The Longleaf Pine Ecosystem
In pre-colonial times, longleaf pine (Pinus palustris Mill.) forests covered nearly 60 million acres of land in the southeastern United States, from southeast Virginia to central Florida, and west to the Gulf region of Texas. Since European settlement the longleaf pine resource has steadily declined. In 1935, 20 million acres were found throughout the South (a 70% reduction), while in 1994 only 3.3 million acres remained (a 95% reduction; Barnett and Dennignton 1992; Boyer 1990; Haywood and Grelen 2000; Jose, Jokela, and Miller 2006; US Forest Service 1999).
The decline in the longleaf pine resource can be attributed to several factors. First, extensive harvesting, particularly between 1900 and 1930, greatly reduced the area occupied by mature longleaf pine (Brown and Nowak 2013) and, hence, the availability of seed sources. Therefore, natural regeneration was both scarce and unpredictable. Second, a lack of understanding of the biological requirements of this species and seedling predation from feral hogs (Wahlenberg 1946) led to the failure of many regeneration efforts. Finally, and perhaps most importantly, many sites were converted to other commercially valuable species (mainly loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii var. elliottii Engelm) or to other land uses such as agriculture (Haywood and Grelen 2000; Wahlenberg 1946).
As a result of this decline, much of the interest today in the restoration of Florida's native ecosystems has focused on longleaf pine. Of the 17.3 million acres of forested land in Florida, there are 2.1 million confirmed acres of longleaf pine. In other words, approximately 12% of Florida's forest area already includes a longleaf pine component (FDACS 2017), which can be beneficial for restoration efforts.
Longleaf pine forests today are managed for a variety of desired benefits including aesthetics, habitat diversity, and quality timber production. The longleaf pine forest is one of the most diverse ecosystems in the world, outside of the tropics. The high-quality timber produced in longleaf pine forests generally has a higher stemwood specific gravity than timber from either loblolly pine or slash pine. The timber from mature longleaf pine trees is strong, rot-resistant, and knot-free (Platt and Rathbun 1993; Schmidtling 1986). Longleaf pine may be a suitable alternative to loblolly pine and slash pine on difficult sites, such as excessively well-drained sandhill soils that do not support acceptable growth rates for these other southern pine species. In addition, longleaf pine is generally considered to be less susceptible (not immume) to southern fusiform rust and therefore a desirable species to plant on high rust hazard sites (Schmidtling and White 1989). However, as interest increases in restoring longleaf pine forests, region-wide screening of longleaf pine seed sources for genetic resistance and susceptibility to fusiform rust resistance and other pests has been advocated (Barnard and Mayfield 2009).
Non-industrial private landowners own approximately 817,000 acres (38%) of the longleaf pine forests in Florida (FDACS 2017), so their management strategies have a large impact on the status of the resource. For non-industrial private landowners whose main land management objective is something other than timber production (i.e., aesthetics, recreation, wildlife habitat, soil or watershed protection), alternative silvicultural systems provide an opportunity to create diversity on their property. The objective of this bulletin is to describe an alternative to even-aged management for longleaf pine in Florida. The alternative approach uses a modified group selection system to create an uneven-aged stand structure, which will provide income from periodic timber harvests, while maintaining continuous tree cover. A discussion of traditional and alternative longleaf pine management practices follows, as well as a description of the process of converting an even-aged stand of mature longleaf pine to an uneven-aged stand.
Management Options for Longleaf Pine
Longleaf pine has traditionally been managed using even-aged silvicultural systems that rely on either natural or artificial regeneration. With this approach, single age class stands are managed over a fixed period of time (rotation) using intermediate cutting techniques (e.g., thinnings) to regulate stand density, control species composition and improve stand vigor and health. Examples of even-aged silvicultural systems used with longleaf pine include the clearcut and shelterwood systems. In the clearcut reproduction system, all mature crop trees are harvested simultaneously at the end of the rotation. The site is then regenerated using either natural or artificial regeneration. The shelterwood reproduction system uses two partial harvests to reduce the basal area of the stand to about 30 ft2/acre (the density which maximizes seed production of longleaf pine). Seed from the remaining mature trees is used to naturally restock the site, assuming a mineral soil seedbed is provided. Once regeneration is established, the parent overstory trees are removed. Both the clearcut and the shelterwood systems have been shown to be effective even-aged systems for regenerating and managing longleaf pine (Croker 1979; Croker and Boyer 1976; Demers and Long 2000).
