It is often useful to categorize species by their predominant strategy of regeneration. These categories are often called regeneration guilds or regeneration groups. Although placement of species into categories will be somewhat artificial (most species exhibit many combinations of reproductive strategies), these categories help silviculturalists design efficient regeneration treatments.

Spanning the range from pioneer to extremely shade-tolerant, Appalachian hardwoods represent most of the reproductive strategies possible in trees. Two systems of categorization are summarized in the following sections:

Kelty (1988) reviewed the reproductive characteristics of the more important Appalachian hardwood species and divided them into three basic regeneration guilds:

1. Pioneer Species
   Yellow-Poplar, Sweet Birch, Black Locust
2. Advance-Growth-Dependent Species: Moderate Shade Tolerance
   Black Cherry, White Ash, Basswood, Red Maple, Sweet Birch, Oaks, Hickories, Black Walnut
3. Advance-Growth-Dependent Species: Extreme Shade Tolerance
   Sugar Maple, Beech, Hemlock

Sutherland and others (2000) classified 62 tree species in the central hardwoods region with respect 16 regeneration attributes that describe flowering, seed production and dispersal, dormancy, germination environment, seedling characteristics, and vegetative reproduction using multivariate techniques. Their analysis resulted in nine guilds arranged into three regeneration strategies:

1. Pioneer
   Spring-dispersed, moist site intolerants; spring-dispersed, moist site tolerants; dry site intolerants
2. Opportunistic
   Long-lived intolerants and intermediates; fast-growing understory tolerant; dispersal-limited (large-seeded or sprout dependent)
3. Persistent
   Large-seeded, advance growth dependent; slow-growing, understory tolerant

Yellow-Poplar, Sweet Birch, Black Locust

The only species of the Appalachian hardwoods that truly fits this description is yellow-poplar. Both yellow-poplar and sweet birch have a number of the traits common to pioneer species that enable them to invade open sites. They both produce seed abundantly, with good seed years occurring frequently. Heavy birch seed crops are produced every 1 to 2 years (BROKEN-LINK Fowells 1965), with heavy crops of yellow-poplar being described variably as nearly annual (Beck and Sims 1983) to somewhat irregular (BROKEN-LINK Fowells 1965). Seed is dispersed by wind, with yellow-poplar being blown for 4 to 5 tree heights; the small seeds of sweet birch can probably be blown even further. In the case of yellow-poplar, seed remains dormant in the forest floor for at least 3 years in abundance, and some remains viable up to 8 years (Clark and Boyce 1964). The dormancy of sweet birch seed is not known precisely; most germinates after the first winter, while some may overwinter perhaps for several years (BROKEN-LINK Fowells 1965; Beck and Hooper 1986). Exposure of mineral soil aids the germination of both species, especially for birch because of the small seed size. Both have very fast juvenile height growth rates and can outgrow most competitors when growing in full sunlight. However, the ability of yellow-poplar to compete with other vegetation is affected by the timing of germination. If cutting occurs by mid-spring at the latest, yellow-poplar will germinate early enough to compete successfully with the flush of weeds that also invades the site. If a cutting takes place in the late spring or summer, yellow-poplar germination is delayed and on some sites the seedlings will not be able to grow fast enough to compete successfully (Trimble and Tryon 1969).

Although black locust lacks some of the characteristics of seed production and dispersal common to pioneer species, it fits best in this category. This is because it generally reproduces by root suckering from a cut or injured tree (BROKEN-LINK Fowells 1965). Locust root suckers are very intolerant and grow rapidly in height, so they compete well in full sunlight. Thus, black locust acts as a pioneer, except that it cannot readily be dispersed onto a new site if it does not already exist there.

The pioneer status of these three species is most apparent following clearcutting. Seedlings of most species decline in numbers following clearcutting, as smaller advance-growth seedlings die from exposure, but yellow-poplar, sweet birch, and black locust increase in numbers and dominate in height development (McGee and Hooper 1970; Beck and Hooper 1986). Only stump sprouts of other species can match the height growth of these species when growing in full sunlight.

