Forest fragmentation heats the planet more than we thought

Forests have long been celebrated as the planet’s air conditioners, drawing carbon dioxide from the atmosphere and pumping moisture into the air. But a new analysis published in Nature Plants suggests that climate planners have been asking only half of the right question. What matters for the cooling power of forests, the study finds, is not merely how much forest remains on Earth, but how that forest is arranged across the landscape. When intact woodland is carved into small, isolated patches, the remaining trees lose much of their capacity to keep the land surface cool, with consequences that ripple far beyond the forest edge.

The research team, led by Yujing Yan, set out to quantify this effect at a scale that had never been attempted before. They generated a global forest map for the year 2020 at a resolution of just 30 metres, fine enough to distinguish individual patches of woodland from the fields, roads and settlements that surround them. From this map they calculated a measure of fragmentation for every forested region on Earth, capturing how thoroughly continuous forest had been broken apart. They then combined this fragmentation metric with environmental variables averaged across the period from 2018 to 2022, allowing them to ask a deceptively simple question: holding everything else constant, does a more fragmented forest run hotter or colder than an intact one?

The answer, across most of the world’s forested land, was hotter. After statistically accounting for the amount of forest cover and for background environmental conditions such as climate and terrain, the researchers found that greater fragmentation was associated with higher land surface temperature in the majority of forest regions. The signal was particularly pronounced in tropical, subtropical and temperate forests, the biomes where most of humanity’s food is grown and where hundreds of millions of people live in close proximity to shrinking woodland. In these regions, the difference between a connected forest and a shredded one translates into measurable differences in the temperature of the ground itself.

The physical mechanisms behind this pattern are well understood in forest ecology, even if their global magnitude has been hard to pin down. Intact forest interiors benefit from a dense, multi-layered canopy that intercepts sunlight before it reaches the ground, and from high rates of evapotranspiration, the process by which trees draw water from the soil and release it as vapour through their leaves, carrying heat away with it. When forest is fragmented, the proportion of edge habitat increases dramatically. Edges are warmer, drier and windier than interiors, and microclimatic changes can penetrate hundreds of metres into a patch. Small patches are essentially all edge, with no protected core left to buffer the interior. Fragmented landscapes also tend to lose the moist air recycling that large, continuous canopies sustain, so the regional evaporative cooling that makes forests such powerful climate regulators weakens.

Yet the study also uncovered a striking exception that reveals how context-dependent forest climate effects can be. In boreal forests, the vast belt of coniferous woodland that circles the Northern Hemisphere across Canada, Scandinavia and Siberia, the relationship reversed. Where mean annual air temperatures fall below roughly zero degrees Celsius, greater fragmentation was associated with cooler land surfaces rather than warmer ones. The explanation lies in the physics of snow and sunlight. In winter, snow-covered ground reflects a large fraction of incoming solar radiation back to space, a property known as high surface albedo. Dense evergreen canopies mask this bright snow from view, so intact boreal forest actually absorbs more sunlight than open land. When the forest is broken up, more snow-covered surfaces are exposed, albedo rises, and the land surface cools. In the far north, this albedo effect can outweigh the loss of evaporative cooling, flipping the sign of the fragmentation effect entirely.

Perhaps the most consequential finding of the analysis concerns the relative importance of fragmentation compared with the variable that has dominated policy discussions for decades: forest area. The researchers found that in 48 per cent of the world’s forested areas, fragmentation had a stronger influence on land surface temperature than forest cover itself. In those regions, its effect was on average 3.23 times greater. In other words, in nearly half of the planet’s forests, the shape and connectivity of the remaining woodland matters more for surface temperature than the sheer quantity of trees. This is a sobering result for a global community that has largely measured forest climate value in hectares.

The implications reach directly into the design of restoration and offset programmes. Tree-planting initiatives have proliferated worldwide, often measured and rewarded by the number of saplings planted or the total area reforested. But the new findings suggest that restoration which increases forest area while producing small, isolated patches may deliver far less cooling than expected. A scattering of woodlots across an agricultural landscape, however valuable for other reasons, does not reproduce the thermal behaviour of a contiguous canopy. If the goal is to retain the climate-regulating services of forests, then protecting large, intact landscapes and preventing further fragmentation should rank alongside, and in many places ahead of, simply adding trees.

This reframing is especially urgent in the tropics, where forest loss and fragmentation are tightly intertwined. Tropical deforestation rarely removes forest uniformly; instead, it nibbles away at the margins of intact blocks, leaving behind increasingly isolated fragments embedded in pasture and cropland. Because the study found the strongest warming effect of fragmentation in tropical and subtropical forests, and because these biomes store the largest carbon pools and drive continental moisture recycling, the cost of ignoring configuration is highest precisely where forests matter most for the global climate system. The researchers argue that forest configuration therefore warrants far greater attention alongside forest area in climate mitigation and conservation planning.

The methodological advance underlying these conclusions is itself noteworthy. Previous studies of fragmentation and microclimate were typically local, comparing a handful of sites with field instruments. By working at 30-metre resolution across the entire globe and combining the fragmentation map with multi-year environmental averages, the team produced an analysis that is both globally comprehensive and locally meaningful. Land surface temperature, measured from satellites, is not identical to the air temperature people experience, but it is a sensitive indicator of the surface energy balance and responds strongly to changes in canopy cover, albedo and evapotranspiration. By controlling for forest cover and background conditions, the analysis isolates the effect of spatial arrangement from the effect of quantity, which is the crux of the new insight.

For policymakers, the message is a call to widen the dashboard. Carbon accounting, biodiversity targets and restoration pledges all currently reward area, while connectivity is treated as a secondary concern, usually in the context of wildlife corridors. This study adds a thermal argument that connectivity is a first-order climate variable. Keeping forests connected, in the phrase that gives the research its title, is not a poetic flourish but a quantifiable climate strategy: in nearly half the world’s forests, the arrangement of the trees does more to determine surface temperature than their number. As the planet warms, the difference between a connected forest and a fragmented one may be one of the cheapest forms of climate adaptation available, provided it is protected before it is carved apart.

Subject of Research: The effect of forest fragmentation on global land surface temperature and forest climate regulation

Article Title: Keeping forests connected

Article References: Keeping forests connected. (n.d.). https://doi.org/10.1038/s41477-026-02452-z

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02452-z

Keywords: forest fragmentation, land surface temperature, climate mitigation, forest restoration, boreal forests, tropical forests, albedo, evapotranspiration, forest cover, conservation planning, remote sensing, Nature Plants