Dendrochronology on the Tavaputs Plateau, Northeastern Utah: Insights on Past Climate, Woodland Demography, and Fremont Archaeology

Long-lived trees and excellent preservation of remnant wood allow examination of late-Holocene climate variability and its relation to woodland tree demography and populations of prehistoric agriculturalists in northeastern Utah using dendrochronological methods. Tree-ring chronologies are developed from Douglas-fir (Psuedotsuga menziesii) and Colorado pinyon (Pinus edulis) on the Tavaputs Plateau covering the last 2,300 years. The climate reconstructions fill an important temporal and spatial gap in our understanding of moisture related climate variability in the region. We investigate the relationships between climate and woodland demography by constructing a 1,500-year record of pinyon establishment and death. Twentieth-century expansion and infill of pinyon/juniper woodlands and more recent widespread die-offs in the early 21st century heighten the importance of understanding these relationships. The climate reconstruction is analyzed in light of the archaeological record of Fremont agriculturalists between approximately AD 550 and 1300, and provides the first glimpse of climate variability throughout the Fremont era in this region.Results of the hydroclimate reconstructions show that multidecadal droughts unlike any observed in the instrumental record occur regularly over the last 2,000 years. Droughts in the mid 12th century and late 13th century are synchronous those found in numerous other records across the southwestern United States. A drought in the early 6th century is especially severe. Analysis of pinyon demography indicates rates of tree establishment, release, and death are highly variable over the last 1,500 years. Broad peaks in tree establishment occur in the 7th and 8th centuries, the 12th and 13th centuries, and again in the 18th, 19th and 20th centuries. Higher tree death rates are related to dry periods, but tree establishment is only weakly associated with wetter periods. Instead, cohorts of suppressed young trees established over decades tend to synchronously experience rapid growth rate increases during wet periods following droughts. Stands appear more susceptible to population turnover as semi-dominant cohorts of trees age and decline. Two critical periods in Fremont archaeology in the region, coincide with significant changes in moisture conditions. These changes follow longer periods of stability suggesting that changes in the predictability of climate conditions may have impacted Fremont agriculturalists in the region