The even-aged approach to management offers a variety of advantages. For example, even-aged management is easy to apply and intermediate cuttings and other cultural practices (i.e. herbicide application) can be applied uniformly across the entire stand (Barnett and Baker 1991). Even-aged systems produce trees that are fairly uniform in size, which simplifies harvesting operations. There are, however, some disadvantages to even-aged management. The uniform stand offers fewer aesthetic, recreational, and wildlife benefits and may not produce significant income for 15 to 20 years (Barnett and Baker 1991).
On public lands, however, there is an increasing interest in developing uneven-aged (multi-aged) stands of longleaf pine. In Florida, the US Forest Service is converting over 250,000 acres of even-aged longleaf pine and mixed longleaf pine-slash pine stands to an uneven-aged structure, using a modified group selection system. While even-aged management systems create stands with one age class of trees, and a diameter distribution that follows a smooth bell-shaped curve, uneven-aged management systems create stands that contain at least three distinct age classes. The diameter distribution of an uneven-aged stand follows a reverse J-shape (Figure 1).
The canopy profile of an even-aged stand is fairly uniform, but the profile of an uneven-aged stand is varied, with trees representing the spectrum from tall canopy dominants to seedlings and saplings (Figure 2).
If the stand is a balanced uneven-aged stand, the ground area occupied by each age class is approximately equal.
Uneven-aged stands are created using the selection system. In the selection system, trees representing a range in size are harvested at fixed intervals (called the cutting cycle, which ranges from 10 to 25 years). Regeneration (either natural or artificial) occurs in the harvested openings. This management approach allows periodic harvests, while maintaining a continuous forest cover. Smaller, lower quality trees are also removed to improve the overall quality of the stand.
There are two main variations of the selection system. The first, referred to as the single tree selection system, involves removal of individual trees across the stand, creating small openings in which species can naturally regenerate (Figure 3).
The second type of selection system is referred to as group selection. In this system, groups of trees are removed in patches (ranging from 0.2 acres to 5 acres in size) throughout the stand (Figure 3), allowing for potential regeneration of the more shade intolerant species such as longleaf pine (provided seedbed requirements are met; Brockway and Outcalt 1998; McGuire, Mitchell, Moser, et al. 2001; Marquis 1978; Matthews 1989). Canopy openings larger than 2 acres may also be referred to as patch clearcuts. However, for the purposes of this paper, they will be referred to as gaps. In both single tree and group selection, canopy openings can be regenerated using either natural or artificial means. These harvested groups mimic canopy gaps created by natural biotic and abiotic events (e.g. insects, windthrow, lightening). Generally, the group selection method has lower logging costs than the single-tree selection management system.
There are numerous advantages to uneven-aged management (using both the single-tree and group selection systems). Stands with a variety of age classes are less likely to suffer catastrophic damage from insect outbreaks, disease, or low intensity fire because, usually, only one age class of trees is susceptible to any one damaging agent. For example, longleaf pine seedlings are susceptible to brown spot needle blight, but saplings and mature trees are not. A brown spot needle blight outbreak in an uneven-aged stand may result in high seedling mortality, but trees in the other age classes will not be damaged. Another advantage of uneven-aged management is that periodic income can be obtained from high-valued products at relatively short intervals over time. Also, due to the continuous presence of large trees (i.e., the entire stand is never totally liquidated), the selection system creates stands with high aesthetic values and diverse wildlife habitat (Barnett and Baker 1991; Farrar and Boyer 1991), while providing excellent site protection. However, both single-tree and group selection systems may involve higher logging costs than even-aged systems, and are sometimes difficult to apply (Barnett and Baker 1991). Also, without careful attention, there is potential for substantial losses due to damage to the residual stand during harvesting operations.
