Definitions and History

Robert Leverett

This book is a compilation of writings by a group of distinguished scientists, naturalists, and environmentalists about an exceedingly rare part of the natural world—the scattered remnants of the original Eastern forests. Today, such forest exists on a tiny fraction of the land, but 500 years ago it was considered inexhaustible.

Isolated pockets of old-growth forest are all that remain. As such they are the subject of intense interest to the scientific, academic, and environmental communities. This book is a measure of that interest.

Some might think that interest in Eastern old growth is an extension of forest preservation movements spawned in the West. Certainly, depletion of the old-growth rainforests of the Pacific Northwest has drawn attention to the status of all public forests. Awareness of Eastern old growth has been heightened by the furor over excessive logging in Western national forests, but most of the authors have connections to Eastern forests that predate the issues surrounding Western old growth.

The uninitiated might think that these remaining patches of original forest are well known, intensively studied, and precisely defined, but spirited debates say otherwise, especially about how old Eastern forests are defined. Consequently, we will begin our tour of Eastern old growth by investigating the problems surrounding definitions and terminology.

What Is an Old-Growth Forest?

As the reader will discover in this book, scientists and forestry professionals apply different meanings to "old growth." To make matters worse, the meanings vary within the professions. These definitional differences, combined with a scarcity of searches for old growth in the field, have led understandably to varying estimates of how much Eastern old growth remains. Current estimates vary from as little as a half million acres to as much as a million and a half. This broad range excludes mature second-growth forests (i.e., previously cut), which some ecologists argue should be included, at least at some future date. The range also excludes scattered stands of "low-quality" noncommercial timber that have survived "unnoticed." When this latter category is thoroughly inventoried, surviving old growth will probably be well above 1.5 million acres. One scientist estimates the figure to be as high as five million.

The term "old growth" implies old trees. However, two criteria, age and naturalness, drive most definitions of old-growth forests. Consequently, it is tempting to conclude that an adequate definition for old growth is a forest of mature trees that has developed naturally. But how old is old, what constitutes natural development, and is it the trees or the forest that is old? A surprising number of definitions have been developed to address these questions.

"Old growth" has been used loosely to describe a forest that has existed since presettlement times and that has experienced little or no direct disruption by Euro-Americans—whether or not the forest has experienced catastrophic natural disturbance and whether or not the trees in it now are old. In this broad sense, "old growth" is roughly equivalent to the terms "primary" or "original."

However, many researchers use "old growth" in a narrower sense. They restrict it to forests in which the trees are old, at least in relation to others of their species. For example, Charles Cogbill, in his chapter on red spruce forest in the upper Northeast, limits old growth to stands that have escaped disturbances long enough for the trees to have a mean age over 150 years. Some researchers eliminate from consideration all stands of tree species that do not live more than 120 years. Peter White counters the tendency to equate exclusively with stands of old trees rather than simply with old, chronologically continuous forests. White expresses the need for a term defining old forests, not necessarily old individual trees. White imposes the requirement that such a forest should have escaped direct anthropogenic disruption.

Researchers may answer the question of what to call old forests of relatively young trees by giving separate meanings to primary and old growth. Primary would refer to forests of any age that have experienced no or only minimal human disturbance. (Whether that definition might allow light logging or grazing or anthropogenic fire varies with the researcher.) On the other hand, old growth would refer to forests that meet certain criteria of age and/or stand development. This is consistent with Lee Frelich's approach. In his chapter, Frelich defines primary and old growth separately. For Frelich and many other researchers, a primary forest may be but is not necessarily old growth. Looking at how researchers have defined old growth, we can place most definitions in one of four categories.

Category 1 definitions are the most demanding and are subscribed to by a majority of the authors of this book. They emphasize the importance of (1) tree age, (2) the lack of human disturbance, and (3) the successional stage of the forest More specifically, they require that an old-growth forest have (1) a high percentage (usually half or more) of its canopy trees over half the maximum life spans for the represented species, (2) at least a few trees near the maximum life spans for the species, (3) no recorded history or discernible signs of direct human disruptions such as planting alien species or cutting, (4) a composition of what ecologists call late successional species, (5) a structure and set of characteristics that are associated with mature, nature-managed forests, and (6) a minimum stand size of usually 5 to 10 acres.

