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Brigham Young University Science Bulletin, Biological Series

Abstract

A mountain brush vegetation typified by Gambel oak (Quercus gambelii) and serviceberry (Amelanchier utahensis) dominates the uplands of Mesa Verde National Park. There is evidence that this brush element is a successional stage that has been maintained by repeated natural fires in a large part of the Mesa Verde landscape. This study was conducted to determine the role of fire in the region's ecology, the nature of the climax pinyon pine (Pinus edulis) and Utah juniper (Juniperus osteosperma) vegetation, and the major successional stages that lead to the climax condition.

Three postburn plant communities and two climax stands overlying residual soils at elevations of about 7,500 feet were studied. The successional stands had been burned through natural causes in 1873, 1934, and 1959. The earliest date was determined by cross-dating trees affected by the fire with the Mesa Verde master chronology, since the fire antedated any written records of the region. Pith dates from pinyon pines in the climax forest stands indicated an age of about four centuries.

Permanent sample plots representative of each stand were established, and the vegetation was analyzed using a modification of the line intercept and macroplot methods. The vascular species that occurred in each sample were noted, and their cover value and frequency were summarized.

Plant succession proceeds rapidly from a pioneer stage of the shade and competition intolerant weeds. Helianthus annuus and Chenopodium pratericola, to a meadow stage dominated by the native grasses, Oryzopsis hymenoides, Sitanion hystrix, and Poa fendleriana. In the most recent burn, several species of perennial exotic grasses have been introduced to check erosion.

After about 25 years, a crown-sprouting brush element, consisting primarily of Quercus gambelii, Amelanchier utahensis, Cercocarpus montanus, and Purshia tridentate becomes the dominant vegetation. At this time, seedlings of Pinus edulis and Juniperus osteosperma are also established. This open shrub stage gradually becomes more dense, and in about 100 years forms a thicket stage. Young trees come up through the shrubs, eventually overtopping them. As the fire sere proceeds toward a climax condition, the brush species are gradually suppressed by the maturing forest. After several centuries, the understory is composed mainly of a sparse shrub component, a grass (Poa fendleriana), prickly-pear (Opuntia polyacantha), and several forbs.

The data tend to support the theory that recurrent fires throughout previous centuries have permitted a chaparral-like, floristically rich shrub vegetation to persist as a fire climax along the uplands of Mesa Verde National Park. Under the present management policies of fire suppression, however, a pinyon-juniper forest is slowly replacing the former extensive shrub vegetation.

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