Introduction
Prior to Euro-American colonization beginning in the late 1700s and subsequent periods of land conversion and intensive resource extraction, the forests of eastern Kentucky’s Cumberland Plateau were part of a nearly contiguous forest covering much of the eastern United States. The Eastern Deciduous Forest, sometimes called “The Great Forest,” was estimated to have covered as much as 380 million hectares (Leverett 1996; Bolgiano 1998), including an estimated 85 – 90% of Kentucky’s total land area (Evans and Abernathy 2008). While those forests would have fluctuated within a range of community associations, structural relations, and successional states, most forest on the Cumberland Plateau would have existed in a state meeting one or more of the definitions of old-growth forest in use today.
Forest clearing for agricultural and industrial use in the Cumberland Plateau from around the mid-1800s to 1930 left little forest untouched, and only a few examples of relatively intact old-growth forests remain in Kentucky (Jones 2005). However, many recovering, mature forests currently exist that might be redeveloping old-growth structure and function. Many existing old-growth forests are recognized as having initiated following major disturbance (Whitney 1994), and models of forest structural development describe forests as proceeding from a regenerating, even-aged distribution toward a multi-aged, old-growth architecture given sufficient time (Oliver and Larson 1996; Frelich 2002). While the specifics may vary by disturbance intensity, species composition, climate, and edaphic conditions, the natural redevelopment of old-growth forest structure, composition, and processes is expected (Frelich 2002).
The purpose of this investigation was to assess the development of old-growth structural characteristics in some of the oldest second-growth hardwood forests of eastern Kentucky.
Forest clearing for agricultural and industrial use in the Cumberland Plateau from around the mid-1800s to 1930 left little forest untouched, and only a few examples of relatively intact old-growth forests remain in Kentucky (Jones 2005). However, many recovering, mature forests currently exist that might be redeveloping old-growth structure and function. Many existing old-growth forests are recognized as having initiated following major disturbance (Whitney 1994), and models of forest structural development describe forests as proceeding from a regenerating, even-aged distribution toward a multi-aged, old-growth architecture given sufficient time (Oliver and Larson 1996; Frelich 2002). While the specifics may vary by disturbance intensity, species composition, climate, and edaphic conditions, the natural redevelopment of old-growth forest structure, composition, and processes is expected (Frelich 2002).
The purpose of this investigation was to assess the development of old-growth structural characteristics in some of the oldest second-growth hardwood forests of eastern Kentucky.
Defining Old-Growth Forests
While the term “old-growth forest” itself may be in common use and evocative of some archetypal visage, it is too general from a scientific or operational perspective to be used without further clarification (Wirth et al. 2009). Generally, most definitions or criteria for assigning or assessing a forest as old-growth can be divided into structural, successional, or age-related considerations (Wirth et al. 2009; Cooper 2011). Frelich and Reich (2003) offer several ecological definitions for old-growth forest that are useful in considerations for the Cumberland Plateau and other regions.
Climax Old-Growth
The climax definition of old-growth forest references the final stage in successional development of the community (Clements 1936; Braun 1950). Hypothesized to be a steady state of community organization in the absence of disturbance, the existence of a true climax community has come into question as the integral relationship between climate change, disturbance, and community structure has come to be better understood.
In terms of forest development, a climax old-growth forest is one that is dominated by shade-tolerant, self-replacing species, and occurs in the absence of significant disturbance that would otherwise allow for more influence by shade-intolerant or mid-tolerant species (Frelich 2002). Understory and midstory species are essentially the same as those in the canopy, such that turnover in the canopy results in a continuity of species composition.
In the Appalachian region, species typifying climax old-growth are sugar maple (Acer saccharum), American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), and black gum (Nyssa sylvatica) (Lorimer 1980).
Sub-climax of Seral Old-Growth
Sub-climax or seral old-growth forests are those that are composed of shade-intolerant or mid-tolerant species, such white oak (Quercus alba), tulip poplar (Liriodendron tulipifera), and mockernut hickory (Carya tomentosa), but otherwise exhibit age and structural characteristics associated with old-growth (Frelich and Reich 2003). Species composition in these forests is understood to be maintained by periodic disturbance, without which the forest succeeds to shade-tolerant, climax-associated species.
Primary Forest
Primary forests are those that have developed in the absence of significant interference from humans through logging, agricultural clearing, or other major manipulation. Structure in primary forests results from a continuous legacy of natural disturbance, regeneration, and stand development (Frelich and Reich 2003). The term virgin forest can be considered synonymous with primary forest.
