Forests play an important role in the earth system, regulating climate, maintaining biodiversity, and provisioning human communities with water, food and fuel. Interactions between climate and forest dynamics are not well constrained, and high uncertainty characterizes projections of global warming impacts on forests and associated ecosystem services. Recently observed tree mortality and forest die-off forewarn an acceleration of forest change with rising temperature and increased drought. However, the processes leading to tree death during drought are poorly understood, limiting our ability to anticipate future forest dynamics. The objective of this dissertation was to improve understanding of drought-associated tree mortality through literature synthesis and tree-ring studies on trees that survived and died during droughts in the southwestern USA. Specifically, this dissertation 1) documented global tree mortality patterns and identified emerging trends and research gaps; 2) quantified relationships between growth, climate, competition and mortality of piñon pine during droughts in New Mexico; 3) investigated tree defense anatomy as a potentially key element in piñon avoidance of death; and, 4) characterized the climate sensitivity of piñon resin ducts in order to gain insight into potential trends in tree defenses with climate variability and change. There has been an increase in studies reporting tree mortality linked to drought, heat, and the associated activity of insects and pathogens. Cases span the forested continents and occurred in water, light and temperature-limited forests. We hypothesized that increased tree mortality may be an emerging global phenomenon related to rising temperatures and drought (Appendix A). Recent radial growth was 53% higher on average in piñon that survived versus died during two episodes of drought-associated mortality, and statistical models of mortality risk based on average growth, growth variability, and abrupt growth changes correctly classified the status of ~70% of trees. Climate responses and competitive interactions partly explained growth differences between dying and surviving trees, with muted response to wet/cool conditions and enhanced sensitivity to competition from congeners linked to growth patterns associated with death. Discrimination and validation of models of mortality risk varied widely across sites and drought events, indicating shifting growth-mortality relationships and differences in mortality processes across space and time (Appendix B). Pre-formed defense anatomy is strongly associated with piñon survivorship over a range of sites and stand conditions. Models of mortality risk that account for both growth and resin duct attributes had≈10¹⁹ more support than models that contained only growth. The greatest improvement in classification was among trees from the 2000s drought, suggesting an enhanced role for tree defense allocation and/or bark beetle activity during recent warm versus historic cool drought. Accounting for defense characteristics and growth-defense allocation is likely to be important for improving representation of drought-associated mortality (Appendix C). Piñon resin duct chronologies contain climate responses that are coherent and distinct from those of radial growth. Growth responds positively and strongly to previous fall and current winter precipitation, and negatively to late spring and early summer temperature. A relatively equal positive resin duct response to winter precipitation and positive response to mid-to-late summer drought suggests that changes in climate will affect tree defense anatomy in complex ways, with the outcome determined by seasonal changes in precipitation and temperature (Appendix D).

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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.

Thesis (Ph. D.)--University of Wyoming, 2006. / Title from PDF title page (viewed on Dec. 20, 2007). Includes bibliographical references.

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Widespread, high levels of tree mortality, termed forest die-off, associated with drought and rising temperatures, are disrupting forests worldwide. Drought will likely become more frequent with climate change, but even without more frequent drought, higher temperatures can exacerbate tree water stress. The temperature sensitivity of drought-induced mortality of tree species has been evaluated experimentally for only single-step changes in temperature (ambient compared to ambient + increase) rather than as a response surface (multiple levels of temperature increase), which constrains our ability to relate changes in the driver with the biological response. Here we show that time-to-mortality during drought for seedlings of two western United States tree species, Pinus edulis (Engelm.) and Pinus ponderosa (Douglas ex C. Lawson), declined in continuous proportion with increasing temperature spanning a 7.7 degrees C increase. Although P. edulis outlived P. ponderosa at all temperatures, both species had similar relative declines in time-to-mortality as temperature increased (5.2% per degrees C for P. edulis; 5.8% per. C for P. ponderosa). When combined with the non-linear frequency distribution of drought duration-many more short droughts than long droughts-these findings point to a progressive increase in mortality events with global change due to warming alone and independent of additional changes in future drought frequency distributions. As such, dire future forest recruitment patterns are projected assuming the calculated 7-9 seedling mortality events per species by 2100 under business-as-usual warming occur, congruent with additional vulnerability predicted for adult trees from stressors like pathogens and pests. Our progressive projection for increased mortality events was driven primarily by the non-linear shape of the drought duration frequency distribution, a common climate feature of drought-affected regions. These results illustrate profound benefits for reducing emissions of carbon to the atmosphere from anthropogenic sources and slowing warming as rapidly as possible to maximize forest persistence.