Defining Old Growth
|Lisha Kill Forest Preserve
leads the way into an Ancient Forest
PHOTO: DYarrow 12/1/01
David Yarrow, June 2002
Significant confusion continues to cloud the status, terms and definitions of old growth forests. A vast audience of diverse, sometimes contrary, interests are learning to recognize, evaluate, classify, and communicate about ancient forests.
One major problem preventing a simple, single definition for old growth is the great variety of forest types in the East. A definition which applies to a mixed hardwood forest cannot be employed to define a more uniform stand of conifers. A hardwood forest has far more diversity and complexity than an evergreen forest, which tends toward a few species with minimal understory and ground-level plants.
A similar diversity of growing conditions adds complexity to any effort to define old growth. Trees grow from the timber line in high elevation alpine mountaintops and high latitude tundra, to seacoast mangrove and cypress swamps. The species that inhabit each growing environment is unique, and the growth potential of each environment in terms of size and age varies tremendously. A definition of old growth must be adaptable to each type of environment and its inherent growth potentials and limitations.
Currently, fourteen criteria employed to evaluate a forest’s classification as old growth. These criteria are not universal, since some types of even virgin forests may not meet all these conditions. So these 14 items are a general list that must be applied with discrimination to each situation. Yet, an old growth forest should meet at least most of these criteria except in the most unusual or extreme circumstances.
- Abundance of Old Trees: Age is the ultimate factor defining ancient forests, but old varies with species. A pitch pine is old at 100 years, but hemlocks and oaks commonly reach 500 years. Similarly, trees growing on mountaintops exposed to harsh wind and weather are short-lived compared to those in sheltered lowlands. So, a precise age for this criterion is needed for each species and environment. A general value for hardwood forests is at least six trees per acre are over 150 years old.
- Abundance of Large Trees: Most people’s image of old growth is a forest of tall trees forming a high canopy. Yet, some species do not reach such gigantic proportions, and some growing conditions restrict the growth rates and potentials. High altitude and mountain top forests must struggle with harsh, restrictive growing conditions, including lack of soil and nutrients, extreme temperatures, high winds, and inadequate moisture. Cliff faces and rocky talus slopes are other restrictive growing environments that result in stunted, even dwarf trees. Nonetheless,
- Majority of Trees in Late Succession Class: This technical criterion is based on forest ecology’s succession communities mentioned earlier under “climax forest.” Old growth forests are dominated by trees and species that fall in later successions—or climax—conditions. Early succession species such as aspen, poplar or paper birch are minimal or absent.
- Diversity of Age Groupings: For most forest types, old growth forest contain trees in every age class, from seedlings to saplings to young, mature and elder trees in various stages of dying and death. Age diversity contrasts with even-aged stands of plantations, tree farms and other man-made, planted forests. On occasion, a forest may suffer large scale, catastrophic disturbance such as wildfire or blowdown, with the result that trees regenerating that stand are all the same age.
- Uneven Canopy Structure: In a mature forest with a diversity of species, ages and sizes of trees, trees don’t top out at a uniform maximum height. The canopy is ragged, with highs and lows. In particular, with different aged trees, the oldest trees continually die and fall over, creating holes and gaps in the canopy.
- Multiple Growth Layers: Most well-developed old growth forests are communities with a complex structure, where large trees form the highest overstory, while smaller trees create a lower understory, and shrubs create an even lower substory. And herbaceous plants cover the ground from ankle to waist high, and the soil surface is carpeted with mosses, liverworts, lichens, and other tiny, often primitive plants. Not every forest type has such complex structure; for example, white pine or balsam fir
- Snags: A snag is a dead tree that remains standing. They provide holes that are homes for birds and small mammals, and their decaying bodies are infested with insects that provide food for woodpeckers, other birds and some rodents. Snags play an important role in creating biodiversity within the old growth community.
- Fallen Logs & Woody Since older trees are not harvested for timber and removed, the old growth forest floor will be littered with downed limbs and fallen trees in every stage of decay. Rotting trees create habitat for many insects and small mammals, and thus assures food sources for other insects, birds and other mammals.
- Undulating Forest Floor: When dead and dying trees blow down and fall over, their roots heave up with a mass of earth, creating a mound and hole. These features gradually weather, softening into a pit and mound topography that is characteristic of an ancient forest. The uneven forest floor is not characteristic of all forest types, however.
- Thick, Humus-rich Soil: In most undisturbed forests, the leaf litter and decaying trees have accumulated into a thick layer of soft, spongy, bark humus—the penultimate icon of fertile topsoil, rich in nutrients and teeming with biological life, including fungi and micro-organisms. However, not every forest type or growing environment develops this criterion. For example, certain conifers or rocky mountaintop or talus slope forests do not achieve this condition.
- Well-developed Herbaceous Layer: A fully developed ancient forest will have a lush layer of plants growing at ground level, with a great variety of species representing members common to that forest type and ecosystem community. In certain situations, such as conifers, or overpopulation by deer, this lowest layer of the forest may be weak, patchy or sparse.
- Abundance of Lichens & Fungi: Recent scientific studies are revealing crucial roles for these smallest organisms in the soil in the breakdown of organic matter and the availability of nutrients to more evolved, complex plants. An undisturbed, healthy old growth forest has a rich diversity of these simple life forms in the soil, or growing on trees, logs, limbs, and rocks.
- Absence of Multi-stem Trees: Trees with multiple trunks are an indication of previous logging activities. Certain species are prolific stump sprouters, most notably oaks, and the presence of multi-stem trees of these species is a virtual guarantee the forest was logged.
- Absence of Human Disturbance: This final criterion is one of the more controversial. Virgin or first forests must fully meet this condition. Signs of human disturbance can include old rotten stumps, stone walls and foundations, barbed wire attached to trees, or a preponderance of species favored by disturbance, such as white ash, black locust or red maple.
However, secondary old growth will have been disturbed in an earlier century or early settlement, yet regenerated sufficiently to meet most other criteria. Some investigators believe certain minimal human disturbances, such as selective logging or roads are acceptable if they don’t compromise the overall health, diversity and integrity of the forest ecosystem.