The Flora of Boulder County, Tim M Hogan, University of Colorado Herbarium
Extending from the short grass prairie of the Great Plains to the alpine tundra of the Continental Divide, Boulder County enjoys a wealth of landscape types. The county covers approximately 750 square miles, rising from 5,000 feet on the eastern plains to 14,000 feet along its western boundary. A variety of natural habitats including grasslands, riparian zones, woodlands, forests, and alpine tundra represent an assemblage of communities that is matched by a diversity of plants and animals that make the county their home.
One thousand five hundred thirty eight (1,538) species of vascular plants found in 135 families and 667 genera are documented for Boulder County. These species account for nearly one half of the flowering plants known from the state of Colorado, and is only slightly less than the entire flora of Alaska.
One reason so many species are known from the county is because Boulder has been well studied by botanists over the years. But the more important reasons are ecological. The easternmost extension of the Continental Divide in North America is along the Indian Peaks. Within a short horizontal distance one can experience a range of floristic regions that would be found on a walk from Mexico to the Arctic. Precipitation ranges from 12 inches on the plains to over 40 inches in the moist forests of the subalpine zone. Substrates include geologically young glacial deposits, shales and sandstones deposited by ancient seas and winds, and some of the oldest granites on the continent. Sheltered foothill canyons where the plains meet the mountains at lower treeline offer refugia for a number of eastern woodland and prairie plants. Many of the broad alpine ridges along the Divide escaped the most recent glacial scouring and serve as another type of refugia for rare alpine species. Boulder County’s rich floristic heritage is harbored in prairie wetlands and pockets of native grasslands on the plains; in open meadows and forest of ponderosa pine in the middle mountains; among old-growth spruce/fir forests near treeline; and upon expanses of windswept tundra where the mountains meet the sky.
Our flora is not only quantitatively abundant, it is also qualitatively rich. A number of rare, threatened, and endangered species occur here. The lady’s tresses orchid (Spiranthes diluvialis) and Bell’s twinpod (Physaria bellii) are federally listed, while many other plants are of special concern. Some plants that are common in the Boulder area such as the Front Range beardtongue (Penstemon virens) and bracted alumroot (Heuchera bracteata) are found nowhere else in the world beyond the southern Rockies – they are endemic. Other species found in Boulder County are at the edge of their range and therefore represent an important element of that species’ genetic diversity. The northernmost occurrence of mountain tail-leaf (Pericome caudata), a southwestern species, is on screes in the Boulder Mountain Park. Also found in the Mountain Park is one of the southernmost localities for paper birch (Betula papyrifera). Other denizens of our local flora reach back into deep time. There is fossil evidence of Oregon grape (Mahonia repens) and waxflower (Jamesia americana) that documents their occurrence in our area from 50 million years ago.
We are fortunate to live amongst such a rich and varied flora – a flora with roots in the Tertiary and with branches that reach across continents. The protection of natural areas serves to preserve the evolutionary lineages and the ecological habitats upon which our native plants and animals depend.[A Checklist of Vascular Plants of Boulder County, Colorado by Curator Emeritus William A. Weber has been published by the University of Colorado Museum of Natural History and is #16 in the Natural History Inventory of Colorado series. Along with a complete list of the Boulder flora, the publication includes a discussion on principal habitats, floristic zonation, weeds, historical plant geography, special floristic patterns, and the significance of anthropogenic change on the flora. It can be purchased at the museum for $5.00.]