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About the Author
Andrew M. Barton is Professor of Biology at the University of Maine in Farmington and author of The Changing Nature of the Maine Woods. William S. Keeton is Professor of Forest Ecology and Forestry, is a Fellow in the Gund Institute for Environment, and currently chairs the IUFRO working group on old-growth forests.
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Introduction: Ecological and Historical Context
Andrew M. Barton
The science of old growth is multifarious, reflecting the complexity of these ecosystems and their considerable variation across the landscape and time. It is also tied to human agency, both the centuries of exploitation of forests and the evolution of the emotive ideas surrounding old growth. The goal of this introduction is to provide context for old-growth science and the diverse set of 14 chapters that follow.
We will start by addressing the perennial question What is old growth? Writing about old-growth ecosystems demands the penance of wrestling with the definition. It is a wickedly difficult but important exercise. We will then remind readers why people care about old-growth forests in the first place, in other words, why this book exists at all. Once we have established the what and why of this book, we will provide some ecological context, first geographically, and then temporally, by examining in detail how the sites supporting two extant eastern old-growth forests have changed over the past 20,000 years. In other words, we will address the questions of how old-growth ecosystems have reached their current state and how this should shape our expectations for the future in a rapidly changing world. Those two paleoecological narratives lead us directly to human history, first to the early North Americans, and then to modern times, in which old growth became an idea, a conservation goal, and a controversy. Although social and political aspects of old growth are not the focus of this book, they provide important background for the science, which affects and is affected by those currents. We will end by exploring different frameworks for making sense of and synthesizing the diversity of chapters to follow.
What Is Old Growth?
Some readers might expect a sharp, clear definition that demarcates old growth from "not old growth." If so, what follows might prove to be a disappointment. Defining old growth has produced a cottage industry for scientists and forest managers striving for clarity and pragmatism. In a recent analysis of old-growth definitions and concepts, Pesklevits et al. (2011) cite more than 15 papers whose purpose is to define and circumscribe the subject. The US Forest Service toiled over definitions for a decade before publishing a guide to old-growth stands on national forests in the eastern United States (Tyrrell et al. 1998). The diversity of definitions, criteria, classifications, and confusions has been characterized vividly, as has the lack of consensus (Spies 2004; Wirth et al. 2009). Pesklevits et al. (2011) argue that defining old growth is a "wicked problem" (Rittel and Webber 1973) in the sense that it is "irreconcilably tricky or perpetually vexing." Wicked has also been used to describe the social problem of solving climate change (Grundmann 2016).
More recently, ecologists have embraced the idea that, "a consensus on the wording of an ecological definition of old growth will never be reached and may not be desirable, given the diversity of forests." (Spies 2004; see also Wirth et al. 2009). In fact, the discipline of ecology is full of terms, such as community, ecosystem, and complexity that are left vague but are operationalized for particular research, sites, or applications. Granted some of the confusion about what constitutes old growth emerges from the reality that "old growth is simultaneously an ecological state, a value-laden social concept, and a polarizing political phenomenon." (Pesklevits et al. 2011). Underlying the diversity of old-growth criteria, however, are real differences in these ecosystems across the local landscape and the continent. The challenge of any definition, therefore, is the tradeoff between generality and acknowledgement of complexity. We propose that the very act of grappling with old-growth forest definitions, as cumbersome as it might be, promotes an understanding of the variation in ecological patterns and processes (see chapter 4, for example).
Despite the variety of definitions, we can identify two intertwined attributes of old growth that are common to many conceptions and are particularly relevant for this book: forests with old trees that have been largely undisturbed by people since their origin. Hunter and White (1997) showed that these two axes should be thought of as continuous, that is, there is no objective threshold of either age or "naturalness" that separates old growth from somewhat old and natural forests. As forests age since their rebirth after the last disturbance, whether natural or anthropogenic, they slowly develop characteristics of old growth, and, by definition, the less intervention by humans, the more an ecosystem can be said to be under the control of nonhuman forces. As will become apparent in this book, the imprint of humans on nature, even old growth, increases apace regardless of remoteness and history, injecting new "wickedness" into defining and characterizing old growth.
