Forest Types Overview
Forest type, or forest cover, refers
to the dominant tree species in the overstory
of a given site. The distribution of forest types
across the landscape is determined by factors
such as climate, soil, elevation, aspect, and
disturbance history (Rogers and others 2001).
A number of Colorado’s forests are characterized
as disturbance driven. The life history of these
forest types evolved with a cycle of natural disturbance
such as wildfire, insect infestations, flooding,
avalanches, windstorms, or disease infections.
These disturbances served to periodically rejuvenate
forests, ensuring a variety of forest types, age
classes and densities across the landscape. Due
to lack of disturbance, the majority of Colorado’s
forests are concentrated in older age classes,
with virtually no significant forest in the zero
to 20-year age classes.
A number of forest types exist throughout Colorado,
with the most extensive being spruce-fir, ponderosa
pine, lodgepole pine, aspen, and piñon-juniper.
A brief description of these types is provided
below. Other forest types in Colorado include
Douglas-fir, southwestern white pine, bristlecone
pine, limber pine, Colorado blue spruce, and the
cottonwood-willow combination found in many riparian
areas.
Learn more about
Douglas-fir.
Learn more about
limber pine.
Learn more
about bristlecone pine.
Learn more about
plains cottonwood.
Learn more
about narroleaf cottonwood.
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Ecoregions
Issues and events that influence forest condition
often occur across forest types, ownerships, and
political boundaries. As a result, scientists,
researchers, and land managers must also find
a way to assess and treat these issues in a boundary-less
way. Ecoregions are often used as a non-political
land division to help researchers study forest
condition. An ecoregion is a large landscape area
that has relatively consistent patterns of topography,
geology, soils, vegetation, climate, and natural
processes (Shinneman and others 2000). Many smaller
ecosystems may reside within an ecoregion.
Colorado contains parts of six major ecoregions
(Bailey 1995), with the most prominent being the
Southern Rockies which occupies most of the state’s
central and western portions and the Great Plains-Palouse
Dry Steppe in the eastern half of the state. Other
ecoregions include the Intermountain Semi-Desert
and Desert, the Nevada-Utah Mountains, and the
Colorado Plateau. Forests are found in all ecoregions
of the state, but the Southern Rockies contain
the most forested area and the greatest variety
of forest types.
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Lodgepole Pine
Lodgepole pine is an important
high-elevation species in the state’s northern
Rocky Mountain region.
Lodgepole pine (Pinus contorta)
is a familiar species in the montane and subalpine
forests of Colorado’s northern Rocky Mountains.
It tolerates a wide variety of climatic and soil
conditions, but achieves its best growth on gentle
slopes and in basins with well-drained, slightly
acidic, sandy, or gravelling loams. Lodgepole
pine is intolerant to shade and thrives in the
aftermath of fire. Many lodgepole produce serotinous
cones, which open in response to extreme heat
and release an abundance of seeds. These long-lived
cones may remain viable for decades, waiting for
a fire to release their seeds. Natural lodgepole
regeneration in open, sunny areas often produces
very dense stands of 20,000 or more trees per
acre. Lodgepole pine is the third most extensive
commercial forest type in the Rocky Mountains
and is used in Colorado for products such as fenceposts,
corral rails, utility poles, paneling, railroad
ties, and pulpwood. Mountain
pine beetle (mpb) and dwarf
mistletoe are the most damaging agents of
lodgepole pine.
Learn more
about lodgepole pine.
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Ponderosa Pine
Ponderosa pine is Colorado’s
dominant lower timberline species and an important
source of wood products.
The ponderosa pine (Pinus ponderosae scopulorum)
is the most widely distributed pine in North America
and occupies a vast area in the West. It is well
adapted to high temperatures and low moisture
and is highly resistant to low-intensity fire.
Ponderosa pine is generally the dominant lower
timberline species in Colorado’s montane
zone, particularly at elevations from 5,800 to
9,800 feet. A long taproot helps the drought-resistant
pine obtain adequate moisture and also decreases
its chances of being uprooted by strong winds.
Ponderosa pine generally evolved with a natural
cycle of frequent, low-intensity fire which eliminated
competing conifer seedlings but allowed mature
pines to survive. Because it is more accessible
than other species, and largely in private ownership,
ponderosa pine has been an important source of
lumber and other wood products (Benson and Green
1987). The advent of aggressive fire suppression
has resulted in increased encroachment of shade-tolerant
Douglas-fir (Pseudotsuga
menziesii) in the understory of ponderosa
pine stands. When fire occurs in these mixed forests,
the understory Douglas-fir tend to carry fire
in ladder fashion into the pine crowns, making
the fire much more likely to reach unnatural,
stand replacement proportions. Forest monitoring
data indicates that ponderosa pine acreage in
Colorado may be declining due to fire suppression
practices that allow encroachment of Douglas-fir.
