Bryan N. Shuman

Late-quaternary vegetation dynamics in north america: Scaling from taxa to biomes

This paper integrates recent efforts to map the distribution of biomes for the late Quaternary with the detailed evidence that plant species have responded individual- istically to climate change at millennial timescales. Using a fossil-pollen data set of over 700 sites, we review late-Quaternary vegetation history in northern and eastern North America across levels of ecological organization from individual taxa to biomes, and apply the insights gained from this review to critically examine the biome maps generated from the pollen data. Higher-order features of the vegetation (e.g., plant associations, physiog- nomy) emerge from individualistic responses of plant taxa to climate change, and different representations of vegetation history reveal different aspects of vegetation dynamics. Veg- etation distribution and composition were relatively stable during full-glacial times (21 000- 17 000 yr BP) (calendar years) and during the mid- to late Holocene (7000-500 yr BP), but changed rapidly during the late-glacial period and early Holocene (16 000-8 000 yr BP) and after 500 yr BP. Shifts in plant taxon distributions were characterized by individ- ualistic changes in population abundances and ranges and included large east-west shifts in distribution in addition to the northward redistribution of most taxa. Modern associations such as Fagus-Tsuga and Picea-Alnus-Betula date to the early Holocene, whereas other associations common to the late-glacial period (e.g., Picea-Cyperaceae-Fraxinus-Ostrya/ Carpinus) no longer exist. Biomes are dynamic entities that have changed in distribution, composition, and structure over time. The late-Pleistocene suite of biomes is distinct from those that grew during the Holocene. The pollen-based biome reconstructions are able to capture the major features of late-Quaternary vegetation but downplay the magnitude and variety of vegetational responses to climate change by (1) limiting apparent land-cover change to ecotones, (2) masking internal variations in biome composition, and (3) obscuring the range shifts and changes in abundance among individual taxa. The compositional and structural differences between full-glacial and recent biomes of the same type are similar to or greater than the spatial heterogeneity in the composition and structure of present-day biomes. This spatial and temporal heterogeneity allows biome maps to accommodate in- dividualistic behavior among species but masks climatically important variations in taxo- nomic composition as well as structural differences between modern biomes and their ancient counterparts.

DISSIMILARITY ANALYSES OF LATE-QUATERNARY VEGETATION AND CLIMATE IN EASTERN NORTH AMERICA

Plant formations different from any extant today apparently were widespread in North America and Europe during the last deglaciation, produced by the independent biogeographic responses of plant taxa to climate change. Dissimilarity analyses of modern and fossil pollen samples in eastern North America show that the unique plant associations centered around the Great Lakes at 14 000 calendar years before present (yr BP), with high dissimilarities during 17 000–12 000 yr BP. The late-glacial fossil pollen assemblages are characterized by (1) high abundances of boreal conifers such as spruce and larch relative to their Holocene values, (2) high abundances of herbaceous types (sedge, sage, and ragweed), (3) high abundances of broad-leaved deciduous types (ash, hornbeam, poplar, hazel, and willow), and (4) the low abundance or absence of pine, alder, and birch. When the fossil pollen samples are assigned to biomes using the affinity score technique, the late Pleistocene pollen samples are assigned to mixed parkland, a biome that is not extant in North America today. The fastest vegetational changes occurred 13 000–11 000 yr BP, when the late Pleistocene vegetation reorganized into the Holocene biomes, which have persisted to today. Simulations by the Community Climate Model, version 1 (CCM1), suggest that late-glacial climates were also unlike modern climates, featuring a “hypercontinental” mixture of colder-than-present winters, warmer-than-present summers, and lower-than-present precipitation. Dissimilarity analyses of the pollen data and CCM1 simulations for 21 000, 16 000, 14 000, 11 000, and 6000 yr BP show that (1) the temporal and spatial distribution of high dissimilarities in the vegetation (relative to present) coincide with dissimilarities in simulated climate, (2) the timing and spatial distribution of changes in the vegetation and simulated climate also agree, and (3) the largest climatic and vegetational changes follow the peak period of dissimilarity from present. Taken together, these three lines of evidence support the hypothesis that the no-analogue plant associations were in equilibrium with orbital- and millennial-scale climate change. Nonclimatic factors such as low atmospheric CO2 concentrations and the presence of now-extinct megafauna species may have increased the openness of the Pleistocene vegetation, but by themselves cannot explain the observed mixture of boreal, temperate, and herbaceous taxa in the no-analogue pollen assemblages, nor can they explain the prevalence of no-analogue pollen samples during the late glacial period.

