Boreal trees, soils and permafrost

Only a handful of tree species dominate the North American boreal forest, yet each has a distinct way of surviving cold, fire, poor soils and frozen ground. This lesson introduces those trees and the shrubs, mosses and lichens beneath them, then turns to the ground itself: the acidic podzols of well-drained uplands, the waterlogged gleysols and organic soils of lowlands, and permafrost, ground that stays frozen for two or more years and that underlies much of the North. Understanding these foundations explains much of what follows, from fire behavior to peatland carbon and the effects of thaw.

Few species, distinct strategies

Black spruce (Picea mariana) is the signature tree of the North American boreal forest. It grows on cold, wet, nutrient-poor sites, including peatlands and permafrost, tolerates shallow rooting and can reproduce by layering, rooting where lower branches touch moss. Its cones are semi-serotinous: many stay closed on the tree for years and open in the heat of fire, releasing a pulse of seed onto the freshly burned ground. White spruce (Picea glauca) prefers better-drained, richer sites such as river terraces and mixedwood uplands, grows larger and longer-lived and relies on mast seed years rather than fire to regenerate. Jack pine (Pinus banksiana) dominates dry, sandy and rocky sites; its cones are strongly serotinous, sealed with resin that melts near 50 °C, so stands are typically even-aged cohorts that established after a fire.

Balsam fir (Abies balsamea) is shade tolerant and abundant in the east, where long fire-free intervals let it build up in the understorey; it is the preferred host of the spruce budworm discussed in lesson 3. Tamarack (Larix laricina), a deciduous conifer, grows in fens and wet sites and drops its needles each autumn. Trembling aspen (Populus tremuloides) is the most widespread tree in North America; after fire or logging it resprouts prolifically from root suckers, so a single genetic individual, a clone, can cover hectares. Paper birch (Betula papyrifera) and balsam poplar also colonize disturbed sites quickly. Broadleaf trees hold more water in their foliage and burn less readily than conifers, a property that matters for fire behavior and management.

Beneath the trees, the boreal understorey is made of ericaceous shrubs such as Labrador tea, blueberries and bog cranberry, willows and alders on wetter sites, feather mosses such as Pleurozium and Hylocomium on upland forest floors, Sphagnum mosses in peatlands, and reindeer lichens (Cladonia and related genera) in open, dry woodlands. Mosses and lichens are not a minor component: in black spruce forests the moss layer insulates the soil, keeps it cold and wet, slows decomposition, stores much of the ecosystem's nitrogen and, through cyanobacteria living on feather mosses, fixes nitrogen from the air. Remove the moss layer, as a severe fire does, and the soil warms and permafrost can thaw.

Boreal soils

Soil formation in the boreal forest is slow because cold limits weathering and decomposition. On well-drained, sandy or loamy uplands under conifers, acidic litter and leaching produce podzols (Podzolic soils in the Canadian classification, Spodosols in the United States system), with a bleached gray eluvial layer beneath the litter and a rust-colored layer below where iron, aluminum and organic matter accumulate. Under aspen and on finer, base-rich parent materials, Luvisolic soils (Alfisols) form, with clay moved downward. In poorly drained lowlands, waterlogging creates gleyed mineral soils, with gray and mottled colors from reduced iron, and organic soils (Organic order; Histosols) where peat at least 40 centimeters deep accumulates. Where permafrost lies within a meter or two of the surface, soils are classified as Cryosols (Gelisols), often churned by freezing and thawing into irregular, mixed layers, a process called cryoturbation.

The thick forest floor, called the organic or LFH layer, is one of the defining features of boreal soils. It accumulates because decomposition is slow: cold, wet, acidic conditions and low-quality litter of conifer needles and mosses limit microbial activity. Nutrients, especially nitrogen, are locked in this organic material, and boreal trees depend heavily on mycorrhizal fungi to obtain them, as BIOL-340 described. This tight nutrient economy explains why boreal forests grow slowly and why disturbance that releases nutrients, such as fire, can boost growth for a time.

Permafrost

Permafrost is ground, soil or rock, that remains at or below 0 °C for at least two consecutive years. It underlies roughly a quarter of the exposed land of the Northern Hemisphere, about half of Canada and about 80 percent of Alaska, according to commonly cited estimates. Above it lies the active layer, which thaws each summer and refreezes each winter; it may be less than 50 centimeters thick under insulating peat in the far north and several meters thick near the southern limit. Permafrost zones are mapped by the share of the landscape underlain by permafrost: continuous (more than 90 percent), discontinuous (50 to 90 percent), sporadic (10 to 50 percent) and isolated patches (less than 10 percent).

