialized. Farmers left the countryside for factory jobs, and abandoned pastures and crop fields began growing back. Researchers studying pairs of abandoned-pasture sites found that woody species were scarce for the first decade after fields were left alone, and substantial woody biomass did not accumulate until roughly fifteen years after abandonment.1Forest Ecology and Management. Forest recovery in abandoned tropical pastures in Puerto Rico Over decades, trees gradually filled in, and today Puerto Rico’s forest cover is dramatically higher than it was in the 1940s.
This regrowth story is one of the most studied examples of tropical forest recovery in the world, partly because Puerto Rico’s detailed census and land-use records make it possible to track exactly when each parcel was abandoned and what grew back. The forests that returned, however, are not replicas of the originals. They are younger, shorter, and composed of a different mix of species, with introduced trees playing a large role alongside returning natives.
Novel Forests and the Role of Introduced Species
One of the most distinctive features of Puerto Rico’s modern tree cover is the prevalence of so-called novel forests, communities assembled from a mix of native and non-native species that have no historical precedent on the island. The African tulip tree (Spathodea campanulata), introduced as an ornamental, has become one of the most common trees in the lowlands and mid-elevations. Rather than simply displacing native species, research shows that Spathodea-dominated forests can actually facilitate the return of native trees by restoring forest structure on land where earlier management practices had eliminated the original vegetation.2New Forests. Structure and species composition of novel forests dominated by an introduced species in northcentral Puerto Rico
This process depends on seed sources. Where remnant patches of old-growth forest or scattered mature native trees survive nearby, native species eventually colonize the understory beneath the introduced canopy. Over time, those native seedlings grow up and diversify the forest, preventing the kind of biological homogenization you might expect from an invasion.3Plant Ecology. Invasion by native tree species prevents biotic homogenization in novel forests of Puerto Rico The implication is counterintuitive: an introduced “weed tree” can serve as a nurse canopy that speeds recovery, as long as native seed sources persist in the landscape. Conserving those scattered old-growth fragments turns out to be critical not just for the trees they contain but for the regeneration they seed across surrounding areas.
Forest Types Across the Island
Puerto Rico’s small land area packs in a surprising range of forest types, driven mainly by elevation and rainfall gradients. Starting at sea level and climbing to the peaks of the Luquillo Mountains or the Cordillera Central, a visitor would pass through several distinct tree communities.
Coastal areas support mangrove forests, dominated by red, white, and black mangrove species adapted to saltwater and tidal flooding. Dry forests hug the rain-shadow southwestern coast around Guánica, where trees are shorter, often deciduous, and adapted to seasonal drought. The moist lowland and mid-elevation forests across much of the island are where the novel forests discussed above are most common. Subtropical wet forests thrive on the wetter northeastern slopes, including the tabonuco forest of El Yunque, named for the tall Dacryodes excelsa trees that anchor its canopy. Higher still, the palo colorado forest transitions into elfin or dwarf cloud forest near the summits.
The Dwarf Cloud Forest at the Top
The cloud forests perched on Puerto Rico’s highest ridges are some of the most unusual tree ecosystems in the Caribbean. Trees in the dwarf cloud forest are genuinely small: the great majority do not exceed about fifteen meters in height anywhere in the Luquillo Mountains.4Caribbean Journal of Science. Dwarf Forest Recovery after Disturbances in the Luquillo Mountains of Puerto Rico Trunks are gnarled, branches are laden with mosses and epiphytes, and the canopy is dense but low. More than half the tree species growing in these summit forests are endemic to Puerto Rico, found nowhere else on Earth, and they play a dominant role in how the forest rebuilds after storms.5Caribbean Journal of Science. Dwarf Forest Recovery after Disturbances in the Luquillo Mountains of Puerto Rico
Ecologists studying these summit plots found that just five species accounted for about 95 percent of stems above a minimum trunk size, and stem density was extremely high, with thousands of stems packed into each hectare.6ResearchGate. Ecological Observations in the Dwarf Cloud Forest of the Luquillo Mountains in Puerto Rico The trees grow slowly, with annual leaf litter production of only about three tons per hectare per year, a fraction of what lower-elevation tropical forests produce. Persistent cloud immersion keeps humidity high and temperatures cool, which slows decomposition and nutrient cycling. These conditions create waterlogged, acidic soils that further limit tree growth, producing the distinctive stunted form.