Yellow-poplar and sweet birch both have alternate modes of reproduction. Yellow-poplar can germinate beneath a partial overstory and exist for a short period as an advance-growth seedling, although this process is not crucial to its establishment as it is for many other species. Sweet birch is unusual for a species exhibiting many pioneer characteristics, in that it is tolerant of shade as a seedling, and can regenerate beneath a fairly dense overstory canopy. Both species can also sprout from seedlings or stumps.

Excerpted from: Kelty 1988
Last updated: 10/3/01

Black Cherry, White Ash, Basswood, Red Maple, Sweet Birch, Oaks, Hickories, Black Walnut

The majority of Appalachian hardwood species fall into this category. The ecological term "gap-phase" that can be applied to these species refers to the usual mode of reproduction in unmanaged forests where seedlings are established beneath small gaps in the canopy caused by windthrow or other natural disturbance. They survive in the understory as advance regeneration until another disturbance enlarges the gap and releases them. The overstory shade is necessary with these species to moderate the temperature extremes and droughty conditions that occur in the upper soil layers in exposed conditions. These kinds of disturbances can be duplicated in shelterwood cuttings, but may occur following thinnings or other partial cuttings as well. These species can be thought of as advance-growth-dependent (even though they do not actually rely completely upon this pathway) to emphasize the fact that abundant regeneration is dependably obtained only from vigorous advance growth. The species in this group range from intolerant to tolerant, with only the extremes (very intolerant and very tolerant) being excluded. There is considerable variation in details of regeneration mechanisms within this category, and three subgroups can be recognized.

Black Cherry

The first subgroup consists only of black cherry, which is the closest of the advance-growth-dependent species to having pioneer characteristics. Frequent cherry seed crops occur every 3 to 4 years (BROKEN-LINK Fowells 1965), and the seed can remain dormant in the litter for 3 years or more (Trimble 1975). Seed dispersal is by birds, so it is more limited than wind-dispersed pioneer species. This species also differs from a true pioneer in that germination is greatly enhanced by partial shade. Unlike yellow-poplar, black cherry seedlings become established in much greater numbers beneath a canopy as compared to open conditions.

In Allegheny cherry-maple stands, the greatest numbers of cherry seedlings become established beneath a fairly heavy overstory (approximately 2/3 of full canopy) (Marquis 1979). These seedlings are intolerant of shade and can survive only a few years beneath a dense overstory, but they are able to respond to overstory removal when fairly small. Seedlings smaller than 2 inches in height do not survive well after overstory removal, but those that have reached 6 inches in height and larger have good survivorship and grow very rapidly in height upon release (Marquis 1982). In contrast, black cherry is sometimes able to become successfully established following clearcutting; this was found to be the case on Appalachian mixed hardwood sites in West Virginia (Trimble et al. 1986). In these cases, most germination is probably from dormant seed stored in the forest floor.

White Ash, Basswood, Red Maple, Sweet Birch

The next subgroup includes white ash, basswood, and red maple. Seeds of ash and maple are wind-dispersed for a moderate distance, but basswood seed dispersal is very limited. Scarification is beneficial for the moderate-sized seeds of these species, but is not critical for germination. Basswood produces good seed crops every 1 to 2 years, with white ash being more irregular. Seed of both can remain dormant up to 3 years in the litter (Trimble 1975). Nearly all red maple germinates the first season, but this is not especially limiting to reproduction because large crops are produced almost every spring. These species can sometimes invade open sites under the right set of conditions of site quality and seed source, but this generally does not provide dependable regeneration. Dense reproduction is only obtained beneath the shelter of a partial overstory. In general, small advance-growth seedlings of these species do not respond as well to release as black cherry. Marquis (1982) found that survival was good for red maple and white ash seedlings at least 6 inches tall, but recommendations based on research with many species in northern hardwood forests are that seedlings should be a minimum of 2 feet tall in order to dependably show good survival and response (Tubbs 1977).

