Tag Archives: University of Colorado

Weber Clouds

Weber: Clouds form on horizons, but what follows?
By William A. Weber Sunday, October 21, 2007

Chris Brown Photography
Chris Brown Photography

Cloud formations are one of Boulder’s most interesting and beautiful environmental features. When the clouds come down below the summits of Eldorado Mountain, Green Mountain and Bear Peak and cover the upper half of the Flatirons, we know it will soon rain in town. When a smooth-capped mass of clouds forms on the western horizon, we know that Chinook winds will blow soon.

In 1961, I helped a university Natural Areas Committee to explain the scientific values and need for preservation in its natural state of the Flatirons-Mesa trail area. Many folks have forgotten what these values were. It is time for us to remember.

The abruptness of the outer foothills creates locally moist microclimates on the east-facing mountainsides partly because of the deep shade afforded by the cliffs and the steep ravines that face northeast. In addition, however, the great height and steep faces of the Flatirons have the frequent effect of stimulating the formation of a cloud veil just in this area, when warm moist air from the southeast meets with colder air from the west.

To the west of the Flatirons-Mesa Trail area, this cloud veil is usually absent. The localized effect of the cloud veil is to raise the relative humidity on the slopes of the Flatirons. This in combination with the more shaded, steep slopes provides localized relatively moist microclimatic conditions.

These more moist conditions provide unique habitats on which many plants not ordinarily suited to our dry climate can survive and reproduce. The effects of this moist situation are reflected even on the back side of Green Mountain and Eldorado Mountain.

The steep gradient of this area has caused a telescoping of the flora and fauna belonging to a great range of habitats, altitudes and temperature tolerances into a relatively small area where they live side-by-side. Because of the steep north and south slopes, this actually results in some of the higher-altitude life forms occurring lower on the north-facing slopes than the low-altitude forms on the south facing slopes. This situation provides unique settings of extraordinary aesthetic and scientific values.

One curious and unique fact about the flora of this strip is that a number of plants typical of the Deciduous Forest Region of Eastern North America occur in very small and scattered stands. On the back of the Flatirons is the only stand of paper birch in Colorado (and the southernmost one in the United States). This is true also of plants such as the hazelnut, wild sarsaparilla, carrion flower, bird-foot violet, wood lily, rattlesnake fern, dwarf creeping raspberry, yellow lady-slipper and several extremely rare orchids.

Other plants in the Flatirons-Mesa Trail area seem to be “living fossils,” relics of the time long ago when the redwood forests were present over all this region. Some relic species are the Oregon-grape and several of the yellow-flowered parsleys common in the lower foothills and largely restricted to this area.

It is the many spots of unusually moist microclimatic conditions that certainly account for the presence of these outpost or relic species of plants in the Flatirons-Mesa Trail area. During the Pleistocene ice ages, when the local climate here was probably cooler and moister, there was apparently a westward movement of the eastern flora along the moist drainage systems as well as an eastward movement of Rocky Mountain species out onto the plains, resulting in the meeting and mixing of these elements along the base of the Front Range.

This wild mixture of unusual geographic vegetation types has all but disappeared in Colorado except for a few moist sites along the Front Range, of which the one represented by the Flatirons-Mesa Trail group is probably the best still remaining.

It would be a real tragedy if these last remnants were destroyed through our ignorance. Filling in the details of their climatic origins and potential for survival in an urban setting would enhance appreciation by the Boulder community of these unique gifts of nature to our open spaces

William A. Weber is a professor emeritus at the University of Colorado at Boulder.

Boulder County Flora

The Flora of Boulder County, Tim M Hogan, University of Colorado Herbarium

Chris Brown Photography
Chris Brown Photography

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.]

Why Caution Is Needed

by Sharon K. Collinge

Chris Brown Photography
Chris Brown Photography

A recent letter to the editor in the Daily Camera criticized the use of the precautionary principle in management of Boulder’s Open Space by stating that “This ‘precaution’ overrides science and data in favor of policies critical of all possible human impacts.”

Put simply, the precautionary principle is an approach that seeks to avoid unintended consequences of particular actions. Rather than ‘overriding’ science and data, this principle explicitly acknowledges the centrality of scientific data to decision-making. Most importantly, it suggests a guiding strategy for managers faced with the uncertainties and knowledge gaps that will always exist in our understanding of a situation.

We will manage Boulder’s Open Space most effectively with a similar approach. Although we do not know everything there is to know about human impacts on native grasslands, forests, and streams, there is ample scientific evidence showing that increased human activities lead to environmental degradation. This warrants a cautious approach to management of our local public lands.
To avoid unintended consequences we must clearly state our intended consequences by asking, “What do we want Open Space lands to look like in the future—say 5, 10, 20, 50 years from now?” “What condition of the natural environment is acceptable?” “How do we ensure that our actions are sustainable?” If we want the status of our Open Space to be the same as it is or even better 20 years from now, then we must avoid actions that fragment, degrade, and destroy the land and its species. That’s exactly why the precautionary principle is vital in managing our valuable Open Space.

Professor Collinge is a member of the Science Advisory Group and teaches at the University of Colorado