The prohibition against human disruption usually applies to the period of and following settlement by Europeans. The limited impact of early Native Americans is generally accepted as part of the natural order. Some scientists impose further conditions—e.g., a net annual growth near zero. This means that the processes of growth, or life, are balanced by those of death.

Another condition sometimes imposed is that the area in question has had trees on it since the beginning of the settlement period. This constraint excludes stands that have regenerated from a catastrophic disturbance after the beginning of European settlement. However, in the minds of some ecologists, forests that receive periodic natural disturbances such as windthrow, fire, or insect damage of a sufficiently severe level as to prevent the development of advanced age should not be excluded from old growth. Charles Cogbill describes forests that fit this class. Most Category 1 definitions are roughly equivalent to those defining a "mature virgin forest."

Category 2 definitions relax the prohibition against direct disturbance by humans. The age criteria are also relaxed. The mature canopy trees usually must be 150 years or older, but no further age requirement is imposed, such as a percentage of the trees near the maximum longevity for the represented species. As with Category 1, Category 2 definitions require the existence of characteristics associated with mature, nature-managed forests. Hence, forests meeting either Category 1 or 2 definitions look old. Some researchers want to include old second-growth forests as old growth provided they have not regrown from clearcutting, an unacceptable level of human disturbance that often produces dramatic changes in species mix.

Category 3 definitions approach old growth from a "stand development" perspective. Beginning from a major disturbance that eliminates all standing trees, a forest goes through stages of development. The first stage is called stand initiation, in which tree growth is not influenced by other standing trees—there are none. The new forest develops somewhat uniformly until the canopy is closed, which inhibits growth of young trees. Thus begins the stem-exclusion phase, in which no new trees are found on the forest floor. In time, random disturbances open up gaps in the canopy and produce opportunities for young trees to start growing. This leads to a stage called understory initiation. When these younger trees reach the canopy, replacing those that have fallen or died, the forest enters the stage of canopy replacement Barring additional major disturbances, and depending on the kinds of trees that began growing after the disturbance that initiated succession, this last stage may last a long time. James Runkle provides an excellent discussion of stand-based definitions in his chapter. This third definition recognizes the physical characteristics that accompany the last phase of stand development, but does not impose them as definitional requirements.

Category 4 definitions utilize economic criteria. The old-growth state is said to be reached when trees slow their growth rates and show signs of advancing age, such as crown die-back and heartwood decay. Definitions falling into this category generally consider trees between the ages of 80 and 150 years to be "overmature," meaning they have reduced economic value.

Category 1, 2, and 3 definitions are grounded in science while Category 4-based definitions are driven by economics. In fact, all these definitions often regard forests as little more than assemblages of trees, discounting the importance of the myriad of life forms that evolve to exploit mature forest niches, particularly those relying on decay. Even so, Category 4 definitions frequently stipulate the existence of characteristics associated with older forests and in this area find common ground with the first three categories.

Old-growth forests are most readily recognized by the structural characteristics they share—to one degree or another. The following characteristics have traditionally been considered indicative of the old-growth phase:

• An abundance of old trees, recognizable by their asymmetrical shapes, relatively long trunks free of low branches (i.e., in-forest as opposed to open-grown shapes), deeply furrowed or plated bark, signs of heartwood decay, large prominent root structures, flattened crowns with protruding dead limbs, large thick limbs, and trunks often showing a twist that develops with age.

• Fallen logs in all stages of decomposition, crisscrossing the forest floor and lying in and across stream beds, covered by moss and lichens.

• Plentiful snags (standing dead trees).

• Canopy gaps large and small, formed from trees that have fallen.

• Undulating forest floor expressed in randomly scattered pits and mounds where trees have fallen over and decomposed (Peter White describes exceptions in his chapter).

• Multiple growth layers: overstory (trees that make up the canopy); understory (trees beneath the canopy); and shrub, herbaceous, and ground layers visible to one degree or another, all reflecting a broad spectrum of ages.