Unfrozen zones within or beneath permafrost are called taliks, and they form under lakes and rivers that do not freeze to the bottom. Ground ice is the key to permafrost's sensitivity. Ice-rich permafrost contains lenses, veins and wedges of nearly pure ice, sometimes more ice than soil by volume. When it thaws, the ground subsides, forming thermokarst: collapse scars in peat plateaus, slumping hillsides, thaw lakes and hummocky terrain. Permafrost temperature has risen across the Arctic over recent decades, often by more than 1 °C at monitored boreholes where the permafrost is coldest, and in the discontinuous zone it is close to 0 °C, poised to thaw.

In Alaska, permafrost underlies nearly all of the North Slope and much of interior and western Alaska, from the continuous permafrost of the Arctic coastal plain to the discontinuous permafrost around Fairbanks, where north-facing slopes and valley bottoms are frozen while south slopes are not. The Trans-Alaska Pipeline, completed in 1977, carries warm oil above ground on vertical supports with heat pipes in permafrost sections so that it does not thaw the ground beneath it, and roads and buildings across interior Alaska show the heaving and sinking that thaw causes.

The thickness of the active layer depends on how much heat reaches the ground in summer and how well the surface insulates. Peat, moss and snow are powerful insulators: a thick moss layer keeps soils cold in summer, whereas deep snow keeps them warm in winter by blocking the cold air. A simple and widely used model, the Stefan equation, predicts the depth of seasonal thaw from the accumulated thawing degree-days and the thermal properties of the soil.

Here \(Z\) is thaw depth in meters, \(k\) the thermal conductivity of the thawed soil in watts per meter per kelvin, TDD the thawing degree-days in degree-days (multiplied by 86,400 seconds per day), \(L\) the latent heat of fusion of water (334,000 joules per kilogram), \(\rho\) the density of water and \(w\) the volumetric water content. In practice an empirical edaphic factor replaces the constants, and depth scales with the square root of thawing degree-days, so doubling summer warmth deepens thaw by about 40 percent, not 100 percent.

# Active layer depth from thawing degree-days with the Stefan equation (synthetic sites).
import numpy as np
L_f, rho_w = 334_000.0, 1000.0                     # J/kg, kg/m3
sites = {                                          # thermal conductivity (W/m/K), volumetric water content
    'mineral soil, thin moss':   (1.2, 0.30),
    'peat plateau, thick moss':  (0.45, 0.55),
    'burned peat, moss removed': (0.6, 0.45),
}
for tdd in (900, 1200, 1500):
    print(f"thawing degree-days = {tdd}")
    for name, (k, w) in sites.items():
        z = np.sqrt(2 * k * tdd * 86400 / (L_f * rho_w * w))
        print(f"   {name:<28} thaw depth {z:4.2f} m")
z1 = np.sqrt(2 * 0.45 * 900 * 86400 / (L_f * 1000 * 0.55)); z2 = np.sqrt(2 * 0.45 * 1800 * 86400 / (L_f * 1000 * 0.55))
print(f"doubling degree-days from 900 to 1800 deepens peat thaw by {100 * (z2 / z1 - 1):.0f} per cent")

thawing degree-days = 900
   mineral soil, thin moss      thaw depth 1.36 m
   peat plateau, thick moss     thaw depth 0.62 m
   burned peat, moss removed    thaw depth 0.79 m
thawing degree-days = 1200
   mineral soil, thin moss      thaw depth 1.58 m
   peat plateau, thick moss     thaw depth 0.71 m
   burned peat, moss removed    thaw depth 0.91 m
thawing degree-days = 1500
   mineral soil, thin moss      thaw depth 1.76 m
   peat plateau, thick moss     thaw depth 0.80 m
   burned peat, moss removed    thaw depth 1.02 m
doubling degree-days from 900 to 1800 deepens peat thaw by 41 percent

The output shows why peat plateaus can hold permafrost far south of where mineral soils would: low conductivity and high water content slow heat flow, so thaw stays shallow. It also shows the danger of losing that insulation. Burned peat with its moss gone thaws markedly deeper under the same summer, and once the active layer no longer refreezes completely, a talik forms and thaw accelerates.

Soil carbon and permafrost carbon

Because decomposition is slow and permafrost freezes organic matter in place, northern soils hold enormous stocks of carbon. The northern circumpolar permafrost region is estimated to contain roughly 1,400 to 1,600 billion metric tons of organic carbon, about twice the carbon now in the atmosphere, much of it in the top three meters and in deep, ice-rich deposits called yedoma in Siberia and Alaska. A soil carbon stock is calculated by multiplying, layer by layer, the layer thickness by its bulk density and its carbon fraction, then summing.

By comparison, a mature black spruce stand might hold 30 to 60 metric tons of carbon per hectare in its trees. Ecosystem carbon in the boreal forest is therefore dominated by what is underground, and processes that expose or thaw that carbon, deep-burning fires, drainage and permafrost thaw, matter more for the carbon balance than changes in tree cover alone. Lesson 11 develops the permafrost carbon feedback that results.