U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007−1096

Vegetation Response to Climate Change in Alaska: 

Examples From the Fossil Record
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Ager, Thomas, 2007, Vegetation response to climate change in Alaska: Examples from the fossil record: 

U.S. Geological Survey Open-File Report 2007-1096, 44 p.
 PREFACE 

Vegetation response to climate change in Alaska: Examples from the fossil record 

 Thomas A. Ager 

 U.S. Geological Survey 

This report was presented as an invited paper at the Fish & Wildlife Service Climate 

Forum held in Anchorage, Alaska on February 21-23, 2007. The purpose of the talk was 

to provide some examples of past climate changes that appear to have caused significant 

responses in Alaskan vegetation. These examples are based on interpretations of dated 

fossil assemblages (pollen, spores and plant macrofossils) collected and interpreted by 

U.S. Geological Survey and collaborating scientists from other scientific organizations 

during the past several decades. 

This presentation has been made available for public distribution at the request of the 

Fish & Wildlife Service Climate Change Forum organizers. The original talk, given as a 

PowerPoint presentation, has been converted to U.S. Geological Survey Open-File 

Report (OF 2007-1096). This presentation received U.S. Geological Survey Bureau 

Approval for public release on April 2, 2007. 

Additional information about the subject matter in this presentation may be obtained by 

contacting the author (address below). 

Dr. Thomas Ager 

Research Geologist 

U.S. Geological Survey 

Mail Stop 980 

Box 25046, Denver Federal Center 

Denver, CO 80225 

(303)236-5728 

tager@usgs.gov 

 
Vegetation response to 

climate change in Alaska: 

Examples from the fossil 

record 

Thomas Ager 

U.S. Geological 

Survey 

Title photo: Vegetation response to climate change in Alaska: Examples 

from the fossil record. 

Contact: Dr. Thomas Ager, Research Geologist 

U.S. Geological Survey, Mail Stop 980, Box 25046 Denver Federal 

Center, Denver, CO 80225 

(303)236-5728 

tager@usgs.gov 

Photo by Thomas Ager, USGS. 

1 
Map of present climate zones of Alaska (after Pewe, 1975). Distribution 

of dominant climate types in Alaska reflects influences of latitude, 

topography, and proximity to oceanic air masses, especially from the 

North Pacific. The highly continental climate of interior Alaska is the 

result of distance from oceanic moisture sources and the precipitation 

shadow effects of the Brooks Range to the north and the Alaska Range 

and other high mountain ranges to the south. 

Reference: Pewe, T.L., 1975, Quaternary geology of Alaska: U.S. 

Geological Survey Professional Paper 835, 1 plate, 2 tables, 145 p. 

2 
Present-day vegetation of Alaska can be divided into three broad types: 

tundra, boreal forest, and coastal forest (modified from Viereck and 

Little, 1972). Tundra types include alpine, arctic, and “subarctic” forms. 

Boreal forest covers lowlands of interior and inland south-central Alaska 

(white spruce, black spruce, larch, balsam poplar, aspen, paper birch, 

willow, and alder). Coastal forest is associated with maritime climates of 

the Gulf of Alaska. In southeastern Alaska, this forest includes western 

hemlock, shore pine, Sitka spruce, mountain hemlock, red cedar, yellow 

cedar, and a few other species. 

Reference: Viereck, L.A., and Little, E.L., Jr., 1972, Alaska trees and 

shrubs: Washington, D.C., USDA Forest Service Agriculture Handbook 

no. 410, 265 p. 

3 
Tundra 

Coastal forest 

Boreal forest 

Modern vegetation communities 

in Alaska: 

This shows examples of the three modern vegetation types of Alaska: 

tundra, boreal forest, and coastal forest. 

Figure courtesy of Dr. Dan Muhs, USGS. 

4 
An extreme warming event that had profound effects on Alaskan 

vegetation occurred during the middle Miocene, when global climates 

became much warmer than today. By examining fossil evidence from 

dated exposures of Miocene sediments, it is possible to reconstruct in 

considerable detail the nature of Miocene vegetation of northwestern 

North America. (Note: the age of the base of the Miocene is 23 million 

years old, and this chart does not show the record for the earliest 

Miocene. Also not shown: the Holocene, representing the last 10,000 

years of history following the Pleistocene. 

Reference: Ager, T.A., 1997, How does climate change influence 

Alaska’s vegetation: insights from the fossil record: U.S. Geological 

Survey Fact Sheet FS-071-97. 