Hydrogen isotope ratios of individual lipids in lake sediments as novel tracers of climatic and environmental change: a surface sediment test

We determined hydrogen isotope ratios of modern lake-waters and individual lipids from surface sediments of 36 lakes in the eastern North America. The lakes selected lie on two transects (south–north transect from Florida to Ontario and east–west transect from Wisconsin to South Dakota) and encompass large temperature and moisture gradients, and a wide range of lake water δD values (>100‰). The study allows a rigorous test of the applicability of using δD values of sedimentary lipids as paleoclimatic and paleoenvironmental proxies. We examined a range of lipids including C17n-alkane, straight chain fatty acids, phytol and sterols in both free extracts and ester-bound fractions in the solvent extracted sediments. Useful isotopic indicators are expected to show a linear correlation and constant fractionation factor between their δD values in surface sediments and modern lake water. Our results demonstrate that several lipid compounds, free and ester-bound palmitic acid (16:0), C17n-alkane, and phytol are useful candidates for paleoclimate reconstructions, in addition to two sterols that have been suggested previously (. Compound-specific D/H ratios of lipid biomarkers from sediments as a proxy for environmental and climatic conditons. Geochim. Cosmochim. Acta 65: 213–222). Authigenic or biogenic carbonate in sediments is conventional material for paleoclimatic study using ocean and lake sediments. However, because majority of lake sediments do not contain suitable carbonate materials for isotopic study, hydrogen isotope ratios of these lipids provide invaluable new sources of paleoclimatic and paleoenvironmental information.

Hydrogen isotope ratios of palmitic acid in lacustrine sediments record late Quaternary climate variations

The rich paleoclimate information preserved in lacustrine sedimentary organic matter can be difficult to extract because of the mixed terrestrial and aquatic inputs. Herein we demonstrate that compound-specific hydrogen isotope analysis of palmitic acid, (PA), a ubiquitous compound in lacustrine sediments, captures the δD signals of lake water. Samples collected across a diverse range of 33 North American lakes show a strong correlation between water and δDPA values. At Crooked Pond, Massachusetts, the δDPA changes in a 14 k.y. sediment record parallel temperature trends inferred from fossil pollen. Downcore changes reveal differences between climatic trends in New England and in Greenland that are consistent with important regional differences in climate controls.

700 yr sedimentary record of intense hurricane landfalls in southern New England

Five intense (category 3 or greater) hurricanes occurring in 1635, 1638, 1815, 1869, and 1938 have made landfall on the New England coast since European settlement. Historical records indicate that four of these hurricanes (1635, 1638, 1815, and 1938) and hurricane Carol, a strong category 2 storm in 1954, produced significant storm surges (>3 m) in southern Rhode Island. Storm surges of this magnitude can overtop barrier islands, removing sediments from the beach and nearshore environment and depositing overwash fans across back-barrier marshes, lakes, and lagoons. In a regime of rising sea level, accumulation of marsh, lake, or lagoon sediments on top of overwash deposits will preserve a record of overwash deposition. We examined the record of overwash deposition at Succotash salt marsh in East Matunuck, Rhode Island, and tested the correlation with historical records of intense storms. Aerial photographs taken after hurricanes in 1954 and 1938 show overwash fans deposited at the site. Analysis of 14 sediment cores from the back-barrier marsh confirmed the presence of these fans and revealed that 4 additional large-scale overwash fans were deposited within the marsh sediments. The four overwash fans deposited since the early seventeenth century at Succotash Marsh matches the historical record of significant hurricane-induced storm surge. These fans were most likely deposited by hurricanes in 1954, 1938, 1815, and either 1638 or 1635. Radiocarbon dating of two prehistoric overwash fans indicated that these were deposited between A.D. 1295– 1407 and 1404–1446 and probably represent intense hurricane strikes. In the past 700 yr, at least 7 intense hurricanes struck the southern Rhode Island coast and produced a storm surge that overtopped the barrier at Succotash Marsh.

EVIDENCE FOR THE CLOSE CLIMATIC CONTROL OF NEW ENGLAND VEGETATION HISTORY

Sediments from lakes in the northeastern United States (“New England”) document climatic changes over the past 15 000 years that may, in turn, explain the long- term history of regional forest development. A rise in New England temperatures ∼14 600 yr BP (calendar years before present) coincided with the initial increase in spruce (Picea spp.) populations after deglaciation. Later temperature fluctuations correlated with changes in spruce forest composition until 11 600 yr BP, when evidence for a shift to warm, dry conditions agrees with the replacement of spruce by pine (Pinus spp.) populations. Raised lake levels indicate increased moisture availability by 8200 yr BP when mesic hemlock (Tsuga canadensis) and beech (Fagus grandifolia) populations replaced the dry-tolerant pines. Cooler-than-modern temperatures, however, persisted until 6000 yr BP and appear to have limited the expansion of hickory (Carya spp.) populations. Similarly, moisture- dependent chestnut (Castanea dentata) populations did not inc...