Typically, forests initiating prior to settlement by Euro-Americans are considered primary forests. However, the applicability of this definition can become muddied when considering the role of anthropogenic fire prior to Euro-American colonization or the loss of species like American chestnut (Castanea dentata) from a human-introduced blight in forests otherwise undisturbed by modern humans.
Secondary Old-Growth
Forests that have been heavily logged or cleared at some time in the past, and in particular since Euro-American settlement, but have redeveloped structural or age characteristics similar to old-growth under one of the above definitions are considered secondary old-growth (Frelich 1995; Frelich and Reich 2003). Many secondary forests in New England are considered secondary old-growth on account of the amount of time of regrowth since initial disturbance associated with European colonization of the region (Dunwiddie et al. 1996). The question of whether old second-growth forests in the Cumberland Plateau region in Kentucky can or should be considered secondary old-growth is unclear and the purpose of this investigation.
Climax Old-Growth
The climax definition of old-growth forest references the final stage in successional development of the community (Clements 1936; Braun 1950). Hypothesized to be a steady state of community organization in the absence of disturbance, the existence of a true climax community has come into question as the integral relationship between climate change, disturbance, and community structure has come to be better understood.
In terms of forest development, a climax old-growth forest is one that is dominated by shade-tolerant, self-replacing species, and occurs in the absence of significant disturbance that would otherwise allow for more influence by shade-intolerant or mid-tolerant species (Frelich 2002). Understory and midstory species are essentially the same as those in the canopy, such that turnover in the canopy results in a continuity of species composition.
In the Appalachian region, species typifying climax old-growth are sugar maple (Acer saccharum), American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), and black gum (Nyssa sylvatica) (Lorimer 1980).
Sub-climax of Seral Old-Growth
Sub-climax or seral old-growth forests are those that are composed of shade-intolerant or mid-tolerant species, such white oak (Quercus alba), tulip poplar (Liriodendron tulipifera), and mockernut hickory (Carya tomentosa), but otherwise exhibit age and structural characteristics associated with old-growth (Frelich and Reich 2003). Species composition in these forests is understood to be maintained by periodic disturbance, without which the forest succeeds to shade-tolerant, climax-associated species.
Primary Forest
Primary forests are those that have developed in the absence of significant interference from humans through logging, agricultural clearing, or other major manipulation. Structure in primary forests results from a continuous legacy of natural disturbance, regeneration, and stand development (Frelich and Reich 2003). The term virgin forest can be considered synonymous with primary forest.
Typically, forests initiating prior to settlement by Euro-Americans are considered primary forests. However, the applicability of this definition can become muddied when considering the role of anthropogenic fire prior to Euro-American colonization or the loss of species like American chestnut (Castanea dentata) from a human-introduced blight in forests otherwise undisturbed by modern humans.
Secondary Old-Growth
Forests that have been heavily logged or cleared at some time in the past, and in particular since Euro-American settlement, but have redeveloped structural or age characteristics similar to old-growth under one of the above definitions are considered secondary old-growth (Frelich 1995; Frelich and Reich 2003). Many secondary forests in New England are considered secondary old-growth on account of the amount of time of regrowth since initial disturbance associated with European colonization of the region (Dunwiddie et al. 1996). The question of whether old second-growth forests in the Cumberland Plateau region in Kentucky can or should be considered secondary old-growth is unclear and the purpose of this investigation.
Structure
Structural characteristics of old-growth forests can vary widely depending on forest type, disturbance regime, climate, edaphic conditions, and other variables. For example, an old-growth boreal forest will have a substantially different structure than an old-growth tropical forest, yet both may be validly considered old-growth (Wirth et al. 2009). Still, a great deal of consistency has been found in the structural characteristics associated with old-growth forests across the Eastern Deciduous Forest and the Central Appalachians (Parker 1989; Martin 1992; Tyrrell and Crow 1994), suggesting a certain unity in pattern and process across the Eastern Deciduous Forest as a whole.
Accepting natural variation and differing ranges of values depending on species composition, forest productivity, and other factors, this suite of characteristics can be used to assess old-growth status or degree of “old-growthness,” which describes the extent to which a forest exhibits the structural and functional characteristics associated with old-growth forests (Bauhus et al. 2009). While the following characteristics are often indicative of old-growth, it is important to note that the presence or absence of some characteristics does not necessary mean that the forest is or is not de facto old-growth per any given definition. Subsequent use of the term “old-growth forest” herein refers to that which is found primarily in the Eastern Deciduous Forest region of North America.
