What permafrost is
Permafrost is ground, soil, rock or ice, that stays at or below the freezing point of water for at least two years in a row. In many cases it has stayed frozen for thousands of years. It is found across much of the far north, in high mountains and in parts of the Southern Hemisphere, and it underlies a large share of the land in the Arctic and the colder parts of the boreal zone.
Permafrost is not necessarily solid ice. Its ice content varies, and some is rich in ice while some is dry frozen ground. Above it lies the active layer, the top part of the ground that thaws in summer and refreezes in winter. Its depth can range from a few tens of centimeters to a few meters depending on the place, the vegetation and the weather.
The boreal edge
Permafrost is classed as continuous, discontinuous or sporadic depending on how much of the ground it underlies. Across the northern edge of the boreal forest and the treeless tundra beyond, permafrost is widespread. Farther south, it occurs in patches, often on cold north-facing slopes, in peatlands and under shady mossy ground, while neighboring warmer spots are free of it. Trees grow across this boundary zone, which is why the term “forest edge” fits: forest gives way to open, stunted woodland and then to tundra.
The northern limit of forest depends on summer warmth and a short growing season, as explained in the article on the alpine treeline and its northern counterpart. Permafrost is one of several factors that help shape that limit locally, though it is not the only cause.
How trees live above frozen ground
Frozen ground creates difficult conditions for trees. Roots cannot grow into it, so they stay in the shallow active layer and are often wide but shallow. The soil is cold, which slows nutrient cycling and slows root growth. And because frozen ground blocks drainage, the thawed layer above is often waterlogged, leaving little oxygen for roots.
Black spruce is a classic species of these sites in North America, and larch is typical in parts of Siberia. They are slow-growing and often small, and a tree of modest size may be quite old. Their growth rings are narrow, and in some regions they record past climate, as described in the article on tree rings. A thick carpet of moss and organic matter insulates the ground and helps keep the permafrost frozen, which in turn helps sustain these wet, cold conditions.
What thaw could change
As air temperatures rise, permafrost warms and in some places thaws. The results depend strongly on the ice content of the ground, the slope, the vegetation and the soil. Several kinds of change are possible:
- Ground collapse. Where the ground contains a lot of ice, thawing can make the surface sink unevenly, forming pits, hollows and ponds, a process called thermokarst. Trees growing on such ground can lean or fall, producing “drunken forest” with tilted trunks.
- Wetter or drier sites. Where thaw produces ponds and bogs, forest can be replaced by wetland. Where thaw improves drainage, soils may dry out, which could favor different vegetation.
- Deeper active layer. A deeper thawed layer gives roots more room and may allow some trees to grow better, but if the soil is nutrient-poor or dry, the benefit is limited.
- Changing tree cover. Some places may see forest expand northward or upslope, while others lose forest. The outcome varies by site, so predictions for any one place are uncertain.
Carbon
Frozen northern soils hold a large amount of organic matter that built up slowly because cold prevented decay. When the ground thaws, microbes can begin to break this material down, releasing carbon dioxide and, in wet places, methane. How much will be released, how fast, and how much will be offset by new plant growth is not well known and remains an active research question. This is a central concern in discussions of soil carbon storage and the global carbon cycle, since a large release would add to warming. The scale and timing should be treated as uncertain.
Fire and permafrost
Fire is a natural part of the boreal forest, and it interacts with permafrost. A fire burns off the insulating moss and organic layer, exposing the ground to sun and warmth and often deepening the active layer for years afterward. In some places, this can start thaw that continues long after the fire. Researchers are also studying whether more frequent or intense fires will change which trees grow back. Broadleaf species such as aspen and birch sometimes replace spruce after severe burns, a change that affects forest fires, the surrounding climate and local wildlife. The longer story of regrowth after fire is in the article on forest succession.
People and infrastructure
People who live in permafrost regions face practical problems when the ground changes. Roads, buildings, pipelines and airstrips built on ice-rich ground can be damaged when it thaws. Rivers and lakes may change, and traditional hunting, travel and harvesting routes over ice and wet ground can become less reliable. Northern communities, including Indigenous peoples who rely on the forest and tundra, are often among the first to see these changes. The wider theme appears in the article on indigenous forest livelihoods.
What we do and do not know
The broad picture is clear: permafrost is warming in many regions and thawing in some. The details are not. Responses differ from place to place, and local conditions such as ice content, soil type and snow cover can matter as much as regional temperature. Monitoring using boreholes, satellites and field plots helps track the changes. Any statement about the future of a specific forest on permafrost should be treated as a possibility, not a forecast.