Because longleaf pine is intolerant of both shade and competition from mature overstory trees, openings created in closed-canopy forests using the single tree selection system may not be large enough for adequate natural regeneration to become established, unless those openings are expanded over time. Seedlings in small openings (<1/3-acre) compete directly with adjacent mature trees for limited site resources (i.e., water, mineral nutrients; Farrar and Boyer 1991). Fine root competition from mature residual trees on good quality sites may extend 50 feet into openings, and may be even higher on poor sites (Boyer 1993). Any established longleaf pine regeneration in these small openings would have slow growth rates (Farrar and Boyer 1991), and may suffer higher mortality from periodic prescribed burning (Gagnon, Jokela, Moser, et al. 2003) than young trees in larger openings.
In spite of these difficulties, however, the single-tree selection system has been used successfully in some open-canopied longleaf pine forests. In fact, gaps as small as ¼ of an acre can be large enough for seedlings to reach maximum growth (Landers, Van Lear, and Boyer 1995), if the surrounding overstory is open enough to allow adequate light to reach the forest floor.
Selection silviculture methods may create an uneven-aged stand similar in structure to old-growth stands of longleaf pine, which are comprised of small, even-aged patches of trees. The diameter distribution of an old-growth longleaf pine forest is somewhat reverse J-shaped, with trees ranging from 3 to 500 years old (Palik, Mitchell, Houseal, et al. 1997).
Longleaf Pine Ecology
The first step in developing a viable group selection management system for longleaf pine involves a review and understanding of the ecology of the species. Specifically, the management strategy must match the habitat, reproduction, germination, and growth requirements of the species. Table 1 summarizes the biological factors that are important for regenerating and managing longleaf pine.
These biological requirements can be met using a modified group selection system. With this modified system, a reverse J-shaped diameter distribution is created and maintained by creating at least three distinct age classes within the stand using area regulation. The following example describes the process of converting an even-aged longleaf pine stand into a balanced uneven-aged stand structure using a modified group selection system.
Conversion of Longleaf Pine Stands from Even-Aged to Uneven-Aged
A hypothetical stand with the following characteristics will be used to illustrate the modified group selection system.
60-year-old naturally regenerated longleaf pine (currently on site)
Site index 70 (base age 50 years)
230 trees per acre, 114ft2/acre basal area (Smith, Larson, Kelty, et. al 1997)
Current standing volume = 16 thousand board feet (MBF - Scribner)/acre (US Forest Service 1929)
Product objective: Pole and sawtimber production (Dennington 1990)
Mature longleaf pine that surround regenerated openings do not have adverse affects on growth and survival of seedlings within the openings.
Trees over 45 years old exhibit a minimal growth response to thinning.
Damage to the residual stand (i.e., scarring and soil compaction) from harvests and thinning operations will be minimal.
Planting of longleaf pine seedlings will be used to regenerate the created gap openings.
Group selection openings will be harvested on a 15-year cutting cycle (Figure 4). Using a 45-year rotation, this amounts to harvesting fifteen acres per cutting cycle.
The fifteen acres will be allocated among five 3-acre gaps within the stand (Figure 5).
Since the age of the current, naturally regenerated stand is 60 years old (the average age of longleaf pine stands in Florida), the ages of the trees harvested in the first three cutting cycles will be 60, 75, and 90 years old, respectively. Once the stand is fully regulated, the difference between the oldest and the youngest trees in the artificially regenerated gaps will be 45 years. The artificially regenerated trees planted in the created gaps will be harvested at age 45. The rotation age for the naturally regenerated and the artificially regenerated trees differs because naturally regenerated stands require longer rotations to produce volumes of sawtimber comparable to that of planted stands (Dennington 1990). For example, at age 60, a naturally regenerated stand would be expected to yield 16 thousand board feet of timber (MBF)/acre (Scribner), while at age 45, a planted stand would yield about 22 MBF/acre (Scribner) (US Forest Service 1929).