Canopy Age
Canopy age is often used as a criterion for determining old-growth forest status. In some cases, the age approach is somewhat arbitrary and can be based more on socio-political rather than ecological considerations (Frelich and Reich 2003). In Kentucky, forest stands are generally considered old-growth if the dominant canopy is older than the period of initial colonization by Euro-Americans near the end of the 1700s. Martin (1992) suggested that to be considered old-growth, the oldest trees in mixed mesophytic forest communities should be at least 200 years old based on the average life expectancy of canopy dominants, while Parker (1989) similarly suggests that old-growth structure in the central hardwood region on the whole should develop by the time the canopy reaches 150 ̵ 200 years. However, forests recovering from a stand-replacing event may need longer than the above time frames to fully recover some old-growth characteristics, and in particular may need much longer to develop a true multi-age canopy structure (Oliver and Larson 1996; Frelich 2002).
Large Diameter Trees
While old trees aren’t necessarily large, nor large trees old (Pederson 2010), old-growth forests tend to contain trees that are relatively large for given site and species constraints. Martin (1992) reported at least seven trees per hectare >75 cm DBH (diameter at breast height, 1.3 m) in the mixed mesophytic forests at Lilley Cornett Woods in Letcher County, Kentucky. Large diameter trees may be larger and more abundant in increasingly mesic, protected cove forests (e.g., coves of the Great Smoky Mountains), while tree size is typically more restricted on xeric, drought-prone, and exposed sites (Stahle and Chaney 1994). Much of the remaining old-growth forest in the eastern U.S. is exemplified by these low-productivity sites, as they were often ignored for timber or agricultural production (Stahle and Chaney 1994).
Large trees play an important role in the ecology of many forests, and can have a major influence on a number of ecosystem processes, including competitive relationships, nutrient dynamics, biomass allocation, and others (Lutz et al. 2012). It is also notable that many of the structural and functional characteristics that distinguish old-growth forests from younger forests, as discussed below, derive from the presence of large trees (Runkle 1991).
Coarse Woody Debris
Coarse woody debris (CWD), also referred to as “coarse woody detritus” or “coarse woody material,” is dead, downed woody material usually delineated as being >10 cm diameter and >1 m in length. On occasion CWD is used to refer to both down and standing dead wood (snags), though I treat the two separately here. Smaller diameter woody material is usually referred to as “fine” woody detritus, material, or debris.
The presence of relatively high volumes of CWD, particularly in larger diameter classes and later stages of decay, is likely one of the characteristics that most distinguishes old-growth forests from second-growth forests (Parker 1989; Martin 1992; Hale et al. 1999; Spetich et al. 1999; Harmon 2009). The larger volumes of CWD observed in old-growth forests are typically the result of the contributions of a few large-diameter trees to the total pool (Shifley et al. 1997). However, distribution and total volume of CWD in a forest can fluctuate considerably based on disturbance history, mortality, and climate (Brown and Schroeder 1999; Harmon 2009), and often increases with forest productivity (Spetich et al. 1999). While old-growth forests are generally assumed to have a greater representation across decay classes than their younger counterparts (Martin 1992; Goodburn and Lorimer 2008), this is not always the case (Shifley et al. 1997; Haney and Lydic 1999).
Coarse woody debris in forests that have been subject to stand replacing events without the removal of logs (e.g., tornados) show a marked spike in CWD volume that decreases with time as decomposition proceeds, and eventually plateaus when background inputs from mortality approximate decomposition (Harmon 2009). Forests subject to logging, either as the primary disturbance or through post-disturbance salvage logging, will similarly exhibit a spike in CWD from logging slash and other residue. However, due to the absence of large decomposing boles, decomposition of the smaller diameter slash will be more rapid and result in a period of very low total CWD until trees grow large enough to provide significant CWD inputs (Spetich et al. 1999).
Coarse woody debris is involved in many ecological processes, including energy flow, nutrient cycling, soil and sediment transport, moisture retention, and providing habitat for a wide array of species, including arthropods, birds, small mammals, herptiles, fungi, and microorganisms (Harmon et al. 1986; Goodburn and Lorimer 1999; McGee et al. 1999; Muller 2003).
The comparatively warmer upper surface, cooler underside, and relative stability of internal moisture and temperature conditions provided by CWD allow for a variety of herpetofauna to utilize CWD for a number of important life history activities, including thermoregulation (both warm and cold-season), avoidance of desiccation, predator avoidance, and successful egg laying and hatching (Whiles and Grubaugh 1993).
