permafrost ecosystems (terrestrial lecture 10)

Question 1

What is permafrost? Where is it? Carbon storage?

Answer 1

• permafrost is permanently frozen ground
• mostly high latitude ecosystems, some alpine
• 24% land in northern hemisphere
• stores ~1300Gt C as peat and methane
• 2+x as much as atmosphere

Question 2

What is the active layer? What is happening to it?

Answer 2

• layer that thaws each summer, refreezing in winter
• global warming thawing permafrost/thickening active layer
• exposes C prev. locked in permafrost to decay
• positive feedback to climate

Question 3

What are the ecosystem controls on active layer thickness (ATL)?

Fisher et al. 2016

Answer 3

Evergreen trees (vs deciduous) = ATL

• winter canopy snow trapping - stops insulative snow layer thereby allowing heat loss from soil
• moss promotion

Moss cover (winter vs summer heat transfer) = thinner ATL

• dry moss in summer minimises heat conduction into soil
• wet/frozen moss promotes head conduction out of soil

Shrub / understorey cover = thinner ATL
- shields irradiance

Organic (peaty) soils
- thicker organic layer reduces ATL

Dry soil
- dry surface soil = reduces ATL

Question 4

Effect of fire on ATL

Answer 4

basically wipes out all ecosystem controls reducing ATL

• destroys canopy, understory foliage, moss
• thins organic layer
• increases surface moisture

Question 5

How do ecosystem controls on ATL interact?

• organic layer thickness & slope
• soil moisture and tree canopy LAI

Answer 5

• organic layer thickness more important on shallower slopes
• good drainage on steeper slopes = organic layer insulation less important
• tree canopy LAI less important on dryer soil
• shading of irradiance less important w insulative dry soils

Question 6

What is thermokast?

What is the effect of recentness of permafrost thaw on methane flux?
How does permafrost thaw affect methane flux compared to non thawed peatland?

How is this measured? How much of methane released from wetland is old carbon?

Answer 6

Thermokast: where thawing of ice-rich permafrost causes land subsidence

No significant difference in methane flux between locations in wetland (how recently collapsed)

Fluxes are substantial compared w non-thawed peatland which released negligible methane

• traps collect methane from different depths, then it’s radiocarbon dated
• deep probe tubes low 14C: old C is decomposing at depth to produce methane
• 14C enriched surface fluxes (have bomb-carbon) shows methane actually released is dominated by modern C
• 8% of CH4 released from wetland comes from the old carbon (sylvic peat)