The aspen parkland vegetation zone in central Canada represents a climatically sensitive region where a combination of temperature, effective moisture, and fire may control the dynamics of the boreal ecotone to the north and the grassland ecotone to the south. However, relatively little is known about the long-term interactions among climate, vegetation, and fire in this region. We present new multi-proxy lake sediment records spanning most of the Holocene for two lakes located at the southern (Jones Lake, 11,500 cal yr BP) and northern (Mallard Lake, 8,500 cal yr BP) ends of this zone in Manitoba Canada. We measured changes in pollen, charcoal, BSi , total P, LOI, and rock magnetic proxies of mineral size.
Before 10,000 BP, the southern site (Jones Lake) was surround by Picea forest, which was replaced by a dry Pinus-grassland assemblage by 9,500 BP. The predominance of herbaceous species persisted until ~4,000 BP when Quercus began to expand and fire severity increased gradually, consistent with increased moisture and fires driven by fuel production rather than aridity. Inorganic sediment deposition was uniformly high, but BSi peaked about 7,000 BP and total P rose steadily through time. The northern site (Mallard Lake) was characterized by a Pinus-grassland assemblage between 8,500-5,500 BP, after which Myriophyllum increased, suggesting that lake levels were lower. After 3,500 BP, herbaceous species declined, Betula increased significantly to a peak of 20-40%, and fire severity increased, consistent with our interpretation of increased moisture and fuel production for Jones Lake at this time. Unlike Jones Lake, Mallard showed a continuous rise in BSi and fine mineral fractions with significant, superimposed centennial-scale variability. Also unlike Jones, total P was constant until ~1,000 BP when large increases in P were observed. These results suggest that regional shifts to drier conditions at ~10,000-9,000 BP followed by wetter conditions ~ 4,000 BP occurred across the entire aspen parkland zone but that within-lake and terrestrial dynamics were more variable at the boreal ecotone.
Earth history is the study of changes in the Earth system over time, focusing not just on sequences of events but on patterns of change. We apply our knowledge of present processes to past events, and also use our knowledge of past events to better understand present processes. In particular, we employ an Earth history approach to understand processes that operate slowly (on a human timescale), or those that only affect the Earth rarely (but dramatically). In the context of geologic time, the rates of anthropogenic environmental change stand out sharply.
Evolution, plate tectonics, and climate change are central to science and are best understood and taught using an Earth history perspective. Earth history and modern observations make a vital combination for teaching about topics with important modern-day ramifications such as the origin of life, geologic hazards, and global warming. Earth history courses and units can be built around relevant concepts like the geologic timescale, the history of science, or regional stratigraphy. Alternatively, more specialized courses on popular topics like dinosaurs represent a last chance to teach science to non-majors. More Earth history teaching resources are available at the Starting Point web site.
This project is intended to replace some of the lectures that would ordinarily be necessary in a survey of Earth history over geologic time. The students will be taking the lecturer’s place in front of the class, presenting some of the material to their colleagues. Students will work in groups on a single era or period. Each student role-plays an expert (such as an oceanographer) and works with teammates playing other sorts of experts (a biologist, a geologist, and an atmospheric scientist). Their presentation will require them to do research. They will be constructing resource lists to keep track of how they learned what they are presenting and beginning a critical analysis of resources found on the World Wide Web. They will also write brief individual summaries of the findings within their sphere. While the students are researching and preparing their presentations, the instructor will start giving lectures on the earliest time units, the ones that are the most complex from an Earth System perspective, modeling the kind of presentation that the students will be doing. Eventually, the students will take the stage, presenting their time units in order. Rubrics for assessing the presentation and the resource list are included.