At least 55 mammal species use downed logs in the southeastern U.S., and CWD may be critical habitat for some small mammals including shrews (Loeb 1993). Logs are used as travel corridors and provide cover for predator avoidance, and, by providing habitat for macroinvertebrates and fungi, are important for feeding and foraging. Several mammal species also use CWD for nesting and denning, including striped skunks (Mephitis mephitis), gray fox (Urocyon cinereoargenteus), weasels (Mustela spp.), black bears (Ursus americanus), and a variety of mice (Mus spp.) and shrews (Soricidae) (Harmon et al. 1986; Wathen et al. 1986; Loeb 1993).
Few birds use downed CWD, with the notable exception of ruffed grouse use of logs for “drumming” (Gullion 1967; Harmon 1986). CWD is also important habitat for a wide array of micro- and macroinvertebrates and fungi that both provide food for a number of taxa and play vital roles in forest nutrient and energy cycling (Harmon et al. 1986; Hanula 1992; Johnston and Crossley 1993).
Large-Diameter Snags
Large-diameter snags (standing dead trees) are frequently missing from young and maturing second-growth forests, but are often typical of old-growth forests (Goodburn and Lorimer 1999; McGee et al. 1999), excepting for low productivity forests where tree diameter may be truncated by edaphic or other conditions. Some studies have found larger frequencies of small-diameter snags in younger forests, most likely related to density-dependent mortality from competition during stem exclusion and demographic transition phases stand development (McComb and Muller 1983; Goodburn and Lorimer 1999; Frelich 2002). The total density or volume of snags can be similar in old-growth and second-growth forests, but this is often due to either residual trees remaining from past partial harvests or the cumulative basal area of smaller snags in the younger forests (McComb and Muller 1983; Goodburn and Lorimer 1999; Hale et al. 1999).
Cavity Trees
Related to snags are cavity trees. While snags are more likely to have cavities than live trees, the latter typically provide more cavities in a forest because live trees are much more frequent (Goodburn and Lorimer 1999; Fan et al. 2003). Cavity formation often occurs through a succession of dead wood utilization by a variety of taxa. Heart rotting fungi create conditions that facilitate wood-eating insects and other fungi, which further provide food for a range of vertebrates. As the wood softens, primary cavity species, usually cavity nesting birds, excavate an initial cavity for use, while secondary cavity species, including birds, bats, squirrels, bees, chipmunks, raccoons, and other taxa, use or enlarge existing cavities (Harmon et al. 1986; Gysel 1961).
Old-growth forests tend to have more cavity trees, and substantially more cavities in trees of larger size classes, than younger forests (Fan et al. 2003; 2005). Large tree cavities are important as the initial diameter of the tree and cavity can be a limiting factor for some cavity nesting birds and other taxa. The greater number and range of sizes of cavities may be why old-growth forests, in general, have a greater number of cavity nesting birds than their younger counterparts (Harmon et al. 1986; Haney and Lydic 1999). Large diameter cavities around 100 cm DBH and greater, which are typically absent in younger forests, have been found to be preferred den sites for black bears (Ursus americanus) in the Southern Appalachians, suggesting the importance of old forests for this species (Wathen et al. 1986, White et al. 2001).
Uneven Age Distribution
Trees in old-growth forests often follow a multi-age distribution, with recruitment either continuous or occurring through multiple recruitment events, or both, depending on the spatial scale under consideration. An uneven-aged distribution results when stand development proceeds in the absence of major disturbance, with tree mortality occurring individually or in small groups (Oliver and Larson 1996; Frelich 2002).
Trees in Multiple Size Classes and the “Reverse-J” Diameter Distribution
Diameter distributions in old-growth forests typically follow a “reverse-J,” roughly inverse exponential distribution, where a large frequency of small diameter trees tapers off to an increasingly lower frequency of large diameter trees, and plot on a log scale as a straight line (Frelich 2002; Gove et al. 2008). Some old-growth forest and other uneven-aged forests have been found to exhibit a “rotated sigmoid” distribution, where the diameter distribution has a hump or plateau in the mid-diameter range (Gove et al. 2008). It has been suggested that this distribution reflects past intermediate-scale disturbance in the stand (Lorimer and Frelich 1984; Leak 1996). While many old-growth forests follow the reverse-J distribution, some even-aged second-growth forests have been found to similarly follow this pattern (Goodburn and Lorimer 1999).
