Rainfall, for instance, develops a somewhat
vertical zoning. Up to 2–3 km of elevation in
the middle latitudes, for example, the rainfall on
mountains slopes increases. Unbroken mountain
ranges are effective barriers to the passage of
moisture (Fig. 9.6). The mountain ranges along
the Pacific Coast of the United States, for example, intercept moisture transported from the
Pacific Ocean by prevailing westerly winds, so
that coastal ranges are moist and inland regions
are dry. However, the effect of mountains in
producing rain depends on the character of the
air blowing against them. Cold air can rise only
sluggishly and may produce only low stratus
(sheetlike) clouds, as along the Peruvian coast
(see Fig. 7.8, p. 74). The heaviest rains in the
world, however, are received on mountain slopes
which lie in the path of warm, buoyant, moistureladen air, as the island of Kauai and the southern
slopes of the Himalayas.
East–west mountain ranges act as temperature
divides. The lowlands on the poleward side of a
mountain range are made colder than they would
otherwise be, and the lowlands on the equatorward side are made warmer.
Mountain climates in mid-latitudes are an
important influence on river flow and floods.
The higher ranges serve as snow storage areas,
keeping back the precipitation until early or
midsummer, releasing it slowly through melting.
This maintains continuous river flow.
The upper tree line in mountainous regions is
caused by the lack of adequate warmth. The
lower tree line found in arid regions is related
to the lack of adequate moisture. This combination restricts the growth of forests in arid regions
to more or less wide bands along the slopes of
mountains (Fig. 9.7).
Soils also change their character with increasing altitude, responding to the changes in climate
and vegetation. Figure 9.8 shows how soil
profiles change with life zones in the western
United States.
Man has caused many changes in the mountain ecosystems by mining, agriculture, grazing,
and fire. The elevational limits of the various
forms of human settlement are similar to the
horizontal limits of the lowlands. The highest
settlements are associated with mining. The
next highest settlements are commonly
supported by the pasturage of domestic animals.
Lower down, the various types of agricultural
settlement appear. Both the animals and the
crops supporting these settlements show fairly
distinct upper limits in any one region. Because
sheep can exist on much scantier pasturage than
cattle, they are driven highest in the mountains.
Cattle and horses usually occupy the richer
pastures lower down. Of the crops, the potato
reaches the highest altitude. Lower down, the
various grains arrange themselves in the expectable sequence: barley, rye, wheat, maize, and
rice, in descending order. The tropical crops
occupy the lower slopes of low-latitude
mountains.
In many areas where agriculture has been the
main objective, it has been a disaster for the
ecosystem. The hill lands of southern China
were stripped of their once-rich soils by careless
land use. The same has happened in southern
Europe, in the Andes, and in the Appalachians
and Ozarks of the United States. There are areas,
however, where steep slopes have been tilled,
using terraces to reduce erosion, for long periods
of time without substantial loss. The best
Fig 9.5 Snowfields above valley glaciers near Mount
McKinley, Alaska. Photograph by Norman Herkenham,
National Park Service
92
9 The Mountain Ecoregions
vertical zoning. Up to 2–3 km of elevation in
the middle latitudes, for example, the rainfall on
mountains slopes increases. Unbroken mountain
ranges are effective barriers to the passage of
moisture (Fig. 9.6). The mountain ranges along
the Pacific Coast of the United States, for example, intercept moisture transported from the
Pacific Ocean by prevailing westerly winds, so
that coastal ranges are moist and inland regions
are dry. However, the effect of mountains in
producing rain depends on the character of the
air blowing against them. Cold air can rise only
sluggishly and may produce only low stratus
(sheetlike) clouds, as along the Peruvian coast
(see Fig. 7.8, p. 74). The heaviest rains in the
world, however, are received on mountain slopes
which lie in the path of warm, buoyant, moistureladen air, as the island of Kauai and the southern
slopes of the Himalayas.
East–west mountain ranges act as temperature
divides. The lowlands on the poleward side of a
mountain range are made colder than they would
otherwise be, and the lowlands on the equatorward side are made warmer.
Mountain climates in mid-latitudes are an
important influence on river flow and floods.
The higher ranges serve as snow storage areas,
keeping back the precipitation until early or
midsummer, releasing it slowly through melting.
This maintains continuous river flow.
The upper tree line in mountainous regions is
caused by the lack of adequate warmth. The
lower tree line found in arid regions is related
to the lack of adequate moisture. This combination restricts the growth of forests in arid regions
to more or less wide bands along the slopes of
mountains (Fig. 9.7).
Soils also change their character with increasing altitude, responding to the changes in climate
and vegetation. Figure 9.8 shows how soil
profiles change with life zones in the western
United States.
Man has caused many changes in the mountain ecosystems by mining, agriculture, grazing,
and fire. The elevational limits of the various
forms of human settlement are similar to the
horizontal limits of the lowlands. The highest
settlements are associated with mining. The
next highest settlements are commonly
supported by the pasturage of domestic animals.
Lower down, the various types of agricultural
settlement appear. Both the animals and the
crops supporting these settlements show fairly
distinct upper limits in any one region. Because
sheep can exist on much scantier pasturage than
cattle, they are driven highest in the mountains.
Cattle and horses usually occupy the richer
pastures lower down. Of the crops, the potato
reaches the highest altitude. Lower down, the
various grains arrange themselves in the expectable sequence: barley, rye, wheat, maize, and
rice, in descending order. The tropical crops
occupy the lower slopes of low-latitude
mountains.
In many areas where agriculture has been the
main objective, it has been a disaster for the
ecosystem. The hill lands of southern China
were stripped of their once-rich soils by careless
land use. The same has happened in southern
Europe, in the Andes, and in the Appalachians
and Ozarks of the United States. There are areas,
however, where steep slopes have been tilled,
using terraces to reduce erosion, for long periods
of time without substantial loss. The best
Fig 9.5 Snowfields above valley glaciers near Mount
McKinley, Alaska. Photograph by Norman Herkenham,
National Park Service
92
9 The Mountain Ecoregions