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We inhabit a changing planet. Most of us realize that our lives are lived on a scale that is insignificant compared to geological time--continents break up and drift apart, mountains rise and are worn away by the elements, and all of human civilization is dwarfed by the vastness of Earth's history. What many of us do not recognize is that, like the continents and the mountains, the Earth's living ecosystems are undergoing constant changes as well. While typically faster than the movement of continents, ecological change occurs at a pace that can be difficult to detect over the span of a human lifetime. Many areas that were covered with moist forests when humans moved into western North America are now shrub-dominated deserts (Thompson et al. 1993), and large expanses of arid grassland seen by the pioneers during the westward expansion of the United States have given way to shrubs and woodland (Hastings and Turner 1965). More recently, popular portrayals of the Lewis and Clark expedition have shown Americans how dramatically the Great Plains have been changed by settlement and agriculture, while portions of the northern Rockies still resemble the landscapes described by these early explorers at the beginning of the 19th century (Ambrose 1996).

Many changes in the patterns of North American land forms and vegetation--collectively referred to as landcover change--have resulted from or been influenced heavily by human activities. In most places across the continent, our activities have become the dominant driver of environmental change. Yet the extensive and influential role of human activity is often overlooked in the literature, as well as in our perception of our surrounding landscapes. Why? Throughout human history, each new generation has accepted the state of the world that they inherited from their ancestors. Environmental impacts that preceded them were viewed as natural, or at least normal, so changes occurring over several generations often went undetected (Reichman and Pulliam 1996). Even today, as we strive to minimize or reverse many of the obvious environmental impacts that are occurring during our own lifetimes, more gradual changes, even the critically important ones like climate change and the loss of biological diversity, are seldom fully appreciated.

A better understanding of the rate and direction of change in the Earth's ecosystems is important for its own sake, but it is also increasingly vital for interpreting current environmental trends and guiding the management of our natural resources. Scientists and resource managers no longer assume that nature exists in a static, unchanging "natural" condition interrupted only by the work of humans. Instead, we view nature as a dynamic system of which humans are a part, recognizing that a variety of forces--ranging from natural disturbances to climatic change, deforestation, and the conversion of native habitats to agriculture--are constantly interacting to determine the pace and direction of change (Pickett and White 1985). This perception of nature has important implications for our understanding of how nature works and our formulation of appropriate responses to emerging environmental problems.

Setting Management Objectives

The perception of ecosystems as dynamic entities, without a single climax or "natural" state, can make the work of land and resource managers more complex. For example, the authorizing legislation for the National Park Service mandates conservation of "natural and historical objects and the wild life therein...unimpaired for the enjoyment of future generations." It is not clear, however, (except in a few cases, such as historic battlefields) whether the "objects" mentioned are those that were present at the time the first descriptions of the landscape were recorded, at the time the park was created, or during some other unspecified period. Similarly, how can biological resources be preserved "unimpaired" if change is a characteristic of the ecosystem itself? Whether we are concerned with forests or wildlife, wetlands or the oceans, identifying appropriate management objectives has become a challenging task, one that is almost impossible to accomplish without an understanding of the natural rate of change and range of variation within natural systems (National Research Council 1992).

Policy makers, and those who advise them, must ask not only whether environmental change is occurring, but also how it fits into the historical context. Certain changes, such as the sustainable harvest of fish and wildlife populations or well-planned logging practices, may fall within the typical range of variation for the ecosystem and require no mitigating management response. Other trends, such as the filling of wetlands and the rapid liquidation of old growth forests, may have no historical precedent, suggesting that decisive action may be necessary to prevent unacceptable ecosystem degradation. Recognizing the difference between these situations constitutes a critical new challenge for environmental scientists, yet without reliable historical records, the differences may be impossible to distinguish, making it difficult to identify an appropriate course of action. Increasingly, applied ecologists are trying to understand the effects of management alternatives in the context of background rates of change in ecological systems, often referred to as the natural range of variation or NRV. A comprehensive history of changes in land use and land cover could identify this variability, enabling policy makers and resource managers to make more informed decisions as they face increasingly complex choices regarding the use and conservation of the resources entrusted to their care.