Multi-layered Canopy
Generally speaking, canopy stratification describes the relative vertical distribution or layering of trees within the forest canopy (Parker and Brown 2000). The vertical and horizontal structure of the canopy, together, are important determinants in growing space availability and light penetration through the canopy (Jennings et al. 1999). Old-growth forest and other uneven-aged forests tend to have greater stratification of their canopies contributing to their greater structural diversity over younger even-aged forests (Frelich 2002).
Large Overstory Basal Area
The basal area (BA) of a stand is the sum of cross-sectional areas of all trees at 1.3 m, or breast-height, over a given area and expressed in m2/ha (or ft2/ac in American forestry). Basal area tends to increase with stand maturity and inversely with stand density, and can vary considerably by forest type with drier or more disturbance-prone forests having lower basal areas than more productive and sheltered forests. Martin (1992) provided a lower threshold for old-growth forests of 25 m2/ha based on values from Lilley Cornett Woods, where BA values ranged from 20.6 to 42.4 m2/ha across all communities. However, total forest BA for mature, and even young, second-growth forests sometimes falls within this same range (Goebel and Hix 1996; Hale et al. 1999).
Overstory Density Approximately 250 Stems/ha
Stem density tends to decrease with age as a function of stand development as basal area is redistributed to increasingly larger diameter trees. Martin (1992) proposed 250 stems/ha > 10 cm DBH as a threshold for old-growth forests based on values ranging from 160 to 315 stems/ha at Lilly Cornett Woods. Parker (1989) found similar values for old-growth throughout the eastern hardwood region ranging from 161 to 427 stems/ha. However, Hart et. al (2012a) found 620 stems > 10 cm DBH/ha in an oak-pine upland forest at Savage Gulf, an old-growth forest on the Cumberland Plateau in Tennessee, and unpublished data from a 2010 inventory of Lilley Cornett Woods found a density of 536 trees > 10 cm DBH/ha (McEwan and Richter 2010), calling into question the usefulness of this metric for assessing old-growth condition.
Herbaceous diversity
Herbaceous diversity may be greater in old-growth forests (Martin 1992), with incomplete recovery in second-growth stands over the historical period (Duffy and Meier 1992). Several factors may contribute to diminished herbaceous diversity following logging, including many species’ short dormancy and consequent lack of persistence in the seed bank, limitations on dispersal (with forest herbs often clonal, gravity-dispersed, or ant-dispersed), inability to compete with r-selected plant species, and changes in microhabitat, among others (Meier et al. 1995; Whigham 2004). However, for considerations of herbaceous diversity in comparisons of forests, differences in community type need to be taken into consideration and not be confounded with differences related to stand age (Harrelson and Matlack 2006).
Pit and Mound Topography
When a large tree falls, its root mass is usually pulled from the soil and lifted perpendicular to the ground along with humus, mineral soil, and rock fragments (Schaetz et al. 1989). The resulting formation is referred to as a “tip-up mound” or, at a larger scale, “pit and mound topography,” and can be an indicator of old-growth forests. The process occurs relative to disturbance frequency, with pits and mounds often evident for centuries after formation (Peterson and Campbell 1993). Tip-up mounds can be missing from second-growth forests due to removal as part of agricultural use prior to abandonment (Whitney 1994). They may also be infrequent as a result of logging alone, where trees large enough to leave substantial tip-up mounds have been missing from the forest during stand development and recovery, creating a lapse in formation. However, there can be a great deal of variability depending on site-specific conditions and history.
Tip-up mounds are important because they create varying moisture, temperature, and nutrient conditions, including the exposure of bare mineral soil, that can affect species richness and distribution by allowing for a diversity of microsites for seedling germination (Schaetzel et al. 1989; Peterson and Campbell 1993; Clinton and Baker 2000). When considered as an ongoing process over the course of millennia, tree uprooting may have important consequences for soil structure, the mixing of soil horizons, and soil carbon and nutrient dynamics.
Canopy Gaps
Gaps in the canopy created by mortality or blow-down of individuals or small groups of trees is a characteristic strongly associated with old-growth forests (Runkle 1985; Martin 1992). The pattern of gaps reflects a history of small-scale disturbance and relates to the development of uneven-aged canopy distributions, canopy layering, coarse woody debris, and other structural elements (Runkle 1985; Frelich 2002). Canopy gaps and gap dynamics are discussed more thoroughly later in this document.
Accepting natural variation and differing ranges of values depending on species composition, forest productivity, and other factors, this suite of characteristics can be used to assess old-growth status or degree of “old-growthness,” which describes the extent to which a forest exhibits the structural and functional characteristics associated with old-growth forests (Bauhus et al. 2009). While the following characteristics are often indicative of old-growth, it is important to note that the presence or absence of some characteristics does not necessary mean that the forest is or is not de facto old-growth per any given definition. Subsequent use of the term “old-growth forest” herein refers to that which is found primarily in the Eastern Deciduous Forest region of North America.