Like all humans, scientists classify continuous phenomena into categories to better understand them. So, developing specific criteria that helps identify or characterize old-growth forests can be seen as an attempt to gain an understanding of the function, variability, and dynamics of forests that have developed over long periods with little human impact. Moreover, at least for some regions or specific forest types, old-growth criteria "may be useful for ... inventorying stands ... prioritizing sites for protection ... determining whether or when forests ... acquire an old- growth condition ..." (Tyrrell et al. 1998).
Many criteria have been proposed, including the following:
trees more than 50 percent of the maximum age of the dominant tree species;
a variety of ages of dominant tree species;
establishment of new individuals by gap-phase dynamics (i.e., the formation of small canopy gaps);
the death of all members of the original cohort that established directly after the last major natural disturbance;
and the presence of large snags and coarse woody debris (e.g., dead trunks and branches) on the forest floor.
Certainly, these criteria effectively describe the stereotypical old- growth forests of the Pacific Northwest, the cove forests of the southern Appalachians (chapter 4), and many of the mixed old-growth forests of the northeastern United States and southeastern Canada (chapter 6) and the northern Lake States region (chapter 7). As helpful as these detailed criteria might be, however, they would eliminate some forest types that are clearly very old and largely undisturbed by humans: centuries-old cedars perched on the cliff-face of the Niagara escarpment (Kelly and Larson 2007), for example, as well as some ecosystems described in this book (e.g., chapter 3). Therein lies the rub: The more specific the definition, the less it applies to the tremendous range of variation in forests and woodlands that meet a commonsense and ecologically important conception of "old growth."
Given that a goal of this book is to better understand the natural patterns and processes of old-growth forests and how these vary across the landscape, we embrace a permissive definition of what constitutes old- growth forest. In other words, we will let our contributors decide what old growth is for the systems in which they work, even if that stretches the boundaries of the most common conceptions. We are convinced that the diversity of ecological phenomena and ecosystems encompassed by this approach justifies its slackness.
Why is Old Growth Important?
Why do individuals and societies care about old-growth forests? Clearly, the science, conservation, and controversy surrounding old growth arose and persist because these ecosystems are valued (Whitney 1987; Davis 1996; Spies and Duncan 2009; Wirth et al. 2009; Maloof 2011). Our goal is to provide a brief summary of the importance of old growth, divided into three categories of values: biodiversity (see Glossary), direct benefits to people, and moral standing. These are interrelated, but we separate them for convenience.
Old-growth forests harbor biodiversity at multiple levels. They provide a storehouse of genetic diversity that has evolved through eons and developed ecologically over centuries. A wide range of species thrive or even depend on the structures, resources, and long-term undisturbed nature of old-growth forests (chapter 11), and destruction would lead to a loss of biodiversity at the forest, regional, and planetary scales, the degree of such harm uncertain (Davis 1996; Spies and Duncan 2009; Wirth et al. 2009). The dependence of the Northern Spotted Owl on old-growth forests in the Pacific Northwest was, of course, the pivotal issue that launched old growth into international consciousness (Spies and Duncan 2009). Finally, old growth is a key stage in the successional processes of forests, and as such is a linchpin in the conservation goal of sustaining a diversity of habitats and ecosystem types (Spies 2004). Our knowledge of the importance of older forests for biodiversity and the complex species interactions therein continues to grow, especially regarding less charismatic, but still important, organisms (chapters 3, 9, 11, and 13).
There are multiple direct benefits of old growth to people, at individual and societal levels. Many derive great recreational pleasure, awe, and psychological and spiritual sustenance from old growth, which offers an experience apart from the heavy imprint of civilization, in a place where organisms have persisted for centuries (Leverett 1996; Moore 2007). Even if they do not actually venture into these places, some people derive well- being by simply knowing that old growth exists and is protected and that they or their descendants could visit them and enjoy them in the future (Loomis 2009). Put simply, most people love forests and trees, especially large, old ones. Old growth also provides essential indirect benefits to society, especially through ecosystem services such as the provision of clean water, as well as through more prosaic enhancements, such as boosting surrounding real estate values.