Understanding Global Change

Technical advances during the past 25 years have unequivocally shown that the entire Earth is undergoing rapid ecological change, and the most obvious and pronounced change is caused by human land use (Vitousek 1994). Perhaps the most remarkable evidence of such change has emerged from the deployment of Landsat and a host of other satellites and airborne sensors. The images acquired through remote sensing technology give us an unprecedented perspective on current land use, and they allow us to track land-use changes during the latter part of the 20th century. Trends detected during the relatively short period of space-based measurements, however, can be difficult to interpret. While the devastating implications of tropical deforestation and other extreme trends are obvious from recent Landsat images (Skole and Tucker 1993), our understanding of more subtle patterns of landcover change, such as the reforestation of the eastern deciduous forests and the spread of shrubs across the arid Southwest, often require a perspective that reaches back before the space age. Longer timelines can be compiled from historical archives. For example, evidence of early landcover change from photographs, accounts from 18th and 19th century land surveys, and paleoecological evidence from fossil pollen and fire scars can be assembled and analyzed through a multidisciplinary land-use history program. By integrating many sources of historical data and developing the scientific and statistical tools for analyzing change, the land-use history program will increase the value of information obtained from modern remote sensing programs by making it possible to interpret these data in the context of a comprehensive, albeit far less detailed, timeline stretching back before the European settlement of North America and, in many cases, back to the last ice age or earlier.

Uncovering Cause and Effect

A descriptive approach to landcover change will identify historic trends and current conditions, but the prediction of future changes in land cover and their effects requires an improved understanding of the cause-and-effect relationships that link human activities with changes in land use and land cover. Much of the impact that humans have had on the environment can be viewed as a series of unplanned experiments, with particular perturbations generating measurable responses in the form of contractions in the ranges of some species and expansions in the ranges of others. Within the context of these temporal dynamics, species extinctions and the spread of nonnative species may be seen as the extreme cases, where biological elements are lost or introduced.

Many of these "experiments" have been run repeatedly throughout human history. As civilizations have expanded and declined, they have left their marks on landscapes, and environmental scientists are developing ingenious techniques for assembling the data needed to assess the results. By analyzing the relationships between changes in land use and land cover at multiple temporal and spatial scales, it is possible to more confidently distinguish human-induced change from background climatic variation and other natural variability. A better understanding of the cause-and-effect relationships that underlie changes in land cover will lead to better predictive models for land-use planning and improved assessment of the likely outcomes of alternative land-use scenarios.

Increasing Environmental Awareness

Understanding landcover change and adopting a dynamic perspective on North American ecosystems have led many specialists to a deeper understanding of the environment and the place of humans in it. A comprehensive land-use history for North America offers many opportunities for extending this understanding to a broader audience, allowing citizens to explore the complex and often striking long-term shifts in land use and land cover that characterize the regions where they live.

Part of the reason that many people have difficulty appreciating the importance of environmental change is that our perception of the world is constrained by the scale at which we live our lives. We typically are preoccupied with events that occur over time scales of a few minutes to a few months and spatial scales ranging from our front lawns to a city block or farm field. Ongoing environmental changes, occurring over extensive areas at rates that vary over many orders of magnitude, are often overlooked when examined at these human scales. Graphical presentations of landcover and land-use change over longer time periods and larger areas provide a powerful introduction to the changing face of North America. The subsequent chapters of this publication provide vivid examples of how maps and graphics can reveal striking patterns over different temporal and spatial scales, challenging us to expand and sharpen our perception of environment. Emerging digital technologies offer unlimited opportunities for extending this approach to museum displays, films, on-line productions, and other educational resources.