5 
Reconstructing Miocene and Pliocene vegetation changes involves 

paleobotany, palynology, geochronology, and stratigraphy. In this 

photo, Dr. James White from the Geological Survey of Canada is 

standing next to a Sequoia (redwood) stump (unfossilized) charred and 

buried by a middle Miocene age basalt flow about 16 million years old. 

The stump is in situ, and its roots penetrate well-preserved peat that 

contains abundant pollen and spores. This site is north of the Arctic 

circle in northeastern Alaska, in the Porcupine River canyon. Dated 

plant fossil evidence from this site and many other sites in Alaska is 

being used to reconstruct the broad outlines of vegetation changes in 

northwestern North America during the Miocene, Pliocene, and 

Quaternary. Today, Sequoia trees grow no farther north than southern 

Oregon. 

Photo by Dr. Thomas Ager, USGS. 

6 
Pine cone preserved in peat layers from between basalt lava flows, 

Porcupine River canyon, northeastern Alaska. Age is middle Miocene, 

about 16 million years old. This cone is probably from an ancestral form 

of lodgepole pine that now grows in western Canada and southeastern 

Alaska. Pollen from this peat and other peat layers contains a rich 

assemblage of northern hardwood and conifer species. This 

assemblage suggests a cool temperate climate for interior and southcentral Alaska during the middle Miocene. 

Photo by Dr. Thomas Ager, USGS. 

7 
Pollen assemblages and plant macrofossils from interior and southcentral Alaska indicate that during the middle Miocene global warming 

event, northern hardwood forests with a rich assemblage of conifers 

covered much of Alaska. Modern-day forests that contain many of the 

tree species growing in Alaska during the middle Miocene can be found 

in China, Korea, northern Japan, the Northeastern United States, and 

the Pacific Northwest of the United States. This photo is of a northern 

hardwood forest in northern Japan. 

Photo by Dr. Jack Wolfe, USGS (deceased). 

8 
Another modern near analog for Alaskan forest vegetation during the 

middle Miocene can be found today in Pennsylvania, Delaware, New 

York, Maryland, Virginia and adjacent States. Pollen and plant 

macrofossils from Alaska indicate that northern hardwoods such as 

hickory, chestnut, oak, basswood, sweetgum, ironwood, holly, beech, 

elm, wing-nut (found in Asia today), ash, and many other broadleaf 

species grew there during the middle Miocene. Conifers that grew in 

interior and south-central Alaska during the middle Miocene included 

pine, spruce, hemlock, fir, Douglas fir, redwood, and dawn redwood. 

Mean annual temperature in interior Alaska was probably about 10 to 

12 degrees Celsius, compared with mean annual temperatures of about 

- 3 degrees Celsius in Fairbanks and - 8 degrees Celsius at Fort Yukon 

today. Mean annual precipitation was probably at least twice or three 

times higher during the middle Miocene than it is now in the interior. 

Photo by Dr. Thomas Ager, USGS. 

9 
A long interval of oscillating climates followed the middle Miocene 

warming event, with a gradual cooling trend that progressively 

eliminated most of the northern hardwoods from Alaska, leaving behind 

a conifer-dominated forest vegetation that was far richer in tree types 

than now exist in the interior. Glaciation of Greenland began about 7-8 

million years ago. Expansion of glacial ice in Antarctica occurred about 

14 million years ago. (Initial glaciation in east Antarctica began during 

the late Eocene.) 

10 
During the late Miocene, many broadleaf tree and shrub species 

associated with cool temperate climates of the mid-Miocene warming 

were eliminated from high latitude North America and Northeast Asia by 

cold climate oscillations. Surviving broadleaf trees and shrubs included 

alders (a fossil leaf of which is shown here, from upper Miocene rocks 

in the Homer area of Cook Inlet, south-central Alaska). 

Photo by Dr. Thomas Ager, USGS. 

11 
Other late Miocene plant fossils from interior and south-central Alaska 

include specimens of willow, fir, pine, and dawn redwood. This rock 

specimen includes a fossil willow leaf below the coin, and a conifer twig 

with needles attached (to the left of the coin). Some of the hardier tree 

types from the Northern Hardwood flora persisted in Alaska, but in 

diminishing importance (for example, elm, hazelnut, wing-nut). 

Photo by Dr. Thomas Ager, USGS. 

