Paul A. Delcourt

Long-Term Forest Dynamics of the Temperate Zone

In a recent summary volume, Huntley and Birks (1983) reviewed the late-Quaternary paleoecological literature from the region of continental Europe situated between the Mediterranean Sea and Fennoscandia, and including the British Isles. This region spans from approximately 10°W to 25°E longitude and from 35°N to 70°N latitude, a geographic sector roughly equal to that in eastern North America for which we have synthesized the literature concerning vegetational changes in the past 20,000 years (Chapters 4 and 5). Together, the summaries available from Europe (including Birks 1986) and from eastern North America cover a substantial portion of the northern Temperate Zone and provide a useful comparison of long-term dynamics of temperate forests.

Quaternary landscape ecology: Relevant scales in space and time

Two primary goals of landscape ecologists are to (1) evaluate changes in ecological pattern and process on natural landscapes through time and (2) determine the ecological consequences of transforming natural land-scapes to cultural ones. Paleoecological techniques can be used to reconstruct past landscapes and their changes through time; use of paleoecological methods of investigation in combination with geomorphic and paleoethnobiological data, historical records, and shorter-term ecological data sets makes it possible to integrate long-term ecological pattern and process on a nested series of temporal and spatial scales. ‘Natural experiments’ of the past can be used to test alternative hypotheses about the relative influences of environmental change, biological interactions, and human activities in structuring biotic communities within landscape mosaics.On the absolute time scale of the Quaternary Period, spanning the past 1.8 million years, current distributional ranges of the biota have taken shape and modern biotic communities have assembled. Quaternary environmental changes have influenced the development of natural landscapes over time scales of centuries to hundreds of thousands of years; human cultural evolution has resulted in the transformation of much of the biosphere from natural to cultural landscapes over the past 5,000 years. The Quaternary extends to and includes the present and the immediate future. Knowledge of landscape changes on a Quaternary time scale is essential to landscape ecologists who wish to have a context for predicting future trends on local, regional, and global scales.

Holocene Ethnobotanical and Paleoecological Record of Human Impact on Vegetation in the Little Tennessee River Valley, Tennessee

Abstract Human occupation and utilization of plant resources have affected vegetation in the lower Little Tennessee River Valley of East Tennessee for 10,000 yr. Changes in Indian cultures and land use are documented by radiocarbon chronologies, lithic artifacts, ceramics, settlement patterns, and ethnobotanical remains from 25 stratified archaeological sites within the Holocene alluvial terrace. The ethnobotanical record consists of 31,500 fragments (13.7 kg) of wood charcoal identified to species and 7.7 kg of carbonized fruits, seeds, nutshells, and cultigens from 956 features. Pollen and plant macrofossils from small ponds both in the uplands and on lower stream terraces record local vegetational changes through the last 1500 to 3000 yr. Human impact increased after cultigens, including squash and gourd, were introduced ca. 4000 yr B.P. during the Archaic cultural period. Forest clearance and cultivation disturbed vegetation on both the floodplain and lower terraces after 2800 yr B.P., during the Woodland period. Permanent Indian settlements and maize and bean agriculture extended to higher terraces 1.5 km from the floodplain by the Mississippian period (1000 to 300 yr B.P.). After 300 yr B.P., extensive land clearance and cultivation by Historic Overhill Cherokee and Euro-Americans spread into the uplands beyond the river valley.

Late-quaternary vegetational dynamics and community stability reconsidered

Abstract Defining the spatial and temporal limits of vegetational processes such as migration and invasion of established communities is a prerequisite to evaluating the degree of stability in plant communities through the late Quaternary. The interpretation of changes in boundaries of major vegetation types over the past 20,000 yr offers a complementary view to that provided by migration maps for particular plant taxa. North of approximately 43°N in eastern North America, continual vegetational disequilibrium has resulted from climatic change, soil development, and species migrations during postglacial times. Between 33° and 39°N, stable full-glacial vegetation was replaced by a relatively unstable vegetation during late-glacial climatic amelioration; stable interglacial vegetation developed there after about 9000 yr B.P. Late-Quaternary vegetation has been in dynamic equilibrium, with a relatively constant flora, south of 33°N on upland interfluves along the northern Gulf Coastal Plain, peninsular Florida, and west-central Mexico.

