Chapter 7: Arctic Tundra and Polar Desert Ecosystems

110 Pages·2005·3.61 MB·English

Chapter 7
Arctic Tundra and Polar Desert Ecosystems
Lead Author
Terry V.Callaghan
Contributing Authors
Lars Olof Björn,F.Stuart Chapin III,Yuri Chernov,Torben R.Christensen,Brian Huntley,Rolf Ims,Margareta Johansson,
Dyanna Jolly Riedlinger,Sven Jonasson,Nadya Matveyeva,Walter Oechel,Nicolai Panikov,Gus Shaver
Consulting Authors
Josef Elster,Heikki Henttonen,Ingibjörg S.Jónsdóttir,Kari Laine,Sibyll Schaphoff,Stephen Sitch,Erja Taulavuori,Kari Taulavuori,Christoph Zöckler
Contents
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244 7.4.Effects of changes in climate and UV radiation levels on
7.1.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244 structure and function of arctic ecosystems in the short
7.1.1.Characteristics of arctic tundra and polar desert ecosystems . .244 and long term . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .292
7.1.2.Raison d’être for the chapter . . . . . . . . . . . . . . . . . . . . . . . . . . .247 7.4.1.Ecosystem structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .292
7.1.3.Rationale for the structure of the chapter . . . . . . . . . . . . . . . . .248 7.4.1.1.Local and latitudinal variation . . . . . . . . . . . . . . . . . . . . .292
7.1.4.Approaches used for the assessment:strengths,limitations, 7.4.1.2.Response to experimental manipulations . . . . . . . . . . . .295
and uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .248 7.4.1.3.Recent decadal changes within permanent plots . . . . . .298
7.2.Late-Quaternary changes in arctic terrestrial ecosystems, 7.4.1.4.Trophic interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .298
climate,and ultraviolet radiation levels . . . . . . . . . . . . . . . . .249 7.4.1.5.Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .303
7.2.1.Environmental history .................................249 7.4.2.Ecosystem function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .305
7.2.2.History of arctic biota . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .250 7.4.2.1.Biogeochemical cycling:dynamics of carbon and
7.2.3.Ecological history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .252 nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .305
7.2.4.Human history related to ecosystems . . . . . . . . . . . . . . . . . . . .252 7.4.2.2.Soil processes and controls over trace-gas exchanges . . .311
7.2.5.Futurechange in the context of late-Quaternarychanges . . . . .253 7.4.2.3.Water and energy balance . . . . . . . . . . . . . . . . . . . . . . . .314
7.2.6.Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .254 7.4.2.4.Summary ......................................314
7.3.Species responses to changes in climate and ultraviolet-B 7.5.Effects of climate change on landscape and regional
radiation in the Arctic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .254 processes and feedbacks to the climate system . . . . . . . . . .315
7.3.1.Implications of current species distributions for future biotic 7.5.1.Impacts of recent and current climate on carbon flux . . . . . . . .315
change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .255 7.5.1.1.Recent changes in carbon dioxide fluxes . . . . . . . . . . . .316
7.3.1.1.Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .255 7.5.1.2.Current circumpolar methane fluxes . . . . . . . . . . . . . . . .317
7.3.1.2.Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .259 7.5.1.3.Relative contributions of methane and carbon dioxide
7.3.1.3.Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .260 to the carbon budget . . . . . . . . . . . . . . . . . . . . . . . . . . . .317
7.3.1.4.Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .262 7.5.2.Current circumpolar water and energy balances . . . . . . . . . . . .318
7.3.2.General characteristics of arctic species and their adaptations 7.5.3.Large-scale processes affecting future balances of carbon,
in the context of changes in climate and ultraviolet-B radiation water,and energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .319
levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .263 7.5.3.1.Permafrost degradation . . . . . . . . . . . . . . . . . . . . . . . . . .319
7.3.2.1.Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .263 7.5.3.2.Changes in circumpolar vegetation zones . . . . . . . . . . . .319
7.3.2.2.Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .264 7.5.4.Projections of future balances of carbon,water,and energy
7.3.2.3.Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .269 exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .321
7.3.3.Phenotypic responses of arctic species to changes in climate 7.5.4.1.Carbon balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .322
and ultraviolet-B radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271 7.5.4.2.Energy and water exchange . . . . . . . . . . . . . . . . . . . . . . .324
7.3.3.1.Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271 7.5.5.Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .324
7.3.3.2.Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .279 7.6.Synthesis:Scenarios of projected changes in the four ACIA
7.3.3.3.Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .282 regions for 2020,2050,and 2080 . . . . . . . . . . . . . . . . . . . . . .327
7.3.4.Genetic responses of species to changes in climate and 7.6.1.Environmental characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . .327
ultraviolet-B radiation levels . . . . . . . . . . . . . . . . . . . . . . . . . . . .284 7.6.2.Vegetation zones and carbon balance . . . . . . . . . . . . . . . . . . . . .327
7.3.4.1.Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .284 7.6.3.Biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .328
7.3.4.2.Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .285 7.7.Uncertainties and recommendations . . . . . . . . . . . . . . . . . . .329
7.3.4.3.Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .286 7.7.1.Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .329
7.3.4.4.Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .286 7.7.1.1.Uncertainties due to methodologies and conceptual
7.3.5.Recent and projected changes in species distributions and frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .329
potential ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .287 7.7.1.2.Uncertainties due to surprises . . . . . . . . . . . . . . . . . . . .331
7.3.5.1.Recent changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .287 7.7.1.3.Model-related uncertainties . . . . . . . . . . . . . . . . . . . . . .331
7.3.5.2.Projected futurechanges in species distributions . . . . . .287 7.7.2.Recommendations to reduce uncertainties . . . . . . . . . . . . . . . . .332
7.3.5.3.Summary ......................................291 7.7.2.1.Thematic recommendations and justification . . . . . . . . .332
7.7.2.2.Recommendations for future approaches to research
and monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .333
7.7.2.3.Funding requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . .334
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .335
Personal communications and unpublished data . . . . . . . . . . . . .335
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .335
244 Arctic Climate Impact Assessment
Summary environmental,geographic,or political biases.This
chapter focuses on biota (plants,animals,and micro-
The dominant response of current arctic species to cli- organisms) and processes in the region north of the
mate change,as in the past,is very likely to be relocation northern limit of the closed forest (the taiga),but also
rather than adaptation.Relocation possibilities vary includes processes occurring south of this boundary that
according to region and geographic barriers.Some affect arctic ecosystems.Examples include animals that
changes are occurring now. migrate south for the winter and the regulation of the
latitudinal treeline.The geographic area defined in this
Some groups such as mosses,lichens,and some herbivores chapter as the present-day Arctic is the area used for
and their predators are at risk in some areas,but produc- developing scenarios of future impacts:the geographic
tivity and number of species is very likely to increase. area of interest will not decrease under a scenario of
Biodiversity is more at risk in some ACIA regions than in replacement of current arctic tundra by boreal forests.
