Doctor of Philosophy

dissertationDisturbances play an integral role in regeneration and succession of many forested ecosystems in the Intermountain West region of the western United States. However, changes in climate have been shown to alter the occurrence, duration, and frequency of disturbances. The research presented here uses a paleoecological approach using multiple proxies from sediment cores from three different sites from the Intermountain West in order to assess the linkages among disturbances, climate, and vegetation composition. From the first site in the central Rocky Mountains, a paleoenvironmental data documents the sensitivity of past quaking aspen occurrence to increased temperatures, while frequent wildfire activity led to the persistence of a quaking aspen. From the second site located in south central Utah, a paleoenvironmental data documents how changes in the position of the El Nio Southern Oscillation dipole transition zone affects moisture availability across the state of Utah, which ultimately influences vegetation composition and wildfire frequency. Lastly, from the third site located on the north slope of the Uinta Mountains in northeastern Utah, paleoenvironmental data assesses the long-term primary control on wildfire activity from the region. The results from this dissertation suggest that disturbance regimes have been in a state of constant change throughout the Holocene as a result of climate variability and in combination these led to changes in vegetation composition. The information contained in this dissertation will be important for natural resource planning and management because it provides context regarding the natural range of disturbance and vegetation variability for three distinctly different forested settings located in the Intermountain West. Forest managers can use paleoenvironmental records as analogs to help place context of how forested ecosystems will respond to climatic changes. By providing forest managers with long-term information about forest composition and disturbance regimes at multiple sites, this dissertation can be used to enhance resource policy making, planning and management

Fire hazard modulation by long-term dynamics in land cover and dominant forest type in eastern and central Europe

Wildfire occurrence is influenced by climate, vegetation and human activities. A key challenge for understanding the risk of fires is quantifying the mediating effect of vegetation on fire regimes. Here, we explore the relative importance of Holocene land cover, land use, dominant functional forest type, and climate dynamics on biomass burning in temperate and boreo-nemoral regions of central and eastern Europe over the past 12 kyr. We used an extensive data set of Holocene pollen and sedimentary charcoal records, in combination with climate simulations and statistical modelling. Biomass burning was highest during the early Holocene and lowest during the mid-Holocene in all three ecoregions (Atlantic, continental and boreo-nemoral) but was more spatially variable over the past 3–4 kyr. Although climate explained a significant variance in biomass burning during the early Holocene, tree cover was consistently the highest predictor of past biomass burning over the past 8 kyr. In temperate forests, biomass burning was high at ~ 45% tree cover and decreased to a minimum at between 60% and 70% tree cover. In needleleaf-dominated forests, biomass burning was highest at ~60 %–65%tree cover and steeply declined at > 65% tree cover. Biomass burning also increased when arable lands and grasslands reached ~15 %–20 %, although this relationship was variable depending on land use practice via ignition sources, fuel type and quantities. Higher tree cover reduced the amount of solar radiation reaching the forest floor and could provide moister, more wind-protected microclimates underneath canopies, thereby decreasing fuel flammability. Tree cover at which biomass burning increased appears to be driven by warmer and drier summer conditions during the early Holocene and by increasing human influence on land cover during the late Holocene. We suggest that longterm fire hazard may be effectively reduced through land cover management, given that land cover has controlled fire regimes under the dynamic climates of the Holocene

Assessing changes in global fire regimes

PAGES, Past Global Changes, is funded by the Swiss Academy of Sciences and the Chinese Academy of Sciences and supported in kind by the University of Bern, Switzerland. Financial support was provided by the U.S. National Science Foundation award numbers 1916565, EAR-2011439, and EAR-2012123. Additional support was provided by the Utah Department of Natural Resources Watershed Restoration Initiative. SSS was supported by Brigham Young University Graduate Studies. MS was supported by National Science Centre, Poland (grant no. 2018/31/B/ST10/02498 and 2021/41/B/ST10/00060). JCA was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 101026211. PF contributed within the framework of the FCT-funded project no. UIDB/04033/2020. SGAF acknowledges support from Trond Mohn Stiftelse (TMS) and University of Bergen for the startup grant ‘TMS2022STG03’. JMP participation in this research was supported by the Forest Research Centre, a research unit funded by Fundação para a Ciência e a Tecnologia I.P. (FCT), Portugal (UIDB/00239/2020). A.-LD acknowledge PAGES, PICS CNRS 06484 project, CNRS-INSU, Région Nouvelle-Aquitaine, University of Bordeaux DRI and INQUA for workshop support.Background The global human footprint has fundamentally altered wildfire regimes, creating serious consequences for human health, biodiversity, and climate. However, it remains difficult to project how long-term interactions among land use, management, and climate change will affect fire behavior, representing a key knowledge gap for sustainable management. We used expert assessment to combine opinions about past and future fire regimes from 99 wildfire researchers. We asked for quantitative and qualitative assessments of the frequency, type, and implications of fire regime change from the beginning of the Holocene through the year 2300. Results Respondents indicated some direct human influence on wildfire since at least ~ 12,000 years BP, though natural climate variability remained the dominant driver of fire regime change until around 5,000 years BP, for most study regions. Responses suggested a ten-fold increase in the frequency of fire regime change during the last 250 years compared with the rest of the Holocene, corresponding first with the intensification and extensification of land use and later with anthropogenic climate change. Looking to the future, fire regimes were predicted to intensify, with increases in frequency, severity, and size in all biomes except grassland ecosystems. Fire regimes showed different climate sensitivities across biomes, but the likelihood of fire regime change increased with higher warming scenarios for all biomes. Biodiversity, carbon storage, and other ecosystem services were predicted to decrease for most biomes under higher emission scenarios. We present recommendations for adaptation and mitigation under emerging fire regimes, while recognizing that management options are constrained under higher emission scenarios. Conclusion The influence of humans on wildfire regimes has increased over the last two centuries. The perspective gained from past fires should be considered in land and fire management strategies, but novel fire behavior is likely given the unprecedented human disruption of plant communities, climate, and other factors. Future fire regimes are likely to degrade key ecosystem services, unless climate change is aggressively mitigated. Expert assessment complements empirical data and modeling, providing a broader perspective of fire science to inform decision making and future research priorities.Peer reviewe

