Longleaf Pine Community Description

Ecosystem Distribution and Extent

Prior to European settlement, the geographic range of longleaf pine forests covered most of the Coastal Plain of the Southeast United States, from southeastern Virginia to eastern Texas and south to south-central Florida (Peet and Allard 1993, Harcombe et al. 1993). Longleaf extended north into the Piedmont providence and mountains of Alabama and Georgia (see map of distribution). Estimates of original acreage of longleaf pine forests range from 60 million (Outcalt and Sheffield 1996) to more than 93 million acres (BROKEN-LINK Frost 1993).

The current geographic distribution of longleaf is similar to the historic one although it has disappeared on the fringes of its original range and is almost extirpated from Virginia (Plocher 1999). Outcalt and Sheffield (1996) documented that longleaf pine currently occupies less than 5% of its original acreage but it is not known how much of the current acreage represents forested areas with native groundcover. In all states except Florida, the majority of remaining stands of longleaf pine are found on private property (Outcalt and Sheffield 1996).

According to Peet and Allard (1993), longleaf pine vegetation is distributed across four physiographic provinces: Coastal Flatlands, Coastal Plain Rolling Hills, Fall-Line Sandhills, the Piedmont and the Interior Uplands. Longleaf woodlands historically comprised the majority of the upland vegetation in the two Coastal Plain provinces and the Sandhills region. In the Piedmont longleaf is, or was, limited to the outer fringes and mountain longleaf is limited to a very few scattered south facing ridge sites in Georgia and Alabama.

Environment

Climate

Longleaf pine forests occur mostly in the Coastal Plain region of the southeastern United States (Peet and Allard 1993, Harcombe et al 1993). Climate of this region is classified as humid subtropical (Trewartha 1968) and is characterized by relatively warm winters, hot summers, abundant rainfall and a long growing season (Twilley et al. 2001, Georgia State Climate Office 1998). Precipitation is distributed throughout the year, but winter and late spring-summer tend to be the wettest periods (Twilley et al. 2001, Chen and Gerber 1990). Winter precipitation is mainly a consequence of frontal cyclonic storms, whereas convection driven thunderstorms account for much of the growing season rainfall (Christensen 1988). In wet years, thunderstorms may occur almost daily from late May through September. Lightning associated with thunderstorms is very common. Indeed the Coastal Plain region has the highest frequency of lightning of any region in North America (Komarek 1968). High lightning frequencies in this region are of fundamental ecological importance since lightning ignitions are thought to have contributed to a historic regime of frequent growing season fires (Komarek 1968, Frost 1998). Within the Coastal Plain, as a whole, there is some geographical variation in climate regime (Christensen 1988, Cook et al. 2001). Areas remote from the coast tend to be cooler and drier. Also the Atlantic and Eastern Gulf Coast sections of the southeastern Coastal Plain tend to be moister than the western Gulf Coast section (Marks and Harcombe 1981, Christensen 1988). The precipitation gradient is especially steep towards the western edge of the range of longleaf pine (Cook et al. 2001).

Soils

Soils of longleaf pine stands are predominantly coarse textured sands, loamy sands or sandy loams that dry rapidly after rain events (Marks and Harcombe 1981, Harcombe et al. 1993, Provencher et al. 2001b). The combination of dry soils, flammable herbaceous dominated groundcover vegetation, and longleaf pine needle litter, creates ideal conditions for fires to start and spread rapidly across the landscape (Frost 1998). Ridges that originated as stranded beach dunes or sandbars near ancient streams are underlain by Entisols, i.e. deep undifferentiated coarse sandy soils. These sites are extremely xeric and are inhabited by sandhill vegetation typified by subcanopy oaks and hickories as described above (Myers 1990). Most other longleaf pine sites occur on Ultisols or Spodosols, soil orders characterized by argillic or spodic horizons, respectively. These subsurface horizons impede the downward flow of water, and, consequently, water accumulates above the subsoil following rainfall events. If the impediment to drainage is located very close to the surface the soil may remain moist for considerable periods since even minor rains produce saturated soils. This will be true to some extent even on flat sites, but moist soil conditions tend to be especially pronounced in seeps or depressions where water accumulates.