others:Beringia (Region 3) has a higher number of threat-
ened plant and animal species than any other ACIA region. 7.1.1.Characteristics of arctic tundra and
polar desert ecosystems
Changes in populations are triggered by trends and
extreme events,particularly winter processes. The southern boundary of the circumpolar Arctic as
defined in this chapter is the northern extent of the closed
Forest is very likely to replace a significant proportion of boreal forests (section 14.2.3).This is not a clear bound-
the tundra and this will have a great effect on the compo- ary but a transition from south to north consisting of the
sition of species.However,there are environmental and sequence:closed forest,forest with patches of tundra,
sociological processes that are very likely to prevent for- tundra with patches of forest,and tundra.The transition
est from advancing in some locations. zone is relatively narrow (30 to 150 km) when compared
to the width of the forest and tundra zones in many,but
Displacement of tundra by forest will lead to a decrease in not all areas.Superimposed on the latitudinal zonation of
albedo,which will increase the positive feedback to the forest and tundra is an altitudinal zonation from forest to
climate system.This positive feedback is likely to dominate treeless areas to barren ground in some mountainous
over the negativefeedback of increased carbon sequestra- regions of the northerntaiga.The transition zone from
tion.Forest development is very likely to also ameliorate taiga to tundra stretches for more than 13400 km around
local climate,for example,by increasing temperature. the lands of the Northern Hemisphere and is one of the
most important environmental transition zones on Earth
Warming and drying of tundra soils in parts of Alaska have (Callaghan et al.,2002a,b) as it represents a strong tem-
already changed the carbon status of this area from sink to perature threshold close to an area of low temperatures.
source.Although other areas still maintain their sink sta- The transition zone has been called forest tundra,sub-
tus,the number of source areas currently exceeds the arctic,and the tundra–taiga boundary or ecotone.
number of sink areas.However,geographic representation The vegetation of the transition zone is characterized by
of research sites is currently small.Future warming of an open landscape with patches of trees that have a low
tundra soils is likelyto lead to a pulse of trace gases into statureand dense thickets of shrubs that,together with
the atmosphere,particularly from disturbed areas and the trees,totally cover the ground surface.
areas thataredrying.It is not known if the circumpolar
tundra will be a carbon source or sink in the long term, The environmental definition of the Arctic does not
but current models suggest that the tundra is likely to correspond with the geographic zone delimited by the
become a weak sink for carbon because of the northward Arctic Circle (66.5º N),nor with political definitions.
movement of vegetation zones thataremore productive Cold watersin ocean currents flowing southward from
than those they displace.Uncertainties are high. the Arctic depress the temperatures in Greenland and the
easternCanadian Arctic whereas the northward-flowing
Rapid climate change thatexceeds the ability of species Gulf Stream warms the northernlandmasses of Europe
torelocate is very likely to lead to increased incidence of (section 2.3).Thus,at the extremes,polar bears and
fires,disease,and pest outbreaks. tundra arefound at51º N in eastern Canada whereas
agricultureis practiced north of 69º N in Norway.
Enhanced carbon dioxide concentrations and ultraviolet-B Arctic lands span some 20º of latitude,reaching 84º N in
radiation levels affect plant tissue chemistry and thereby Greenland and locally,in eastern Canada,an extreme
havesubtle but long-termimpacts on ecosystem processes southernlimit of 51º N.
that reduce nutrient cycling and have the potential to
decrease productivity and increase or decrease herbivory. The climate of the Arctic is largely determined by the
relativelylowsolar angles with respect to the earth.
7.1.Introduction Differences in photoperiod between summer and winter
become moreextreme toward the north.Beyond the
The Arctic is generallyrecognized as a treeless wilder- Arctic Circle,the sun remains abovethe horizon at mid-
ness with cold winters and cool summers.However, night on midsummer’s day and remains below the hori-
definitions of the southern boundary vary according to zon at midday on midwinter’s day.
Chapter 7 • Arctic Tundra and Polar Desert Ecosystems 245
Climatically,the Arctic is often defined as the area where in the southern part of the tundra zone.In many areas of
the average temperature for the warmest month is lower the Arctic,continuous permafrost occurs at greater
than 10 ºC (Köppen,1931),but mean annual air temper- depths beneath the soil surface and degrades into discon-
atures vary greatly according to location,even at the tinuous permafrost in the southern part of the zone.
same latitude (see Chapter 2).They vary from -12.2 ºC at Active-layer depth,the extent of discontinuous perma-
Point Barrow,Alaska (71.3º N) to -28.1 ºC at the summit frost,and coastal permafrost are very likely to be partic-
ofthe Greenland Ice Sheet (about 71º N) (Weller,2000) ularly sensitive to climatic warming (section 6.6).
and from 1.5 ºC at 52º N in subarctic Canada to 8.9 ºC Permafrost and active-layer dynamics lead to topographic
at 52º N in temperate Europe.The summer period,or patterns such as polygons in the landscape.Topography
period of most biological activity,progressively decreases plays an important role in defining habitats in terms of
from about 3.5 to 1.5 months from the southern bound- moisture and temperature as well as active-layer dynam-
ary of the Arctic to the north,and mean July temperature ics (Brown et al.,1980;Webber et al.,1980),such that
decreases from 10–12 ºC to 1.5 ºC.In general,annual arctic landscapes are a mosaic of microenvironments.