• Undisturbed soils with a relatively thick humus layer in some forest types.

• Large trees for the growing conditions.

• Well-developed herbaceous layer with a diverse composition, especially in neutral soils.

• Abundance of lichens and fungi, particularly in acid-based soils.

• Majority of tree species that fall into the late successional class and a conspicuous absence of multiple-stemmed trees (called coppices).

• Absence of signs of human disturbance.

• A mosaic of age groupings left as imprints from many natural disturbances of varying sizes.

It is important to emphasize that these features are far from universal. Some stands of old growth will possess all of them; others will possess only a few. Possession of the characteristics depends on forest type, local climate (which, along with soils, tends to determine forest type), and disturbance history. The absence of a characteristic can be highly confusing to anyone with checklist in hand. Mixed-mesophytic old-growth forests exhibit remarkable species diversity while old-growth boreal forests exhibit limited diversity. For example, the lush coves of the Great Smoky Mountains of North Carolina and Tennessee are remarkably diverse; but fire-successional stands of red, white, or jack pine in the Northeast and Midwest can be uniform in age, have sparse understories, and exhibit low diversity in ground plants. Such forests would be excluded as old growth by some definitions. Dry forests with an abundance of pine often have a higher percentage of snags than their wetter deciduous counterparts. High-altitude spruce-fir stands may have few truly old trees. The list of exceptions is exceedingly long.

Some ecologists exclude from old growth (over the protests of others) forest types that are in a state of transition, changing from one species mix to another. Transitional forests in which short-lived, light-loving species are being replaced by longer-lived, shade-tolerant species are seldom classified as old growth, although they are certainly primary forests if they have not experienced disruption by Euro-Americans. However, some transitional forests are populated by long-lived species such as white pine that reach extraordinary sizes. Such stands are often considered old growth. In truth, such old successional stands probably need a definition all their own. They most closely match fire-successional stands of Douglas-fir in the West, which is quintessential old growth to many.

In an attempt to reduce confusion over semantics, scientists have introduced other old-forest terms. They include: (1) virgin (meaning no direct human disturbance), (2) primeval (from a very early time), (3) original or presettlement (existing before settlement by Europeans), (4) climax (meaning the stage in a successional process characterized by stability of species), and (5) ancient.

Before leaving the subject of definitions, I should point out that a fifth class of old growth appears to be evolving. Resource managers speak of artificially creating old-growth forest as part of a management scheme that retains timber harvesting as the top priority. Bizarre experiments like dynamiting the tops out of trees to create snags and drilling holes in healthy young trees to create wildlife cavities are illustrative of the methods. In the future we will likely see a number of experiments attempting to speed up natural processes. This "designer old growth" will be devoid of old trees; it will thwart the fulfillment of natural processes and consequently will have limited biological and scientific value, to say nothing of being an eyesore.

We hope that in the future there will be a true fifth category—old growth restored slowly and naturally. We do not know whether heavily logged forests will ever regain all the characteristics of original forests, and the process of recovery will take centuries. But as many writers of this volume indicate, we must set aside forests to recover to the extent possible.

Despite disagreements over definitions, efforts to inventory remnants of the original forest have gained momentum. The combined efforts of groups and individuals have added many acres to the inventory. Lack of previous detection of these pockets of old forest can be traced to several factors.

1. The perception was that remnants of the original forest had been thoroughly inventoried. This belief had been expressed both by authors writing for popular nature guides and by experienced scientists writing for prestigious journals. There was nothing left to find.

2. Stereotypes of the original forest precluded the ready identification of old growth, even by relatively experienced researchers. One stereotype is an open, park-like forest of large, mature, widely spaced trees. Another is of a homogeneous stand of decadent trees shading an understory and forest floor devoid of all but a few plant species—a veritable botanical desert.

3. Low priority has been given to inventorying and preserving residual old-growth pockets by the public agencies overseeing our forests. This factor is especially significant since much of the surviving old growth is on public lands.

4. Information on Eastern old-growth forests has been generally inaccessible. Consequently, popular writers over-relied on relatively few sources, which reinforced the perception that all old growth in the East had been found.