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Colorado Flora: Western Slopes

Colorado Flora: Western Slopes

by William A. Weber, Ronald C. Wittmann (Joint Author)
Reflecting the conclusions of current taxonomic research and recognizing new species found in the state, these thoroughly updated guides offer the most complete and authoritative reference to the plants of Colorado. Both volumes explain basic terminology; discuss plant geography; and describe special botanical features of the mountain ranges, basins, and plains.


Reflecting the conclusions of current taxonomic research and recognizing new species found in the state, these thoroughly updated guides offer the most complete and authoritative reference to the plants of Colorado. Both volumes explain basic terminology; discuss plant geography; and describe special botanical features of the mountain ranges, basins, and plains. Interesting anecdotes and introductions are given for each plant family, and hints on recognizing the largest families are provided as well. Each volume includes a complete glossary, indices to common and specific names, and hundreds of illustrations. Ideal both for the student and scientist, Colorado Flora: Eastern and Western Slopes, Third Edition remain essential to readers interested in Colorado's plant life.

Editorial Reviews

The companion to Weber's (curator, U. of Colorado Museum Herbarium) Colorado flora: Western Slope (U. Press of Colorado, 1987), this guide provides keys to 2,560 types of vascular plants. Weber discusses plant geography, special botanical features of the mountain ranges, basins, and plains, as well as basic terminology. Illustrated with b&w line drawings and 64 color plates. Acidic paper. Paper edition (unseen), $19.95. Annotation c. Book News, Inc., Portland, OR (booknews.com)

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Univeristy Press of Colorado
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Colorado Flora: Western Slope

By William A. Weber, Ronald C. Wittmann

University Press of Colorado

Copyright © 2001 William A. Weber and Ronald C. Wittmann
All rights reserved.
ISBN: 978-0-87081-553-9


Scope of the Book

This book has as its subject the vascular plants — ferns, gymnosperms and flowering plants — native and naturalized on the entire hydrologic Western Slope of Colorado — from the Continental Divide to the Utah, Wyoming, and New Mexico borders. It is a rich and varied territory, lacking only the level plains of the Eastern Slope, but possessing a rich variety of rock types: granites, limestones, sandstones, and volcanics exposed at high and low altitudes. There are deep canyons, river valleys, natural lakes, high plateaus, and a substantial core of alpine tundra. The forests are equally varied, with ponderosa pine, spruce and fir, piñon pine and juniper, and white fir. Sagebrush, serviceberry and oak clothe the high plateaus and desert shrubs the lower steppe. An ephemeral spring flora blooms in the arid "adobes" of the Colorado River valley in April and May, as well as on the high sagebrush plateaus which are often impenetrable then because of roads blocked by snowbanks or deep in mud.

The Western Slope

The Western Slope of Colorado has as its eastern boundary the alpine tundra of the Continental Divide of the Rocky Mountains, with a topography and flora identical to that of the Eastern Slope. It is here that the greatest overlap of the flora occurs. The Rocky Mountains are a formidable barrier to migration of plant species from west to east. As we leave the high spine of the Rocky Mountains we encounter very different types of topography, and the flora changes to resemble more that of the Great Basin. Many species common to the lower altitudes of each slope are able to maintain continuity in their ranges because of the low divides in southern Wyoming and northern New Mexico. Small concentrations of Eastern Slope species occur in Archuleta County and probably may be found elsewhere along the "soft underbelly" of Colorado and in New Mexico.

The elevation of the Western Slope ranges from over 14,000 ft. (ca. 4,250 meters) in the Saguache Range, down to 4,500 ft. (ca. 1,400 meters) near Grand Junction. The area is drained by five river systems: the Yampa, White, Colorado (including Dolores), Gunnison, and San Juan. The high country includes the main Rocky Mountain chains near the Continental Divide, the Elk and West Elk Mountains, the San Juan Volcanic region, and several great plateaus: White River, Blue Mountain, Grand Mesa, Blue Mesa, Cochetopa and Uncompahgre. The Gunnison basin is a major intermountain basin or "park."

