FIRE HISTORY AND VEGETATION PATTERN IN MESA VERDE NATIONAL PARK, COLORADO, USA

Abstract

Pifion-juniper woodlands (Pinus edulis, Juniperus osteosperma, and J. sco- pulorum) and petran chaparral communities (Quercus gambelii, Amelanchier utahensis, Cercocarpus montanus, and other tall shrub species) cover much of the Colorado Plateau in the southwestern United States. Long-term fire history and successional dynamics are poorly understood in these vegetation types. Therefore, we lack a suitable historical context for interpreting the ecological significance of large fires and dramatic vegetative changes that have occurred recently in these ecosystems. For example, in Mesa Verde National Park, located in southwestern Colorado, four large intense fires in the last 50 years have threatened significant cultural and resources and have caused debate over whether Mesa Verde's fire regime has been significantly altered by human activities in the last century. In this study, we dated prehistoric fires in shrublands dominated by Gambel oak (Quercus gambelii) by aging stems that resprouted after fire. We mapped the spatial extent of all fires >10 ha that occurred during the last 150 years within a 6600-ha, shrub-dominated portion of Mesa Verde National Park. The turnover time (years required to burn an area equal to the entire shrubland zone) was -100 years under the natural fire regime of the mid- to late 19th century. Fire occurrence was reduced substantially during the first half of the 20th century, but the current fire regime (since about 1950) appears to be similar to that of the 19th century-despite a continuing policy of total fire suppression. The natural fire turnover time in pifion-juniper woodlands of Mesa Verde is about 400 years. A sharp boundary exists between pinon-juniper woodlands at slightly lower elevations in the southern portion of the park and petran chaparral at slightly higher ele- vations in the north. This pattern is explained, in part, by more extensive fires in the northern area, which favor resprouting shrubs and eliminate the fire-sensitive pifion and juniper. The less frequent occurrence of large fires, and resulting persistence of woodland in the southern portion of the park, may be due in part to barriers to fire spread (cliffs and sparsely vegetated slopes) to the south and west of the pifnon-juniper woodlands. Our findings demonstrate that fire frequency and extent in Mesa Verde during the last 50 years have not been greatly different from the natural fire regime of the late 1800s. Therefore, the recent large fires in the park, and the vegetative responses to those fires, appear to be within the historic range of variation for this ecosystem.