In addition to behaving as a pioneer species, sweet birch can be included with these species because it is tolerant of shade (similar in degree to red maple and basswood) and can become established under a fairly dense overstory. It often germinates on decaying stumps and logs, and thus is not limited by the lack of a mineral soil seedbed on relatively undisturbed sites.

Oaks, Hickories, Black Walnut

The third subgroup consists of species with heavy, animal-dispersed seed--the oaks, hickories, and black walnut. Regeneration of these species is more irregular and difficult than of any other group. Seed production of black walnut and hickory is somewhat irregular, with good seed crops occurring every 2 to 3 years. Good acorn crops are produced very sporadically in all the important oak species, occurring at 2 to 10 year intervals (Watt 1979). During years with poor seed production, predation by insects and mammals may eliminate the entire crop (Watt 1979). The seed of these species is dispersed only as far as small mammals carry them, and best germination occurs when seeds are buried beneath the litter, bringing them in contact with mineral soil.

Of the "gap-phase" species, the oaks are the most dependent upon the advance regeneration pathway. Oaks have slow initial height growth even with full sunlight, because much of the early growth goes into establishing the root system. Thus, they do not compete successfully with many intolerant species. Because they are only moderately tolerant of shade, the seedling shoots cannot survive for long periods beneath an overstory. However, the oaks have the ability to produce sprouts repeatedly when the shoot dies back or is damaged, so they can persist for extended periods in the understory as seedling sprouts (Watt 1979). As with other species, oak advance-growth seedlings or seedling sprouts respond better to release from overstory shading if they are large and vigorous. The minimum size is greater for the oaks than for other species, although; only those that have reached a height of 4.5 feet or greater have a high probability of surviving and competing well with other vegetation upon release (Sander 1972). Furthermore, seedlings with flat-topped crowns do not show rapid growth compared with more vigorous stems having definite recognizable leaders (Carvell 1979). Obtaining this large, vigorous, oak advance growth is the major stumbling block in oak regeneration. Increasing the light levels reaching the understory is the major way of stimulating height growth of oaks, but this usually produces conditions that are also favorable to many faster growing competing species.

Less information on natural regeneration characteristics is available for hickory and black walnut, because hickory is not a highly desired species, and black walnut is so desirable that most research has dealt with plantation management. In general, these species have many of the same root and shoot growth characteristics as oak. Black walnut is even less tolerant, so it is necessary to provide full or nearly full sunlight when seedlings are young. However, walnut also suffers from competition with many faster growing species when growing in high light conditions, especially on poor sites. Also, mature stands usually contain only scattered walnut trees, so the seed source for natural regeneration is often poor. Natural walnut regeneration is thus very unpredictable, requiring just the right combination of seed source, moderate-sized canopy gaps, and sufficiently rich sites.

Sugar Maple, Beech, Hemlock

Two of the Appalachian hardwood species--sugar maple and beech--and one important conifer associate--hemlock--are tolerant enough to become established and survive for considerable periods beneath intact overstories. They achieve this by different mechanisms. Overstory sugar maples are good seed producers, with heavy crops occurring at 2- to 5-year intervals (Fowells 1965), and the large seeds can become established on undisturbed leaf litter. Seedlings cannot exist indefinitely under a completely closed canopy, but new establishment is frequent enough where a seed source exists that there are generally abundant seedlings in the understory. Beech often reproduces by root suckering, sometimes producing dense understory thickets. They can persist for many years at low light levels and still respond vigorously to release.

The small seeds of hemlock require either exposed mineral soil or the moist conditions of decaying logs or stumps for germination. Hemlock is so tolerant that it can persist for decades with very little growth under low light conditions. The adequacy of seed source can be deceptive for these species because they often occur primarily in the lower crown classes of mature stands, with faster growing species in the upper canopy. Trees in the lower canopy positions are generally not vigorous enough to produce seed, so the mere presence of these species in a mature stand does not assure adequate seed production (Marquis 1979).

