Holocene Vegetation, Drought, and Fire Variability in the Northern Great Basin, Oregon
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Date
2024-01-09
Authors
Saban, Chantel
Journal Title
Journal ISSN
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Publisher
University of Oregon
Abstract
The Northern Great Basin of Oregon is an area of diverse ecologies organized along elevational gradients and variable water sources. At the lowest elevations are the remnants of Pleistocene pluvial lakes, now deflated alkaline playas. Sagebrush (Artemisia tridentata) steppe dominates the region, and anywhere there is water at or very near the surface, marshes are present. At higher elevations open dry-forest systems begin appearing, composed primarily of Pinus ponderosa, but also including Pinus contorta and Juniperus occidentalis. Populus tremuloides is also found in greater abundance at these mid-elevation areas. High-elevation sites often host mixed conifer forests, subalpine forests, and some alpine conditions, with white-bark pine (Pinus albicaulis) found at some of the highest peaks. During the Pleistocene atmospheric conditions were cooler than present day, and evapotranspiration was much lower, resulting in the formation of large lakes. There were also glaciers present in some places, as well as locations too cold and dry to form glaciers.Climatic conditions began changing rapidly beginning ca. 12,000 years ago (Mehringer 1987; Wigand 1987). Maximum insolation continued warming the planet and peaked by 11,000 years ago most of the continental ice sheets were rapidly retreating while montane glaciers in the NGB had already retreated (Osborn and Beavis, 2001). By 9000 years ago maximum air temperatures and increased aridity resulted in the Northern Great Basin pluvial lakes desiccating and many vegetation communities shifting upward in elevation. Such climate changes would also cause fire event frequency to also change during this time, resulting in conditions and disturbance timings very different than the current day.
Towards the end of the early Holocene the catastrophic eruption of Mount Mazama in ca. 7640 cal yr BP (Egan, 2015) would again alter vegetation communities and fire events to varying degrees depending on locations relative to the main eruption blast zone. Despite periodic droughts climatic conditions in the NGB have generally cooled through the late Holocene, with vegetation communities again responding. (Benson et al., 1997; Minckley et al., 2007; Marsicek et al., 2018).
With an emphasis on the Holocene, questions behind this dissertation were driven by asking 1) by how much did vegetation communities change in the Northern Great Basin responding to changes in climate and fire, 2) which taxa changed the least, which the most, and was it climate or fire that drove those changes, 3) by how much is it possible to observe regional or local drought severity, and 4) by how much and when did climatic timing in the Northern Great Basin differ from Central and Southern Great Basin regions, if at all? To address these broad questions three locations were identified as good study sites. These three locations are within 40 km of each other but at different elevations. Differing elevations were sought for the purpose of attempting to determine what the rate of ecological change was for each location.
There are few records showing continuous ecological and fire records at different elevations in the NGB from the early Holocene through to today, but of the records that do remain a rich history of variable timing for fire histories and ecological community structures are sharply delineated and preserved (Gruell, 1995; Minckley et al., 2007).
Environmental conditions were reconstructed using traditional and novel methods for three sites identified as ideal for contributing ecological perspectives that would overlap in time. To reconstruct ecological settings, pollen was the primary data source for all three sites. Pollen assemblages provide a view of the climatic conditions at a given point in time, but in some cases may not reflect the full context of conditions as other variables such as tephra or charcoal may alter interpretations. Iin the case of coprolite pollen, a false sense of what vegetation is present on the landscape and in what abundances can occur. When available, carbon and nitrogen concentrations show how climate affected lake productivity, and charcoal provides insights on the fire-adapted landscape and how vegetation responded to changing arid conditions and fire events over time.
Chapter 2 examines the late Pleistocene through early Holocene environmental conditions at a low-elevation site by contrasting the regional pollen signal preserved in the sediments of Paisley Caves to the more focused and hyper-local pollen found in chronologically contemporaneous coprolites produced by medium to large-sized mammals as they moved across their ancient landscapes. The results show several consistent differences in pollen assemblage composition in the coprolites compared to the sediments, consistent with the coprolite producers favoring certain environments prior to depositing coprolites in a cave.
Chapter 3 examines the history of a rare mid-elevation freshwater lake in the NGB. Dog Lake is a landslide-formed lake whose lake level fluctuates annually, but remained very low during the early Holocene, followed by a period of low lake productivity and lower vegetation cover between 8700 and 8200 cal yr BP, then deepened to a point it resembled depths seen today. Using pollen, C and N concentrations, plant macrofossils, and charcoal, we found when lake productivity was low resulting from increased aridity in the early Holocene, there was also fewer fire episodes than expected from climate, likely due to low fuel availability and probably fewer ignition events. Fire frequencies increased with cooling temperatures and increased effective moisture during the middle Holocene.
Chapter 4 describes the fire and hydrological history of White Pine Marsh (WPMA), a high-elevation site located in a small cirque valley at the northern terminus of the Warner Mountains. The site is in a mesic, mixed conifer forest with the perennial marsh having formed after the Mazama eruption and subsequent deposition of tephra in the basin. Sediments also show the fire history was also altered by the tephra. Charcoal showed fires were more frequent and increased in intensity during the early Holocene, abruptly changing to lower intensity and longer fire intervals post-Mazama. Pollen showed mixed conifer conditions since 9500 cal yr BP with Pinus ponderosa always dominant with variable presence of Abies.
This dissertation includes published and unpublished co-authored material. At the time of writing Chapter 2 is in press at Quaternary Review. Co-authors include Daniel Gavin, Erin Herring, and Dennis Jenkins. Herring processed the coprolites and provided analysis descriptions. Jenkins provided site expertise to this paper. Gavin and Saban conceptualized the study and devised the methodology of analysis. Both Gavin and Saban analyzed the sedimentary lithological components. Saban wrote the original manuscript with Gavin’s help in the analysis and visualization of the data. Gavin also reviewed, edited, and contributed to the final manuscript.
In chapter 3 Saban and Gavin conceptualized the study and analyzed sediments. Saban analyzed the pollen and charcoal. Analysis and data visualizations were significantly aided by Gavin while Saban wrote the original manuscript. Gavin further reviewed, edited, and contributed to the final manuscript. In chapter 4 Saban and Gavin conceptualized the study and devised the research methodology as well as analyzed the sediments, while Saban processed and analyzed pollen and charcoal. Saban wrote the original paper, and Gavin reviewed, edited, and contributed to the final manuscript.
Description
Keywords
coprolite, fire, Holocene, lake, Northern Great Basin, pollen