Catfish Pond is a small lake in central Illinois. I have analyzed the pollen and diatom records from a core taken in 1996. Three AMS dates have been obtained along the core. The diatom assemblages of the deepest levels contain mostly Eunotia monodon and large (80 microns+) Pinnularia species, including P. maior and P. viridis. These levels are also rich in pine, oak, ash, and sedge pollen. Sediments dated to the late Wisconsinan glacial period, just over 22,000 14C years ago also contain a lot of spruce pollen, indicating a cooler climate. Eunotia monodon accounts for over 60% of the diatom valves counted, up from ~ 20% in earlier levels. Between 6,000 and 200 years ago, the pollen assemblages indicate that the landscape was a mosaic of oak-hickory forest and prairie. Eunotia monodon decreases to less than 10% of valves counted and the diatom flora becomes more diverse, including substantial amounts of Stauroneis phoenicenteron and Neidium affine. After 200 years ago, sedimentation rate and ragweed pollen percentages increased, indicating homesteading and associated intense agriculture in central Illinois. The diatom assemblages have become even more diverse, dominated in the upper levels by Aulacoseira italica.
Throughout the record, there is a tight correlation between Eunotia monodon as a percentage of diatom valves counted and coniferous tree pollen as a percentage of terrestrial pollen types (r2 = 0.7924, p < 0.0005). Perhaps both the diatom flora and tree taxa are responding synchronously to climate change or perhaps there is a more direct link. Coniferous tree litter tends to decompose slowly and to acidify the soil of the catchment, which would in turn decrease the pH and increase the DOC content of the lake, which might favor Eunotia monodon over its competitors.
Pittsburg Basin, in south-central Illinois, contains a sediment record extending from the present back to the end of the late Illinoian glaciation, when central Illinois was covered with Picea/Pinus forest. During the last interglaciation, a temperate deciduous forest more diverse than Holocene Quercus/Carya forest replaced the Illinoian late-glacial boreal forest. Prairie pollen types and the charcoal/pollen ratio, indicating fire frequency, temporarily increased. Then forest, with high Juniperus percentages, became dominant once more, as the charcoal/pollen ratio dropped. After the last interglaciation, the charcoal/pollen ratio increased again and prairie and wetland surrounded Pittsburg Basin through the entire Wisconsinan glacial age. The area was still prairie in late Wisconsinan time, but with some Picea and Pinus. During the Holocene, the region has been a mixture of prairie and Quercus/Carya forest. During the last interglaciation, Pittsburg Basin was surrounded by vegetation different from that surrounding it during the present interglaciation. Rather than indicating substantial differences in climate between analogous phases of different glacial/interglacial cycles, this variation may be due to changes in fire frequency, which could be caused by small changes in climate, human activity, or differences in soil.
We investigated the effects of strip width, slope position, and soil scarification in a split–split plot design on the regeneration of northern hardwoods in northwestern Massachusetts. Whole plots of 20 and 40 m in width were cut in 1954 in a second growth forest dominated by Betula papyrifera. Slope position and soil scarification were the split and split–split plot treatments, respectively. We measured height for all tree species present in randomly located 4 m2 plots beginning in 1955 and at irregular intervals over the following 42-year period. We measured all trees in the cut strips in 1996. Prunus pensylvanica was the dominant species initially, but had nearly disappeared from the cut strips by 1996. Soil scarification significantly increased initial establishment of B. papyrifera, but density and basal area of this species did not differ by soil treatment in 1996. Tree composition in cut strips was weakly correlated with soil moisture, soil scarification, and initial tree density immediately following cutting, but high spatial variation in species composition and low replication made it difficult to detect any significant correlations among the distribution and abundance of different species and selected environmental variables. The canopy of the cut strips is even-aged; establishment of most canopy trees occurred within 5 years following cutting. A comparison of successional trends in adjacent uncut strips with the trends in the cut strips indicates that cutting has altered the sequence of successional changes in forest composition increasing the abundance of some species that were of low importance prior to cutting. In 1996, Acer rubrum and A. saccharum are replacing B. papyrifera in the canopy of the uncut strips. The canopy of the cut strips consists of a diverse and spatially varying mixture of intermediate hardwoods including Quercus rubra, Fraxinus americana, Betula lenta, Acer rubrum, B. papyrifera, and an understory of late successional hardwoods.
Last updated 7 July 2004.