Advance-growth seedlings of sugar maple, beech, and hemlock can respond to release from overstory shading if they meet the 2-foot minimum height applicable to other species, but their height growth is so slow that they will be surpassed in height by many other species. They will still survive because of their great tolerance of shade, but will be relegated to the lower crown classes of the new stand. If they are to reach the overstory at heights comparable to competing intolerant and midtolerant species, they must be of sapling size or greater as advance growth, so that they have a considerable headstart on the other species (Marquis 1981, Kelty 1986).

Pioneer guilds contain species that regenerate where disturbances are either frequent or severe. Seasonally flooded areas, abandoned fields or areas subjected to high-intensity fires are most likely to be colonized by pioneer guilds. Seeds come from outside the disturbed site or are stored on site as buried seed or in an above-ground seed bank. (Sutherland and others 2000)

Spring-dispersed, moist site intolerants

Betula nigra, Populus deltoides var. deltoides, Populus grandidentata, Populus tremuloides, Salix nigra

Short-lived pioneers that colonize sites rapidly and are too shade intolerant to replace themselves These species are closest to the traditional definition of pioneers: they mature early, have rapid growth rates and are generally short-lived. Seed crops are copious and widely dispersed by wind; regeneration potential is limited mainly by the conditions necessary for germination and establishment. Full sunlight and mineral soil seedbeds are essential for successful germination and establishment. (Sutherland and others 2000)

Spring-dispersed, moist site tolerants

Acer rubrum, Acer saccharinum, Ulmus americana, Ulmus rubra, Ulmus thomasii

These species have many pioneer characteristics but they are neither as short-lived nor as intolerant as those intolerant. These trees are common early successional species that establish in a variety of situations as long as sufficient light is available to maintain growth. persistent pioneers that colonize frequently disturbed sites more slowly and are shade tolerant. (Sutherland and others 2000)

Dry site intolerants

Juniperus virginiana, Pinus echinata, Pinus rigida, Pinus virginiana

This group includes highly competitive intolerants that colonize dry uplands and abandoned fields. Species begin seeding early, but unlike trees in the other pioneer guilds, seed crops are infrequent and dispersal is limited to nearby areas. Although these species may establish and survive under pine or hardwood canopies on dry or nutrient poor sites, regeneration is most favored where disturbances create large openings. (Sutherland and others 2000)

Opportunistic guilds contain species that use several reproductive mechanisms and thus are capable of regenerating following a range of disturbance types. Frequent seeding and extended seed viability ensure that seeds are continually available; sprouts from damaged individuals may also provide an additional source of propagules. Shade tolerant opportunists maintain seedling or sprout banks that readily respond to canopy openings. (Sutherland and others 2000)

Long-lived intolerants and intermediates 

Betula alleghaniensis, Betula lenta, Juglans cinerea, Juglans nigra, Liquidambar styraciflua, Liriodendron tulipifera, Pinus strobus,
Magnolia acuminata, Platanus occidentalis

This group includes some of the fastest growing species in the central hardwood forest. They are commonly found on mesic sites and are long-lived and highly competitive. Once they are established they are capable of occupying a site for a long time. Large canopy openings and disturbed seed beds favor germination and all but Pinus strobus are vigorous sprouters. Platanus occidentalis, Juglans spp. and Betula spp. reproduction depends on continued high light levels for successful recruitment into the canopy. Seedlings and sprouts of the other species are more tolerant of overhead shade and new cohorts may survive for several years in the understory. (Sutherland and others 2000)

Fast-growing understory tolerant 

Acer negundo, Celtis occidentalis, Cornus florida, Fraxinus americana, Fraxinus nigra, Fraxinus pennsylvanica, Morus rubra, Prunus serotina

This guild contains the more shade tolerant opportunistic species. They have abundant seed available for germination because good seed crops are produced frequently and seeds remain viable for several years. Understory light levels are sufficient for germination and reproduction quickly becomes sapling-size. Although saplings may persist for several years in the shaded understory they soon must be released by some form of canopy opening in order to survive. Some of these species may form a subcanopy layer that either persists or gradually dies out depending on overstory composition.  (Sutherland and others 2000)