Climate change poses a particular threat to these forests. Cloud base height matters enormously: if the altitude at which clouds regularly form rises even modestly, ridgetop forests that currently sit inside the cloud layer could lose the moisture bath they depend on. Measurements taken over several years at the Luquillo Mountains found that the lowest cloud bases most frequently formed at elevations between roughly 740 and 960 meters, which is higher than cloud base altitudes measured at six other Caribbean sites by about 200 to 600 meters.7Copernicus Publications (Atmospheric Chemistry and Physics). Analyzing cloud base at local and regional scales to understand tropical montane cloud forest vulnerability to climate change That already-high baseline means Puerto Rico’s cloud forests operate close to the edge. Any upward shift in cloud formation could dry out the summits and fundamentally alter the tree community.
Hurricanes and How Trees Respond
No discussion of Puerto Rico’s trees makes sense without hurricanes. Major storms have hit the island roughly every sixty years on average historically, and their effects on forest structure are profound. The Luquillo Mountains have been struck by three major hurricanes with modern monitoring in place: Hugo in 1989, Georges in 1998, and María in 2017. The data from these storms tell a compelling story about how different tree species cope with extreme wind.
Hurricane María was in a class by itself. It killed twice as many trees as Hugo and, across nearly all species studied, broke two to twelve times more stems than either of the earlier storms.8Nature Communications. Hurricane María tripled stem breaks and doubled tree mortality relative to other major storms The average increase in stem breakage was more than threefold. One of the most striking findings was that dense wood, which had been protective during Hugo, offered no defense against stem breakage during María. Species with heavy, hard trunks were less likely to uproot during María, but they snapped at the same rate as lighter-wooded species.9Nature Communications. Hurricane María tripled stem breaks and doubled tree mortality relative to other major storms The implication is that above a certain wind threshold, even the toughest wood gives way.
Species identity still matters, though. Pioneer trees that colonize gaps after disturbance, like Cecropia schreberiana, are highly susceptible to stem breakage, while the late-successional tabonuco (Dacryodes excelsa) tends to lose crown branches rather than snap at the trunk.10Biotropica. Variation in Susceptibility to Hurricane Damage as a Function of Storm Intensity in Puerto Rican Tree Species Crown loss is generally less lethal than a broken trunk, so tabonuco trees often survive storms that kill their neighbors.
Puerto Rico’s dry forests on the southwest coast tell a different recovery story. In Guánica Forest, trees respond to hurricane defoliation by sprouting aggressively from the trunk and branches. After one major storm, the number of trees with visible sprouts increased more than tenfold, and sprout survival two years later exceeded 86 percent.11Journal of Biogeography. Structural response of Caribbean dry forests to hurricane winds: a case study from Guánica Forest, Puerto Rico Sprouting even occurred on trees that showed no visible wind damage at all, suggesting that defoliation alone triggers the response. This capacity for vegetative regrowth is one reason dry forests can look devastated after a hurricane yet bounce back within a few years.
What More Frequent Hurricanes Would Mean for Forest Carbon
Tropical forests store enormous quantities of carbon in their wood, and hurricanes redistribute that carbon by killing trees and accelerating decomposition. Modeling work based on the Luquillo forest found that at the historical hurricane frequency of about once every sixty years, the forest maintained a steady-state biomass of roughly 81 metric tons of carbon per hectare. If hurricanes struck twice as often (every thirty years), that dropped to about 61 tons. At a frequency of every ten years, biomass plummeted to around 33 tons per hectare.12Treesearch (USFS). Modeling the Effects of Increased Hurricane Frequency on the Tropical Forest Carbon Cycle Net primary productivity, the rate at which the forest grows new biomass, did not change much between scenarios. The forest kept trying to grow at roughly the same pace; it simply never had time to accumulate large trees before the next storm knocked them down.