precipitation in the Arctic is low,decreasing from about Topographic differences of even a few tens of centime-
250 mm in southern areas to as low as 45 mm in the ters (e.g.,polygon rims and centers) are important for
northern polar deserts (Jonasson et al.,2000),with determining habitats,whereas larger-scale topographic
extreme precipitation amounts in subarctic maritime differences (meters to tens of meters) determine wind
areas (e.g.,1100 mm at 68º N in Norway).However, exposure and snow accumulation that in turn affect plant
owing to low rates of evaporation the Arctic cannot be communities and animal distribution.Topographic differ-
considered arid:even in the polar deserts,air humidity ences become more important as latitude increases.
is high and the soils are moist during the short growth
period (Bovis and Barry,1974).In the Arctic context, Disturbances of ecosystems are characteristic of the
“desert”refers to extreme poverty of life. Arctic.Mechanical disturbances include thermokarst
induced by permafrost thaw (section 6.6.1);freeze–thaw
The Arctic is characterized by the presence of continuous processes;wind,sand,and ice blasts;seasonal ice oscilla-
permafrost (section 6.6.1),although there are excep- tions;slope processes;snow load;flooding during thaw;
tions such as the Kola Peninsula.Continuous and deep changes in river volume;and coastal erosion and flood-
(>200 m) permafrost also exists south of the treeline in ing.Biological disturbances include insect pest outbreaks,
large areas of Siberia extending south to Mongolia. peaks of grazing animals that have cyclic populations,and
The depth of the active (seasonally frozen) layer of the fire.These disturbances operate at various spatial and
soil during the growing season depends on summer tem- temporal scales (Fig.7.1) and affect the colonization and
peratures and varies from about 80 cm near the treeline survival of organisms and thus ecosystem development.
to about 40 cm in polar deserts.However,active-layer
depth varies according to local conditions within land- Arctic lands areextensivebeyond the northern limit of
scapes according to topography:it can reach 120 cm on the tundra–taiga ecotone,encompassing an area of
south-facing slopes and be as little as 30 cm in bogs even approximately 7567000 km2,including about
Fig.7.1.Timescale of ecological processes in relation to disturbances (shown as breaks in horizontal lines) in the Arctic.The schemat-
ic does not showresponses projected as a result of anthropogenic climate change (based on Oechel and Billings,1992;Shaver et al.,
2000;Walker D.and Walker,1991).
246 Arctic Climate Impact Assessment
On average,it does not exceed 300 km,and in some
regions (e.g.,the lower reaches of the Kolyma River),
the tundra zone extends only 60 km from the treeline to
the coast.In such areas,the tundra zone is very likely to
be highly vulnerable to climate warming.
Thevegetation of the Arctic varies from forest tundra in
the south,where plant communities have all the plant life
forms known in the Arctic and have continuous canopies
in several layers extending to more than 3 m high,to
polar deserts in the north,where vegetation colonizes
5% or less of the ground surface,is less than 10 cm high,
and is dominated by herbs,lichens (symbionts of algae
and fungi),and mosses (Fig.7.3).Species richness in the
Arctic is low and decreases toward the north:there are
about 1800 species of vascular plants,4000 species of
cryptogams,75 species of terrestrial mammals,
240 species of terrestrial birds,3000 species of fungi,
3300 species of insects (Chernov,2002;Matveyeva and
1Cushion forb,lichen,and moss tundra Chernov,2000),and thousands of prokaryotic species
2Graminoid and forb tundra (bacteria and Archaea) whose diversity in the tundra has
3Prostrate dwarf-shrub tundra only recently started to be estimated.However,the
4Erect dwarf-shrub tundra
Arctic is an important global pool of some groups such as
5Low- and high-shrub tundra
mosses,lichens,springtails (and insect parasitoids:
6Cold evergreen needleleaf forest
Hawkins,1990;Kouki et al.,1994,Price et al.,1995)
7Cold deciduous forest
8Cool evergreen needleleaf forest because their abundance in the Arctic is higher than in
9Cool mixed forest other biomes.Net primary production (NPP),net
10 Cool–temperate evergreen needleleaf and mixed forest ecosystem production (NEP),and decomposition rates
11 Temperate evergreen needleleaf forest
are low.Food chains are often short and typically there
12 Temperate deciduous broadleaf forest
are few representatives at each level of the chain.Arctic
13 Temperate grassland and xerophytic shrubland
14 Barren soils aregenerallyshallowand underdeveloped with low
15 Ice productivity and immature moor-type humus (Brown et
al.,1980).Substantial heterogeneity of the soil cover,
Fig.7.2.Present-day natural vegetation of the Arctic and
neighboring regions from floristic surveys.Vegetation types owing to numerous spatial gradients,has an important
1to 5 areclassified as arctic,whereas types 6 to 8 are classi- influence on the microtopographical distribution of the
fied as boreal forest (Kaplan et al.,2003). soil biota (invertebrates,fungi,and bacteria) that will
possibly amplify any negative effects of climate change.
2560000 km2in the former Soviet Union and
Scandinavia,2480000 km2in Canada,2167000 km2in The Arctic has a long history of human settlement and
Greenland and Iceland,and 360000 km2in Alaska (Bliss exploitation,based initiallyon its rich aquatic biological
and Matveyeva,1992).Figure 7.2 shows the distribution resources and morerecentlyon its minerals and fossil
of arctic and other vegetation types based on a classifica- hydrocarbons.At the end of the last glacial stage,humans
tion byWalker D.(2000) and mapped by Kaplan et al. migrated from Eurasia to North America across the ice-
(2003).The distribution of arctic landmasses is often free Bering land bridge and along the southerncoast of
fragmented:seas separate large arctic islands (e.g., Beringia (ca.14000–13500 years BP;Dixon,2001).