The Eurasian Modern Pollen Database (EMPD), version 2

The Eurasian (née European) Modern Pollen Database (EMPD) was established in 2013 to provide a public database of high-quality modern pollen surface samples to help support studies of past climate, land cover, and land use using fossil pollen. The EMPD is part of, and complementary to, the European Pollen Database (EPD) which contains data on fossil pollen found in Late Quaternary sedimentary archives throughout the Eurasian region. The EPD is in turn part of the rapidly growing Neotoma database, which is now the primary home for global palaeoecological data. This paper describes version 2 of the EMPD in which the number of samples held in the database has been increased by 60 % from 4826 to 8134. Much of the improvement in data coverage has come from northern Asia, and the database has consequently been renamed the Eurasian Modern Pollen Database to reflect this geographical enlargement. The EMPD can be viewed online using a dedicated map-based viewer at https://empd2.github.io and downloaded in a variety of file formats at https://doi.pangaea.de/10.1594/PANGAEA.909130 (Chevalier et al., 2019)Swiss National Science Foundation | Ref. 200021_16959

The Eurasian Modern Pollen Database (EMPD), version 2

The Eurasian (nee European) Modern Pollen Database (EMPD) was established in 2013 to provide a public database of high-quality modern pollen surface samples to help support studies of past climate, land cover, and land use using fossil pollen. The EMPD is part of, and complementary to, the European Pollen Database (EPD) which contains data on fossil pollen found in Late Quaternary sedimentary archives throughout the Eurasian region. The EPD is in turn part of the rapidly growing Neotoma database, which is now the primary home for global palaeoecological data. This paper describes version 2 of the EMPD in which the number of samples held in the database has been increased by 60% from 4826 to 8134. Much of the improvement in data coverage has come from northern Asia, and the database has consequently been renamed the Eurasian Modern Pollen Database to reflect this geographical enlargement. The EMPD can be viewed online using a dedicated map-based viewer at https://empd2.github.io and downloaded in a variety of file formats at https://doi.pangaea.de/10.1594/PANGAEA.909130 (Chevalier et al., 2019).Peer reviewe

Long-Term Perspectives on Aspen-Fire Dynamics

In the US Rocky Mountains, aspen is considered a keystone species that supports communities of high biological and genetic diversity. Generally, aspen communites can be characterized as either stable or seral, each supporting distinct regimes. While multiple possible aspen regimes have been postulated, they are based on limited,observational tree-ring-based studies. New datasets are necessary to elucidate how aspen vegetation dynamics and fire regimes will respond to directional climate change that has the potential to reorganize vegetation communities and promotes shifts in fire regimes over longer time scales. Paleoecological reconstructions provide fundamental information of baseline environmental conditions that contributes to a fuller understanding of the broad-scale, long-term patterns of past disturbance and vegetation changes in aspen-dominated systems. Specifically, lake sediment studies relying on pollen and charcoal provide information about past vegetation composition and fire histories in many conifer-dominated systems. However, information specific to aspen ecosystems are underrepresented in environmental reconstructions, despite their ecological significance. Our objective is twofold: 1) Using pollen records, how can we distinguish stable from seral aspen communities in long-term ecological reconstructions? and 2) By combining pollen and charcoal records, can we determine the possible role of fire in promoting, maintaining, or inhibiting aspen- dominated communities? We examine paleoecological data from several subalpine lakes in the central Rockies to help us understand climate-mediated aspen- re dynamics over centennial to millennial timescales

MS

thesisThe Medicine Bow Range of southeastern Wyoming acts as a conduit for vegetation migration from Colorado into southeastern Wyoming. Macroscopic charcoal and pollen from Long Lake, Wyoming were used to reconstruct past fire and vegetation changes in conjunction with local, regional and broad-scale climatic changes. Distinct changes in pollen influx, along with high peak magnitudes (calculated using from CharAnalysis), were used to identify high fire severity fire episodes. The late Glacial period experienced cold and dry conditions, with low fire occurrence because there was low connectivity among the vegetation. As the climate shifted to warmer conditions in the early Holocene, conifers began to migrate into the region, creating a greater fuel source

Millennial-scale fire dynamics in temperate forest of central Europe

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