Parallel climate and vegetation responses to the early Holocene collapse of the Laurentide Ice Sheet

Parallel changes in lake-level and pollen data show that the rapid decline of the Laurentide Ice Sheet (LIS) between 10,000 and 8000 cal yr BP triggered a step-like change in North American climates: from an ice-sheet-and-insolation-dominated climate to a climate primarily controlled by insolation. Maps of the lake-level data from across eastern North America show a reorganization of climate patterns that the pollen data independently match. Raised lake-levels and expanded populations of moist-tolerant southern pines (Pinus) document that summer monsoons intensified in the southeastern United States between 9000 and 8000 cal yr BP. Simultaneously, low lake-levels and an eastward expansion of the prairie illustrate an increase in mid-continental aridity. After the Hudson Bay ice dome collapsed around 8200 cal yr BP, lake-levels rose in New England, as populations of mesic plant taxa, such as beech (Fagus) and hemlock (Tsuga), replaced those of dry-tolerant northern pines (Pinus). Available moisture increased there after a related century-scale period of colder-than-previous conditions around 8200 cal yr BP, which is also recorded in the pollen data. The comparison between pollen and lake-level data confirms that vegetations dynamics reflect climatic patterns on the millennial-scale.

The anatomy of a climatic oscillation: vegetation change in eastern North America during the Younger Dryas chronozone

Century-scale climate changes reshaped circulation patterns over the North Atlantic and adjacent regions during the last glacialto-interglacial transition. Here, we show that vegetation across eastern North America shifted dramatically at the beginning and end of the Younger Dryas chronozone (YDC: 12,900–11,600 cal yr B.P.), when changes in ocean circulation rapidly cooled and then warmed the North Atlantic sea-surface. On both the site-specific scale and the continental-scale, vegetation changed only gradually during the millennia before (15,000–13,000 cal yr B.P.) and after (11,000–9000 cal yr B.P.) the YDC, but climate changes ca 12,900 and 11,600 cal yr B.P. altered the vegetation on both spatial scales within centuries. Plant associations changed and some taxa rapidly migrated hundreds of kilometers (>300 km within B100 yr). In limited regions near the North Atlantic coast, abrupt cooling ca 12,900 cal yr B.P. resulted in a return to earlier vegetation types. Elsewhere, however, the vegetation patterns during the YDC were distinct from those of both earlier and later intervals. They indicate abrupt, ‘non-reversing’ seasonal temperature changes that were probably related to atmospheric circulation changes during the YDC, rather than to the direct influence of North Atlantic sea-surface temperatures. Atmospheric circulation patterns during the YDC were unique within the last 21,000 yr because of a unique combination of climate controls. Insolation, ice sheet extent, and atmospheric composition were significantly different from their full-glacial states, even when the North Atlantic returned to near full-glacial conditions. The YDC vegetation patterns demonstrate (1) rapid ecological responsiveness to abrupt climate change and (2) spatially varied patterns of YDC climate change. r 2002 Elsevier Science Ltd. All rights reserved.

A continuous climatic impact on Holocene human population in the Rocky Mountains.

Ancient cultural changes have often been linked to abrupt climatic events, but the potential that climate can exert a persistent influence on human populations has been debated. Here, independent population, temperature, and moisture history reconstructions from the Bighorn Basin in Wyoming (United States) show a clear quantitative relationship spanning 13 ka, which explains five major periods of population growth/decline and ∼45% of the population variance. A persistent ∼300-y lag in the human demographic response conforms with either slow (∼0.3%) intrinsic annual population growth rates or a lag in the environmental carrying capacity, but in either case, the population continuously adjusted to changing environmental conditions.

Postglacial climate reconstruction based on compound‐specific D/H ratios of fatty acids from Blood Pond, New England

We determined hydrogen isotope ratios of individual fatty acids in a sediment core from Blood Pond, Massachusetts, USA, in order to reconstruct climate changes during the past 15 kyr. In addition to palmitic acid (C16n-acid), which has been shown to record lake water D/H ratios, our surface sediments and down core data indicate that behenic acid (C22n-acid), produced mainly by aquatic macrophytes, is also effective for capturing past environmental change. Calibration using surface sediments from two transects across eastern North America indicates that behenic acid records δD variation of lake water. Down core variations in δD values of behenic acid and pollen taxa are consistent with the known climate change history of New England. By evaluating the hypothesis that D/H fractionations of long chain even numbered fatty acids (C24-C32n-acids) relative to lake water provide independent estimates of relative humidity during the growing season, we find that differences between lake-level records and isotopically inferred humidity estimates may provide useful insight into seasonal aspects of the hydrologic cycle. Combined analyses of D/H of short and long chain fatty acids from lake sediment cores thus allow reconstructions of both past temperature and growing season relative humidity. Comparison of δD records from two lakes in New England provides critical information on regional climate variation and abrupt climate change, such as the 8.2 ka event.