Person: Oswald, William
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Oswald
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William
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Oswald, William
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Publication A Climatic Driver for Abrupt Mid-Holocene Vegetation Dynamics and the Hemlock Decline in New England(Ecological Society of America, 2006) Foster, David; Oswald, William; Faison, Edward; Doughty, Elaine; Hansen, BarbaraThe mid-Holocene decline of eastern hemlock is widely viewed as the sole prehistorical example of an insect- or pathogen-mediated collapse of a North American tree species and has been extensively studied for insights into pest–host dynamics and the consequences to terrestrial and aquatic ecosystems of dominant-species removal. We report paleoecological evidence implicating climate as a major driver of this episode. Data drawn from sites across a gradient in hemlock abundance from dominant to absent demonstrate: a synchronous, dramatic decline in a contrasting taxon (oak); changes in lake sediments and aquatic taxa indicating low water levels; and one or more intervals of intense drought at regional to continental scales. These results, which accord well with emerging climate reconstructions, challenge the interpretation of a biotically driven hemlock decline and highlight the potential for climate change to generate major, abrupt dynamics in forest ecosystems.Publication Early Holocene Openlands in Southern New England(Ecological Society of America, 2006) Faison, Edward; Foster, David; Oswald, William; Doughty, Elaine; Hansen, BThe pre-historical vegetation structure in temperate forest regions is much debated among European and North American ecologists and conservationists. Frans Vera’s recent hypothesis that large mammals created mosaics of forest and openland vegetation in both regions throughout the Holocene has been particularly controversial and has provoked new approaches to conservation management. Thirty years earlier, American paleoecologists Herb Wright and Margaret Davis debated whether abundant ragweed pollen at Rogers Lake, Connecticut at 9500 yr BP signified local forest openings or long-distance transport of pollen from Midwestern prairies. Using new pollen records from Harvard Forest and the North American Pollen Database, we address this question and offer insights to the openland discussion. Ragweed and other forbs exceed 3.5% at five sites in a restricted area of southern New England between 10 100 and 7700 yr BP. Strong evidence suggests this pollen originated from the landscapes surrounding these sites (supporting Davis), as ragweed pollen percentages do not increase with longitude from New England to the Midwest. Ragweed pollen percentages are also unrelated to basin size and therefore unrelated to the proportion of extraregional pollen in New England. High forbs values were associated with increases in oak, decreases in white pine, and relatively high charcoal values. Modern pollen records with similar forb and tree percentages occur along the Prairie Peninsula region of the Upper Midwest. However, the closest analogue to the southern New England early Holocene assemblages comes from Massachusetts’ Walden Pond in the early 18th century. These results and the affiliation of ragweed for open, disturbed habitats suggest that oak–pine forests with large openings persisted for over 2000 years due to dry conditions and perhaps increased fire frequency. This conclusion is corroborated by independent lake level and climate reconstructions. Because these early Holocene openlands have no detectable analogue in New England for the past 7000 years before European settlement, we suggest that all important openlands today are almost exclusively a legacy of Colonial agriculture and should be managed accordingly. Nonetheless, our results may have implications for forest dynamics accompanying projected climate change to more arid conditions in New England over the next century.Publication Postglacial climate reconstruction based on compound-specific D/H ratios of fatty acids from Blood Pond, New England(Wiley-Blackwell, 2006) Hou, Juzhi; Huang, Yongsong; Wang, Yi; Shuman, Bryan; Oswald, William; Faison, Edward; Foster, DavidWe 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 (C16 n-acid), which has been shown to record lake water D/H ratios, our surface sediments and down core data indicate that behenic acid (C22 n-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 dD variation of lake water. Down core variations in dD 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-C32 n-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 dD 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.Publication Centennial-scale compound-specific hydrogen isotope record of Pleistocene–Holocene climate transition from southern New England(Wiley-Blackwell, 2007) Hou, Juzhi; Huang, Yongsong; Oswald, William; Foster, David; Shuman, BryanNortheastern North America experienced major climate shifts during the Pleistocene –Holocene transition. However, there have been no high-resolution isotopic records of climate change from this region. Here, we present a centennial-scale record of climate change during the transition based on D/H ratios of behenic acid (C22 nacid) or dDBA from a sediment core in Blood Pond, Massachusetts. Surface calibrations from a transect of 19 lakes in eastern North America show that dDBA values track mean annual atmospheric temperature variations. The abrupt climate events observed in Blood Pond records show remarkable similarity with Greenland ice core d18O records during the Pleistocene. During the early Holocene, the northeastern North America dDBA record was more variable than Greenland, possibly due to the close proximity of the Laurentide ice sheet, and impact of freshwater outbursts as the ice sheet rapidly retreated. Citation: Hou, J., Y. Huang, W. W. Oswald, D. R. Foster, and B. Shuman (2007), Centennialscale compound-specific hydrogen isotope record of Pleistocene–Holocene climate transition from southern New EnglandPublication Pollen Morphology and Its Relationship to the