Peatlands and northern wetlands

Seen from the air, much of the boreal forest is not forest at all. Bogs, fens, ponds and lakes fill the hollows between ridges and cover vast flat lowlands, and in parts of northern Canada and Alaska wetlands occupy more than half of the land. Peatlands, wetlands where dead plant material accumulates faster than it decays, are the most important of these. They store a disproportionate share of the world's soil carbon, regulate water flows, emit methane, support distinctive plants and animals and are increasingly affected by drainage, roads, mining and permafrost thaw. This lesson explains how peatlands form and work, measures their carbon, and introduces the animals, especially beavers, that engineer northern wetlands. Aquatic food webs of lakes and rivers are left to BIOL-460.

What makes a peatland

Peat forms where the production of plant matter exceeds its decomposition, usually because waterlogging keeps the soil short of oxygen and cold temperatures and acidity slow microbes further. Over thousands of years, peat accumulates at rates of roughly 0.2 to 1 millimetre per year, so a peatland 3 metres deep may have taken 5,000 to 10,000 years to form. Most northern peatlands began forming after the retreat of the ice sheets, between about 12,000 and 8,000 years ago, often by the infilling of shallow lakes or by paludification, the waterlogging and peat growth that spreads across poorly drained forest soils. Canada's wetland classification defines peatlands as having at least 40 centimetres of peat; shallower wetlands include marshes, swamps and shallow open water.

Peatlands fall into two broad types, defined by where their water comes from. Fens receive water that has flowed through or over mineral soil, groundwater or surface runoff, so they are less acidic and richer in nutrients and support sedges, brown mosses, shrubs and often tamarack. Bogs have built up so far above the surrounding water table that they receive water and nutrients only from precipitation; they are acidic, often with a pH below 4.5, and nutrient-poor, and they are dominated by Sphagnum mosses, ericaceous shrubs such as Labrador tea and leatherleaf, and stunted black spruce. Many peatlands change from fen to bog as peat accumulates and the surface rises above the influence of groundwater.

Sphagnum: the peat builder

Sphagnum mosses are often called ecosystem engineers because they create the conditions that favour themselves. Their cell walls contain compounds that resist decay, their large dead hyaline cells hold many times their weight in water, and they acidify their surroundings by exchanging hydrogen ions for nutrient cations. The result is a wet, acidic, nutrient-poor environment in which few vascular plants thrive and decomposition is very slow. Bog surfaces form a microtopography of drier hummocks and wetter hollows occupied by different Sphagnum species. Carnivorous plants such as pitcher plants and sundews, which obtain nitrogen from insects, are characteristic of these nutrient-poor surfaces, and a peat bog's pH, water table and vegetation are tightly linked.

Within peat, ecologists distinguish an upper acrotelm, where the water table fluctuates and oxygen enters at times, so decomposition is relatively fast, and a lower catotelm, permanently waterlogged and anoxic, where decomposition slows to a trickle. Most peat that survives the acrotelm is effectively preserved in the catotelm for millennia, which is why peat cores record past vegetation, climate, fire and even atmospheric lead from the industrial era.

Peatland carbon

Northern peatlands cover only about 3 per cent of the world's land surface but store an estimated 400 to 550 billion tonnes of carbon, often summarized as roughly a quarter to a third of all soil carbon, although estimates vary with mapping methods and depth. Canada holds the largest area of peatland of any country after Russia, much of it in the Hudson Bay Lowlands of Ontario and Manitoba, which form one of the largest continuous peatland complexes in the world, along with large peatland regions in the Mackenzie basin and northern Alberta, Saskatchewan and Quebec. Alaska and the western Great Lakes states also have extensive peatlands, including the large patterned peatlands of northern Minnesota around Red Lake.

# Peat carbon stock and long-term accumulation rate from a synthetic core (Sable River fen-to-bog sequence).
import numpy as np
rng = np.random.default_rng(55)
depth_top = np.arange(0, 300, 10) / 100              # 10 cm slices to 3 m, depths in m
bd = np.clip(0.06 + 0.035 * depth_top + rng.normal(0, 0.008, depth_top.size), 0.04, None)   # g/cm3
c_frac = np.clip(0.47 + rng.normal(0, 0.015, depth_top.size), 0.4, 0.55)
stock = (0.10 * bd * 1000 * c_frac).sum()            # kg C per m2 (thickness 0.10 m, bd in kg/m3)
basal_age = 8200                                     # calibrated radiocarbon age at the base, years
print(f"total carbon stock: {stock:.0f} kg C per m2 ({stock * 10:.0f} t C per ha)")
print(f"mean long-term carbon accumulation: {stock * 1000 / basal_age:.0f} g C per m2 per year")
print(f"mean vertical peat accumulation: {3000 / basal_age:.2f} mm per year")
area_km2 = 120                                        # peatland area in the watershed
print(f"carbon in {area_km2} km2 of similar peatland: {stock * area_km2 * 1e6 / 1e9:.1f} million tonnes")

total carbon stock: 158 kg C per m2 (1581 t C per ha)
mean long-term carbon accumulation: 19 g C per m2 per year
mean vertical peat accumulation: 0.37 mm per year
carbon in 120 km2 of similar peatland: 19.0 million tonnes

A long-term average carbon accumulation rate of around 15 to 30 grams per square metre per year is typical of northern peatlands. That is small per year, but over millennia and millions of square kilometres it built one of the planet's largest carbon reservoirs. The present-day carbon balance of a peatland, measured with flux towers and chambers, varies from year to year: wet, cool years tend to favour accumulation, while drought, fire and drainage can turn peatlands into carbon sources as the acrotelm dries and oxygen reaches old peat.