12 
Poplar, cottonwood, alder, and willow are broadleaf trees and shrubs 

that survived the late Miocene and Pliocene cooling events in Alaska. 

All of these plants survive today in the boreal forest of Alaska except 

cottonwood, which grows in the coastal forest. 

Photo by Dr. Thomas Ager, USGS. 

13 
Late Miocene forests of interior and southern Alaska included many tree 

types that now grow along the Pacific Northwest coast. This photograph 

shows a stand of mature western hemlock trees that grow in 

southeastern and parts of south-central Alaska today. Western 

hemlock and other coastal trees once grew in interior Alaska, far inland 

from the coast. This implies that the climate of interior Alaska was 

warmer and wetter than it is today, and winters were far less severe. 

During the latest Miocene and Pliocene, the rise of the Alaska Range 

resulted in the development of a more continental climate in interior 

Alaska. This gradually eliminated conifer types from the interior that 

now grow along the southern or southeastern coasts of Alaska (for 

example, hemlock, pine, and cedar). 

Photo by Dr. Thomas Ager, USGS. 

14 
Map of Pleistocene glacial extent in Alaska. Blue represents the 

maximum glacial limits known in Alaska and may represent late 

Pliocene and (or) early Pleistocene glaciations. Red approximates the 

glacial limits during the late Wisconsin glaciation, ca. 25,000 -10,000 yr 

B.P. Black areas depict present-day glaciers and icefields, which cover 

about 4 percent of the present land area of Alaska. During the late 

Wisconsin, glaciers covered about 30 percent of Alaska. During the 

maximum glaciation(s) of Alaska, glaciers covered about 50 percent of 

Alaska. 

Map courtesy of Dr. William Manley (Institute of Arctic and Alpine 

Research, University of Colorado) and Dr. Darrell Kaufman (University 

of Northern Arizona). 

15 
The present-day icefields in the mountains adjacent to the Gulf of 

Alaska represent the largest area of glaciers in the northern hemisphere 

outside of Greenland. Yet this impressive glacial landscape represents 

just a small remnant of icefields that existed in past glaciations. During 

the late Wisconsin glacial interval, ice covered about 30 percent of 

Alaska (plus vast areas of the continental shelf in the Gulf of Alaska). 

Little is known of the details of glacial history in the Gulf of Alaska 

region, and relatively little research has focused on it. 

Photo by Dr. Thomas Ager, USGS. 

16 
Figure courtesy of Dr. Dan Muhs, USGS.

Oxygen isotopes in foraminifera from deep-sea cores 

Normalized o

xygen

isotope c

omposition 

track glacial-interglacial cycles: 

Present Last interglacial interglacial -3 

Penultimate 

-2 

-1 

0 

1 

2 

0 100 200 300 400 

Last 

glacial 

glacial 

Age, in thousands of years before present 

(Martinson et al., 1987, QR) 

Now we will examine the more recent history of climate changes in 

Alaska, and examine the effects of major cooling and warming events of 

the past ca. 2.5 million years. Oxygen isotope curves derived from 

foraminifera from deep ocean cores can be interpreted to reflect 

changes in sea water temperatures and global ice volumes over 

hundreds of thousands to millions of years. This curve shows several 

interglacial or interstadial warm events separated by much longer 

intervals of cold, dry glacial climates. The marine oxygen isotope curves 

suggest that about 50 glacial-interglacial cycles have occurred during 

the past ca. 2.5 million years. The most climatically extreme glacialinterglacial cycles have occurred during the past ca. 950,000 years. In 

Alaska and northwestern Canada, paleobotanical evidence has been 

documented from about eight interglacial events that occurred during 

the past 2.2 million years. That record is incomplete, but the pollen 

evidence from these 8 interglacials shows a remarkably consistent 

history of spruce-dominated boreal forest during interglacial times. This 

contrasts markedly with early Pliocene forests of Alaska that contained 

more conifer types, including hemlock, fir, spruce, larch, and pine. Cold 

climate intervals vastly reduced forest vegetation, and tundra dominated 

the landscapes for most of the past 2.5 million years. 

Reference: Martinson, D.G., Pisias, N.G., Hayes, J.D., Imbrie, J., 

Moore, T.C., Jr., and Shackleton, N.J.,1987. Age dating and the orbital 

theory of ice ages: Development of a high-resolution 0 to 300,000 year 

chronostratigraphy: Quaternary Research, v. 27, p. 1-29. 

