Chapter 12: The Great Plains And Praires

How Does Climate Change Influence Alaska's Vegetation?
Insights from the Fossil Record

Plant fossils, such as leaves, wood, cones, pollen, and seeds, provide important evidence of how Alaska's vegetation has responded to climate changes over time periods of centuries to millions of years. Long-term trends of global temperatures have been reconstructed from oxygen isotope measurements of microscopic fossils (foraminifera) in the Pacific Ocean (fig. 1). This temperature curve can be used to compare the major changes in Alaskan vegetation with global climate changes spanning the past 20 million years.

Figure 1. Pacific Ocean seawater temperature trends derived from measurements of oxygen isotopes in fossil marine organisms provide the climatic framework for interpreting the Alaskan fossil plant record.

USGS studies of the Alaskan fossil record of plants include data from many natural exposures and sediment cores (fig. 2). These data provide the basis for reconstructing the record of past vegetation changes over millions of years of Earth history. The fossil record shows that dramatic changes in high latitude vegetation have occurred many times in the past, primarily in response to global climate changes. The record further suggests that the magnitude of ecological response to global climate change is greater at high latitudes than at low latitudes. During the Miocene, a major global warming event occurred between 17 to 14.5 Ma (million years ago). In Alaska and in other high latitude regions of the world, this warming event profoundly changed vegetation from a conifer-dominated forest with few temperate hardwoods to a temperate forest containing many tree and shrub species now found far to the south in Asia and North America, such as oak, hickory, beech, chestnut, walnut, wing-nut, elm, holly, basswood, hazelnut, and sweetgum. In order for temperate vegetation to grow at such high latitudes, the mean annual temperature of interior Alaska must have been about 25-30° F warmer than today.

Global cooling began about 14.5 Ma, and its influence on Alaskan vegetation was abrupt and dramatic. Temperate trees disappeared, leaving behind a simpler forest vegetation of hardy birch, alder, and pine species until a more complex conifer-dominated forest developed after about 12 Ma. A series of cool climate pulses during the middle and late Miocene and late Pliocene eliminated additional tree species from Alaskan forests. Minor warming events about 11, 9, and 7 Ma reversed these trends slightly for brief periods, but the overall trend during the Miocene and Pliocene was the steady reduction in tree and shrub species, leaving hardy plant types that could grow in Alaska's progressively cooler climate. By the middle Pliocene, about 3 Ma, Alaskan vegetation began to resemble modern Alaskan boreal forests. Middle Pliocene forests were composed primarily of pine, spruce, larch, birch, alder, and willow, and with the exception of pine, these compose the vegetation found today in Alaska's boreal forests.

Figure 2. U.S. Geological Survey's global change drilling project at Fort Yukon, Alaska recovered a detailed record of climate and environmental change for interior Alaska spanning much of the past 16 million years.

Global climate changes during the past 2.5 million years have been extreme, oscillating between relatively long periods (ca. 100,000 years) of predominantly cold, dry glacial climates and shorter intervals (ca. 10-20,000 years) of warmer, moister interglacial climates. During full glacial intervals, mean annual temperature in interior Alaska declined about 9 to 15° F below modern temperatures. During cold periods, glaciers expanded from the 5 percent of Alaska they now cover to 30 to 50 percent of the present area of the State. When glaciers expanded and temperatures declined, tundra vegetation spread to lower elevations and forests were reduced to small areas in the eastern interior, and some tree species may have been eliminated entirely from Alaska (fig. 3).
During warm interglacial periods such the one we are currently experiencing (the Holocene, spanning the past 10,000 years), boreal forests of spruce, larch, poplar, birch, alder, and willow spread throughout most of interior Alaska. During the warmest part of the previous interglacial (about 130,000 to 120,000 years ago), average growing season temperatures in Alaska appear to have been at least 5° F warmer than today. Spruce-dominated boreal forests spread north of the Brooks Range and west to the Bering Sea coast, areas where lowland tundra vegetation now grows. Warmer summer temperatures allowed trees to grow at higher altitudes than today. The expansion of forests into higher elevations greatly reduced the area covered by upland tundra communities for thousands of years.
During periods of less extreme cool climates (interstadials), Alaskan vegetation was dominated by shrub and herb-shrub tundra communities; boreal forest vegetation was restricted to a few areas in the interior lowlands of Alaska. The fossil record provides important evidence of how past climate changes influence Alaskan vegetation. Examples of both colder and warmer than modern climatic conditions of the past can be found in the geologic record, and the ecological responses to climate changes of different magnitudes can be studied. This valuable record of the past is useful for testing the results of computer simulations of climate change on global and regional scales.

