A biome is a large, naturally occurring community of flora and fauna occupying a major habitat, defined primarily by its specific climate and geography. Colorado’s topography, featuring an ascent from the Great Plains to the high peaks of the Rocky Mountains, creates significant ecological diversity. This variation is driven by “life zones,” where temperature and moisture levels change with increasing elevation. For every 1,000 feet gained in altitude, the average temperature drops and precipitation often increases, creating distinct ecological gradients that support unique plant and animal communities.
Plains and Grasslands
The lowest-elevation biome in the state is the plains and grasslands, characterized as a shortgrass steppe. This region is semi-arid, with low annual precipitation that typically falls as summer thunderstorms. The ecosystem is adapted to limited moisture and high winds.
The dominant vegetation consists of warm-season grasses that tolerate drought and heavy grazing, such as blue grama (Bouteloua gracilis) and buffalo grass (Buchloë dactyloides). These grasses grow close to the ground, forming a dense, resilient sod layer that conserves soil moisture. Historically, this biome supported bison and pronghorn, which shaped the landscape through their grazing patterns.
Today, the shortgrass steppe remains home to animals like the black-tailed prairie dog, whose colonies create habitat for other species, including the burrowing owl. Pronghorn (Antilocapra americana) roam the open plains, utilizing their speed to evade predators. This ecosystem relies on a natural cycle of grazing and fire to maintain its structure.
Montane Forests
The montane forest is typically found at elevations ranging from 6,000 to 9,000 feet. This zone experiences warmer and generally drier conditions compared to higher forests, but has more consistent moisture than the grasslands below.
The characteristic tree of the lower montane zone is the Ponderosa Pine (Pinus ponderosa), which thrives on sunny, south-facing slopes. These trees are recognizable by their reddish-orange bark and long needles that grow in bundles of two or three. Douglas Fir (Pseudotsuga menziesii) becomes more prevalent on cooler, north-facing slopes and in moister canyon bottoms.
Natural fire ecology plays an important role in maintaining the health of the Ponderosa Pine forests, which are adapted to frequent, low-intensity surface fires. These fires clear out underbrush and smaller trees, resulting in open, park-like stands. Stands of Quaking Aspen (Populus tremuloides) are also common, particularly in areas recovering from past disturbances.
Subalpine and Alpine Zones
The subalpine and alpine zones represent the high-elevation ecosystems of Colorado’s mountains. The subalpine zone extends from 9,000 feet up to the treeline, typically found between 11,000 and 12,000 feet. This zone has a cooler, moister climate, a shorter growing season, and heavy winter snowpacks.
The forest canopy is dominated by two cold-tolerant species: Engelmann Spruce (Picea engelmannii) and Subalpine Fir (Abies lasiocarpa). As elevation increases, the trees become stunted and deformed due to the stress of high winds, intense sunlight, and deep snow. At the treeline limit, these species form dense, low-growing mats known as Krummholz, a German term meaning “crooked wood.”
Above the Krummholz zone lies the alpine tundra, defined by the absence of upright trees. This zone experiences high winds, intense ultraviolet radiation due to the thin atmosphere, and a growing season lasting only six to ten weeks. Permafrost can be present in the soil, limiting root growth and stressing plant life.
The flora of the alpine tundra is composed of specialized, low-growing perennial plants, such as cushion plants, sedges, and grasses that hug the ground for warmth. Wildlife includes the American Pika, which stores vegetation for winter, and the Yellow-bellied Marmot, which hibernates for up to eight months. The White-tailed Ptarmigan, a bird that changes its plumage color to match the seasonal snow cover, is also a resident.
Semi-Arid Shrublands and Plateaus
The semi-arid shrublands and plateaus are found primarily in western Colorado, often called the Western Slope. This biome is shaped by the rain shadow effect, where the Rocky Mountains block moisture-laden air from the Pacific, resulting in lower precipitation and higher temperatures. The landscape features mesas, canyons, and plateaus.
The lower elevations are characterized by xeric, or dry-adapted, plant communities, including expansive sagebrush scrublands dominated by Artemisia. This dense, gray-green shrub provides forage and cover for mule deer and other wildlife. The open nature of the terrain reflects the arid climate and porous soils.
At slightly higher elevations, the landscape transitions into Pinyon-Juniper Woodlands. These open forests are defined by the co-dominance of Pinyon Pine (Pinus edulis) and Utah Juniper (Juniperus osteosperma). The trees are widely spaced, allowing for an understory of drought-tolerant grasses and shrubs to persist in the rocky, nutrient-poor soil.