Visualizing Environmental Science

132 CHAPTER 6 Ecosystems and Evolution

Average monthly precipitation in cm

14

12

10

8

6

4

2

0

Months

10

15

20

5

0

–5

–10

–15

–20

–25

–30

Average monthly temperature in

°C

JFMAMJJASOND

Freezing

point

Based on data from http://www.worldclimate.com

Paul Nicklen NG/Images Collection

flat topography (or surface features), and the layer of

permafrost, produces a landscape of broad, shallow lakes

and ponds, sluggish streams, and bogs.

Tundra has low primary productivity (see Figure 5.8)

and supports relatively few species compared to other

biomes, but the species that do occur there

often exist in great numbers. Mosses, lichens,

grasses, and grasslike sedges are the dominant

plants. Stunted trees and shrubs grow only in

sheltered locations. As a rule, tundra plants

seldom grow taller than 30 cm (12 in).

Animals adapted to live year-round in the

tundra include lemmings, voles, weasels, arctic

foxes, snowshoe hares, ptarmigan, snowy owls, and musk

oxen. In summer, caribou migrate north to the tundra to

graze on sedges, grasses, and dwarf willow. Dozens of bird

species also migrate north in summer to nest and feed

on abundant insects. Mosquitoes, blackflies, and deer-

flies survive winter as eggs or pupae and appear in great

numbers during summer weeks.

Tundra recovers slowly from even small disturbances.

Oil and natural gas exploration and military use have

caused damage to tundra likely to persist for hundreds of

years (see the Chapter 17 case study).

Climate change is beginning to affect the arctic tun-

dra. As the permafrost melts, conifer trees (cone-bearing

evergreens) are replacing tundra vegetation. The trees

have a lower reflectivity than snow, ice, or tundra vegeta-

tion, causing additional warming, an example of a posi-

tive feedback mechanism. In addition, the permafrost

is melting, and boreal forest is moving northward, two

other results of a warming climate.

Tundra

Tundra (or arctic tundra) occurs in the extreme

northern latitudes where the snow melts sea-

sonally (Figure 6.4). The Southern Hemi-

sphere has no equivalent of the arctic tundra

because it has no land in the corresponding

latitudes. A similar ecosystem located in the higher eleva-

tions of mountains, above the tree line, is called alpine

tundra.

Arctic tundra has long, harsh winters and short sum-

mers. Although the arctic tundra’s growing season is

short, the days are long. Above the Arctic Circle, the sun

does not set at all for many days in midsummer. There is

little precipitation, and most of the yearly 10 to 25 cm (4

to 10 in) of rain or snow falls during summer months.

Most tundra soils formed when glaciers began

retreating after the last ice age, about 17,000 years ago.

These soils are usually nutrient poor and have little de-

tritus, such as dead leaves and stems, animal droppings,

or remains of organisms. Although the tundra’s surface

soil thaws during summer, beneath it lies a layer of perma-

frost, permanently frozen ground that varies in depth and

thickness. Permafrost impedes drainage, so the thawed

upper zone of soil is usually waterlogged during summer.

Limited precipitation, combined with low temperatures,

tundra The treeless

biome in the far north

that consists of boggy

plains covered by

lichens and mosses;

it has harsh, cold

winters and extremely

short summers.

ÀV̈VÊÌ՘`À>ÊUʈ}ÕÀiÊÈ°{Ê

Because of the tundra’s short growing season and permafrost,

only small, hardy plants grow in the northernmost biome

that encircles the Arctic Ocean. Photographed in the

Yukon Territory, Canada. The Ogilvie Mountains are in the

background. Climate graph shows monthly temperatures (line)

and precipitation (bars) for Fort Yukon, Alaska.

Interpreting Data

Analyze the data and infer if the average monthly

temprature in the Alaskan tundra is at or near

freezing. Give the number of months, if any, that the

average monthly temperature is below freezing.