Differences in subsurface drainage can produce pronounced differences in longleaf pine ground layer vegetation composition (Walker and Peet 1982, Peet and Allard 1993, Streng et al. 1993, Provencher et al. 2001, Glitzenstein et al. 2001, 2003). Flat sites with shallow subsoils are generally inhabited by wet savannas whereas seeps and shallow depressions support bogs. As the impermeable soil horizon recedes further beneath the surface the surface soil remains drier for longer periods of time since a greater quantity of rain is required before capillary action can enhance surface soil moisture. These drier, better-aerated soils are more conducive to woody plant growth and, even when frequently burned, often support abundant shrubs (Glitzenstein et al. 2003). Since they occupy level topography and incorporate an important shrub component, these infrequently saturated sites have are often referred to as flatwoods (Abrahamson and Hartnett 1990).

Soil texture and drainage are two environmental parameters with important direct effects on species composition of longleaf-associated vegetation (Glitzenstein et al. 2001, Marks and Harcombe 1981, Harcombe et al. 1993, Gilliam et al. 1993, Kirkman et al. 2001, Provencher et al. 2002). These and other environmental parameters affect plant species composition independently of or in addition to any fire regime. Experimental work by Glitzenstein et al. (2001) in South Carolina demonstrated that species characteristic of sandhill or subxeric sites were most prominent on the higher parts of a manipulated gradient with the deepest surface soils. In contrast, species characteristic of moist flatwoods occurred mostly in the lower, poorly drained sections of the gradients. In addition, for any given drainage condition there was a significant effect of soil texture.

Ecosystem Description

Old-growth Condition and Forest Structure

On a landscape-scale, structure of pre-European settlement longleaf pine ecosystems was one dominated by widely spaced large trees that created an open canopy forest. Comments of many early explores of the region included mention of the openness of the forest under the canopy trees. Bartram (1791) applied the term “park-like” to large areas of scattered large canopy longleaf pine trees in the lower Coastal Plain. For a site in North Carolina, Ruffins (1861) described a “thick canopy of green” … “over the open space below” and for a large wet moist he termed a “great savanna”, he noted that “nearly the whole” was “destitute of trees, and nearly so of bushes, and of any shrubs of as much as two feet high”.

Old-growth stands of longleaf pine are rare in the modern landscape. Varner and Kush (2004) estimate that in 2004 old-growth longleaf stands occupy 0.004% of the extant range and 0.00014% of the pre-European settlement extent. These relics provide visual insight into the original longleaf forest. In addition, the USDA Forest Service has endeavored to define old-growth conditions for many longleaf pine and other important forest ecosystems. Landers and Boyer (1999) describe the old-growth condition for upland longleaf pine forests, woodlands and savannas. Harms (1996) does the same for similar but wetter ecosystems. Walker (1999) provides an overview of processes important in maintaining old-growth conditions in longleaf pine forests.

Recent research on some of the remaining old-growth sites, provides quantified descriptions of what may represent original structure of longleaf pine forests (Platt et al.1988, Noel et al. 1998). The quantified descriptions suggest open canopied forests with widely-spaced, single species patches of trees of multiple ages (sizes). The absence of fire and logging alters the habitat structure and species composition of the forest (Gilliam and Platt 1999, Noel et al. 1998). In addition, long periods without fire (e.g., fire suppression) results in large increases in fuel making re-introduction of fire problematic (see Effects of Fire Suppression). Currently there are some attempts at burning long-suppressed stands, in an effort to restore the original forest without killing all of the mature trees (Varner et al. 2003, Kush et al. 2000).

Major Species

In the modern landscape, well-maintained, frequently burned, longleaf pine sites are characterized by a sparse, often nearly monospecific canopy of longleaf pine itself, along with a diverse ground layer vegetation of grasses, forbs, and short, rhizomatous shrubs (Wells 1942, Bridges and Orzell 1989, Taggart 1990, Harcombe et al. 1993, Peet and Allard 1993). This matches the descriptions of Bartram (1791), one of the earliest botanists to comments on regional floral diversity of longleaf pine forests. On a scale of meters to tens of meters, longleaf pine groundcover is the most diverse vegetation in North America (Peet et al. 1983, Peet and Allard 1993, Walker and Peet 1983).