Taxonomy of the Genus Sarracenia (Sarraceniaceae)(New England Botanical Club, 2011) Ellison, Aaron; Oswald, William; Doughty, Elaine; Ne'eman, Gidi; Ne'eman, RidaDespite nearly a century of research, the systematic relationships among North American pitcher plants in the genus Sarracenia (Sarraceniaceae) remain unresolved. In this study we analyzed pollen morphology of the 11 currently recognized species of Sarracenia and examined how variations in key pollen characteristics relate to our current understanding of the taxonomy of this genus. We used principal components analysis to explore variations in pollen grain size (equatorial diameter and length) and shape (number of colpi) among Sarracenia species, and used cluster analysis to compare systematic groupings of Sarracenia based on floral, vegetative, and pollen characters. We compared these results with a previously published phylogeny based on molecular data. Groupings based on pollen characteristics alone did not align completely with those based on molecular or all morphological data. In clusters based on pollen alone and those using all morphological characters, S. purpurea and S. rosea formed a single group, and S. flava, S. alata, and S. leucophylla grouped together consistently. The pollen morphology of S. jonesii and S. alabamensis differed substantially from that of S. rubra, supporting the current systematic treatment of the genus that recognizes these three taxa as distinct species.Publication A record of Holocene environmental and ecological changes from Wildwood Lake, Long Island, New York(Wiley-Blackwell, 2010) Oswald, William; Foster, David; Doughty, Elaine; MacDonald, DanaAnalyses of pollen, charcoal, and organic content in a lake-sediment core from Wildwood Lake, Long Island, New York, provide insights into the ecological and environmental history of this region. The early-Holocene interval of the record (~9800-8800 cal. a BP) indicates the presence of Pinus rigida-Quercus ilicifolia woodlands with high fire activity. A layer of sandy sediment dating to 9200 cal. a BP may reflect a brief period of reduced water depth, consistent with widespread evidence for cold, dry conditions at that time. Two other sandy layers, bracketed by carbon-14 dates, represent a sedimentary hiatus from ~8800 to 4500 cal. a BP. This discontinuity may reflect the removal of some sediment during brief periods of reduced water depth at 5300 and 4600 cal. a BP. In the upper portion of the record (<4500 cal. a BP), subtle changes at ~3000 cal. a BP indicate declining prevalence of Quercus-Fagus-Carya forests and increasing abundance of Pinus rigida, perhaps due to reduced summer precipitation. Elevated percentages of herbaceous taxa in the uppermost sediments represent European agricultural activities. However, unlike charcoal records from southern New England, fire activity does not increase dramatically with European settlement. These findings indicate that present-day Pinus rigida-Quercus ilicifolia woodlands on eastern Long Island are not a legacy of recent, anthropogenic disturbances.Publication A Record of Lateglacial and Early Holocene Environmental and Ecological Change from Southwestern Connecticut, USA(John Wiley & Sons, 2009) Oswald, William; Foster, David; Doughty, Elaine; Faison, EdwardAnalyses of a sediment core from Highstead Swamp in southwestern Connecticut, USA, reveal Lateglacial and early Holocene ecological and hydrological changes. Lateglacial pollen assemblages are dominated by Picea and Pinus subg. Pinus, and the onset of the Younger Dryas (YD) cold interval is evidenced by higher abundance of Abies and Alnus viridis subsp. crispa. As climate warmed at the end of the YD, Picea and Abies declined and Pinus strobus became the dominant upland tree species. A shift from lacustrine sediment to organic peat at the YD-Holocene boundary suggests that the lake that existed in the basin during the Lateglacial interval developed into a swamp in response to reduced effective moisture. A change in wetland vegetation from Myrica gale to Alnus incana subsp. rugosa and Sphagnum is consistent with this interpretation of environmental changes at the beginning of the HolocenePublication W. W. Oswald et al. reply(Springer Science and Business Media LLC, 2020-07-20) Oswald, William; Foster, David R.; Shuman, Bryan N.; Chilton, Elizabeth S.; Doucette, Dianna L.; Duranleau, Deena L.Publication Conservation Implications of Limited Native American Impacts in Pre-Contact New England(Springer Science and Business Media LLC, 2020-01-20) Oswald, William; Foster, David; Shuman, Bryan N.; Chilton, Elizabeth S.; Doucette, Dianna; Duranleau, Deena L.; DuranleauAn increasingly accepted paradigm in conservation attributes valued modern ecological conditions to past human activities. Accordingly, disturbances, including prescribed fire, are employed by land managers to impede forest development in many potentially wooded landscapes under the interpretation that openland habitats were created and sustained by human-set fire for millennia. We test this paradigm using paleoenvironmental and archaeological data from New England. Despite the region’s dense population, anthropogenic impacts to the landscape before European contact were limited and fire activity was independent of changes in human populations. Whereas human populations reached maxima during the Late Archaic (5000-3000 ybp) and Middle-Late Woodland (1500-500 ybp) periods, lake-sediment charcoal records indicate elevated fire activity only during the dry early Holocene (10,000-8000 ybp) and following European colonization. Pollen data depict closed forests from 8000 ybp to the onset of European deforestation, and archaeological evidence of pre-contact horticultural activity is sparse. Climate largely controlled fire severity in New England during the postglacial interval, and widespread openlands developed only after deforestation for European agriculture. Land managers seeking to emulate pre-contact conditions should deemphasize human disturbance and focus on developing mature forests; those seeking to maintain openlands should apply the agricultural approaches that initiated them four centuries ago.