Contrary to common usage, the Rocky Mountains, strictly speaking, do not occupy most of the Western Slope. The region is one of long, deeply excavated river valleys: The Green, Yampa, and White in the northern third; the Colorado (or Grand, as it has been called) and the Gunnison and Eagle in the central third; and the San Juan and Animas in the southern third. These river systems are separated by a variety of mountain ranges and plateaus, each tending to have a distinctively different flora.

The Northern Section

The northern portion includes, on the west, the Blue Mountain Plateau of Dinosaur National Monument, a region of high desert, deep canyons, extensive river benches, riparian riverbanks, and alkaline flats; a jumble of lower hills separate this area from the White River drainage. Eastward, the Park Range, flanked by a major urban development centered around Steamboat Springs, separates the Western Slope from North Park, which belongs to the Eastern Slope. In the eastern part of the Yampa drainage extensive strip mining of coal, and a huge coal-burning power plant, have altered the aspect of the vegetation.

This part of the Western Slope contains discrete floristic aspects. The Park Range is noteworthy for harboring perhaps up to 100 species not found elsewhere in Colorado, and representing types belonging to a Northern Rocky Mountain element: Azaleastrum albiflorum, Drymocallis glandulosa, Erocallis triphylla, Mimulus lewisii and M. moschatus, Trillium ovatum, and Viola purpurea. The Yampa River drainage has an extensive riparian zone in which the Pacific Northwestern "yampa", Perideridia gairdneri, and a few northern species of Carex, occur. The flanks and summits of the Blue Mountain Plateau support a number of species characteristic of the northwestern high deserts: Cercocarpus ledifolius, Danthonia unispicata, Eriogonum heracleoides, Fritillaria pudica, Lewisia rediviva, and Ranunculus jovis; and several truly endemic species: Bolophyta ligulata, Eriogonum tumulosum, and Oenothera acutissima. Between the White River and Colorado Drainages lie the Roan Cliffs and the Piceance Basin, a remarkable plateau area famous for the oil shales in its horizontal rock strata, as well as for one of the important concentrations of narrowly endemic high desert plant species in Colorado: Aquilegia barnebyi, Astragalus lutosus, Lesquerella congesta and L. parviflora, Penstemon debilis, Physaria obcordata, Thalictrum heliophilum, as well as a few species found only here and in one or two similar places (disjunct) in the Great Basin: Ceanothus martinii, Argillochloa dasyclada, and Nuttallia argillosa. If oil shale production had been allowed to proceed as planned, these rare species probably would have been exterminated.

The Middle Section

The region between the Colorado and the Gunnison River drainages is also extremely varied, with several discrete floristic assemblages. The Elk Mountains, an essentially east-west-trending mountain range is particularly exciting for us because, along with being the richest mountain flora on the Western Slope, the occurrence of sedimentary rocks (sandstone, limestone, marble) at very high altitudes and forming extremely steep slopes, physically recalls comparable areas in the Canadian Rocky Mountains. In fact, several species "jump" to the Elks from the Canadian Rockies, among them Arnica alpina, Braya glabella, Erigeron lanatus, and E. humilis.

The Gunnison River Basin is a large open sagebrush "park" with several endemic species of its own: Astragalus anisus, A. microcymbus, Boechera gunnisoniana, and B. pallidifolia. The Gunnison Basin ends with Blue Mesa and the spectacular Black Canyon of the Gunnison in which plateau and cliff plants abound, including the endemic Gilia penstemonoides.

The lower Gunnison River is bordered on the east by great hills of soft, yellowish Mancos shale deposits, locally called "adobes" that stretch from Montrose all the way to Grand Junction. These adobes support, in some years characterized by having good winter precipitation, wonderful arrays of ephemeral spring annuals, many of them endemic, including: Lomatium concinnum, Psilostrophe bakeri, Oreocarya elata, O. paradoxa, Acrolasia humilis, Abronia argillosa, Camissonia eastwoodiae, and C. walkeri. In exceptional years, masses of the white-flowered Stanleya albescens cover great expanses of alkaline flats.

Dividing the Colorado from the Gunnison River drainage loom the cliffs of the Grand Mesa, a huge, almost level plateau covered by a subalpine forest with many small lakes and wet meadows. Although nothing of exceptional floristic specialty occurs here, the flora is rich, and several species characteristic of the high Rockies, such as Primula parryi, Psychrophila leptosepala, Frasera speciosa, and Bistorta bistortoides, are present. The mesa is snow-covered for much of the year.