While the value of a land-use history to the environmental sciences is clear, scientific inquiry often has been divorced from or in conflict with historical narrative. One of the world's most influential ecologists, Robert MacArthur, wrote that "unraveling the history of a phenomenon has always appealed to some people and describing the machinery of the phenomenon to others. In both processes generalizations can be made and tested against new information so both are scientific, but the same person seldom excels at both" (MacArthur 1972, p. 239). More recently, the escalating severity of environmental problems and the difficulties involved in trying to solve them have led an increasing number of scientists to attempt to do both, often with startling insight. Commonly, the scientific objective of understanding the "machinery" or cause-and-effect relationships that drive environmental issues requires an unraveling of historical factors. For example, concerns over increasing concentrations of carbon dioxide in the atmosphere, and the implications this has for climate change, have led scientists to examine air trapped in rock and ice thousands of years ago. Analyses of the "ancient air" have allowed scientists to reconstruct a record of atmospheric CO₂ concentrations reaching back 160,000 years (reviewed in Vitousek 1994). This history has demonstrated that the build-up of CO₂ and other greenhouse gases since the industrial revolution is unprecedented, at least since the last ice age. For many people, this finding suggests that the ever-increasing production of greenhouse gasses, with their potential to warm the atmosphere and disrupt climatic patterns, is indeed a serious problem that demands serious attention. Similarly, the reconstruction of fire histories, records of changing climates, and studies of shifts in plant communities give us a deeper perspective on current environmental trends and help us interpret their meaning and importance.

Of course, historical approaches can be misused in scientific inquiry, particularly if we refer to historical information to shore up dubious data, or to "explain away" or downplay controversial scientific observations, or to make observations fit with our previous expectations. Uncritical use of historical arguments has hindered the development of ecological science in the past (Peters 1991), underscoring the need for rigorous, repeatable, and clearly documented approaches to reconstructing the history of land-use and landcover change. Rather than weaving together a historical narrative to "paint a picture" of environmental change, a science-based history of land use will provide a means for placing current conditions and recent trends into a broader temporal context. Compilation of historical trends will allow us to begin to associate cause and effect, exploring the relationship between human activities and environmental change. It also will help us identify the most important questions for future scientific research.

Compiling a history of land use and land cover is a large undertaking, one already begun by many scholars working in a variety of fields (for examples, see Shepard 1967; Delcourt and Delcourt 1991; Pielou 1991; Turner and Meyer 1994; Loveland and Hutcheson 1995; subsequent chapters in this volume). Nevertheless, the effort to aggregate information from across the continent and to construct a relatively seamless timeline from studies focusing on different time periods and employing a variety of data sources and analytical approaches is a complex endeavor. Clearly, an international effort will be required, not only because different countries occupy the continent, but also because records of early changes, particularly from the time of European settlement, exist in museums and archives throughout the world. Researchers from universities, government agencies, and private institutions have begun this task. Completing it will require a clear set of objectives, a coordinated effort that encourages creative approaches, and substantial new investments. The costs will be repaid many times over through a deeper appreciation of the environment and improved environmental management.

The relationship between human civilization and nature is a complex and changing aspect of all ecosystems, and our understanding of that relationship is itself dynamic (see, for example, Cronon 1995). Recognition of the role of the human within the ecosystem, obvious and inescapable when most people hunted and gathered food or tilled the earth, has been largely lost in the developed world, where links to nature are distant and abstract for the majority of the human population. We often see ourselves as being a step or more removed from nature, harvesting resources, recreating, monitoring impacts, interpreting conditions from the sidelines, and administering first aid to resuscitate an infirm world, when necessary. Donald Worster (1993) suggests that the way we use the land reflects our understanding of nature and our perceptions of ourselves. If widespread environmental degradation is an indication that our understanding of nature is narrow, it also suggests that so too is our perception of our own role in the functioning of natural systems.

An appreciation of history can widen and deepen this understanding. In 1906, American philosopher George Santayana warned that those who cannot remember the past are condemned to repeat it. But unlike the people who have faced social and political crises throughout the comparatively short history of civilization, those who ignore environmental history will not be blessed with the opportunity to repeat it. Instead, they will likely suffer through an increasingly difficult future, facing complex problems without the opportunity to consult the record of environmental change that stretches back, beyond the limits of human memory. Compiling this history and developing the tools to analyze, interpret, and share it broadly are important and inspiring missions as we enter the next millennium.

I thank H.R. Pulliam, former Director of the National Biological Service, for many stimulating discussions during the formative period of the Land Use History of North America project and for his unwavering support of this effort throughout his tenure with the U.S. Department of the Interior. Participants in the 1995 Patuxent workshop contributed ideas that have deepened my appreciation of historical data and the relationship between human land-use and landcover change. I also thank the LUHNA investigators who contributed their time and vision, as well as the chapters that appear in this volume, and I am grateful to C. Allen, T. Crews, J. Grahame, N. Haddad, and H. Sparrow for their helpful comments on a previous version of this manuscript.

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