Separately, island-wide measurements of second-growth forests show that Puerto Rico’s recovering forests accumulate biomass more slowly on average than second-growth forests in other parts of the Neotropics.13Biotropica. Patterns and controls on island‐wide aboveground biomass accumulation in second‐growth forests of Puerto Rico The reasons likely include the relatively recent age of most regrowth stands, the prevalence of introduced species that may not build biomass as efficiently as old-growth natives, and the hurricane cycle itself, which periodically resets the carbon clock.
Mangroves and Blue Carbon on the Coast
Puerto Rico’s mangrove forests occupy a different ecological niche from the upland forests but play an outsized role in carbon storage relative to their area. Mangroves trap sediment and organic material in their waterlogged soils, locking away carbon that might otherwise enter the atmosphere. A comparison of two mangrove sites on the island found that soil carbon stocks in the top half-meter differed substantially depending on hydrology: a semi-enclosed lagoon site with restricted tidal exchange stored about 166 metric tons of carbon per hectare, while a more tidally flushed site stored roughly 115 tons per hectare.14Soil Science Society of America Journal. Blue carbon storage and environmental influences in mangroves, Puerto Rico
The difference comes down to how much organic matter stays put. Lagoons with limited tidal flushing retain more debris and have higher standing biomass, which means more material accumulates in the soil over time. Open-coast mangroves lose more of their organic matter to tidal export. For coastal managers, this means that protecting enclosed or semi-enclosed mangrove systems could yield disproportionate carbon benefits compared with mangrove patches on well-flushed shorelines.
Shade Coffee and Trees as Agricultural Partners
Coffee has been grown in Puerto Rico’s highlands since the eighteenth century, and traditional farms rely on shade trees planted above the coffee shrubs. These agroforestry systems blur the line between farm and forest. Surveys comparing shade and sun coffee plantations found that shade farms harbored significantly more birds, including insect-eating species, as well as higher abundances of insects across most orders studied.15Agriculture, Ecosystems & Environment. Species abundance and potential biological control services in shade vs. sun coffee in Puerto Rico Certain lizard species also partitioned between plantation types: forest-adapted anoles preferred the shade farms, while open-habitat species were more common in sun coffee. Even sun plantations supported considerable biodiversity, though shade farms were richer overall.
Beyond biodiversity, shade trees contribute meaningfully to carbon storage on coffee farms. A study of coffee agroforestry systems found that farms with shade trees had substantially larger total carbon pools, and the shade trees themselves accounted for nearly all the farm-to-farm variation in how much carbon was stored. Other carbon pools, like the coffee plants themselves and the soil organic layer, remained relatively constant regardless of whether shade trees were present.16Agriculture, Ecosystems & Environment. The importance of shade trees in promoting carbon storage in the coffee agroforest systems For a crop that covers millions of hectares across the tropics, the carbon implications of choosing shade over full-sun management are considerable.
Trees and the Urban Heat Island
Puerto Rico is one of the most densely populated islands in the Caribbean, and the San Juan metropolitan area is home to over a million people. Urban trees provide services that go well beyond aesthetics. Research comparing temperature trends at urban, grassland, and forested monitoring sites in northeastern Puerto Rico found that tree canopy cover meaningfully reduced daytime warming. The relationship was strong: vegetation in the upwind direction, especially within about 180 meters of a temperature sensor, was the best predictor of local cooling, explaining roughly 94 percent of the temperature variation.17International Journal of Climatology. The relationship between land cover and the urban heat island in northeastern Puerto Rico
In practical terms, that means a neighborhood’s temperature depends heavily on whether trees stand between it and the prevailing trade winds. Removing a patch of upwind forest can raise local temperatures in a measurable way, while strategic planting in those same zones can provide targeted cooling. Given that Puerto Rico’s climate is warm year-round, and air conditioning loads drive a substantial share of energy demand, urban canopy decisions have both comfort and economic consequences.
