A LEVEL GEO
Arctic Tundra
ARCTIC TUNDRA
occupies some 8 million km^2
northern Canada, Alaska & Siberia
extends from the northern edge of the boreal coniferous forest to the Arctic Ocean
southern limit = 10°C July isotherm
climatic conditions = severe, becomes more extreme with latitude
8 / 9 months - has negative heat balance
average monthly temperatures below freezing
summer = ground is permanently frozen, top metre for thawing
winter = Sun remains below the horizon, temperatures plunge to below -40°C
long hours of daylight in summer - provides compensation for brevity of the growing season
mean annual precipitation is low
plants and animals
few have adapted to this extreme environment
biodiversity is low
apart from a few dwarf species, ecosystem is treeless
southern areas
Low Arctic
conditions are less severe
vegetation provides a continuous ground cover
northern areas
High Arctic
plant cover is discontinuous
extensive areas of bare ground
WATER CYCLE
limited transpiration -> sparseness of vegetation cover & short growing season
low rates of evaporation -> much of the Sun's energy [summer] is expended melting snow
ground temperatures remain low -> inhibit convection
limited groundwater & soil moisture stores
permafrost is a barrier = infiltration, percolation, recharge & groundwater flow
accumulation of snow and river/lake ice -> [winter months]
melting of snow, river & lake ice + uppermost active layer of permafrost [spring / early summer] -> results in sharp increase of river flow
extensive wetlands, ponds and lakes [summer]
temporary store of liquid water -> permafrost impedes drainage
low annual precipitation = 50 - 350 mm
precipitation falling as snow
small stores of moisture - low temperatures -> reduce absolute humidity
CARBON CYCLE
the permafrost is a vast carbon sink
estimated = 1600 GT of carbon
accumulation of carbon is due to low temperatures -> slow decomposition of dead plant material
amount of carbon in soils is 5x greater than in the above-ground biomass
flux of carbon is concentrated in the summer when the active layer thaws
plants grow rapidly in the short summer
long hours of daylight -> allow plants to flower and fruit within a few weeks
net primary productivity (NPP) = <200 grams/m^2/year
tundra biomass is small = between 4 and 29 tonnes/ha -
depending on density of vegetation cover
[growing season] -> plants input carbon-rich litter to the soil
activity of micro-organisms increases -> releasing CO2 to the atmosphere through respiration
CO2 & methane (CH4) emissions are not just confined to the summer
[winter] -> pockets of unfrozen soil and water in the permafrost act as sources of CO2 & CH4
snow cover may insulate microbial organisms -> allow some decomposition despite low temperatures
[in the past] -> permafrost functioned as a carbon sink
[now] -> global warming has raised concerns that it is becoming a carbon source
outputs of carbon has increased -> higher temperatures have simulated plant growth and greater uptake of CO2