Scale and frequency of natural disturbances in the northeastern US: implications for early successional forest habitats and regional age distributions
Introduction
Historical land-use changes in the eastern US, as in many other parts of the world, have caused extreme swings in forest habitat conditions. In the late 19th and early 20th centuries, much of the northeast was dominated by young forest stands as a legacy of extensive logging, land clearing, fuelwood utilization, repeated fires on cutover land, and widespread farm abandonment. For example, >75% of the forest in central Massachusetts was less than 30 years old in 1885 (Foster et al., 1998). Government surveys in 1908 revealed that 56% of the forest land in 12 eastern states was classified as “cutover land” (Whitney, 1994, p. 192). Subsequent declines in the rate of forest utilization allowed many forests in eastern US to recover (McKibben, 1995). As these regenerating forests have matured, a number of animal populations also recovered that were uncommon a hundred years ago (Kendeigh, 1946, Whitney, 1994).
Concurrent with the maturation of second-growth stands, the abundance of early successional habitats has declined in much of the northeast. Seedling–sapling stands currently represent 4–18% of forests in the region (Trani et al., 2001). Yet the effects of the reduction in habitat on wildlife species that require young forests have received relatively little attention. As Askins (2001) has pointed out, shrublands, clearcuts, and thickets are “unpopular habitats” among the general public. There is also a widespread notion, even among some conservation groups, that wildlife species dependent on early successional habitats are “weedy generalists” that thrive in human-dominated habitats and therefore require no special conservation measures. However, substantial population declines have occurred among early successional obligates. For example, of the 126 neotropical migrant bird species in the northeast, 74 require disturbance-generated habitats or young forests, and these species are scarce or absent in mature and old-growth stands (Smith et al., 1993). Thompson and DeGraaf (2001) pointed out that no breeding bird species are dependent upon uneven-aged stands, whereas many species require even-aged habitats. Among the disturbance-dependent bird species throughout eastern North America, Hunter et al. (2001) predicted that if early successional habitats continue to decline, many species will be extirpated from portions of the eastern US and others risk extinction. Fourteen of these species are federally listed as endangered or threatened, and 18 others are on a national watch list (Hunter et al., 2001).
Broader-level landscape considerations also are important in maintaining suitable conditions. Because early successional species utilize habitats that persist only for a short time, continual turnover of stands somewhere on the landscape is necessary. In addition to the total amount of young-forest habitat, the scale of disturbance also is an important consideration. For example, prairie warblers (Dendroica discolor) and yellow-breasted chats (Icteria virens) will utilize moderate or large patches of habitat, but apparently avoid small openings (Annand and Thompson, 1997).
The wide historical swings in habitat conditions have prompted ecologists to consider appropriate historical benchmark conditions in the amount of early and late-successional habitats. Although natural disturbance regimes in the post-glacial era have fluctuated (e.g., Anderson et al., 1986), managing landscapes within a natural range of variability may help sustain population viability by maintaining landscape characteristics to which the regional plants and animals have become adapted (Seymour and Hunter, 1999, Thompson and DeGraaf, 2001). An understanding of natural disturbance regimes is also needed to fulfill the broader goals of ecosystem management on public lands (Thomas, 1996). Therefore, the purpose of our paper is to review evidence on the frequency, severity, and scale of natural disturbances in the major forest regions of the northeastern US. In regions with sufficient evidence, we provide estimates of the relative amount of young and old forest in presettlement times, but the temporal scope of the paper includes evidence spanning a broader period of several thousand years to characterize the range of variability.
A distinction is made in this paper between natural disturbances (caused by lightning, windstorms, insect outbreaks, etc.) and anthropogenic disturbances caused by Native Americans and European settlers, although these causes are often not distinguishable using historical or scientific evidence. Some disturbances in modern times also are discussed in situations where these events clarify the potential scope and impacts of natural disturbance, even though they may be operating on landscapes considerably affected by humans. However, analysis of recent anthropogenic disturbances such as logging, land clearing, and invasion of exotic species lies beyond the scope of this paper. Some types of anthropogenic disturbance (e.g., logging, grazing, prescribed burning) may be useful in conserving species at risk and are covered in other papers in this issue (cf. Foster and Motzkin, 2003, DeGraaf and Yamasaki, 2003, Litvaitis, 2003).
Section snippets
Methods of analyzing disturbance frequency
There are six principal methods or sources that can be used to investigate natural disturbance regimes. These include analysis of sedimentary pollen and charcoal (e.g., Patterson and Backman, 1988, Clark and Royall, 1995a), presettlement land survey records (Siccama, 1971, Cogbill, 2000), early descriptions by travelers, naturalists, and foresters (Day, 1953, Whitney, 1994), reconstructions of disturbance history in old-growth stands (Lorimer and Frelich, 1989, Chokkalingam, 1998), modern
Pine–oak barrens
Pine–oak barrens in the northeast are commonly dominated by pitch pine (Pinus rigida) with varying amounts of canopy oaks (e.g., Quercus alba), scrub oaks (e.g., Q. ilicifolia), and ericaceous shrubs (e.g., Gaylussacia and Vaccinium spp.). They are commonly associated with the sandy soils of the Coastal Plain (Lull, 1968) or similar soils inland; they may also be found on rocky ridges and hilltops. Although pine barrens typically occupy droughty, nutrient-poor sites, average annual
Eastern oak forests
Forests dominated by oaks and hickories (Carya spp.) are dispersed across eight distinct physiographic and soil regions in the northeast, from unconsolidated sandy soils of the coastal plain to the predominantly stony loam and sandy loam soils on steep slopes of the Allegheny Mountains. However, the boundaries of the region correspond well with climatic variables, especially a mean frost-free period of 150–180 days (Lull, 1968).
Oaks of all species made up 35–75% of the witness trees in
Northern hardwood forests
Northern hardwood forests, dominated by American beech, sugar maple (Acer saccharum), and yellow birch (Betula alleghaniensis), occur in a climatic zone with a frost-free period of 120–150 days (Lull, 1968). In New England and northern New York, northern hardwoods are found on spodosols of the New England Upland physiographic province and at moderate elevations along the Appalachian mountain chain. Northern hardwoods also dominate the Allegeheny Plateau of southern New York and northern
Spruce–northern hardwood forests
Northern hardwood forests with a substantial admixture of spruce (Picea spp.) and other conifers are common on a variety of habitats in northern New England and New York. Spruce–hardwood forests are best discussed separately from other northern hardwood forests because of differences in both physical environment and disturbance regime. Spruce and fir (Abies balsamea) in particular are much more vulnerable to windthrow, insect epidemics, and crown fires than most of the associated species.
Conclusions
Natural disturbance regimes vary markedly across the northeastern US, being influenced primarily by geographic location, forest type, and local habitat conditions. In presettlement times, there seemed to be a gradient in the frequency of stand-replacing disturbances from coastal regions to interior zones. From Delaware to southern Maine, coastal regions often had the highest disturbance frequencies because of the large expanses of sandy pine–oak barrens near the coast, abundant populations of
Acknowledgements
We thank David Foster, Bill Patterson, John Livaitis, and an anonymous reviewer for helpful suggestions on the paper. We would also like to thank David Foster, Bob Seymour, Charlie Cogbill, Bill Patterson, and the many others with whom we have had stimulating discussions over the years about disturbance dynamics of northeastern forests. The authors received support during the writing of this paper from the School of Natural Resources, University of Wisconsin-Madison, Maine Agricultural and
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