17 
Map of present-day western Alaska, the Bering Sea, the Arctic Ocean, 

and eastern Siberia. During glacial times, so much of the Earth’s water 

was stored on land as glacial ice and snow that sea level dropped 

dramatically. During the most recent glaciation (about 27,000 -11,500 

years ago), sea level dropped about 125 meters below present sea 

level. 

18 
Lower sea levels during glacial times exposed continental shelves in 

many parts of the world. When the lowering of global sea level reached 

its maximum (100 to 125 m below present sea level), a vast area of 

shallow sea floor was exposed between western Alaska and Siberia (as 

well as large areas north of Siberia). This large land connection 

between northwestern North America and northeastern Asia (the Bering 

land bridge) has served as an important route that allowed animals and 

plants to spread between North America and Eurasia. During major 

glaciations, Alaska was isolated from the rest of North America by the 

Cordilleran and Laurentide ice sheets, and became an eastern 

extension of northeastern Asia. Glacial climates of the Bering land 

bridge and adjacent land masses were dry and cold, but summers were 

warm enough to support grassy tundra or tundra/steppe vegetation. 

19 
Do modern analogs exist for the past full-glacial vegetation of the 

Bering land bridge and interior Alaska? No exact modern analog may 

exist, but there may be near analogs such as this Kobresia-herb 

vegetation in northern Mongolia. Other near-analog vegetation exists 

today on Wrangel Island north of northern Siberia. Fossil pollen and 

plant macrofossil evidence from Alaska and Yukon Territory indicate 

that forest vegetation was essentially eliminated from the landscape. 

However, genetic studies of spruce populations in western North 

America suggest that some spruce trees probably survived in Alaska or 

Yukon Territory during the last major glacial interval. Small populations 

of spruce trees may have survived in sheltered valleys, but so far, no 

convincing fossil evidence has been found to show where such trees 

may have survived. 

Photo by Dr. Steve Young, Center for Northern Studies. 

20 
Grass-sedge-herb tundra adjacent to the Greenland ice sheet may be 

another reasonable analog for full-glacial vegetation on the Bering land 

bridge and interior Alaska and Yukon. 

Photo by Dr. Todd Hinkley, USGS. 

21 
This late winter photograph (taken in Mongolia) of a Przewalski’s horse 

in Mongolia probably approximates the conditions that prevailed during 

full glacial time on the Bering land bridge and in interior Alaska: thin 

snow cover and sparse, grassy vegetation. 

Photo by Dr. Richard Reading, Wildlife Conservation Biologist, Denver 

Zoo. 

22 
Fossil plant and pollen assemblages from Alaska and Yukon Territory 

suggest that interior regions were treeless, or nearly so. Genetic 

evidence from modern spruce populations in Alaska suggest that boreal 

spruce may have survived in unglaciated parts of eastern Alaska and 

Yukon during the late Wisconsin. If so, they were probably scattered, 

small populations that may have been stunted, low-growing trees such 

as these spruce near the altitudinal limits of tree growth in the coast 

mountains of southeastern Alaska. 

Photo by Dr. Thomas Ager, USGS. 

23 
Soon after deglaciation, much of the herb-dominated tundra vegetation 

was rapidly displaced by shrub-herb tundra composed largely of dwarf 

birch, willow, blueberry, labrador tea, crowberry, along with cottongrass 

and other sedges, grasses, and tundra herbs. This type of vegetation 

was well established in Alaska by about 15,500 yr B.P. 

Photo by Dr. Thomas Ager, USGS. 

24 
By about 2.2 million years ago, interglacial vegetation in interior and 

inland south-central Alaska was composed of boreal forest trees and 

shrubs that exist in these regions today, during the present Holocene 

interglacial. Major trees and shrubs common in interglacials include 

spruce, larch, birch, poplar (and probably aspen), alder, willow, 

bayberry, and heaths (blueberry, huckleberry, cranberry, Labrador tea, 

and crowberry). During glaciations, boreal forest vegetation was either 

eliminated from interior Alaska by cold, dry temperatures, or their 

populations were greatly reduced and restricted in their distribution. 

In south-central and other mountainous areas of Alaska, forest 

vegetation was eliminated by the expansion of glaciers and glacial lakes 

in lowlands during late Pliocene-Pleistocene ice ages. 

Photo by Dr. Thomas Ager, USGS. 