A 15-year cutting cycle allows for adequate growth between harvests, to ensure operable volumes, and it falls within the recommended range of cutting cycles for this species. Sawtimber revenues occurring at 15-year intervals may be preferable to pulpwood revenues occurring at 20–25 year intervals. The large (3-acre) gaps that are created with this management system should reduce detrimental edge effects that may impact seedling survival and growth within the openings (Farrar and Boyer 1991, Outcalt and Outcalt 1994). Skid trails should be oriented in the direction of the planted rows and be as wide and straight as possible to facilitate future harvesting and thinning operations, and to minimize damage to trees in the residual stand. The size and the location of the gaps, and the positioning of the skid trails parallel to the planted rows, simplify both marking and timber harvesting operations.
Site disturbance resulting from harvesting should sufficiently scarify the soil for planting. If there is excessive woody vegetation in the openings, a prescribed surface fire may be used to reduce competing vegetation. Some additional site preparation (i.e., herbicide application) may also be required prior to planting in areas with heavy understory competition or oak (Quercus spp.) encroachment (Brockway and Outcalt 2000; Grace and Platt 1995).
Although longleaf pine can be regenerated by both natural and artificial means, artificial regeneration by planting is recommended for this modified group selection system because of the sporadic nature of longleaf pine seed crops, the bare mineral soil requirement for natural germination, and the heavy seed predation pressures commonly found with this species (Table 1). These three factors make natural regeneration of longleaf pine somewhat unreliable. Artificial regeneration also helps to ensure prompt and uniform seedling establishment (US Forest Service 1999). Spacing and density can be manipulated to meet specifications for desired products. When relying upon natural regeneration for longleaf pine, canopy openings are limited to about 0.7 acres to ensure adequate seedfall throughout the created gap (effective seed dispersal distance of longleaf pine is about 100 ft). However, there is no biological limit to the size of openings when artificial regeneration is used.
Another advantage of artificial regeneration is that it provides an opportunity to use genetically improved seedlings. Improved longleaf pine seed is being developed, which has higher survival rates and earlier emergence from the grass stage than unimproved seed sources (Schmidtling 1986).
Artificial regeneration of longleaf pine can result from direct seeding or planting of bare-root or containerized seedlings. Containerized seedlings are more expensive, but usually have higher survival rates due to decreased transplant shock, which results in a greater ability for the seedlings to compete with understory vegetation. In general, containerized longleaf seedlings are easier to plant, have an extended planting season, and they initiate height growth earlier than direct seeded or bare-root seedlings (Barnett, Lauer, and Brissette 1989; Boyer 1984; Boyer 1988). The high survival and rapid growth rates of containerized seedlings result in better yields, which in turn offsets the higher initial planting costs associated with their use. Containerized longleaf pine seedlings (minimum root collar diameter 0.4 in.) (Barnett, Lauer, and Brissette 1989) can be planted (either by hand or machine) at a density ranging from about 450 to 700 trees per acre (e.g. approximately 8 x 12 to 6 x 10 ft spacing). Staggered spacing can be used to create a more natural appearance (Figure 6).
Although containerized seedlings can be planted throughout the year, survival is best when they are planted under moderate weather conditions. The traditional planting season for longleaf pine in Florida occurs from November through February, although there has also been successful planting of containerized longleaf pine seedling in late summer. Additional information on planting Southern pines is available in a number of other extension publications (Croker and Boyer 1976; Dennington and Farrar 1983; Duryea 1998).
To provide more operable volumes and revenues, thinnings should be timed to coincide with group selection harvests. A low thinning (which removes smaller or poorly formed trees) beginning at about age 30 years is recommended for the remainder of the stand (residual basal area should be about 80 ft2/acre; Dennington (1990)).
Prescribed surface fires can be used on a two to three-year burning cycle to minimize fuel loads and to control understory and woody vegetation (Duryea 2000). Regular competition control is necessary to encourage growth and survival of longleaf pine seedlings. A three-year burn cycle is currently being used successfully on the National Forests in Florida to maintain longleaf pine-wiregrass (Aristida stricta Michx.) ecosystems (US Forest Service 1999). Although longleaf pine is fairly resistant to fire damage, the seedlings are susceptible to injury during the initial stages of active height growth. Therefore, fire should be postponed during this stage and not resumed until seedlings are over 3 feet in height. To minimize damage to longleaf pine seedlings, cool prescribed burns at this stage of development should occur during the winter months. When the trees are taller than 10 to 15 feet, low intensity burns could be prescribed in other seasons, assuming that fuel loads are low to moderate.