Canopy Age
Canopy age is often used as a criterion for determining old-growth forest status. In some cases, the age approach is somewhat arbitrary and can be based more on socio-political rather than ecological considerations (Frelich and Reich 2003). In Kentucky, forest stands are generally considered old-growth if the dominant canopy is older than the period of initial colonization by Euro-Americans near the end of the 1700s. Martin (1992) suggested that to be considered old-growth, the oldest trees in mixed mesophytic forest communities should be at least 200 years old based on the average life expectancy of canopy dominants, while Parker (1989) similarly suggests that old-growth structure in the central hardwood region on the whole should develop by the time the canopy reaches 150 ̵ 200 years. However, forests recovering from a stand-replacing event may need longer than the above time frames to fully recover some old-growth characteristics, and in particular may need much longer to develop a true multi-age canopy structure (Oliver and Larson 1996; Frelich 2002).
Large Diameter Trees
While old trees aren’t necessarily large, nor large trees old (Pederson 2010), old-growth forests tend to contain trees that are relatively large for given site and species constraints. Martin (1992) reported at least seven trees per hectare >75 cm DBH (diameter at breast height, 1.3 m) in the mixed mesophytic forests at Lilley Cornett Woods in Letcher County, Kentucky. Large diameter trees may be larger and more abundant in increasingly mesic, protected cove forests (e.g., coves of the Great Smoky Mountains), while tree size is typically more restricted on xeric, drought-prone, and exposed sites (Stahle and Chaney 1994). Much of the remaining old-growth forest in the eastern U.S. is exemplified by these low-productivity sites, as they were often ignored for timber or agricultural production (Stahle and Chaney 1994).
Large trees play an important role in the ecology of many forests, and can have a major influence on a number of ecosystem processes, including competitive relationships, nutrient dynamics, biomass allocation, and others (Lutz et al. 2012). It is also notable that many of the structural and functional characteristics that distinguish old-growth forests from younger forests, as discussed below, derive from the presence of large trees (Runkle 1991).
Coarse Woody Debris
Coarse woody debris (CWD), also referred to as “coarse woody detritus” or “coarse woody material,” is dead, downed woody material usually delineated as being >10 cm diameter and >1 m in length. On occasion CWD is used to refer to both down and standing dead wood (snags), though I treat the two separately here. Smaller diameter woody material is usually referred to as “fine” woody detritus, material, or debris.
The presence of relatively high volumes of CWD, particularly in larger diameter classes and later stages of decay, is likely one of the characteristics that most distinguishes old-growth forests from second-growth forests (Parker 1989; Martin 1992; Hale et al. 1999; Spetich et al. 1999; Harmon 2009). The larger volumes of CWD observed in old-growth forests are typically the result of the contributions of a few large-diameter trees to the total pool (Shifley et al. 1997). However, distribution and total volume of CWD in a forest can fluctuate considerably based on disturbance history, mortality, and climate (Brown and Schroeder 1999; Harmon 2009), and often increases with forest productivity (Spetich et al. 1999). While old-growth forests are generally assumed to have a greater representation across decay classes than their younger counterparts (Martin 1992; Goodburn and Lorimer 2008), this is not always the case (Shifley et al. 1997; Haney and Lydic 1999).
Coarse woody debris in forests that have been subject to stand replacing events without the removal of logs (e.g., tornados) show a marked spike in CWD volume that decreases with time as decomposition proceeds, and eventually plateaus when background inputs from mortality approximate decomposition (Harmon 2009). Forests subject to logging, either as the primary disturbance or through post-disturbance salvage logging, will similarly exhibit a spike in CWD from logging slash and other residue. However, due to the absence of large decomposing boles, decomposition of the smaller diameter slash will be more rapid and result in a period of very low total CWD until trees grow large enough to provide significant CWD inputs (Spetich et al. 1999).
Coarse woody debris is involved in many ecological processes, including energy flow, nutrient cycling, soil and sediment transport, moisture retention, and providing habitat for a wide array of species, including arthropods, birds, small mammals, herptiles, fungi, and microorganisms (Harmon et al. 1986; Goodburn and Lorimer 1999; McGee et al. 1999; Muller 2003).