Svalbard,Novaya Zemlya,Severnaya Zemlya,New As earlyas about 12200 years BP,areas north of the
Siberian Islands,and Wrangel Island) and the landmasses Fennoscandian Ice Sheet in northernmost Finnmark
of the Canadian Archipelago and Greenland.Similarly, (Norway) had been settled (Thommessen,1996).Even
the Bering Strait separates the arctic lands of Eurasia and earlier Paleolithic settlements (ca.40000 years BP) have
North America.Large mountains suchas the east–west been recorded in the easternEuropean Arctic (Pavlov et
running Brooks Range in Alaska and the Putorana Plateau al.,2001).The impacts of these peoples on terrestrial
in Siberia separate tundra and taiga.Such areas of relief ecosystems aredifficult to assess but were probably small
contain outposts of boreal species on their southern given their small populations and “hunter-gatherer”way
major slopes that are likely to expand northward and of life.The prey species hunted by these peoples included
higher-elevation areas that are likely to act as refuges for the megafauna,such as the woolly mammoth,which
arctic-alpine species.The Taymir Peninsula is the only became extinct.The extent to whichhunting mayhave
continuous landmass that stretches 900 km from the been principally responsible for these extinctions is a
northern tundra limit to taiga without geographic barri- matter of continuing debate (Stuart et al.,2002) but this
ersto the dispersal of animals and plants (Matveyeva and possibility cannot be excluded (Alroy,2001).It is also
Chernov,2000).The width of the tundra zone varies uncertain to what extent the extinction of the megafauna
greatly in different parts of its circumpolar distribution. may have contributed to,or been at least partly a result
Chapter 7 • Arctic Tundra and Polar Desert Ecosystems 247
Fig.7.3.Growth forms of arctic plants (modified from Webber et al.,1980 and T.Polozova,pers.comm.,2005).
of,the accelerated northward movement of trees and 7.1.2.Raison d’être for the chapter
shrubs and consequent changes in vegetation structure
(section 7.2).Although estimates of the population densi- The Arctic is experiencing dramatic environmental
ty of megafaunal species have large uncertainties,it seems changes that are likely to have profound impacts on arctic
unlikelythatmegafaunal populations were sufficient to ecosystems.The Arctic is outstanding among global
constrain the spread of woody taxa in response to favor- biomes in that climate change dominates the major
able environmental change. factors affecting biodiversity (Sala and Chapin,2000).
Present-dayarctic biota arealso relatively restricted in
During the last 1000 years,resources from terrestrial range and population size compared with their Quater-
ecosystems have been central to the mixed economies of nary situation.For example,when the treeline advanced
the Arctic:many inland indigenous communities still northward during the early Holocene warming,a low-
derive most of their protein from subsistence activities ered sea level allowed a belt of tundra to persist around
such as caribou/reindeer hunting (Berkes and Fast, the Arctic Basin,whereas any future northward migration
1996).During this period,increasing trade between of the treeline is very likely to further restrict tundra
peoples of temperate latitudes and arctic indigenous areas because sea level is projected to rise.Arctic ecosys-
peoples is likely to have affected a few target animal tems are known to be vulnerable to disturbances
species,suchas the reindeer that was domesticated in (Crawford,1997b;Forbes et al.,2001;Walker D.and
Fennoscandia and Russia,ermine hunted for fur,and Walker,1991) and to havelong recovery times:subarctic
birds of prey used for hunting as far away as the eastern birch forest defoliated by insects can take 70 years to
Mediterranean.However,the most dramatic impacts recover (Tenow and Bylund,2000).Current and
occurred after World War II as a result of the exploita- projected environmental changes arelikely to create
tion of minerals and oil and fragmentation of the arctic additional stresses and decrease the potential for ecosys-
landscape byinfrastructure (Nellemann et al.,2001). tem recovery from natural disturbances,while providing
Vlassova(2002) suggested thatindustrial activities and thresholds for shifts to newstates (e.g.,disturbance open-
forestry have displaced the Russian forest tundra south- ing gaps for invasion of species new to the Arctic).
ward bydeforesting 470000 to 500000 km2of land that
nowsuperficiallyresembles tundra.Although this esti- Changes in arctic ecosystems and their biota areimpor-
mate has been challenged as greatly exaggerated tant to arctic residents in terms of food,fuel,and culture
(because northern taiga areas have been included in that (Chapter 12) and arelikely to have global impacts
estimate’sdefinition of forest tundra),sucheffects have because of the manylinkages between the Arctic and
occurred locally in the Yamal Peninsula and the estimate more southerly regions.Several hundreds of millions of
highlights a need for reappraisal.Knowledge of possible birds migrate to the Arctic each year and their success in
past interactions between humans and the environment the Arctic determines their success and impacts atlower
that may have shaped present-day arctic ecosystems is latitudes (section 7.3.1.2).Physical and biogeochemical
limited,but shows that any future increases in popula- processes in the Arctic affect atmospheric circulation and
tion density and human activity arelikelyto modify the the climate of regions outside of the Arctic (section 7.5).
projected responses of arctic ecosystems to changes in It is known that ecosystems have responded to past envi-
climate and ultraviolet (UV) radiation levels. ronmental changes (section 7.2) and that environmental
248 Arctic Climate Impact Assessment
changes are presently occurring in the Arctic (Chapman West Greenland and the eastern Canadian Arctic have
and Walsh,1993 as quoted in Weller,2000;Dye,2002; decreased by 0.25 to 1 ºC per decade (Chapman and
Fioletov et al.,1997;Chapters 2,5,and 6).This under- Walsh,1993,quoted in Weller,2000).Over a longer
standing indicates that there are very likely to be respons- period,from 1954 to 2003,the annual increase and
es of arctic ecosystems to projected future and ongoing decrease in temperatures have been slightly less:about
climate change.It is also known that current levels of 2to 3 ºC for the whole period (Chapter 1,Fig.1.3).
ultraviolet-B (UV-B) radiation,as well as higher levels, Temperature increases in Fennoscandia over the past
can affect subarctic plants (Gwynn-Jones et al.,1997; century have been small,ranging from about 1 ºC in the
Johanson et al.,1995;Phoenix et al.,2000).Arctic plants west to near 0 ºC in the east (Lee et al.,2000).