The jewel of the Colorado River drainage is the spectacular Colorado National Monument, which forms the lower northern base of the great north-south-trending Uncompahgre Plateau. The plateau itself has been heavily grazed and offers relatively little botanical excitement, except for its highest part, where Pinus ponderosa forms open stands, and a dense understory of the manzanita, Arctostaphylos patula covers many acres, creating an aspect of the California chaparral, to which it belongs. The plateau is separated from the Colorado National Monument by a low divide or pass called Unaweap canyon. Unaweap Canyon is noteworthy because of the only stand of Oreobatus deliciosus on the Western Slope, and for a spring, recently having come under protection of Nature Conservancy, that supports a rare butterfly and several uncommon plant species. The very hard quartzite cliffs of Unaweap Canyon have populations of two very rare plants: Heuchera rubescens and Leptodactylon watsonii.

Colorado National Monument is a floristic as well as a geological treasure-house. Excellent stands of pinon-juniper woodland, canyon and cliff vegetation, and desert hills and flats present a comprehensive picture of the characteristic flora of the Colorado Plateaus region. All of the characteristic shrubs: Atriplex, Brickellia, Chrysothamnus, and Cercocarpus, are within easy reach of the road. Gently sloping sandstone pavements support a rich assemblage of characteristic herbs and grasses, as well as some fine endemics such as Oreocarya osterhoutii, Penstemon cyanocaulis, and Mirabilis glandulosa.

The Southern Section

This leaves only the San Juan Volcanic system and the "soft underbelly" of Colorado to be discussed. The San Juan Mountains are extremely rugged and difficult of access except for a dew well- trodden trails and jeep roads. Timberline is very high, and in many areas tundra is not well-developed. Steep slopes and huge cliffs are deterrents to botanical exploration. Parts of the range were explored during the mining days, particularly around Ouray, and in the most accessible mining areas of the La Plata Mts. Although the flora is not unique, there are a few notable endemics: Besseya ritteriana and Trifolium attenuatum are representative. Although not endemics, there are some choice species that have been found only once: the maidenhair fern, Adiantum aleuticum, and the Arctic-Alpine Bupleurum triradiatum. In recent years there have been several field studies and collections made by University of Colorado workers, but the great bulk of the area has not been adequately investigated. Recently, interest has been shown by the botanists at Fort Lewis College in Durango, who are collecting actively.

Mesa Verde National Park has had a long history of floristic studies. The flora is now quite well known. A catalog is available there, and a local herbarium is housed at the research center. The Park is home to a few notable endemics which, of course, are well protected. Among these are Hackelia gracilenta, Sclerocactus mesa-verdae, Astragalus schmolliae, and A. humillimus. The Mancos shales covering much of the Southern Ute Reservation support a rich spring flora in good years, including some endemics, but permission is required from the Ute Reservation to botanize there.

The "soft underbelly" of western Colorado includes the area from Pagosa Springs to Durango, where open hillsides of black shale support a small but very interesting variety of local endemics and rare plants, including Lesquerella pruinosa, Rhamnus smithii, and Townsendia glabella. Near Ignacio the exceedingly rare cactus, Pediocactus knowltonii, described from just across the New Mexico State line, may or may not actually occur in Colorado. Several species otherwise known only from the Eastern Slope occur here, including Oxalis violacea, Sanicula marilandica, and Cylindropuntia imbricata.

Middle Park

Of the four major mountain "parks", strung in a line from north to south in Colorado, Middle Park is the only one belonging to the Western Slope. It contains Grand Lake, the source of the Colorado River, which drains westward through Byers Canyon and out through Gore Canyon. In the eastern and southern part, Middle Park is characterized by typical mountain topography and spruce-fir forests, replaced at lower altitudes by sagebrush (Seriphidium vaseyanum) benches, but the center has a special area of "adobe" bluffs and undulating hills, sedimentary formations characteristic of the extreme western lowlands that have been trapped between the mountain ranges. These adobes support several narrowly endemic species, including Astragalus osterhoutii and Penstemon penlandii.