As described in chapter 14, ecosystem services can even accrue at a planetary level, for old-growth ecosystems store high levels of carbon, effectively sequestering it from the atmosphere where it traps heat. Recent research has overturned the conventional wisdom that old trees and forests are carbon neutral, revealing that they often continue to accrue additional carbon regardless of age (chapter 14). Finally, old-growth forests provide a unique research laboratory for scientists, allowing them to investigate the workings of nature with relatively few confounding human impacts compared to other ecosystems. Such research can inform our quest for improving management and conservation of all forests (Spies 2009).
Many religions, spiritual principles, and philosophies declare an ethical basis for the protection of old-growth forests, regardless of utilitarian human purpose. In other words, they give old-growth forests independent moral standing. These principles are based on connections to deities, sacredness, the special role of trees, and human stewardship. We refer the reader elsewhere for more in-depth discussions of these issues, which help explain the long-standing adoration of humans for old trees and forests (Whitney 1987; Albanese 1990; Proctor 2009).
Geographic and Ecoregional Context
The old-growth forests discussed in this book occur across an enormous swath of North America, including the Deep South (chapters 2 and 3), southern Appalachians (chapter 4), central Appalachians (chapter 5), northeastern United States and southeastern Canada (chapter 6), northern Lake States region (chapter 7), and Canadian boreal zone (chapter 8). In the United States, this area encompasses east to west more than 2,500 kilometers and 28° longitude (67° to 95°) and north to south more than 3,500 kilometers and 23° latitude (25° to 48°). The boreal forest alone circumscribes an even larger area. Within these geographic boundaries are five Koppen-Trewartha climate zones (Belda et al. 2014): tropical (Aw), humid subtropical (Cf), temperate continental with a warm summer (Dca), temperate continental with a cool summer (Dcb), and boreal (E). Plant hardiness zones (based on minimum winter temperatures on an 11-point scale) range from 2 in southern Florida to 10 in the northern boreal forest (Daly et al. 2012; McKenney et al. 2014).
This geographical and climatic range supports a tremendous diversity of ecoregions. Ecoregional classifications attempt to take largely continuously varying patterns of ecosystems across the landscape and divide them into discrete units, which are organized in a hierarchical scheme. The goal is to simplify the complexity of nature in a way that facilitates our understanding and its management. As a reference for this book, we use an ecoregional classification for North America developed jointly by Mexico, the United States, and Canada (CEC 2006), which operates at a fine scale (Level III) nested into progressively more coarse scales (Levels II and I). The old-growth ecosystems in this book occur within Tropical Wet Forests, Eastern Temperate Forests, Northern Forests, the Hudson (Bay) Plain, and Taiga (Level I). These five are divided into 15 ecoregions at Level II and then further into 61 at Level III. Plate 1 provides a map of the ecoregional classification for all three levels, with Level III denoted with three digits.
The Nature Conservancy's 5,000-acre Big Reed Reserve in northern Maine is one of the largest tracts of continuous old growth in New England (Barton et al. 2012, 68). The forest is a mix of spruce-fir, northern hardwoods, and northern white-cedar swamps. Sugar maples (Acer saccharum; figure 1–1), yellow birches (Betula alleghaniensis), and cedars (Thuja occidentalis) one meter in diameter are common. Scattered white pines tower above the canopy. Moss and lichens cover the lower trunks of trees. The forest floor is shady and moist, crisscrossed with decomposing dead wood, and effusive with fungi. Because large natural disturbances are rare, early-successional species, such as aspen (Populus tremuloides and P. grandidentata), pin cherry (Prunus pensylvanica), and white birch (Betula papyrifera) are hard to find, which contrasts sharply with the vast acreage of early- to mid-successional timberlands surrounding the reserve. Although the trees are not enormous, one senses that Big Reed is every bit the "forest primeval," forever unchanging.