may be particularly sensitive to increases in UV-B irradi-
ance because UV-B radiation damage is not dependent on Precipitation has also changed.The duration of the
temperature whereas enzyme-mediated repair of DNA snow-free period at high northern latitudes increased by
damage could be constrained by low temperatures 5to 6 days per decade and the week of the last observed
(Björn,2002;Li et al.,2002a,b;Paulsson,2003). snow cover in spring advanced by 3 to 5 days per decade
between 1972 and 2000 (Dye,2002).Stratospheric
For all of these reasons,understanding the relationships ozone has been depleted over recent decades (e.g.,by a
between ecosystems and the arctic environment is maximum of 45% below normal over the high Arctic in
important.Although many aspects of its environment spring;Fioletov et al.,1997).This has probably led to an
are changing concurrently (e.g.,climate,pollution, increase in surface UV-B radiation levels in the Arctic,
atmospheric nitrogen deposition,atmospheric concen- although the measurement period is short (section 5.5).
trations of carbon dioxide (CO ),UV-B radiation Scenarios of future change project that mean annual
2
levels,and land use),the specific mission of this chapter temperatures in the Arctic will increase by nearly 4 ºC
is to focus on the impacts of changes in climate and by 2080 (section 4.4.2) and that spring (April) UV-B
UV-B radiation levels on arctic terrestrial ecosystems radiation levels will increase by 20 to 90% in much of
and their species and processes. the Arctic by 2010–2020 (Taalas et al.,2000).
7.1.3.Rationale for the structure of the The assessment of impacts on terrestrial ecosystems
chapter presented in this chapter is based on existing literature
rather than new research or ACIA modeling activities.
The effects of climate are specific to species,the age and Existing long-term experimental manipulations of tem-
developmental stages of individuals,and processes from peratureand/or UV-B radiation relied on earlier scenar-
metabolism to evolution (Fig.7.1).Although there are ios of climate and UV-B radiation change (IPCC,1990).
many ways in which to organize an assessment of climate However,the most recent scenarios (Chapters 4,5,and
and UV-B radiation impacts,this chapter follows a logi- 6) areused to provide a context for the assessment in
cal hierarchy of increasing organizational biological com- this chapter,and to modify projections of ecosystem
plexity to assess impacts on species,the structure of responses based on earlier scenarios where appropriate.
ecosystems,the function of ecosystems,and landscape The ACIA climate scenarios (section 4.4) are also used
and regional processes.Abasic understanding of biologi- directlyto illustrate the responses of some species to
cal processes related to climate and UV-B radiation is projected climate changes.
required beforethe impacts of changes in these factors
on terrestrial ecosystems can be assessed (Smaglik, 7.1.4.Approaches used for the assessment:
2002).Consequently,this chapter progresses from a strengths,limitations,and uncertainties
reviewof climate and UV radiation controls on biologi-
cal processes to an assessment of the potential impacts of This chapter assesses information on interactions
changes in climate and UV-B radiation levels on process- between climate,UV-B radiation levels,and ecosystems
esatthe species and regional levels.Some effects of from a wide range of sources including experimental
climate change on ecosystems maybe beneficial to manipulations of ecosystems and environments in the
humans,while others may be harmful. field;laboratory experiments;monitoring and observa-
tion of biological processes in the field;conceptual mod-
The changes in climate and UV-B radiation levels that eling using past relationships between climate and biota
are used in this chapter to assess biological impacts are (paleo-analogues) and current relationships between cli-
of twotypes:those already documented (section 2.6) mate and biota in different geographic areas (geographic
and those projected byscenarios of futurechange in analogues) to infer futurerelationships;and process-
UV-B radiation levels (section 5.7) and climate (section based mathematical modeling.Where possible,indigenous
4.4) derived from models.Mean annual and seasonal knowledge (limited to published sources) is included as
temperatures havevaried considerably in the Arctic since an additional source of observational evidence.Relevant
1965 (Chapman and Walsh,1993 as quoted in Weller, information from indigenous peoples on arctic tundra
2000;section 2.6.2.1).Mean annual temperatures in and polar desertecosystems is given in Chapter 3.
westernparts of North America and central Siberia have
increased by about 1 ºC (up to 2 ºC in winter) per Each method has uncertainties and strengths and these
decade between 1966 and 1995 while temperatures in are discussed in section 7.7.By considering and compar-
Chapter 7 • Arctic Tundra and Polar Desert Ecosystems 249
ing different types of information,it is hoped that a tions east and west of the Mackenzie River (Ehrich et al.,
more robust assessment has been achieved.However,the 2000;Fedorov and Goropashnaya,1999),the latter most
only certainties in this assessment are that there are vari- probably in the Canadian Archipelago.The latter conclu-
ous levels of uncertainty in the projections and that even sion is supported by the phylogeography (relationship
if an attempt is made to estimate the magnitude of these between genetic identity and geographic distribution) of
uncertainties,surprise responses of ecosystems and their the Paranoplocephala arcticaspecies complex,a cestode
species to changes in climate and UV-B radiation levels parasite of Dicrostonyxspp.,indicating that two subclades
are certain to occur. probably survived the LGM with their host in the
Canadian High Arctic (Wickström et al.,2003).More
7.2.Late-Quaternary changes in arctic controversial are suggestions that elements of the arctic
flora and fauna may have survived the LGM on nunataks
terrestrial ecosystems,climate,and
(hills or mountains extending above the surface of a
ultraviolet radiation levels
glacier) in glaciated areas of high relief such as parts of
In order to understand the present biota and ecosystems Greenland,Svalbard,and Iceland (Rundgren and
of the Arctic,and to project the nature of their respons- Ingolfsson,1999).Although a recent molecular genetic
es to potentially rapid future climate change,it is neces- study of the alpine cushion plant Eritrichium nanum
sary to examine at least the last 21000 years of their his- (Stehlik et al.,2001) provides strong evidence for sur-
tory.This period,which is part of the late Quaternary vival of that species on nunataks within the heart of the
Period,extends from the present back to the last glacial European Alps,similar studies of arctic species have so far
maximum (LGM),encompassing the Holocene,or post- not supported the hypothesis of survival on nunataks in
glacial period,that spans approximately the last 11400 areas such as Svalbard (Abbott et al.,2000) that experi-
years.A review of this period of the history of the biota enced extreme climatic severity as ice sheets extended to
and ecosystems found in the Arctic today also must margins beyond the current coast during the LGM.