Grass species composition and abundance is especially important in longleaf ecosystems because of this plant family’s relationship to availability of fine fuel that promotes fire in these forests. Species of wiregrass (Aristida), little bluestems (Schizachyrium) and bluestems (Andropogon) are of particular importance. Wiregrass (Aristida stricta and/or Aristida beyrichiana; see Peet 1993) is generally the dominant grass species in the eastern section of the outer Coastal Plain, from southern North Carolina to eastern Mississippi (Peet 1993). However, there is a curious “gap” in the distribution of wiregrass in central South Carolina (Peet 1993). Herbarium records (Peet 1993) and historical observations (Elliott c. 1810 unpublished correspondence to Henry Muhlenberg, Ruffin 1843, BROKEN-LINK Ravenel 1850) suggest that this “wiregrass gap” is not a function of land use history but a long-standing biogeographic feature. Within the wiregrass gap, little bluestem (Schizachyrium scoparium) is generally the prevalent grass species on mesic to dry sites whereas toothache grass (Ctenium aromaticum) or bushybeard (Andropogon glomeratus) are dominant in wet savannas. Bluestem grasses also tend to replace wiregrass in the inner Coastal Plain and piedmont areas of the Carolinas, Georgia and Alabama (BROKEN-LINK Frost 1993). Lastly, little bluestem is also the dominant grass in longleaf pine woodlands west of the range of wiregrass in western Mississippi, Louisiana, and eastern Texas (Bridges and Orzell 1989, Harcombe et al. 1993).

It should be emphasized that these generalizations concerning regional shifts in grass dominance may be true only at a coarse scale. Numerous grass and shrub species are capable of assuming dominance on any given site, and lack or reduced importance of wiregrass and bluestems does not necessarily indicate a history of disturbed soils or agricultural abandonment. This is particularly true of wet savannas in which muhly grass (Muhlenbergia expansa) and various dropseeds (e.g. Sporobolus curtissii, S. teretifolius) may dominate or co-dominate along with wiregrass and toothache grass (Taggart 1990, Hermann 1995, Weakley 1998).

Considering canopy trees only, longleaf pine occurred historically in almost pure stands (Schwarz 1907, Harper 1962, Chapman 1909, 1932, Heyward 1939, Frost 1993, Harcombe et al. 1993) and this is still the case for the best, fire-maintained sites. There are, however, a number of exceptions to this generalization (Heyward 1939). “Sandhill” oaks (Quercus laevis, Q. incana, and Q. margaretta), and, in some areas, sand hickory (Carya pallida) can form a more or less continuous subcanopy on dry ridge sites. While the importance of these subcanopy “scrub” oaks and hickories may have been enhanced by altered fire regimes (BROKEN-LINK Myers 1990), there is good historical evidence that they occurred commonly in presettlement forests (Catesby 1771, Ravenel 1850, Watson 1926, Heyward 1939). Another exception is in wet savannas where pond pine (Pinus serotina), slash pine (Pinus elliottii) or loblolly pine (Pinus taeda) may co-dominate, depending on geography and local site conditions (Watson 1926, Heyward 1939). At the very wettest extreme pond cypress (Taxodium distichum var. nutans) replaces the pines. Relative canopy dominance of longleaf pine also varies geographically. This is particularly evident in the transitional zones between the main longleaf pine areas and the mixed pine-oak-hickory communities prevalent to the north and west (Frost 1993, Harcombe et al. 1993). In eastern Texas there is a region of mixed longleaf-loblolly pine forest transitional to the loblolly pine zone that forms the western limit of southeastern pine dominated woodlands (BROKEN-LINK Frost 1993, Harcombe et al. 1993).

Associated Ecosystems

An interesting and unresolved question concerns the role of fire, and fire regime, in determining the boundary between longleaf pine and hardwood dominated plant communities. If fires are frequent longleaf pine and associated species may move down-slope into at least the drier hardwood dominated habitats. On the other hand, reductions in fire frequency, or fire exclusion, leads to invasions by hardwood trees into longleaf uplands (see Effects of Fire Suppression). Historically, it is probable that the boundary between the two habitats fluctuated depending on recent fire history. It is also likely that upper slope hardwood-dominated communities depended on at least occasional fires to limit encroachment of fire-intolerant trees of lower slope habitats (see: Hardwood Hammocks).