The comparatively warmer upper surface, cooler underside, and relative stability of internal moisture and temperature conditions provided by CWD allow for a variety of herpetofauna to utilize CWD for a number of important life history activities, including thermoregulation (both warm and cold-season), avoidance of desiccation, predator avoidance, and successful egg laying and hatching (Whiles and Grubaugh 1993).
At least 55 mammal species use downed logs in the southeastern U.S., and CWD may be critical habitat for some small mammals including shrews (Loeb 1993). Logs are used as travel corridors and provide cover for predator avoidance, and, by providing habitat for macroinvertebrates and fungi, are important for feeding and foraging. Several mammal species also use CWD for nesting and denning, including striped skunks (Mephitis mephitis), gray fox (Urocyon cinereoargenteus), weasels (Mustela spp.), black bears (Ursus americanus), and a variety of mice (Mus spp.) and shrews (Soricidae) (Harmon et al. 1986; Wathen et al. 1986; Loeb 1993).
Few birds use downed CWD, with the notable exception of ruffed grouse use of logs for “drumming” (Gullion 1967; Harmon 1986). CWD is also important habitat for a wide array of micro- and macroinvertebrates and fungi that both provide food for a number of taxa and play vital roles in forest nutrient and energy cycling (Harmon et al. 1986; Hanula 1992; Johnston and Crossley 1993).
Large-Diameter Snags
Large-diameter snags (standing dead trees) are frequently missing from young and maturing second-growth forests, but are often typical of old-growth forests (Goodburn and Lorimer 1999; McGee et al. 1999), excepting for low productivity forests where tree diameter may be truncated by edaphic or other conditions. Some studies have found larger frequencies of small-diameter snags in younger forests, most likely related to density-dependent mortality from competition during stem exclusion and demographic transition phases stand development (McComb and Muller 1983; Goodburn and Lorimer 1999; Frelich 2002). The total density or volume of snags can be similar in old-growth and second-growth forests, but this is often due to either residual trees remaining from past partial harvests or the cumulative basal area of smaller snags in the younger forests (McComb and Muller 1983; Goodburn and Lorimer 1999; Hale et al. 1999).
Cavity Trees
Related to snags are cavity trees. While snags are more likely to have cavities than live trees, the latter typically provide more cavities in a forest because live trees are much more frequent (Goodburn and Lorimer 1999; Fan et al. 2003). Cavity formation often occurs through a succession of dead wood utilization by a variety of taxa. Heart rotting fungi create conditions that facilitate wood-eating insects and other fungi, which further provide food for a range of vertebrates. As the wood softens, primary cavity species, usually cavity nesting birds, excavate an initial cavity for use, while secondary cavity species, including birds, bats, squirrels, bees, chipmunks, raccoons, and other taxa, use or enlarge existing cavities (Harmon et al. 1986; Gysel 1961).
Old-growth forests tend to have more cavity trees, and substantially more cavities in trees of larger size classes, than younger forests (Fan et al. 2003; 2005). Large tree cavities are important as the initial diameter of the tree and cavity can be a limiting factor for some cavity nesting birds and other taxa. The greater number and range of sizes of cavities may be why old-growth forests, in general, have a greater number of cavity nesting birds than their younger counterparts (Harmon et al. 1986; Haney and Lydic 1999). Large diameter cavities around 100 cm DBH and greater, which are typically absent in younger forests, have been found to be preferred den sites for black bears (Ursus americanus) in the Southern Appalachians, suggesting the importance of old forests for this species (Wathen et al. 1986, White et al. 2001).
Uneven Age Distribution
Trees in old-growth forests often follow a multi-age distribution, with recruitment either continuous or occurring through multiple recruitment events, or both, depending on the spatial scale under consideration. An uneven-aged distribution results when stand development proceeds in the absence of major disturbance, with tree mortality occurring individually or in small groups (Oliver and Larson 1996; Frelich 2002).
Trees in Multiple Size Classes and the “Reverse-J” Diameter Distribution
Diameter distributions in old-growth forests typically follow a “reverse-J,” roughly inverse exponential distribution, where a large frequency of small diameter trees tapers off to an increasingly lower frequency of large diameter trees, and plot on a log scale as a straight line (Frelich 2002; Gove et al. 2008). Some old-growth forest and other uneven-aged forests have been found to exhibit a “rotated sigmoid” distribution, where the diameter distribution has a hump or plateau in the mid-diameter range (Gove et al. 2008). It has been suggested that this distribution reflects past intermediate-scale disturbance in the stand (Lorimer and Frelich 1984; Leak 1996). While many old-growth forests follow the reverse-J distribution, some even-aged second-growth forests have been found to similarly follow this pattern (Goodburn and Lorimer 1999).