Using prescribed fire in the larger openings could be difficult in some cases. One of the primary fuels for prescribed fire is pine needles. Needle fall from surrounding trees may drop off significantly towards the center of the gaps. Depending on the type of understory vegetation, this could result in fire not carrying well across the opening. In these instances, chemical control of competing vegetation may be required.
The group selection system as described above would also have the same potential for landowners interested in converting loblolly or slash pine stands (natural or artificially regenerated) to longleaf pine (Franklin 2009; Hu, Knapp, Wang, et. al 2016). For example, rather than harvesting a mature slash pine stand all at one time and regenerating it to longleaf pine, a more gradual species conversion process could be prescribed using the group selection method in conjunction with planting longleaf pine in the group openings. An advantage of this approach would be that portions of the mature, overstory canopy would remain intact, thereby supporting other ecosystem benefits and services (e.g., mature stand structure attributes, aesthetics, wildlife habitat, understory community (Kirkman, Mitchell, Kaeser, et. al 2007), while also providing needle cast for fuel to carry prescribed fire through the area (Jack, Hiers, Mitchell, et. al 2010). More detailed information on this process can be found at: http://www.americaslongleaf.org/media/2625/converting_loblolly_to_longleaf.pdf (Franklin 2009).
There are many advantages associated with using a modified group selection system with longleaf pine. There are, however, a few potential problems with the system. The main disadvantage is that it has not been rigorously field-tested. Potential decreases in yields due to increased competition between the overstory and understory trees, as well as decreased volume production from periodic prescribed burning (Matthews 1989), may make this method less productive than even-aged management systems. Therefore, landowners primarily interested in commodity production may find this management approach less appealing than even-aged systems (McGuire, Mitchell, Moser, et al. 2001).
The group selection silvicultural system represents an uneven-aged management strategy useful for longleaf pine. Although traditional even-aged management systems will continue to be used with this species, the group selection system offers many additional potential advantages, including periodic income from high-quality timber, continuous presence of large trees on site, diverse wildlife habitat, soil and watershed protection, and high aesthetic and recreation values. The stands created using this system are based on sound biological principles and they can be used to develop diversity in forest structure at the landscape level, and restore a native ecosystem that once dominated much of the southeastern United States.
Non-industrial private landowners interested in managing their forestland using the modified group-selection approach are encouraged to consult with a professional forester. The forester can facilitate implementation of these guidelines by evaluating the site, estimating current timber volumes, drafting a timber sale contract, and assisting with regeneration efforts. They will also be able to evaluate landowner opportunities for federal cost share assistance programs for non-industrial forest landowners. Additional information on forest management, including incentives programs can be found on the University of Florida Forestry Information website at http://www.sfrc.ufl.edu/Extension/ffws/home.htm
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Biological characteristics of longleaf pine important in developing a management plan. (Adapted from 3, 7, 8, 12, 14, 43).
Temperature: 60–70°F hot summers/mild winters
Rainfall: 43–69" annually
Sandy, acidic, low organic matter—Ultisols, Entisols, Spodosols
Wet, poorly-drained sites to dry, rocky mountain ridges
Heavy seed crops (50 to 60 seeds/cone) every 5 to 7 years
Maximized in stands with 30ft2/acre basal area
Dominant and codominant trees with diameters at breast height > 10"
Late October through early November
4200 clean seeds/lb, winged
Wind, maximum distance 120'
Bare mineral soil, low competition. Prescribed fire within one year of planting/seedfall
One week after reaching bare mineral soil
Stemless grass stage lasting 2 to many years; intolerant of competition and shade; growth comparable to slash pine and loblolly pine on cultivated and fertilized sites
Brown-spot needle blight, seed predators, grazing animals, and feral hogs
Resistant, except during early height growth