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Item Impacts of forest mortality on streamflow in whitebark pine forests within the greater Yellowstone ecosystem(Montana State University - Bozeman, College of Letters & Science, 2024) Rautu, Teodora Stefana; Co-chairs, Graduate Committee: Brian V. Smithers and Danielle E. M. UlrichIncreasing forest mortality across the western U.S. raises concerns about its impact on streamflow. The hydrologic role of whitebark pine (Pinus albicaulis Engelm.) is of particular interest given its ongoing decline and prevalence at the upper treeline where precipitation is highest. Understanding the link between disturbed whitebark pine forests and streamflow is essential for better informing water resource management. In Chapter One, I investigated streamflow changes in two Wyoming whitebark pine watersheds: Upper Wind River (53% area affected by beetle outbreak) and Buffalo Fork (53% area affected by beetle outbreak and fire). Streamflow significantly increased post-beetle for Upper Wind River but did not significantly change post-disturbance for Buffalo Fork, attributed to the fire's limited spatial extent and post- beetle effects potentially occurring in the pre-disturbance period. In Chapter Two, I integrated Leaf Area Index into a hydrologic model to reflect changing canopy conditions and assessed water balance variables that drove the observed changes in streamflow in Chapter One. I found that an increase in annual precipitation primarily led to the increase in observed streamflow more so than forest mortality, and snowpack and snowmelt were consistent predictors of streamflow metrics. My findings suggest monitoring snow dynamics for accurate real-time and future streamflow forecasting. In Chapter Three, I used streamflow field data and the same hydrologic model to assess the impact of increasing tree mortality on streamflow within a whitebark pine- dominated watershed in Big Sky, Montana. After simulating mortality levels ranging from 0-90% for one year, tree mortality did not substantially impact streamflow until the 90% mortality level where annual flow and late summer flow substantially increased. Considering that mortality levels between 25-50% are more representative of whitebark pine mortality in one year, the lack of substantial impacts on snowpack and streamflow at the 25-50% mortality levels challenges the traditional assumption that whitebark pine mortality would lead to reduced snowpack and reduced late summer flow in open watersheds with 30% forest cover. Future studies should assess the multi-decade impacts of whitebark pine mortality on hydrologic processes and consider species differences in evapotranspiration as other subalpine species replace whitebark pine.Item A forest entombed in ice: a unique record of mid-Holocene climate and ecosystem change in the northern Rocky Mountains, USA(Montana State University - Bozeman, College of Letters & Science, 2022) Stahle, Daniel Kent; Chairperson, Graduate Committee: David McWethy; This is a manuscript style paper that includes co-authored chapters.Across the high alpine of the northern Rocky Mountains small vestiges of perennial ice have endured for thousands of years. These ice patches reside hundreds of meters above modern treeline, with some persisting through mid-Holocene warmth and others establishing at the onset of a cooler period that began around 5,000-5,500 years BP. Recent warming-driven melting at the margins of one ice patch high on the Beartooth Plateau of northern Wyoming exposed over 30 intact mature whitebark pine (Pinus albicaulis) tree boles, all > 25 cm in diameter. We extracted cross-sectional samples from the stems of 27 preserved logs, and radiocarbon dated annual growth rings from 11 of these trees, anchoring the chronology to a date range spanning 5,947 to 5,436 years BP + or - 51.3 years. From this fossil wood chronology, we developed estimates of warm-season, annual, and biennial average temperatures for upper-elevation treeline during the mid-Holocene. To identify the predominant climate-growth relationships of the subfossil trees, we sampled live whitebark pine trees growing at an adjacent treeline site approximately 120 m lower in elevation. Temperature was found to be the major driver of variability in tree growth at the modern treeline location, with trees producing narrower (wider) rings during periods of cooler (warmer) growing season temperatures. Using linear and non-linear transfer functions based upon the stable statistical relationship between modern tree growth and temperature, we reconstructed past temperature estimates from the ice patch subfossil ring-width chronology. Our results provide estimates of mid-Holocene warm-season (and biennial) average temperatures ranging from 5.7-6.5 °C (-0.44-0.26 °C) respectively. A multi-century regional cooling trend beginning around 5,650 years BP resulted in average temperatures declining below a warm-season (biennial) critical threshold of ~5.8 °C (-0.34 °C), likely leading to the eventual death of the whitebark pine stand and subsequent formation of the ice-patch. This high-quality paleo-ecological dataset reveals a major shift in the alpine and forest ecotone on the Beartooth Plateau following the mid-Holocene warm period and offers further insight on the thermal limits of whitebark pine trees in the Greater Yellowstone Ecosystem.Item Characteristics of whitebark pine (Pinus albicaulis) growth & defense in disturbance-prone, high-elevation, montane ecosystems of the northern Rocky Mountains(Montana State University - Bozeman, College of Letters & Science, 2022) Kichas, Nickolas Earl; Chairperson, Graduate Committee: David McWethy; This is a manuscript style paper that includes co-authored chapters.Whitebark pine (Pinus albicaulis) is a high-elevation conifer, recognized as a foundation species due to the numerous ecological benefits it provides in subalpine environments. In whitebark pine and other conifers, resin-based defenses have long been recognized as the primary mechanism by which trees respond to bark beetle attacks and several studies have linked resin duct properties to survivorship during periods of increased beetle activity. Utilizing a unique dataset of whitebark pine collected on the Flathead Indian Reservation in northwestern Montana, we set out to investigate the following research questions: (1) Are there differences in physiology (tree growth and resin duct anatomy) between trees that persisted through recent mountain pine beetle outbreaks and trees that died? (2) Does constitutive resin chemistry differ between whitebark and co-occurring lodgepole pine and are there relationships between tree growth, resin duct anatomy and resin chemistry? (3) Does competition influence constitutive resin chemistry in either whitebark or lodgepole pine? and (4) Is whitebark pine growth and/or resin duct anatomy constrained by warmer and/or regionally drier conditions? We found that whitebark pine trees that have persisted through recent stand-level disturbance produced fewer but larger resin duct structures with greater duct area compared to trees that died. We also detected important differences in the chemical composition of resin between whitebark and lodgepole pine that generally support field observations, whereby under endemic scenarios mountain pine beetle preferentially select lodgepole pine, while under outbreak scenarios, beetles successfully colonize whitebark pine trees. We found complex relationships between tree growth, resin duct anatomy and constitutive resin chemistry that present beetles with many permutations of resin-based defenses, while competition, particularly with Engelmann spruce (Picea engelmannii) can further influence constitutive resin chemistry. Lastly, we found that whitebark pine across our study sites are experiencing increased growth and defense under warmer and regionally drier conditions. Whitebark pine at our study sites exhibit differing strategies in the allocation of resources toward growth and defense. Our results support the idea that maintaining genetic variability promotes diverse response strategies to a complex array of biophysical stressors that might leave a species vulnerable to extinction across its range.