25 
Normalized o

xygen

isotope c

omposition 

The last interglacial period was the last time the Earth was 

as warm or warmer than the present, about 125,000 years ago: 

20 40 60 80 100 120 140

Last 

glacial 

What were climate 

and vegetation like 

the last interglacial 

Present


interglacial
 Last 

interglacial 

-3 

-2 

-1 

in Alaska during 

0 

period? 

1 

2 

0 

Age, in thousands of years before present 

(Martinson et al., 1987, QR) 

Oxygen-isotope curve derived from analysis of foraminifera in deep 

ocean cores provides a sea-water temperature history for the past ca. 

150,000 years. It shows that the last interglacial lasted about 20,000 

years, and the present interglacial has been underway for 10,000 years. 

For 120,000 of the past 150,000 years, glacial or interstadial climates 

have dominated. Interstadials are intermediate between cold, full glacial 

climates and relatively warm interglacial climates. In Alaska, forest 

vegetation was widespread only during interglacials and was greatly 

reduced or possibly eliminated during glacial times. 

Reference: Martinson, D.G., Pisias, N.G., Hayes, J.D., Imbrie, J., 

Moore, T.C., Jr., and Shackleton, N.J., 1987. Age dating and the orbital 

theory of ice ages: Development of a high-resolution 0 to 300,000-year 

chronostratigraphy: Quaternary Research, v. 27, p. 1-29. 

Figure courtesy of Dr. Dan Muhs, USGS. 

26 
Boreal forest composed of white spruce, black spruce, alder, willow, 

balsam poplar, and aspen cover much of interior and inland southcentral Alaska today. It spread across interior and much of western 

Alaska between about 10,000 - 6,000 years ago. This photograph was 

taken in Copper River basin, with Wrangell Mountains in background. 

Photo by Dr. Thomas Ager, USGS. 

27 
Vegetation map of Alaska today (after Viereck and Little, 1972), to bring 

our focus on the Cook Inlet area of south-central Alaska, where alpine 

and shrub tundra, boreal forest, and coast forest vegetation grow today 

in close proximity and in fact overlap in some areas. Cook Inlet is 

located northwest of the Gulf of Alaska as shown here. 

Reference: Viereck, L.A., and Little, E.L., Jr., 1972. Alaska trees and 

shrubs: Washington, D.C., USDA Forest Service Agriculture Handbook 

no. 410, 265 p. 

28 
During maximum glacial events of the Pleistocene, it is likely that 

glaciers expanded all the way to edge of the continental shelf, leaving 

few areas of exposed land to act as refugia for plants and animals. 

The extent of glacial ice in south-central Alaska during the most recent 

major glaciation, the late Wisconsin glacial interval, is not well 

understood, however. It may have been less extensive than shown 

here. Areas of south-central Alaska that escaped total burial by glacial 

ice supported mostly alpine and graminoid-herb tundra vegetation. Such 

areas included part of southwestern Kodiak Island and parts of the 

western Kenai Peninsula. 

29 
Glaciers covered most of the Cook Inlet region during the last 

glaciation, but glacial retreat began about 18,000 years ago. 

Deglaciation was interrupted by at least three brief readvances of 

glaciers, with each advance less extensive than its predecessors. 

Photo by Dr. Austin Post, USGS. 

30 
Map of upper Cook Inlet region today, with hatchured semicircles 

showing maximum terminal positions of the last glacial advance in Knik 

and Turnagain Arms (Elmendorf Stade, ca. 16,000-14,000 yr B.P.). 

Pollen records from the Anchorage lowland (A.L., site 1) and eastern 

Turnagain Arm (Girdwood, site 4) record vegetation changes in the 

region since about 14,500 yr B.P. 

The history of vegetation development in the Anchorage lowland (site 1) 

reflects the drier, colder climates associated with areas in the 

precipitation shadows formed by the Chugach and Kenai Mountains. 

31 
In the Anchorage lowland, deglaciation and marine inundation were 

followed by isostatic rebound of the lowland to a position above sea 

level by about 14,000 yr B.P. Sedge-birch tundra colonized the 

lowland, followed quickly by birch-sedge-shrub-herb tundra, then birchwillow-alder-fern tundra and shrubland. Poplar trees and willows 

colonized by about 10,000 yr B.P., and boreal spruce colonized the 

area by about 8,800 yr B.P. 

Chart based on data from Dr. Thomas Ager, USGS. 

