Bark beetles (Dendroctonus spp.) and wildfire are significant disturbance agents in conifer forests of western North America. Understanding the occurrence, the ecological significance and how these two disturbances interact over long time scales and under different climatic regimes is import to contextualize current forest dynamics. Methodologies exist for reconstructing past fire episodes over the Holocene (last ~10,000 years) using sedimentary charcoal from lake sediments. However, no methodology currently exists for assessing bark beetle disturbances over similar timescales using lake sediments, despite their ecological significance. The ecological response of subalpine forests to a spruce beetle infestation is generally that subalpine fir (Abies lasiocarpa) and other understory components increase in abundance and productivity following mortality of mature spruce. The invigorated growth of the understory is attributed to increased availability of sunlight, moisture and nutrients. The 'release' of the understory component following severe outbreaks is well documented in historic stand inventory data as well as in tree ring records. These changes in stand structure are reflected in the pollen record. Using pollen records obtained from six subalpine basins in Utah, a calibration study was performed emphasizing changes in pollen deposition concurrent with several 20th century spruce beetle (D. rufipennis) outbreaks. These changes generally reflect an exchange of spruce pollen for fir and other understory taxa. The research supported here will apply the calibration set to pollen data from three Holocene-length sediment cores retrieved from the Wasatch Plateau, Markagunt Plateau and Aquarius Plateau, located in central and southern Utah. The Markagunt Plateau site is the most temporally resolved core and will be dated and analyzed at multidecadal resolution to further refine forest dynamics associated with spruce beetle infestations.
This project will provide an important step forward in using lake sediments to reconstruct non-fire disturbances, specifically for spruce beetle in subalpine forests. Developing long term records of forest ecology and disturbance are important resources for land managers and their respective agencies. The research conducted here will provide an important baseline of understanding with which to interpret recent bark beetle outbreaks in western conifer forests. The support of this research through a Doctoral Dissertation Research Improvement award will enable a promising student to establish a strong environmental research career.
During recent decades in western North America, native bark beetle populations increased substantially resulting in unprecedented levels of tree mortality. In particular, high elevation ecosystems dominated by conifers such as spruce and pine have been especially hard hit. The outbreaks have been attributed to a complex interaction of environmental variables, including climate warming and unnaturally high stand densities due in part to fire suppression strategies and livestock grazing. In the wake of these recent bark beetle outbreaks, important concerns have surfaced that highlight major uncertainties for society and landscapes. In addition to millions of hectares of standing dead trees, aesthetic losses, water quality issues, and perceived risk of elevated wildfire susceptibility underscore the need for greater understanding of bark beetle disturbance processes. Long term environmental records are vital scientific resources for understanding the ecological role and outcomes of forest disturbances. This research project contributes directly to the data and information needs identified by public land stewardship agencies such as the USDA Forest Service and National Park Service. We used paleoecological proxies such as pollen and charcoal preserved in lake sediment cores to assess the landscape impacts to severe spruce beetle (Dendroctonus rufipennis) infestations. The goal of this project was to improve understanding of how terrestrial disturbances are recorded in lake sediments to develop a methodology that would enable reconstructions of bark beetle disturbance over long timescales (for example, the last 10,000 years). We collected and examined sediment cores collected from six subalpine lake basins in south-central Utah. All of the watersheds investigated for this study experienced at least 80% mortality in mature Engelmann spruce from spruce beetle disturbances during the 20th century. To establish a chronology for the six sediment cores, we used established dating techniques to constrain the historic (last few hundred years) and long term portions of the record. The sediment samples then underwent a series of chemical and sieving procedures conducted in the laboratory to isolate pollen and other plant remains. The residues were then mounted on glass slides and examined with light microscopes. Pollen grains, for example, are generally identifiable to the genera-level (e.g. spruce or fir) though exceptions do exist. In each of our sediment samples, at least 300 individual pollen grains were identified and counted. This helps us understand what plants were growing around our study sites in the past. Upon examining pollen from the historical portions of the sedimentary cores, the spruce beetle outbreak was clearly evident. We observed that pollen of host trees (Engelmann spruce) dropped off significantly during the spruce beetle epidemic. At the same time, pollen of plants that are non-host to spruce beetle and remain on the landscape after the attack, such as understory subalpine fir, shrubs, and herbs, became more abundant. The trade-off between host and non-host plants was observable in all six sediment cores that we examined and provided a framework for analyzing pollen records over centennial to millennial timescales. Our analysis of longer records using this methodology was encouraging but will benefit from other types of evidence. This is because we found that the pollen signal associated with historic outbreaks was not directly comparable with the temporal resolution of longer records.