Implications for future climate change

The study of past climates and ecological changes in Alaska are an important key to understanding the likely consequences of future climate changes in high latitude ecosystems. Future climate changes, whether triggered by human-induced changes in the atmosphere or by natural climate cycles will result in changes in the species composition and distribution of vegetation types. On the basis of the fossil record and climate history of Alaska, we can expect that future periods of cooler, drier climate will result in shrinkage of forest boundaries, lowering of altitudinal tree line, and expansion of tundra vegetation into lower elevations. A future change to warmer, moister climates will result in expansion of Alaska's forests into areas now occupied by tundra. The past record also shows that the magnitude of future global scale climate changes and ecological responses will be greater at high latitudes than at lower latitudes.

http://pubs.usgs.gov/fs/fs-0071-97/

Alaska - Vegetation, Wildlife and People

Soils suitable for tillage are located in the lowlands and valley areas where they may have originated from river deposits. They contrast with the shallow soils on the steeper slopes and uplands. Permafrost is common north of the Alaska Range, thawing out only at the surface in summer creating waterlogged conditions. Roads and buildings are subject to damage in these areas.

Extensive coniferous forests adorn the slopes of the south-eastern mountains. In the interior the forest cover thins out giving way to grasslands and marsh. North of the Brooks Range are the treeless plains of the tundra, supporting mosses, lichens and grasses capable of surviving the severe climatic conditions which are present in those areas. The caribou live here as well as wild fowl and small fur-bearing mammals, which are essential to the native economy.

River fish, such as salmon and sea mammals such as the seal and the whale, are also captured. In the south the larger mammals, including the brown bear and prolific salmon streams are an attraction to sportsmen.

Alaska has the smallest population of all the American States. About a fifth is native Indian and Eskimo. Their standard of living, though improving, is lower than that of the white population. Many depend on a subsistence economy. The white population enjoys a high living standard; many come for short and profitable periods on contract, often during the summer months.

The temporary population gives the State a predominance of males and a per capita income, which is one of the highest in the USA. Alaska's settlements are small and scattered widely with some concentration along the Gulf Coast. The three largest centres, Anchorage, Fairbanks and Juneau, are service centres for numerous resource-oriented communities, fishing settlements and timber camps. To the north and west of the railroad and highway system, on the coast and in the river valleys are a large number of native settlements, which depend on subsistence hunting and fishing.

Less than 10 per cent of tillable land in Alaska is used, partly due to the expense of making the land productive. The principle farming area is north of Anchorage in the Matanuska Valley. Eggs, potatoes, dairy products, lettuces and cabbages are the most important items produced for local consumption. Farming is also found around Fairbanks. Moderate climate and large areas of grassland near the Gulf of Alaska have encouraged cattle farming.

Vast forests are located in two main areas. The coastal forests of the southeast produce 90 per cent of the State's wood, mainly western hemlock and sitka spruce. Both are situated near tidewater, which enables transport to the pulp mills at Ketchikan and Sitka easy.

Although gold has declined since the initial goldrush in the 1890s, it still remains the most valuable mineral resource. Coal is also found throughout the State, the most important field near Fairbanks. Other minerals found include gravel, sand, lead and mercury. There are also deposits of iron in the southeast.

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