examine a spatial domain that is not restricted to the
present arctic regions.At the LGM,many of these Direct evidence of the severity of the full glacial climate
regions were submerged beneath vast ice sheets,where- in the Arctic comes from studies of ice cores from the
as many of the biota comprising present arctic ecosys- Greenland Ice Sheet and other arctic ice sheets (section
tems werefound atlower latitudes. 2.7) thatindicate full glacial conditions with mean annu-
al temperatures 10 to 13 ºC colder than during the
7.2.1.Environmental history Holocene (Grootes et al.,1993).Paleotemperature
reconstructions based upon dinoflagellate cyst assem-
At the LGM,vast ice sheets accumulated not only on blages indicate strong seasonal temperature fluctuations,
many high-latitude continental areas but also across with markedly cold winter temperatures (de Vernal and
some relatively shallow marine basins.The beds of rela- Hillaire-Marcel,2000;de Vernal et al.,2000).
tively shallow seas such as the North Sea and Bering Sea
were exposed as a result of a global sea-level fall of The LGM was,however,relatively short-lived;within a
approximately 120 m,resulting in a broad land connec- fewmillennia of reaching their maximum extent many of
tion between easternSiberia and Alaska and closureof the ice sheets weredecaying rapidlyand seasonal temper-
the connection between the Pacific and Arctic Oceans. atures had increased in many parts of the Arctic.Deglaci-
The reduction in sea level also exposed a broad strip of ation was not,however,a simple unidirectional change;
land extending northward from the present coast of instead a series of climatic fluctuations occurred during
Siberia.Most,if not all,of the Arctic Ocean basin may the period between about 18000 and 11400 years BP
havebeen covered by permanent sea ice. thatvaried in intensity,duration,and perhaps also in
geographic extent.The most marked and persistent of
Although details of the extent of some of the ice sheets these fluctuations,the Younger Dryas event (Alley,2000;
continue to be a controversial matter (see e.g.,Astakhov, Peteet,1993,1995),was at least hemispheric in its
1998;Grosswald,1988,1998;Lambeck,1995;Siegert et extent,and was marked bythe reglaciation of some
al.,1999),it is certain that the majority of land areas regions and readvances of ice-sheet margins in others.
north of 60º N were ice-covered.The principal excep- Mean annual temperatures during this event fell substan-
tions werein easternSiberia,Beringia,and Alaska, tially;although not as lowas during the glacial maxi-
although there is some geological evidence to suggest that mum,they were nonetheless 4 to 6 ºC cooler than at
smaller ice-free areas also persisted in the high Arctic,for present over most of Europe (Walker M.J.,1995),
example in the northernmost parts of the Canadian and as muchas 10 to 12 ºC colder than atpresent in the
Archipelago (Andrews,1987) and perhaps even in north- northern North Atlantic and the Norwegian Sea (Koç et
ernand northeastern Greenland (Funder et al.,1998). al.,1996),as well as in much of northern Eurasia
This evidence is supported byrecent molecular genetic (Velichko,1995).The end of the Younger Dryas was
studies of arctic species;for example,a study of the marked by a very rapid rise in temperatures.At some
dwarf shrub Dryas integrifoliaindicates glacial occurrences locations,mean annual temperature rose by more than
in the high Arctic (Tremblayand Schoen,1999) as well as 5ºC in less than 100 years(Dansgaard et al.,1989).
in Beringia,and a study of the collared lemming The most rapid changes probably were spatially and tem-
Dicrostonyx groenlandicusindicates separate glacial popula- porally transgressive,with the global mean change thus
250 Arctic Climate Impact Assessment
occurring much less rapidly.Nonetheless,in many areas between 9 and 27% higher during periods of low solar
summer temperatures during the early Holocene rose to output (cool periods) than during periods of high solar
values higher than those at present.Winter conditions output (Rozema et al.,2002;see also section 5.4.1).
remained more severe than today in many higher-latitude
areas,however,because the influence of the decaying ice 7.2.2.History of arctic biota
sheets persisted into the early millennia of the Holocene.
During the LGM,when most land areas in the Arctic
Despite higher summer temperatures in the early to were ice-covered,biomes able to support the elements
mid-Holocene in most of the Arctic,Holocene climate of the arctic biota,including some species that are now
has not differed qualitatively from that at present. extinct,were extensive south of the Fennoscandian Ice
Following the general thermal maximum there has been Sheet in Europe (Huntley et al.,2003).Similar biomes
amodest overall cooling trend throughout the second apparently were extensive south of the Eurasian ice
half of the Holocene.However,a series of millennial sheets of northern Russia,eastward across Siberia and
and centennial fluctuations in climate have been super- the exposed seabed to the north,and via Beringia into
imposed upon these general longer-term patterns Alaska and the northern Yukon (Ritchie,1987),although
(Huntley et al.,2002).The most marked of these they were much more restricted south of the Laurentide
occurred about 8200 years BP and appears to have been Ice Sheet in central and eastern North America (Lister
triggered by the catastrophic discharge of freshwater and Bahn,1995).The most extensive and important of
into the northern North Atlantic from proglacial lakes these glacial biomes,the steppe–tundra,has been inter-
in North America (Barber et al.,1999;Renssen et al., preted and referred to by various authors as “tundra–
2001).A reduction in strength,if not a partial shut- steppe”or “Mammoth steppe”(Guthrie,2001;Walker D.