Presettlement landscape heterogeneity: Evaluating grain of resolution using General Land Office Survey data

General Land Office Survey (GLOS) records from the A.D. 1840s provide data for quantitative characterization of presettlement vegetation across western Mackinac County, Michigan, located within the mixed conifer-northern hardwoods forest region. We analyzed data from land survey plat maps and 1958 bearing, witness, and line trees from 162 surveyed section and quarter-section corners in order to map vegatation cover types at a level of spatial resolution appropriate for characterizing landscape heterogeneity using standard landscape ecological metrics. As also demonstrated by a number of both classic and contemporary plant-ecological studies, the distribution of landforms, soils properties, hydrology, and location of fire breaks all contribute to the heterogeneity in vegetation observed at a landscape scale in the region. Through a series of spatial landscape analyses with differing grain of resolution, in this study we determine that a grid cell size of 65 ha (0.5 mi×0.5 mi or 0.25 mi2) to 259 ha (1 mi2) gives a conservative characterization of landscape heterogeneity using standard metrics and is therefore appropriate for use of GLOS data to study historical landscape changes.

Paleoecology of central Kentucky since the last glacial maximum

Pollen grains and spores, plant macrofossils, and sponge spicules from a 7.2-m sediment core from Jackson Pond dating back to 20,000 yr B.P. are the basis for new interpretations of vegetational, limnological, and climatic changes in central Kentucky. During the full-glacial interval (20,400 to 16,800 yr B.P.) upland vegetation was closed spruce forest with jack pine as a subdominant. Aquatic macrophyte and sponge assemblages indicate that the site was a relatively deep, open pond with low organic productivity. During late-glacial time (16,800 to 11,300 yr B.P.) spruce populations continued to dominate while jack pine declined and sedge increased as the vegetation became a more open, taiga-like boreal woodland. Between 11,300 and 10,000 yr B.P., abundances of spruce and oak pollen oscillated reciprocally, possibly reflecting the Younger Dryas oscillation as boreal taxa underwent a series of declines and increases at the southern limit of their ranges before becoming extirpated and replaced by deciduous forest. In the early Holocene (10,000 to 7300 yr B.P.) a mesic deciduous woodland developed; it was replaced by xeric oak-hickory forest during the middle Holocene between 7300 and 3900 yr B.P. Grass increased after 3900 yr B.P., indicating that the presettlement vegetation mosaic of mixed deciduous forest and prairie (the “Kentucky Barrens”) became established in central Kentucky after the Hypsithermal interval. Sponge spicules increased in number during the Holocene, reflecting reduced water depths in the pond. Sediment infilling, as well as climatic warming and the expansion of fringing shrub thickets, increased nutrient and habitat availability for freshwater sponges.

Late Quaternary paleoclimates and biotic responses in eastern North America and the western North Atlantic Ocean

Abstract Broad-scale late Quaternary paleoclimatic changes are interpreted from palynological records across eastern North America and from assemblages of marine Foraminifera in the western North Atlantic Ocean. Quantitative reconstructions of past dominance for major tree taxa at eight fossil-pollen sites, located south of the maximum glacial extent in the southeastern United States, illustrate dramatic responses of tree populations to climatic change during the transition from Pleistocene to Holocene conditions. A series of maps depicting changes in inferred boundaries of predominant airmasses during the late Quaternary are based upon paleoclimatic interpretation of paleovegetation maps for eastern North America, and CLIMAP maps of winter sea-surface temperatures for the western North Atlantic Ocean. In the eastern United States, at 18, 000 B.P. strong zonal circulation resulted in a fixed position of the atmospheric Polar Frontal Zone along 33°N latitude. The Pacific Airmass extended from 33°N latitude north to the southern margin of the Laurentide Ice Sheet, with the Maritime Tropical Airmass dominating south of 33°N across the Gulf Coastal Plain, the Gulf of Mexico, and the equatorial Atlantic Ocean. The oceanic Polar Frontal Zone extended northeastward from Cape Hatteras to 37°N in the western North Atlantic Ocean. Late glacial climatic amelioration was first reflected in vegetational changes south of the Laurentide Ice Sheet beginning at 16,500 B.P. Late glacial weakening in zonal atmospheric flow resulted in the penetration of the Maritime Tropical Airmass farther to the north during the summer season. Increased summer warmth and rain produced substantial melting of the southern flank of the Laurentide Ice Sheet. The early Holocene interval, from 12,500 to 8,000 B.P., bracketed the time of maximum seasonality of solar radiation, ca. 11,000 B.P. During the early Holocene, enhanced meridional air flows resulted in increased seasonality of temperatures. By 10,000 B.P., the oceanic Polar Frontal Zone was displaced northward from the Gulf Stream. Anomalously cold, maritime climatic conditions persisted along the central Atlantic coast, reflecting the position of the cold-marine Labrador Current between the Atlantic Seaboard and the Gulf Stream. During the mid-Holocene, or Hypsithermal, interval, from 8,000 to 4,000 B.P., prevailing westerlies expanded the midcontinental region of warmth and aridity. For the last 4,000 years, the modern climatic regime has been characterized by increased meridional flow.