32 
Alpine tundra (early summer) in the southwestern Talkeetna Mountains 

of south-central Alaska provides a likely modern analog for the tundra 

vegetation that colonized much of south-central Alaska during and 

immediately following deglaciation. Tundra vegetation became 

shrubbier over time, as herbaceous plants were partly displaced by 

willow and heath species. 

Photo by Dr. Thomas Ager, USGS. 

33 
The first trees to colonize Cook Inlet were members of the poplar genus 

(Populus): balsam poplar (Populus balsamifera), and perhaps aspen 

(Populus tremuloides) and cottonwood (Populus trichocarpa). The 

history of these tree species is incomplete, because the pollen they 

produce is thin-walled and is often not preserved in the fossil record. 

Photo by Dr. Thomas Ager, USGS. 

34 
Alders quickly colonized south-central Alaska around 10,000 yr B.P. 

This shrub is Sitka alder, which is the dominant alder type in coastal 

regions of southeastern and south-central Alaska. 

Photo by Dr. Thomas Ager, USGS. 

35 
Black spruce colonizing peat bog in Anchorage lowland near 

Earthquake Park, upper Cook Inlet. Larger trees growing on welldrained ridges adjacent to the bog include white spruce trees and 

deciduous trees such as balsam poplar and paper birch. 

Photo by Dr. Thomas Ager, USGS. 

36 
Boreal forest in south-central Alaska in September. Boreal forest 

vegetation colonized upper Cook Inlet by about 8,800 yr B.P. 

Photo by Dr. Thomas Ager, USGS. 

37 
Map of upper Cook Inlet region today, showing position of pollen 

records from eastern Turnagain Arm (Girdwood, site 4). In contrast to 

the Anchorage lowland, which has a relatively cool and dry climate, the 

upper Turnagain Arm area has a much wetter and somewhat warmer 

climate. These varied climate conditions have influenced the history of 

vegetation development in the region following deglaciation. 

38 
Map of Prince William Sound and vicinity. Ongoing research in this 

region involves pollen analysis of peat cores obtained from bogs from 

sites scattered across this map area. 

39 
The maritime climate of eastern Turnagain Arm did not favor 

colonization by boreal forest vegetation. Following deglaciation about 

14,000 yr B.P., herb-shrub tundra vegetation was the initial colonizer 

following glacial retreat, followed by alders during the early Holocene. 

Forestation in eastern Turnagain Arm was delayed until the late 

Holocene (ca. 3,000 yr B.P.), when mountain hemlock and Sitka spruce 

migrated into the area from the east. 

Chart based on data obtained by Dr. Thomas Ager, USGS. 

40 
Comparison of mean annual precipitation in a west to east transect from 

Anchorage lowland to northwestern Prince William Sound. Boreal 

forest vegetation occurs where precipitation is low (a result of 

precipitation shadows inland from coastal mountain ranges). Pacific 

coastal forest and alder thickets occupy areas with high precipitation 

and milder winter temperatures. 

41 
Sitka alder and abundant ferns colonized much of south-central Alaska 

during the early Holocene, particularly along the coast. In many areas, 

alders and ferns were dominant elements of the vegetation until they 

were partly displaced by expanding coastal forests of mountain hemlock 

and Sitka spruce during the past 3,200 years. 

Photo by Dr. Thomas Ager, USGS. 

42 
Mountain hemlock and Sitka spruce forest grow in eastern Turnagain 

Arm and adjacent Prince William Sound. Mountain hemlocks growing 

in peat bogs and muskegs tend to form slow-growing, stunted forms, 

such as those seen in the foreground. From the time of deglaciation to 

the establishment of coastal forest in this region, about 11,000 years 

elapsed. Vegetation change in response to climate change can be slow 

if there are no nearby seed sources of plants that are adapted to the 

new climatic conditions. Photographed near Girdwood in upper 

Turnagain Arm. 

Photo by Dr. Thomas Ager, USGS. 

43 
Sitka spruce can attain ages of 600 years or more and sometimes grow 

to awe-inspiring size. This tree and many others like it were seen on 

southwestern Hinchinbrook Island in Prince William Sound. Although 

coastal forest vegetation has been established in some areas of southcentral Alaska for about 3,000 years, other areas suitable for the growth 

of trees are still unforested. The process of colonization of coastal 

areas by forest vegetation is still underway in many areas. Late-arriving 

trees such as western hemlock and yellow cedar have colonized only 

limited areas of the south-central Alaskan coast and will continue to 

spread westward. 

Photo by Dr. Thomas Ager, USGS. 

44