Early Holocene Openlands in Southern New England

Abstract

The 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.

Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation

“…Several pollen records from eastern North America show an increase in hardwood deciduous taxa immediately after the Sporormiella-indicated megafaunal decline, including increases in palatable and shade-tolerant woody species (74,75,78), and a more closed vegetation, consistent with release from browsing pressure. The continued postextinction presence of light-demanding oak (Quercus alba) indicates that the surviving large herbivores could have maintained a certain degree of openness of the landscape (42), which has also been ascribed to the effect of dry climate and anthropogenic fires (79). Similarly, the nowendangered grass balds of the southern Appalachian mountains are hypothesized to be remnants of past herbivory (later maintained by Native American burning) (80).…”

Environmental predictors of forest expansion on open coastal barrens

“…Faison et al (2006) suggest that, prior to European settlement, open shrub dominated habitats would have occurred in small isolated patches. As Europeans spread across the northeastern United States these shrublands expanded in response to the abandonment of large agricultural fields as well as prescribed burning of forested landscapes (Foster and Motzkin 2003).…”

“…In eastern North America, there is very little paleoecological evidence indicating the occurrence of large areas of open shrub dominated habitat prior to European settlement, so the emergence of these habitats is likely a result of the various land use practices conducted by early European settlers (Faison et al 2006). Faison et al (2006) suggest that, prior to European settlement, open shrub dominated habitats would have occurred in small isolated patches.…”

“…Saldarriaga et al 1988;Bakker et al 1996;Asselin et al 2006). Disturbances such as fire, grazing, windthrow or human-caused forest clearing may create and/or maintain barrens in otherwise forested landscapes (Oosting and Anderson 1939;Russell and Davis 2001;Foster and Motzkin 2003;Asselin et al 2006;Faison et al 2006). Determining the successional status of such habitats and the mechanisms that maintain them is crucial to the task of preserving their biodiversity in the face of changing climates and disturbance regimes.…”

“…These openings can persist for many years and even decades before forest expansion occurs (Laycock 1991;Ehrenfeld et al 1995;Mallik 1995;Latham et al 1996;Bradley et al 1997;Jäderlund et al 1997;Mallik 2003;Suding et al 2004;Faison et al 2006). Other rocky habitats that persist as islands in a forested landscape can last for hundreds of years without significant development of soil and succession into forest (Stark et al 2003(Stark et al , 2004.…”