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Fire severity, frequency, and extent are expected to change dramatically in coming decades in response to changing climatic conditions, superimposed on the adverse cumulative effects of various human-related disturbances on ecosystems during the past 100 years or more. To better gauge these expected changes, knowledge of climatic and human influences on past fire regimes is essential. We characterized the temporal and spatial properties of fire regimes in ponderosa pine forests of the southern San Juan Mountains of southwestern Colorado by collecting 175 fire-scarred tree samples from nine sites across a wide range of topographic settings. All tree rings and fire scars were dated using standard dendrochronological techniques. Fire-free intervals were statistically modeled using the Weibull distribution to provide quantitative measures that characterized the historical range of variation in pre-EuroAmerican fire regimes.Fires during our reference period were more frequent in the low elevation ponderosa pine forests (6-10 yr) than in the high elevation, mixed conifer forests (18-28 yr). Fires at lower elevations were predominantly low-severity, isolated fires. Fires during some years (e.g., 1748) were spatially extensive throughout the entire mountain range. Intervals that delimited significantly long fire-free periods ranged from 10-19 yr (low elevation) to 27-50 yr (high elevation). Fire histories were similar between the eastern and western portions of the mountain range, although we found significant evidence of topographic isolation on fire regimes at one site. Pre-1880 fires primarily occurred in the dormant season, and we found no temporal changes in past fire seasonality. We found no compelling evidence that Native Americans influenced fire regimes in our study sites.We found a hiatus in fire occurrence between 1750 and 1770 that we believe was likely related to weakened El Niñ o-Southern Oscillation activity, an extended series of cool-phase Pacific Decadal Oscillation events, and weakened monsoonal moisture, all possibly entrained in an invasive air mass typical of locations that are more northerly. In addition, pre-1880 fires occurred during years of severe drought, conditioned by above average moisture conditions in preceding years. The 20th century is characterized by a near complete absence of fires (fire-free interval of Ͼ100 yr), suggesting future wildfires may be more widespread and ecologically severe compared to pre-1880 fires.

Piñ on-juniper woodlands (Pinus edulis, Juniperus osteosperma, and J. scopulorum) 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 natural 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 Ͼ10 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 ϳ100 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 piñ on-juniper woodlands of Mesa Verde is about 400 years. A sharp boundary exists between piñ on-juniper woodlands at slightly lower elevations in the southern portion of the park and petran chaparral at slightly higher elevations 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 piñ on 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 natural barriers to fire spread (cliffs and sparsely vegetated slopes) to the south and west of the piñ on-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.

Six large wildfires have burned in Mesa Verde National Park during the last 15 years, and extensive portions of burns were invaded by non-native plant species. The most threatening weed species include Carduus nutans, Cirsium arvense, and Bromus tectorum, and if untreated, they persist at least 13 years. We investigated patterns of weed distribution to identify plant communities most vulnerable to post-fire weed invasion and created a spatially explicit model to predict the most vulnerable sites. At the scale of the entire park, mature piñon–juniper woodlands growing on two soil series were most vulnerable to post-fire weed invasion; mountain shrublands were the least vulnerable. At a finer scale, greater richness of native species was correlated with greater numbers of non-native species, indicating that habitats with high native biodiversity are at the greatest risk of weed invasion. In unburned areas, weed density increased with greater soil nitrogen and phosphorus, and lower salinity. In burned areas weed density correlated with soil nitrogen status and textural class. We also evaluated the effectiveness of a variety of weed mitigation methods; aerial seeding of targeted high-risk areas with native grasses was the most effective treatment tested. We recommend a conservative mitigation plan using natives grass seed on only the most invasible sites.

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