Final Report: Historical and recent fire regimes in pinon-juniper woodlands of Mesa Verde, Colorado, USA

Floyd-Hanna, Lisa; Hanna, David; Romme, William

We investigated the historic fire regimes in pinyon-juniper woodlands on the Mesa Verde cuesta utilizing stand and age structures. All pinyon trees in eight stands were aged and stand age was extrapolated to the surrounding landscapes using digital imagery, creating a time-since-fire map of the 1995 landscape. Six sampled stands were over 400 years, while two were between 200 and 300 years. We also determined pinyon and juniper size structure in 23 other stands, grouping them into 4 types of distributions (inverse Gaussian, exponential, beta and normal), suggestive of considerable heterogeneity in the old woodlands. Stand-replacing fire with a rotation of 400 years or longer characterized this pinyon-juniper landscape before 1995; low-severity surface fires apparently have never been an important component of the fire regime in Mesa Verde. Superposed epoch analysis revealed that large fires occur following significantly low precipitation in May or in the winter (October-March) preceding a summer fire season. Since the mid-1990's, a severe drought has characterized climate in this region, but the recent drought is similar to previous drought periods since 1950. A combination of canopy fuel build-up during two wet decades before 1995 and current drought conditions has resulted in unprecedented fire activity which is outside the range of natural variation when compared with the reference period 1700-1900. We apparently are witnessing a unique period in the ecological history of the southwest, a period when vegetation patterns are being altered over extensive areas within a very short time. We emphasize however, that these changes are not due to fire suppression or other direct human intervention, but rather result from natural ecological responses to climatic variability. Our data provide no ecological justification for aggressive management activities such as mechanical fuel reduction or prescribed burning, except in the immediate vicinity of vulnerable cultural resources.

Final Report: Historical and recent fire regimes in pinon-juniper woodlands of Mesa Verde, Colorado, USA

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