Multi-layered Canopy
Generally speaking, canopy stratification describes the relative vertical distribution or layering of trees within the forest canopy (Parker and Brown 2000). The vertical and horizontal structure of the canopy, together, are important determinants in growing space availability and light penetration through the canopy (Jennings et al. 1999). Old-growth forest and other uneven-aged forests tend to have greater stratification of their canopies contributing to their greater structural diversity over younger even-aged forests (Frelich 2002).
Large Overstory Basal Area
The basal area (BA) of a stand is the sum of cross-sectional areas of all trees at 1.3 m, or breast-height, over a given area and expressed in m2/ha (or ft2/ac in American forestry). Basal area tends to increase with stand maturity and inversely with stand density, and can vary considerably by forest type with drier or more disturbance-prone forests having lower basal areas than more productive and sheltered forests. Martin (1992) provided a lower threshold for old-growth forests of 25 m2/ha based on values from Lilley Cornett Woods, where BA values ranged from 20.6 to 42.4 m2/ha across all communities. However, total forest BA for mature, and even young, second-growth forests sometimes falls within this same range (Goebel and Hix 1996; Hale et al. 1999).
Overstory Density Approximately 250 Stems/ha
Stem density tends to decrease with age as a function of stand development as basal area is redistributed to increasingly larger diameter trees. Martin (1992) proposed 250 stems/ha > 10 cm DBH as a threshold for old-growth forests based on values ranging from 160 to 315 stems/ha at Lilly Cornett Woods. Parker (1989) found similar values for old-growth throughout the eastern hardwood region ranging from 161 to 427 stems/ha. However, Hart et. al (2012a) found 620 stems > 10 cm DBH/ha in an oak-pine upland forest at Savage Gulf, an old-growth forest on the Cumberland Plateau in Tennessee, and unpublished data from a 2010 inventory of Lilley Cornett Woods found a density of 536 trees > 10 cm DBH/ha (McEwan and Richter 2010), calling into question the usefulness of this metric for assessing old-growth condition.
Herbaceous diversity
Herbaceous diversity may be greater in old-growth forests (Martin 1992), with incomplete recovery in second-growth stands over the historical period (Duffy and Meier 1992). Several factors may contribute to diminished herbaceous diversity following logging, including many species’ short dormancy and consequent lack of persistence in the seed bank, limitations on dispersal (with forest herbs often clonal, gravity-dispersed, or ant-dispersed), inability to compete with r-selected plant species, and changes in microhabitat, among others (Meier et al. 1995; Whigham 2004). However, for considerations of herbaceous diversity in comparisons of forests, differences in community type need to be taken into consideration and not be confounded with differences related to stand age (Harrelson and Matlack 2006).
Pit and Mound Topography
When a large tree falls, its root mass is usually pulled from the soil and lifted perpendicular to the ground along with humus, mineral soil, and rock fragments (Schaetz et al. 1989). The resulting formation is referred to as a “tip-up mound” or, at a larger scale, “pit and mound topography,” and can be an indicator of old-growth forests. The process occurs relative to disturbance frequency, with pits and mounds often evident for centuries after formation (Peterson and Campbell 1993). Tip-up mounds can be missing from second-growth forests due to removal as part of agricultural use prior to abandonment (Whitney 1994). They may also be infrequent as a result of logging alone, where trees large enough to leave substantial tip-up mounds have been missing from the forest during stand development and recovery, creating a lapse in formation. However, there can be a great deal of variability depending on site-specific conditions and history.
Tip-up mounds are important because they create varying moisture, temperature, and nutrient conditions, including the exposure of bare mineral soil, that can affect species richness and distribution by allowing for a diversity of microsites for seedling germination (Schaetzel et al. 1989; Peterson and Campbell 1993; Clinton and Baker 2000). When considered as an ongoing process over the course of millennia, tree uprooting may have important consequences for soil structure, the mixing of soil horizons, and soil carbon and nutrient dynamics.
Canopy Gaps
Gaps in the canopy created by mortality or blow-down of individuals or small groups of trees is a characteristic strongly associated with old-growth forests (Runkle 1985; Martin 1992). The pattern of gaps reflects a history of small-scale disturbance and relates to the development of uneven-aged canopy distributions, canopy layering, coarse woody debris, and other structural elements (Runkle 1985; Frelich 2002). Canopy gaps and gap dynamics are discussed more thoroughly later in this document.