alaskan tundra Flashcards
location
occupied 8 million km2 in northern Canada, Alaska, Siberia
- extends from northern edge of boreal coniferous forest to the Arctic Ocean + its southern limit approximates 10•C July isotherm (climatic limit of tree line)
- Climatic conditions in Alaskan tundra are severe with mean temperatures below -15•C
what type of heat balance does the tundra have
for 8 or 9 months a year, has negative heat balance with average monthly temperatures below freezing
- as result, ground permanently frozen with only top metre so thawing during Arctic summer
what underlies much of Alaskan tundra + is important feature
permafrost
In winter, when for several weeks Sun remains below horizon, temperatures can plunge below
-40•C
‘…’ in summer provide some compensation for shortness of growing season.
long hours of daylight
- there is also low mean annual precipitation
few plants + animals have adapted to this extreme environment
- low biodiversity
- ecosystem is treeless (apart from few dwarf species)
- in southern areas, low Arctic - conditions less severe + vegetation provides continuous ground cover
- further north in High Arctic, plant cover is discontinuous with extensive areas of bare ground
main features of water cycle in Alaskan Tundra
- low annual precipitation (less then 100mm in most places) with most precipitation falling as snow
- small stores of moisture in atmosphere due to low temperatures which reduce absolute humidity
- limited transpiration as sparseness of vegetation cover + short growing season of only abt 3 months
- low rates of evaporation (much of Sun’s energy in summer is expended melting snow so that ground temperatures remain low + inhibit convection
- also, surface + soil water are frozen for most of year
- limited groundwater + soil moisture stores (permafrost is barrier to infiltration, percolation, recharge + groundwater flow)
- accumulation of snow + river/lake ice during winter months (melting of snow, river + lake ice + uppermost active layer of permafrost in spring + early summer, results in sharp increase in river flow)
- extensive wetlands, ponds + lakes on tundra during summer (this temporary store of liquid water is due to permafrost which impedes drainage)
How many lakes does Alaska have
3 million
- extensive wetlands lie in valleys (e.g Yukon River), in deltas + along coast (especially of Bering Sea)
carbon cycle in tundra
permafrost is vast carbon sink
- accumulation of carbon due to low temperatures which slow decomposition of dead plant material
- overall amount of carbon in Alaskan tundra soils is 5x greater than in above-ground biomass
- flux of carbon is concentrated in summer months when active layer thaws
plants in carbon cycle
grow rapidly in short summer (e.g sedges, crow berry + moss)
- long hours of daylight allow them to flower + fruit within few weeks
- Alaskan tundra biomass is small, ranging between 4 + 29 tonnes/ha depending on density of vegetation cover
- during growing season, tundra plants input carbon-rich litter to soil
- activity of microorganisms increases, releasing CO2 to atmosphere through respiration
- but CO2 + CH4 emissions are not just confined to summer
- even in winter, pockets of unfrozen soil + water in permafrost act as sources of CO2 + CH4
- snow cover may insulate microbial organisms + allow some decomposition despite low temperatures
-in past, permafrost functioned as carbon sink, but today…
global warming has raised concerns in Alaska that is becoming carbon source
- at moment, evidence is unclear
- while outputs of carbon from permafrost have increased in recent decades, higher temperatures stimulated plant growth in tundra + greater uptake of CO2
- this in turn has increased amount of plant litter entering store
- so it’s possible that despite warming Alaskan climate, carbon budget in tundra today remains close to balance
physical factors, seasonal changes + stores + flows of water
flows + stores of water in Alaskan tundra are influenced by temperature , relief , rock permeability
- average temperatures are below freezing for most of year so water stored as ground ice in permafrost layer
- during short summer, shallow active layer (top metre)
thaws + liquid water flows on surface
- meltwater forms millions of pools + shallow lakes which scatter tundra landscape
- poor drainage: water cannot infiltrate soil bc of permafrost at depth
- in winter, sub-zero temperatures prevent evapotranspiration
- in summer, some evapotranspiration occurs from standing water, saturated soils + vegetation
- humidity is low all year round +precipitation sparse
- low permeability due to permafrost + Precambrian igneous + metamorphic rocks which dominate geology of tundra in Alaska
- ancient rock surface which underlies tundra has been reduced to gently undulating plain by hundreds of millions of years of erosion + weathering
- minimal reliefs + chaotic glacial deposits impede drainage + contribute to waterlogging during summer months
physical factors, seasonal changes + stores + flows of carbon
- carbon mainly stored as partly decomposed plant remains frozen in permafrost
- most of this carbon has been locked away for at least past 500,000 years
- low temperatures, unavailability of liquid water for most of year + parent rocks containing few nutrients, limit plant growth
- thus total carbon store of biomass is relatively small
- averaged over year, photosynthesis + NPP are low, with growing season lasting barely 3 months
- but there is some compensation for short growing season in long hours of daylight in summer
- low temperatures + waterlogging slow decomposition + respiration + flow of CO2 to atmosphere
- owing to impermeabiliry of permafrost, rock permeability, porosity + mineral composition of rocks exert little influence on cycles
oil + gas production + carbon + water cycles in Alaska
North Slope of Alaska, between Brooks Range in South + Arctic Ocean in North, is vast wilderness of Arctic tundra
oil + gas were discovered here at Prudhoe Bay in
1968
