GC54A-01
Effects of Past Climate Changes on Ecosystem Biogeochemical Cycles in Rocky Mountain Forests and Lakes
Ongoing climate trends will likely alter how forest ecosystems produce important goods and services, in part, by changing ecosystem responses to disturbances, such as fires and land-use. Disturbances induce forest succession and thus dramatically change the flow of water and nutrients through a given ecosystem. However, long-term ecosystem responses to disturbance, especially regarding nutrient pools and cycling rates, are poorly documented, and less is known about the effects of century-scale climate trends on these responses especially with respect to moisture. Here, we show biogeochemical responses to repeated (>20) episodes of disturbance and succession in a single ecosystem under a range of climatic conditions over 2000 years. Our lake sediment record shows regular fluctuations in the flux of base cations and other macronutrients from lodgepole pine ( Pinus contorta) forests in northern Colorado following catastrophic stand-replacing fires. Post-fire elemental fluctuations are consistent with ecosystem theory regarding the re-equilibration of biomass and nutrient pools during succession, but show systematic variation that has been previously undocumented. The time span of post-fire re-equilibration correlates positively with measures of fire severity, which is consistent with hypotheses that seed dispersal and soil recovery likely slow re-growth after large or severe fires. Likewise, dry conditions during the Medieval Climatic Anomaly (MCA, 1200-500 yrs BP) altered elemental fluctuations and, thus, generated post-fire pulses of lake eutrophication that were not evident during other periods. The interaction of climate and disturbance, therefore, has important consequences for ecosystem function and services, including the quality of aquatic environments.
GC54A-02
Sedimentological and Paleoecological Records From the Central and Western Canadian Arctic in Relation to ice Core Paleoclimatic Data
A series of lake sediment cores spanning the Holocene from across the central and western Canadian Arctic document ecosystem responses to climatic variations on several timescales. Sediment parameters (grain size, LOI, magnetic susceptibility) analyzed at very high resolution are used to quantify hydroclimatic variability at the watershed scale. Pollen, diatom, biogenic silica and chironomid series measured at high resolution show that the responses of terrestrial and aquatic organisms to millennial-scale climate variations are coeval with those variations interpreted from Greenland and Canadian ice core records. Climatic change caused large fluctuations in terrestrial production, as measured by pollen influx and concentrations, although biodiversity changes were less significant. Diatom stratigraphies are more complex, and show not only production changes but also diversity changes. These fluctuations are ultimately controlled by climate, but the direction and magnitude of changes in diatom assemblages are often determined by local factors such as habitat availability, lake size and nutrient status. Interpretation of chironomid records in relation to diatom-derived proxies for primary production indicate the combined impact of food source and climate on chironomid populations. Climate variations during the Holocene and the associated ecological impacts are sometimes comparable in rate and magnitude to those occurring now, suggesting paleoecological records from Arctic lakes can be used to better predict future impacts of climatic change.
GC54A-03
Late Holocene Decline of Beech Populations in the Central Great Lakes Region: Drought- Induced Vegetation Change in a Humid Region.
A large decline in beech populations ( Fagus grandifolia) has been well-documented from pollen records in southeastern Michigan and Southern Ontario between 1000 and 600 BP. These records reveal that declines in beech pollen were generally associated with increases in oak ( Quercus) and pine ( Pinus). The beech decline probably extended eastward into western Pennsylvania and New York, although beech populations in Upper Michigan remained unaffected or even expanded during this time period. The causes of these forest changes and their spatial patterning is not completely understood, although they have been variously attributed to anthropogenic disturbance, climatic cooling, or drought. Recent paleoclimate evidence from the region indicates that the most severe droughts of the last 2000 years occurred between 1000 and 700 BP. However, direct attribution of drought as a cause of the decline has been problematic because of uncertainties associated with comparison of radiocarbon-dated chronologies. We have conducted tandem investigations of pollen, charcoal, hydroclimate proxies (testate amoebae, humification), and a temperature proxy (d18O of Sphagnum cellulose) from the archives contained in three Sphagnum-dominated peatlands of the region. Two of these peatlands were located within the region of the beech decline (eastern, lower Michigan) and the other was outside the decline region (Upper Michigan). Our results reveal that a series of large droughts, likely the combined result of decreased summer precipitation and warm temperatures, were associated with the beech decline. Large wildfires were also associated with the droughts. Neither droughts nor beech declines were recorded in Upper Michigan. High-resolution analysis of beech pollen and hydroclimate proxies at sites that experienced the droughts reveals complex dynamics at multidecadal timescales between 1000-700 BP, with large fluctuations in beech pollen, available moisture, and charcoal concentrations. Our study demonstrates the potential of using the tandem application of pollen, charcoal, and a wide variety of vegetation-independent climatic proxies to assess ecological responses to climate variability at multidecadal timescales. Although moisture variability is not generally recognized as a driver of ecological change in humid regions, our data indicate that ecologically significant thresholds have been crossed in the recent past.