down,of the thermohaline circulation in the northern et al.,2001;Yurtsev,2001).The vegetation of this biome
North Atlantic and Norwegian Sea was also associated comprised a no-analogue combination of light-
with this event,as well as with the series of less severe demanding herbaceous and dwarf-shrub taxa that are
climatic fluctuations that continued throughout the found today either in arctic tundra regions or in the
Holocene (Bianchi and McCave,1999). steppe regions that characterize central parts of both
North America and Eurasia (Yurtsev,2001).Evidence of
The most recent of these climatic fluctuations was that an abundance of grazing herbivores of large body mass,
of the “Little Ice Age”(LIA),a generally cool interval some extant (e.g.,reindeer/caribou – Rangifer tarandus;
spanning approximately the late 13th to early 19th cen- muskox – Ovibos moschatus)and others extinct (e.g.,
turies (section 2.7.5).At its most extreme,mean annu- giant deer or “Irish elk”– Megaloceros giganteus;woolly
al temperatures in some arctic areas fell by several mammoth – Mammuthus primigenius;woolly rhinoceros –
degrees.Sea ice extended around Greenland and in Coelodonta antiquitatis),associated with this biome sug-
some yearsfilled the Denmark Strait between Green- gests thatit was much more productive than is the con-
land and Iceland (Lamb H.H.,1982;Ogilvie,1984; temporarytundra biome.This productive biome,domi-
Ogilvie and Jonsdottir,2000;Ogilvie and Jonsson, nated by non-tree taxa,corresponded to a no-analogue
2001),the Norse settlement of Greenland died out environment that was relatively cold throughout the
(Barlowet al.,1997;Buckland et al.,1996),and the year,with a growing season short enough to exclude
population of Iceland was greatly reduced (Ogilvie, even cold-tolerant boreal trees from at least the majority
1991;Sveinbjarnardóttir,1992).Although there was of the landscape.The “light climate”,however,was that
greattemporal climate variability (on decadal to cen- of the relativelylower latitudes (as low as 45º N in
tennial timescales) within the LIA,and spatial variability Europe) at which this biome occurred,rather than that
in the magnitude of the impacts,it was apparently a of the present arctic latitudes;the greater solar angle and
period of generallymoresevereconditions in arctic and consequent higher insolation intensities during the sum-
boreal latitudes;the marked impacts upon farming and mer months probably made an important contribution
fisheries (Lamb H.H.,1982) imply similar impacts on to the productivity of the biome.
other components of the arctic ecosystem.Since the
early 19th century,however,there has been an overall The productive steppe–tundra and related biomes were
warming trend (Overpeck et al.,1997),although with muchmore spatially extensive during the last glacial
clear evidence of both spatial variability and shorter- stage than is the tundra biome today(Fig.7.4).The last
term temporal variability (Maxwell,1997).The magni- glacial stage was thus a time when many elements of the
tude of this recent warming is comparable to that of the present arctic biota thrived,almost certainly in greater
warmest partof the Holocene,at least in those parts of numbersthan today.Fossil remains of both arctic plants
the Arctic that have experienced the most rapid warm- (see e.g.,West,2000) and mammals (see e.g.,FAUN-
ing during the last 30 years or so. MAP Working Group,1996;Lundelius et al.,1983;
Stuart,1982) found atnumerous locations attest to their
The solar variability thought to be responsible for the widespread distribution and abundance.Similar conclu-
LIA,and for other similar centennial to millennial cli- sions have been reached on the basis of phylogeographic
matic fluctuations,probably also affected the ozone studies of arctic-breeding waders(Kraaijeveld and
layer and UV-B radiation levels.Ultraviolet-B irradiance Nieboer,2000).Species such as red knot (Calidris
at ground level absorbed by DNA could have been canutus)and ruddy turnstone (Arenaria interpres)are
Chapter 7 • Arctic Tundra and Polar Desert Ecosystems 251
(a) (b)
Cushion forb,lichen,and moss tundra
(c)
Graminoid and forb tundra
Prostrate dwarf-shrub tundra
Erect dwarf-shrub tundra
Low- and high-shrub tundra
Cold evergreen needleleaf forest
Cold deciduous forest
Cool evergreen needleleaf forest
Cool mixed forest
Cool–temperate evergreen needleleaf and mixed forest
Temperate evergreen needleleaf forest
Temperate deciduous broadleaf forest
Temperate grassland and xerophytic shrubland
Barren
Ice
Fig.7.4.Northern vegetation during the mid-Holocene simu-
lated byforcing the BIOME4 vegetation model with output
from (a) the Institut Pierre-Simon Laplace Coupled Model 1
atmosphere–ocean general circulation model (AOGCM) and
(b) the HadCM2 AOGCM;and (c) reconstructed from pollen
data (Bigelowet al.,2003;Kaplan et al.,2003).
inferred to have had much larger populations and more little or no evidence of genetic differentiation that might
extensivebreeding areas during glacial stages,although indicate past population fragmentation,and Fedorov et
others,suchas dunlin (C.alpina),exhibit evidence of al.(1999a) inferred thatEurasian true lemmings (Lemmus
range fragmentation during glacial stages leading to the spp.) experienced no effective reduction in population
evolution of distinct geographically restricted infraspe- size during recent glacial–interglacial cycles.
