CHAPTER 1
Concepts of Forest Ecology

A forest is an ecological system dominated by trees and other woody vegetation. More than simply a stand of trees or a community of woody and herbaceous plants, a forest is a complex ecological system, or ecosystem, characterized by a layered structure of functional parts. Ecology is the study of ecological systems and their interacting abiotic and biotic components. Forest ecology, therefore, addresses the structure, composition, and function of forests. In forest ecology, we study forest organisms and their responses to physical factors of the environment across forested landscapes. Forests are widespread on land surfaces in humid climates outside of the polar regions. It is with forests in general, and with the temperate North American forest in particular, that this book is concerned.

There are many ways to study forest ecosystems. Most simply, a forest may be considered in terms of the trees that give the forest its characteristic aboveground appearance or physiognomy. Thus, we think of a beech-sugar maple forest, a ponderosa pine forest, or of other forest types, for which the naming of the predominant trees alone serves to characterize the forest ecosystem. Forest types are often considered to be composed of forest stands, which are trees in a local setting possessing sufficient uniformity of species composition, age, spatial arrangement, or condition to be distinguishable from adjacent stands (Ford-Robertson 1983).

A broader concept of a forest may take into account the interrelationships that exist between forest trees and other organisms. Certain herbs and shrubs are commonly found in beech-sugar maple forests, and these may differ from those found in ponderosa pine or loblolly pine forests. Similar interrelationships may be demonstrated, for example, for birds, mammals, arthropods, mosses, fungi, and bacteria. Thus, part of the forest ecosystem is the assemblage of plants and animals living together in a biotic community. The forest community, then, is an aggregation of plants and animals living together and occupying a common area. It is thus a more organismally complex unit than the forest type.

A third approach is to focus on geographic or landscape ecosystems. This approach is centered conceptually and in practice on whole ecosystems and not just their parts. When our primary focus is real live chunks of Earth space, that is, landscapes and waterscapes (oceans, lakes, rivers; hereafter included as parts of a landscape), we can effectively study their parts (e.g., organisms, soils, and landforms) while recognizing that each is but one part of a functioning whole. We emphasize this focus on ecosystems rather than on the individual organisms and species that are parts of them.

In the past, the forest stand or the species has been the focus in natural resource fields such as forestry and wildlife. However, we are really managing whole forest ecosystems, despite their incredible complexity, because the diverse biota is inseparable from the physical environment that supports it. A consideration of the field of ecology from this viewpoint provides an overall perspective.

ECOLOGY

Broader fields of scientific inquiry are difficult to limit and define, and ecology is one of the most indistinct. In 1866, Ernst Haeckel proposed the term oecology, from the Greek oikos meaning home or place to live, as the fourth field of biology dealing with environmental relationships of organisms. Thus, ecology literally means "the knowledge of home," or "home wisdom." Since its introduction, the term has been applied at one time or another to almost every aspect of scientific investigation involving the relationship of organisms to one another or to the environment (Rowe 1989). Haeckel's organismal focus of ecology has since been redefined and expanded to include the physical aspects of the environment that provide life for those organisms (Hagen 1992; Golley 1993). Thus, Rowe (1989, p. 230) suggests:

Ecology is, or should be, the study of ecological systems that are home to organisms at the surface of the earth. From this larger-than-life perspective, ecology's concerns are with volumes of earth space, each consisting of an atmospheric layer lying on an earth/water layer with organisms sandwiched at the solar-energized interfaces. These three-dimensional air/organisms/earth systems are real ecosystems-the true subjects of ecology.

This approach to ecology emphasizes whole ecosystems as well as organisms, both volumetric and having structure and function.

LANDSCAPE ECOSYSTEMS

The British botanist-ecologist Arthur Tansley (1935) introduced the term ecosystem, writing with an emphasis on "the whole 'system,' including not only the organism complex but also the whole complex of physical factors." He also noted that from the point of view of the ecologist, ecosystems "are the basic units of nature on the face of the Earth." Tansley was a biologist and vegetation ecologist, and so his idea of ecosystem was centered on organisms (species or communities) rather than geographic or landscape entities. With this bioecosystem approach, "ecosystem" derives its meaning from particular plant or animal organisms of interest, and an "abiotic" environment defined by the organisms as relevant or not is considered with lesser emphasis. In this approach, every organism defines its own ecosystem, nearly infinite in number and difficult to study and use as a basis for management and conservation.

On the other hand, others (e.g., Rowe 1961a and Troll 1968, 1971) view ecosystems centered on geographic or landscape units (i.e., geoecosystems) of which organisms are but one important structural component (Rowe and Barnes 1994). We term these units landscape ecosystems in part to differentiate them from geology-based units of study (e.g., Huggett 1995). Landscape ecosystems are geographic objects, with a defined place on the Earth. Landscape ecosystems have three dimensions (volume) just as organisms do, including landforms and biota at the Earth's surface as well as the air above them and the soils below them (Figure 1.1). Other terms have been introduced to express the same idea, but are less commonly used, such as the ecotope (Troll 1963a, 1968) and the ecoterresa (Jenny 1980). This geographic/volumetric concept has been discussed and adapted by professional and academic ecological societies (Christensen et al. 1996), and is useful to field ecologists, naturalists, foresters and other land managers, and natural resource professionals. The concept is described in detail in Chapters 2 and 11.

In addition to being geographic and volumetric, landscape ecosystems are hierarchical, extending downward from the largest ecosystem we know, the ecosphere (Cole 1958), through multiple levels of ecological organization (Figure 1.2). These levels include macrolevel units of continents and seas, each of which contains mesolevel units of regional ecosystems (major physiographic units and their included organisms), which in turn contain local ecosystems (Hills 1952), the smallest level of homogeneous environment with organisms enveloped in it. We therefore conceive the ecosphere and its landscapes as ecosystems, large and small, nested within one another in an ecological hierarchy, having processes at each level with their own spatial and temporal scales (see Chapter 2).

FIGURE 1.1 The three-dimensional, volumetric nature of a landscape ecosystem. Ecosystems comprise the atmosphere (macroclimate as well as the climate affected by surface relief), landforms and soils (underlain by ground water and bedrock), and the biota that provide a physical connection between the air and the Earth.

Source: Bailey (2009) / Springer Nature.

FIGURE 1.2 Objects of study from the most inclusive (ecosphere) to the least inclusive levels of organization (cell and organelle and molecule below it). Note that each higher level envelops the lower ones as parts of its whole. Some corresponding fields of study are also shown. Aggregates of organisms, such as populations and communities, are components of ecosystems at all scales. Like other components such as atmosphere, landform, and soil, they do not appear in this diagram of first-order objects of study, but are shown in Figures 1.3 and 1.4.

Source: Modified from Rowe (1961a).

Two landscape ecosystems in Figure 1.3 (Rowe 1984b) illustrate characteristic ecosystem differences in hilly or mountainous terrain. The two ecosystems are distinguished by different geomorphologies (convex upper slope versus concave lower slope, and high versus low topographic slope position) that mediate microclimate, soil water, and nutrient availability. The vertical dashed line is placed at an ecologically significant boundary that spatially separates the two ecosystems. Organisms in these ecosystems are sandwiched between the air and the Earth. Also shown in Figure 1.3 are the traditional fields of study in which individuals seek to understand each of the ecosystem components, although forest...