Of course, we would be wrong. Twenty-thousand years ago, this area was under a continental ice sheet nearly two kilometers thick that stretched from the Arctic to New Jersey (Borns et al. 2004). About that time, astronomical cycles tilted the climate system toward warming, and, by 12,000 to15,000 years ago, across the long span of Maine, the ice sheet retreated, raw land was exposed, life recolonized, and the gradual process of primary succession began to build new forests (Barton et al. 2012, 29). Pollen embedded in the sediment layers on the bottom of ponds and in small, damp hollows in and near Big Reed tell the story of vegetation change since deglaciation (Anderson 1986; Schauffler and Jacobson 2002; Dieffenbacher-Krall and Nurse 2005; Barton et al. 2012, 79). The first well-established vegetative cover was open tundra with grasses and herbs, succeeding to open taiga, and eventually closed spruce forests. From about 9,000 to 7,000 years ago, during an unusually warm period (mid-Holocene Hypsithermal), rapid warming and drier conditions led to a decline in spruce and predominance of pines and oaks. Charcoal deposited in pond sediments during this time suggests that fires were common during this era, which might have amplified favorable conditions for these species. Charcoal fragments are rare in sediments and soil at Big Reed since that time, suggesting that fire has played very little role for a long time.
Moderate cooling then shifted the forest to hemlock (Tsuga canadensis), yellow birch, and beech (Fagus grandifolia). Beech was a laggard of sorts, migrating from southern glacial refugia into Maine long after other tree species with which it freely mixes today. A temperate northern hardwood-hemlock forest persisted until about 1,500 years ago, with one notable exception. About 5,400 years ago, hemlock suffered an abrupt decline, not just in Maine but across its entire range, the result possibly of severe drought, or a pest outbreak, or both. After about 1,000 years, hemlock recovered to its predecline abundance. Surprisingly, not until about 1,000 years ago did spruce reassert its place as one of the most abundant trees in Big Reed Reserve and all of interior Maine. If we use 25 years (the average age of sexual maturity for common tree species in the reserve) as generation time, then, the current old-growth forest at Big Reed Reserve has been around for a mere 40 generations.(Continues…)
Excerpted from "Ecology and Recovery of Eastern Old-Growth Forests"
Copyright © 2018 Andrew M. Barton and William S. Keeton.
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Table of Contents
Foreword Preface Chapter 1. Introduction: Ecological and Historical ContextAndrew Barton Chapter 2. Old-Growth and Mature Remnant Floodplain Forests of the Southeastern United StatesLoretta Battaglia and William Conner Chapter 3. Fire-Maintained Pine Savannas and Woodlands of the Southeastern United States Coastal Plain Robert Peet, William Platt, and Jennifer Costanza Chapter 4. Old-Growth Forests in the Southern Appalachians: Dynamics and Conservation FrameworksPeter White, Julie Tuttle, and Beverly Collins Chapter 5. Topography and Vegetation Patterns in an Old-Growth Appalachian Forest: Lucy Braun, You Were Right!Julia Chapman and Ryan McEwan Chapter 6. Old-Growth Disturbance Dynamics and Associated Ecological Silviculture for Forests in Northeastern North AmericanAnthony D'Amato, Patricia Raymond, and Shawn Fraver Chapter 7. Historical Patterns and Contemporary Processes in Northern Lake States Old-Growth LandscapesDavid Mladenoff and Jodi Forrester Chapter 8. Is Management or Conservation of Old Growth Possible in North American Boreal Forests?Daniel Kneeshaw, Philip Burton, Louis De Grandpre, Sylvie Gauthier, and Yan Boulanger Chapter 9. Forest-Stream Interactions in Eastern Old-Growth ForestsDana Warren, William Keeton, Heather Bechtold, and Clifford Kraft Chapter 10. Belowground Ecology and Dynamics in Eastern Old-Growth Forests Timothy Fahey Chapter 11. Biological Diversity in Eastern Old GrowthGregory McGee Chapter 12. Eastern Old-Growth Forests under Threat: Changing Dynamics due to Invasive OrganismsJohn Gunn and David Orwig Chapter 13. Silviculture for Eastern Old Growth in the Context of Global ChangeWilliam Keeton, Craig Lorimer, Brain Palik, and Frederik Doyon Chapter 14. Source or Sink? Carbon Dynamics in Eastern Old-Growth Forests and Their Role in Climate Change MitigationWilliam Keeton Chapter 15. Conclusion: Past, Present, and Future of Old-Growth Forests in the EastWilliam Keeton and Andrew Barton Glossary Contributors About the Editors Index