cific taxa.Phylogeographic studies of other arctic taxa
show individualistic responses (see Weider and Hobaek, In the context of their late-Quaternary history,the arctic
2000 for a recent review).Some species,such as Arctic biota atpresent are relatively restricted in range and
char (Salvelinus alpinus;Brunner et al.,2001),and gen- population size.Although tundra areas wereof even
era,such as whitefish (Coregonusspp.;Bernatchez et al., smaller extent during the early part of the Holocene
1999),exhibit evidence of sub-taxa whose origins are than atpresent,as a result of greater northward exten-
apparentlyrelated to recurrent isolation of populations sion of the treeline (Huntley,1997;Huntley and
throughout the alternating glacial and interglacial stages Bradshaw,1999;MacDonald et al.,2000),that reduction
of the Pleistocene.Collared lemmings (Dicrostonyxspp.), in extent was small in magnitude compared to that
however,apparently parallel C.alpinain exhibiting experienced atthe end of the last glacial stage,during
genetic differentiation principally as a consequence of which they were much more extensive than at any time
the relatively recent geographic isolation of populations since.Similarly,while extant arctic taxa at the lower tax-
during the last glacial stage (Fedorovand Goropashnaya, onomic levels often exhibit considerable diversity that
1999;Fedorov et al.,1999b).Other species,such as the can be related to their late-Quaternary history,the biota
polar bear (Ursus maritimus;Paetkau et al.,1999),exhibit as a whole has suffered a recent reduction in overall
252 Arctic Climate Impact Assessment
diversity owing to the extinctions of many species,and ranges at rates of between 0.2 and 2 km/yr (Huntley and
some genera,that did not survive into the Holocene. Birks,1983;Ritchie and McDonald,1986).They exhib-
Ofat least 12 large herbivores and six large carnivores ited individualistic responses with respect to their distri-
present in steppe–tundra areas at the LGM (Lister and butions and abundance patterns in response to climatic
Bahn,1995;Stuart,1982),only four and three,respec- patterns that differed from those of today.Milder win-
tively,survive today.Of the surviving species,only two ters and more winter precipitation in western Siberia
herbivores (reindeer/caribou and muskox) and two car- during the early Holocene,for example,allowed
nivores (brown bear – Ursus arctos and wolf – Canis lupus) Norway spruce (Picea abies)to dominate in areas where
occur today in the arctic tundra biome.Present arctic Siberian fir (Abies sibirica)and Siberian stone pine (Pinus
geography also imposes extreme migratory distances sibirica)have become important forest components dur-
upon many tundra-breeding birds owing to the wide ing the later Holocene (Huntley,1988,1997;Huntley
separation between their breeding and wintering areas and Birks,1983).Throughout northern Russia,the arctic
(Davidson N.et al.,1986;Wennerberg,2001),render- treeline had advanced more or less to the position of the
ing many of them,in common with much of the arctic present arctic coastline by about 10200 years BP,
biota,extremely vulnerable to any further climatic although the lower sea level at that time meant that a
warming (Evans,1997). narrow strip of tundra,up to 150 km wide at most,
persisted north of the treeline (MacDonald et al.,2000).
7.2.3.Ecological history Subsequently,as sea level continued to rise during the
early Holocene,tundra extent reached a minimum that
Although relatively few in overall number,paleo- persisted for several millennia.For tundra species,
ecological studies of the late Quaternary Period have including tundra-breeding birds,the early Holocene thus
been conducted in many parts of the Arctic (see e.g., seems likely to have been a time of particular stress.
Anderson and Brubaker,1993,1994;Lamb H.F.and This stress may,however,have been in part relieved by
Edwards,1988;MacDonald et al.,2000;Ritchie,1987). enhanced productivity in these areas,compared to mod-
In areas that were by then ice free,the transition to the ern tundra ecosystems,as a consequence of the warmer
Holocene was marked by evidence of rapid ecological summers and higher insolation intensity.
response.Elsewhere,in proximity to the decaying ice
sheets,therewas a lag between the global changes and In glaciated areas of the Arctic,such as northern
the ecological changes because of the regional influence Fennoscandia and much of arctic Canada,peatlands
of the ice sheets.Although the precise nature of the eco- became extensive only after the mid-Holocene (see e.g.,
logical changes depended upon location,the overall pic- Lamb H.F.,1980;Vardy et al.,1997) in response to the
ture was one of widespread rapid replacement of the general pattern of climatic change toward cooler and
open,discontinuously vegetated tundra and polar desert regionally moister summer conditions.The same cooling
thatcharacterized most ice-free areas during the late- trend led to the southward retreat of the arctic treeline,
glacial period by closed tundra.This was in turn replaced whichreached more or less its present location in most
by shrub tundra and subsequently by arctic woodlands or regions by about 4500 years BP (MacDonald et al.,
northern boreal forest in southern areas of the Arctic. 2000).The consequent increase in tundra extent proba-
In areas thatwereunglaciated at the LGM (e.g.,Alaska), blyrelieved the stress experienced by tundra organisms
the ecological transition began earlier,coinciding with during the early Holocene,although the cooler,less pro-
the first rapid climatic warming recorded in Greenland ductiveconditions,and the increasing extent of seasonal-
about 14700 yearsBP (Björcket al.,1998;Stuiver et al., lywaterlogged tundra peatlands,mayhave offset this at
1995).In Alaska,tundra was replaced by shrub tundra least in part.While the early Holocene was a time of
during the late-glacial stage,and the first forest stands permafrost decayand thermokarst development,at least
(of balsam poplar – Populus balsamifera)werealreadypres- in some regions (Burn,1997),the extent of permafrost
ent before the transition to the Holocene (Anderson and has increased in many areas during the later Holocene
Brubaker,1994).South of the Arctic,the extensive areas (see e.g.,Kienel et al.,1999;Vardy et al.,1997).
of steppe–tundra thatwerepresent atthe LGM were
rapidly replaced by expanding forests.Only in parts of 7.2.4.Human history related to ecosystems
northernmost Siberia may fragments of the steppe–
tundra biome havepersisted into the Holocene,support- Recentlydiscovered evidence (Pavlov et al.,2001) shows
ing the last population of woolly mammoths that persist- that Paleolithic “hunter-gatherers”were present about
ed as recently as 4000 years BP (Vartanyan et al.,1993). 40000 years BP (long before the LGM) as far north as
66º34' N in Russia,east of the Fennoscandian Ice Sheet.
Theearly Holocene was characterized by higher summer Although it seems likely that humans did not range as far
insolation intensities at northern latitudes than at pres- north during the glacial maximum,it is clear that they
ent.The warmer summer months enabled trees to expanded rapidlyinto the Arctic during the deglaciation.
extend their ranges further northward than at present;
positivefeedback resulting from the contrasting albedo Humans entered North America via the Bering “land
of forest compared to tundra (sections 7.4.2.4 and bridge”and along the southerncoast of Beringia about
7.5.4.2) probably enhanced this extension of the forest 14000 to 13500 years BP (Dixon,2001).These so-called
(Foley et al.,1994).Boreal forest trees expanded their Clovis hunters were hunter-gatherers who had developed