Floristic Zones

Floristic zones are rough estimates of the altitudinal coverage of the easily recognizable plant communities. They are not always consistent in mountain regions because of a phenomenon called environmental compensation. There is a telescoping effect of altitude combined with slope exposure that makes it possible for a species to grow up to a higher altitude on a south slope than it does on a north slope, and vice versa. On a south-facing slope, species of low altitudes may climb very high, and on north-facing slopes high altitude species may reach very low altitudes because of various factors which define their ecological requirements. Statements of the floristic zone occupied by a species should always be allowed some latitude. An especially protected cool, north-facing canyonside may actually harbor typically alpine species, while an alpine tundra site with easily warmed sedimentary rock substrate and soil churned up by gophers may support typical subalpine vegetation.

A well known phenomenon of mountain floras is the transportation of high altitude species downstream to lower than usual altitudes by spring floods. These waifs may survive for a year or two, but usually are selected against by the climatic norms. Mosses and lichens of high altitudes, however, are often found in cool, moist pockets on north-facing slopes of canyonsides, where they have probably survived for millennia.

In this book I use a loosely construed group of floristic zones: Desert-steppe is the treeless semi-arid canyon-side, river bench, or talus of the lower river basins. Riparian woodlands are wetlands along the major streams. Piñon-juniper refers to the plant community dominated by these species (there seems to be nothing gained by changing the name to piñon-red cedar although technically these are not Juniperus). Sagebrush refers to sites dominated by various species of Seriphidium (formerly part of Artemisia). Montane refers to the middle-altitude, relatively dry forested zone, Subalpine to the spruce-fir zone below the limit of trees, and Alpine to the area above the limit of trees. Ruderal is a term used for sites much disturbed by the activities of man.

I prefer not to assign altitudinal limits because the Western Slope, with its great and abrupt topographic gradients, is full of compensating environments. One should always expect to find plants growing above or below the limits given by the book. In floristic books, assigning limits to altitudinal distribution is simply an invitation for someone to prove the datum wrong.

Plant Geography

The Colorado Rocky Mountain region is like a huge flattened wheel with a hub — the Rocky Mountain chain — running north to south and presenting a potential highway for plant species to move along it. Thus, it is not surprising that the majority of our high mountain species are also found to the north and to the south at least as far as the terminus of the range near Santa Fé, New Mexico. I call this the Rocky Mountain element. Because the Rocky Mountains are less continuous to the south and because of events in geoclimatic history, we have relatively few species, if any, of a Mexican mountain element, although at one time many of our species did extend into the Mexican cordillera and are still present in small patches there.

Continuing with the concept of the Rockies as a hub of a wheel with radiating spokes, we see elements of other floristic regions extending into Colorado along the river valleys (the spokes), from the Great Basin, Uintah Basin, Colorado Plateau, Rio Grande Valley, the Chihuahua desert, the Northern Rockies, and the northern and southern Great Plains. At least one genus, Psychrophila, seems to be an Andean-Australasian group of which few other vestiges remain, including the lichen Toninia bullata. Lupines and paintbrushes also occur commonly in the South American Andes.

The northern Rocky Mountain element is especially interesting because it is restricted to the Park Range north of Steamboat Springs. Here a number of species common in the Northern Rocky Mountains and Pacific Northwest survive in this very mesic mountain area and must have once extended north across what is now the Wyoming Desert. Characteristic species include Azaleastrum albiflorum, Drymocallis glandulosa, Perideridia gairdneri, and Trillium ovatum.

The Southern, or Colorado, Rocky Mountains have virtually no so-called Amphi-Atlantic connections characteristic of the floras of northern Europe and northeastern North America. Nevertheless, there are some curious plant distributions involving Greenland and the Rockies, such as Sisyrinchium montanum, Draba aurea, and Festuca saximontana. This is not surprising, since Greenland was yet another great high-mountain mass before the Pleistocene, probably harboring a large reservoir of species common to Asia and western America. But some Rocky Mountain species occur in isolated pockets around the Great Lakes. At one time the populations were believed to have ranged across North America and were eliminated by glacial action, surviving only on ice-free nunataks. Whether this is so is still a matter of controversy.


Excerpted from Colorado Flora: Western Slope by William A. Weber, Ronald C. Wittmann. Copyright © 2001 William A. Weber and Ronald C. Wittmann. Excerpted by permission of University Press of Colorado.
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