Learn more
about ponderosa pine.
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Spruce-Fir
The spruce-fi r forest
type is the most extensive in the state.
The slow-growing Engelmann spruce (Picea
engelmannii), in association with the smaller,
narrow-crowned subalpine fir (Abies lasiocarpa),
forms the vast green vistas so typical of Colorado’s
high country or subalpine zone. The spruce-fir
combination often results in a climax-type forest
at high elevations, despite the existence of many
uneven-aged stands. This is because both species
are shade tolerant and tend to quickly repopulate
shaded gaps in the forest. At lower elevations,
Engelmann spruce forests can contain more moisture
than any other stand type in the state. Snow packs
may exist well into summer and water yield from
these forests is very important. The return interval
for naturally-occurring fires within the spruce-fir
forest may be 300 years or longer. Thin bark and
the persistence of dead lower limbs increases
the spruce’s susceptibility to fire as well
as the likelihood of intense crown fires and tree
mortality. In the case of a stand-replacing fire,
it may be as many as 300 years before a spruce-fir
forest again occupies the site. Spruce-fir is
the most extensive type in the state and also
the most publicly owned (Benson and Green 1987).
Learn more
about Engelmann spruce.
Learn more
about Colorado blue spruce.
Learn more
about subalpine fir.
Learn more about
white fir.
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Aspen
Aspen represents the state’s
only widespread, native deciduous tree.
The aspen’s (Populus tremuloides)
delicate leaves and vibrant fall color are often
displayed as a symbol of Colorado itself. Aspen
represents the state’s only widespread,
native deciduous tree and is most commonly found
in the western two-thirds of the state. Aspen
can occur over a wide variety of sites ranging
from dry, high-elevation grasslands to poorly
drained meadow sites. The sun-loving aspen regenerates
through sprouts or suckers that are produced by
its roots in response to increased soil or root
temperatures. Because of this sprouting response,
aspen is often first to colonizes forest clearings,
burns, or other disturbed sites. As stands grow,
and shade on the site increases, conifer species
begin to invade and may eventually replace the
aspen. The open canopy and high light levels of
aspen stands lead to a more lush understory than
that of neighboring conifers. Drier groves generally
consist of grasses, with some wildflowers and
medium-sized shrubs such as ninebark or cinquefoil.
Wetter forests have an abundance of wildflowers,
including meadow rue, arnica, lupine, and paintbrush,
and sometimes a thicker shrub layer consisting
commonly of snowberry or wild rose. Combined with
dead and downed material, this ground layer provides
cover for small rodents and a source of food for
birds. Throughout its lifecycle, aspen also provides
critical habitat for wildlife such as cavity-nesting
birds and those, like the warbling vireo, who
are dependent on deciduous trees for breeding
(Kingery 1998).
Learn more about
Colorado's aspen.
Learn about the numerous
insects and diseases affecting Colorado's aspen.
What's
Happening in Colorado's Aspen Forests? (1 MB PDF)
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Piñon-Juniper
Pine nuts and juniper berries provide an important
source of winter food for wildlife in Colorado’s
pinion-juniper woodlands.
Piñon pine and juniper woodlands are
widespread in the lower elevations of Colorado’s
West Slope. While the U.S. Forest Service distinguishes
32 piñon and 23 juniper plant communities,
Colorado piñon pine (Pinus edulis)
and Utah juniper (Juniperus osteosperma)
are the most common species. Annual precipitation
is typically from 10 to about 15 inches in piñon-juniper
or “P-J” woodlands, and tree species
in these communities have developed resistance
to both drought and cold. Juniper tends to grow
at lower elevations and in more arid areas
as
its scaled foliage allows it to conserve water
more effectively than piñon pine. Juniper-dominated
woodlands tend to include open savannas of
scattered
trees without a significant shrub component,
except in areas where big sagebrush has become
dominant
as a consequence of grazing. Woodland communities
have expanded considerably over the course
of
this century in many parts of the Colorado Plateau,
including New Mexico, Arizona, and Utah. Tree
densities have increased in some areas to the
point that larger proportions of piñon-juniper
woodland now support damaging crown fires.
Learn more about
piñon pine.
Learn more about
Rocky Mountain juniper.
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