Methane

Peatlands are also the largest natural source of methane in the northern latitudes. In waterlogged, anoxic peat, methanogenic archaea produce methane as the final step of decomposition, and it escapes by diffusion, by ebullition as bubbles, or through the aerenchyma, the air channels, of sedges and other wetland plants. Methane is a much more powerful greenhouse gas than carbon dioxide over short timescales, about 80 times more powerful over 20 years and about 28 to 30 times over 100 years. Fens with high water tables and sedges usually emit more methane than drier bogs. Emissions rise steeply with temperature, a relationship described by the Q10 coefficient, the factor by which a rate increases for a 10 °C rise.

# Temperature dependence of methane flux with a Q10 model, and the effect of a warmer summer (synthetic).
import numpy as np
F_ref, T_ref = 60.0, 10.0                            # mg CH4 per m2 per day at 10 C peat temperature
Q10 = 3.0                                            # typical of wetland methane production
days = np.arange(150)                                # May to September
T_now = 4 + 10 * np.sin(np.pi * days / 150)          # peat temperature at 10 cm, degrees C
flux_now = F_ref * Q10 ** ((T_now - T_ref) / 10)
flux_warm = F_ref * Q10 ** ((T_now + 2 - T_ref) / 10)
print(f"season total now: {flux_now.sum() / 1000:.1f} g CH4 per m2")
print(f"season total if peat is 2 C warmer: {flux_warm.sum() / 1000:.1f} g CH4 per m2 (+{100 * (flux_warm.sum() / flux_now.sum() - 1):.0f}%)")
print(f"CO2-equivalent over 100 years (GWP 28): {flux_warm.sum() * 28 / 1000:.0f} g CO2e per m2")

season total now: 9.9 g CH4 per m2
season total if peat is 2 C warmer: 12.3 g CH4 per m2 (+25%)
CO2-equivalent over 100 years (GWP 28): 345 g CO2e per m2

The model ignores the water table, which matters as much as temperature: a warmer but drier summer can lower methane emissions while raising carbon dioxide losses. Real projections couple temperature, water table, vegetation and permafrost. Still, the calculation explains why scientists watch northern wetlands closely in a warming climate.

Peatlands and permafrost

In the discontinuous permafrost zone, permafrost survives mainly beneath peat plateaus and palsas, raised peat mounds whose insulating peat keeps the ground frozen. As the climate warms or fires remove the insulating moss, these frozen cores thaw and collapse into wet, sedge-dominated collapse scar bogs and fens. Studies in the southern Northwest Territories, northern Alberta and interior Alaska have documented the loss of a large share of peat plateau area since the mid-twentieth century. Thaw changes hydrology, tree cover, carbon and methane fluxes: thawed sites often emit more methane, and newly flooded margins can release old carbon, while new Sphagnum growth slowly accumulates peat again.

Beavers, engineers of northern wetlands

The North American beaver (Castor canadensis) is the most important animal engineer of boreal wetlands. By damming streams, beavers flood forests, create ponds and meadows, raise water tables, trap sediment and increase wetland area and habitat diversity for waterfowl, amphibians, fish and moose. Beaver ponds store water during droughts and can act as firebreaks; studies have shown green, wet corridors of beaver-modified streams persisting through severe fires. Beavers were trapped to near extinction in much of their range during the fur trade, which shaped the economy and exploration of northern North America, and their recovery over the twentieth century has transformed many landscapes.

Beavers are now expanding north onto the Arctic tundra of Alaska and the western Canadian Arctic, where satellite imagery shows thousands of new ponds since about 2000, apparently enabled by longer ice-free seasons and expanding shrubs that provide food and building material. Their dams flood tundra, thaw permafrost beneath ponds and alter streams used by fish, with consequences that researchers and northern communities are only beginning to understand.

Threats and protection

Canadian peatlands face drainage and excavation for horticultural peat, mostly in Quebec, New Brunswick, Alberta and Manitoba; disturbance by oil sands mining, roads, seismic lines and pipelines in Alberta; hydroelectric flooding in Quebec, Manitoba and Labrador; and mining interest in the Hudson Bay Lowlands. In Ontario's Ring of Fire, a chromite and nickel deposit beneath the Lowlands, road and mine proposals have been debated for more than a decade. In June 2025 Ontario passed Bill 5, the Protect Ontario by Unleashing our Economy Act, which allows cabinet to designate special economic zones exempt from some provincial rules and replaced the province's Endangered Species Act with a narrower Species Conservation Act; the province has said the Ring of Fire would become such a zone only after consultation, and several Treaty 9 First Nations have opposed the law or vowed legal challenges. Peatland scientists warn that disturbing the Lowlands could release large amounts of carbon.