Dispersal-limited (large-seeded or sprout dependent) 

Aesculus glabra, Aesculus octandra, Cercis canadensis, Diospyros virginiana, Gleditsia triacanthos, Nyssa sylvatica var. sylvatica, Robinia pseudoacacia, Sassafras albidum

This guild contains species with very different successional roles. For example, Sassafras albidum and Robinia pseudoacacia are old-field pioneers whereas Aesculus octandra and Diospyros virginiana are understory tolerants. Although some species fit the profiles of other guild types, they were considered opportunistic because they produce seeds frequently and germination occurs on a wide range of seed beds. Established individuals have fast growth rates in open conditions (Diospyros virginiana is an exception), and all but Sassafras albidum and Gleditsia triacanthos are tolerant of shade. Seeds are large and have a limited dispersal range, but colonization of new areas is possible because birds, small rodents or livestock often act as dispersal agents. Large seeds also provide food reserves that allow new seedlings to establish on a broad range of sites. Survival rates are usually high because tap roots or wide-spreading root systems support vigorous growth. Root suckering is also common in this guild and many new cohorts of Sassafras albidum, Robinia pseudoacacia and Gleditsia trzacanthos arise on or adjacent to previously occupied sites. The buckeyes (Aesculus spp.) have the most limited regeneration potential in this guild. Their large seeds quickly lose viability if they are not kept moist so germination is restricted to mesic, litter-covered sites.  (Sutherland and others 2000)

 

 

Persistent guilds contain species that develop and maintain advance regeneration. Many persistent species have large seed reserves that enable germinants to establish in shade or on dry sites. Large cohorts often develop as a result of mast seeding or other occasional events (e.g., wet years). Species with moderate understory tolerance may survive with a cycle of die back and resprouting, whereas more tolerant species maintain seedling or sapling banks. Small canopy gaps maintain the regeneration pool for these species, but individuals do not enter the overstory until more significant canopy disturbances occur. (Sutherland and others 2000)

Large-seeded, advance growth dependent 

Carya cordiformis, Carya glabra, Carya laciniosa, Carya ovata, Carya tomentosa, Quercus coccinea, Quercus palustris, Quercus rubra, Quercus velutina, Quercus alba, Quercus bicolor, Quercus macrocarpa, Quercus prinus, Quercus stellata

Oaks and hickories develop and maintain "advance growth" regeneration. New cohorts arise from established seedlings or sprouts that are released by canopy openings: Seeds germinate well in the understory, but seedlings have intermediate shade tolerance so survival under a closed canopy is limited to a few years. Seedlings actually persist as "seedling sprouts" with repeated episodes of dieback and resprouting. In spite of the large, gravity-dispersed seeds, this guild often colonizes new areas with seeds cached and forgotten by birds and rodents. Vigorous sprouts also develop from cut or damaged stems. Seedling sprouts or other sprout origin stems: exhibit fast height growth which makes this group more competitive in canopy openings than the other persistent species. Hickories and white oaks are slow-growing and more shade tolerant than red oaks. Hickory seeds germinate in the spring following a period of cold stratification unlike white oak. (Sutherland and others 2000)

Slow-growing, understory tolerant

Acer nigrum, Acer saccharum, Carpinus caroliniana, Fagus grandifolia, Ostrya virgimana, Oxydendrum arboreum, Tilia americana, Tilia heterophylla, Tsuga canadensis

This is a highly versatile group. Good seed crops are produced at regular intervals and a variety of seedbeds are suitable for germination. Seedlings grow slowly and survive in the shaded understory for many years. As small canopy gaps form, individuals are gradually recruited into the overstory. Seedling or sapling banks are common and Ostrya virginiana and Tilia spp. also maintain a seed bank. (Sutherland and others 2000)