Long-Term Monitoring in the Luquillo Mountains
Much of what scientists know about Puerto Rico’s trees comes from two permanent forest plots in the Luquillo Experimental Forest that have been measured continuously for decades, making them among the longest-running tropical forest monitoring sites anywhere. These plots have tracked individual trees through multiple hurricane cycles, droughts, and land-use changes, providing the kind of before-and-after data that is almost impossible to collect after the fact.18Forests. Insights on Forest Structure and Composition from Long-Term Research in the Luquillo Mountains Without that continuous record, researchers would have no baseline against which to judge how a storm like María compared with Hugo, or whether species composition was shifting before any particular disturbance hit.
El Yunque National Forest, which encompasses the Luquillo Mountains, is the only tropical rainforest in the U.S. National Forest System. Its relatively easy accessibility from San Juan and its protected status have made it a magnet for tropical ecology research, with hundreds of studies conducted on its slopes. The resulting dataset is dense enough that ecologists can test ideas about tropical forest dynamics here that would be impossible to examine in more remote or less-studied forests.
Disease Threats on the Horizon
Puerto Rico’s trees face biological threats beyond wind and drought. Myrtle rust, caused by the fungal pathogen Austropuccinia psidii, infects plants in the myrtle family, which includes ecologically and economically important species like guava, allspice, and the many native Eugenia species found across the island. Genetic analysis of the pathogen across the Americas and Hawaii identified a “pandemic biotype” present in Puerto Rico, the same genetic cluster responsible for myrtle rust outbreaks across Central America, the Caribbean, Florida, Hawaii, Australia, and as far as New Caledonia and Indonesia.19Forest Pathology. Genetic diversity of the myrtle rust pathogen (Austropuccinia psidii) in the Americas and Hawaii: Global implications for invasive threat assessments Because this single genotype infects a wide range of host species across many regions, the risk to Puerto Rico’s native myrtles is real and ongoing.
The island’s position as a trade hub between the mainland United States, Central America, and the broader Caribbean also makes it vulnerable to new introductions. Plant material moves through Puerto Rican ports regularly, and the warm, humid climate is hospitable to a wide range of tropical pests and pathogens. Monitoring for new arrivals is a persistent challenge.
Genetic Isolation and Island Biogeography
Island trees face genetic pressures that mainland populations do not. Populations on Puerto Rico can be small and isolated from conspecifics on other islands, which limits gene flow and can reduce genetic diversity over time. A genetic study of Pterocarpus officinalis, a native swamp tree found across the Caribbean, examined populations from Puerto Rico and nearby islands using molecular markers. The results showed high genetic differentiation among populations, with substantial variation in observed heterozygosity from one site to another.20SpringerLink / PubMed Central. Genetic diversity and gene flow in a Caribbean tree Pterocarpus officinalis Jacq.: a study based on chloroplast and nuclear microsatellites High genetic differentiation means that individual island populations have been evolving somewhat independently, which makes each local population irreplaceable from a conservation standpoint. Losing the Puerto Rican population of a Caribbean-wide tree species would not just reduce numbers; it would eliminate a genetically unique lineage.
This matters practically because many conservation decisions treat widespread species as low-priority. If a tree grows on ten islands, the thinking goes, losing it on one is not catastrophic. But when gene flow between islands is low and each population carries its own genetic identity, that assumption breaks down. For Puerto Rico’s native trees, maintaining local populations is not just about keeping a species present on the island. It is about preserving the genetic raw material that might help the species adapt to future conditions across its entire range.