PALEOECOLOGICAL INSIGHTS ON CONSERVATION OF BIODIVERSITY:A FOCUS ON SPECIES, ECOSYSTEMS, AND LANDSCAPES

Environmental changes of the Quaternary have driven changes in biodiversity at every level of organization from genetic variation within populations of individual species to gradients in composition of biotic communities and heterogeneity of vegetation mosaics across landscapes. In the southern Appalachian Mountains, increased seasonality of climate during the changeover from glacial to interglacial conditions in the late Pleistocene and Holocene resulted in landscape instability that fostered a fine-grained, heterogeneous mosaic of habitats supporting a diverse biota. Ecotones between alpine, boreal, and temperate ecosystems in the Appalachians shifted dramatically through the Holocene in response to the passage of critical thresholds of temperature and changing disturbance regimes. Regional projections of future greenhouse-gas-induced climatic warming indicate that alpine tundra may be lost between 44° and 57° N and Picea rubens–Abies fraseri forests may become extinct in the southern Appalachians. Mont...

Late Quaternary paleoecology and paleoenvironments of the Central Mississippi Alluvial Valley

A radiocarbon-dated sediment core from Powers Fort Swale, located in the Western Lowlands of southeastern Missouri, provides sedimentological and palynological evidence for dynamic changes in fluvial regime and vegetation in the Central Mississippi Alluvial Valley during the late Pleistocene and Holocene intervals. During the late Wisconsinan glacial maximum, glacial meltwater carried by the Mississippi River maintained active braided streams. Sand grains in full-glacial and late-glacial sediments of Powers Fort Swale show evidence of reworking by eolian activity, with late Pleistocene formation of sand dunes continuing until 11,600 yr B.P. within the Western Lowlands. The sedimentological sequence indicates a progressive, stepwise loss of glacial meltwater and coarse sediment supply during the late-glacial interval, resulting in abandonment of braided-stream terraces. The initial, partial diversion of Mississippi River flow occurred in the late-glacial interval, possibly as early as 16,300 yr B.P., as the Mississippi River shifted from the Western Lowlands through the Bell City-Oran Gap in Crowleys Ridge and into the Eastern Lowlands. By 14,500 yr B.P., the complete diversion of meltwater through this gap coincided with a change in the Powers Fort sediment core from a spruce-dominated to an oak-dominated pollen assemblage. At 11,500 yr B.P., sedimentological changes reflected a changeover to a meandering river regime. The transition to pollen assemblages characteristic of Holocene temperate swamp forest occurred at 9500 yr B.P.

Late-Quaternary dynamics of temperate forests: Applications of paleoecology to issues of global environmental change

Abstract Paleoecological evidence recently summarized from 162 fossil-pollen sites in eastern North America provides new insights concerning the nature and rate of response of temperate forest ecosystems to late Pleistocene and Holocene environmental changes. Across this subcontinental region (25°N to 60°N; 50°W to 100°W), temperate forests have changed in composition, location and area occupied in adjustment to major episodes of climatic cooling and warming during glacial-interglacial cycles of the Quaternary. Forest taxa have migrated differentially, reflecting their individualistic life-history characteristics, dispersal and competitive abilities, and tolerance thresholds to environmental changes, as well as the geographic distribution of corridors and barriers to plant migration. Gradient analysis and ecological ordination of paleovegetational data illustrate that: (1) both positions and breadth of major vegetational ecotones have shifted latitudinally over the past 20 ka; (2) good modern analogues exist for certain full-glacial warm-temperate and boreal forests; (3) during the transition from Pleistocene to Holocene conditions, mixed conifer-northern hardwoods forests, spreading across newly deglaciated terrain, lacked good analogues within the modern vegetation; and (4) most cool-temperate deciduous forest communities north of 35°N developed in the Holocene. Forest clearance and cultivation by Native Americans along principal riverways resulted in a transformation from natural to cultural landscapes during the mid- and late Holocene intervals. Fragmentation of temperate forests accelerated with the onset of EuroAmerican settlement and technologic developments after the Industrial Revolution.