GC54A-04
Coupled Pollen, Spore, and Macrofossil Hudson River Marsh Paleoecological Analysis with X-Ray Fluorescence Elemental Analysis to Study Estuarine Ecosystem Response to Anthropogenic and Climatic Changes
Stockport Flats (41.3N, 73.8W) and Tivoli North Bay (42.0N, 73.9W) are the two northernmost Hudson River National Estuarine Research Reserve freshwater tidal marshes in New York. Our paleoecological records based on pollen, spores, macrofossils, and loss-on-ignition (LOI) of marsh sediment cores at these two sites suggest significant local and regional anthropogenic changes and climatic variability, including the Medieval Warming Period. We implement the use of a field portable X-Ray Fluorescence Spectroscopy (Innov-X, USA) as an independent proxy to provide more information about chronology, watershed land-use changes, and estuarine processes. Over the last 200 years, there is a pronounced decrease in organic matter, a shift in vegetation, and an increase in invasive species such as Phragmites australis, Lythrum salicaria, and Typha angustifolia. Coupling of more traditional chronological measurements, such as Ambrosia pollen rise and radiometric dating (C-14, Cs-137, and Pb-210), with heavy metals profiles (Pb, Cr, Cu, and Zn) using the XRF unit provides additional time horizon markers, as these metals have distinct peaks in the 1960s and toward the present. Dates from the XRF profiles near the top of the core help to confirm the timing and rate of vegetation changes, especially the spreading of the invasive species. Discrete metal peaks using the XRF help to quickly determine the degree of disturbances and resolution of the cores as analysis of Cs-137 profile is much slower. Sediment proxies, including Ca, K, Ti/S, and Fe/S increase while Sr and Zr decrease toward the top of the core, probably representing higher erosion from land-use changes concurrent with lithologic shifts, LOI decline, and invasive species expansion. Sulfur concentration increases many orders of magnitude especially in the Stockport core and may be a good proxy of salinity, an indicator of drought and seawater rise. This information is valuable to compare with the vegetation changes to understand how salinity and hydrology anomalies may have affected these wetlands in the past.
GC54A-05
Ecological Impact of Climate Change on Leaf Economic Strategies Across the Paleocene- Eocene Thermal Maximum, Bighorn Basin, Wyoming
Deciphering the ecological impacts of climate change is a key priority for paleontologists and ecologists alike. An important ecological metric in vegetated settings is the leaf economics spectrum, which represents an adaptive continuum running from rapid resource acquisition to maximized resource retention. This spectrum is comprised of a large number of coordinated traits, including leaf mass per area (LMA), leaf lifespan, photosynthetic rate, nutrient concentration, and palatability to herbivores. Here we apply a recently developed technique for reconstructing LMA to a suite of four isotaphonomic fossil plant sites spanning the Paleocene-Eocene thermal maximum (PETM) in the Bighorn Basin, Wyoming, USA. This technique is based on the biomechanical scaling between petiole width and leaf mass, and it has been calibrated with 65 present-day sites from five continents and tested on two well-known Eocene fossil localities (Bonanza, Utah and Republic, Washington). There are no significant differences in LMA among plants across the PETM. This stasis is present despite a backdrop of extreme climate change during the PETM in this region, including a three-to-four-fold increase in atmospheric CO2, an ~5 °C rise in temperature, and possible drying. Moreover, quantitative measurements of insect herbivory show, on average, a two-fold increase during the PETM relative to before and after the event. We interpret our results to suggest that leaf-economic relationships can, in some situations, partially decouple. More specifically, our documented increase in insect herbivory during the PETM with no concomitant decrease in LMA implies that during this interval less carbon was being captured by plants per unit of investment. Because the rate and magnitude of climate change during the PETM is similar to present-day anthropogenic changes, our results may provide clues for predictions of ecological impacts in the near future.