Distribution, Environment, and Associated Species

Ecosystem Distribution and Extent

The range of loblolly pine encompasses the Southern Mixed Forest and Outer Coastal Plain Mixed Forest provinces defined by (Bailey 1995). Loblolly pine occurs mainly in the southern Piedmont and Coastal Plain physiographic sections from East Texas to Virginia, but is also found at the southern extremities of the Ridge and Valley, and Cumberland Plateau sections of the Appalachian Highlands province (Eyre 1980). It does not grow in the Mississippi River valley which forms a barrier of 30 to 200 km between the western and eastern populations. There are currently more than 19.8 million hectares (49 million acres) of loblolly pine forest in the 13 southern states. Of the (1,952 million m³) (68,918 million ft³) of loblolly-shortleaf pine timber volume, 75% occurs in naturally regenerated stands and 25% occurs in plantations (Conner and Hartsell 2002). In contrast to this widespread distribution in the modern landscape, loblolly pine was probably limited to wetter sites that burned infrequently in presettlement forests. The arrival of Europeans precipitated a series of major land use changes, which in turn fostered a significant expansion of loblolly pine across the region. Today, loblolly pine is the most important timber species in the United States, and loblolly pine forests provide a variety of ecosystem services including wildlife habitat, watershed protection, and carbon storage (Schultz 1997). Fire was historically an important ecological process in these forests, and prescribed burning is still widely used for silvicultural purposes, habitat enhancement, and fuel reduction.

Environment

Climate

Climate within the range of loblolly pine is humid with long hot summers and mild winters. Rainfall is evenly distributed throughout the year, but heavy downpours and summer droughts are common (Peterson 2002). Tropical air masses originating over the Gulf of Mexico and South Atlantic produce more moderate temperatures and higher rainfall along the coast than in the interior. Growth of loblolly pine is impaired by late spring frosts near the northern edge of its range, and by inadequate rainfall during the growing season at its westernmost extent (Baker and Langdon 1990).

Average annual temperatures in the outer Coastal Plain vary from 16° C (61° F) in Virginia to 21° C (70° F) in north Florida (Bailey 1995). Forests benefit from a long growing season in this climate, with frost-free periods ranging 200 days in Virginia to 300 days along the Gulf Coast (Hunt 1967). Precipitation ranges from 102 cm (40 inches) per year at the western extent of loblolly pine in Texas to 153 cm (60 inches) per year in Florida (Bailey 1995), and is generally lowest in fall and highest in summer when convective thunderstorms develop (Peterson 2002). Thunderstorms typically occur 80-130 days annually in Florida and 80-100 days annually on the Gulf Coastal Plain (White et al. 1998). Coastal areas are also subject to hurricanes that impact forests through high winds that topple trees, and heavy rains that can cause severe flooding. Hurricane season extends from late summer into fall, and at least 2 hurricanes per year have reached the southeastern US over most of the past 50 years (Myers and Van Lear 1998).

The interior portions of the loblolly pine range are slightly cooler, with average annual temperatures ranging from 14° (57° F) to 18° (64° F) C in the Piedmont section, and 13° (55° F) to 16° (61° F) C in the Southern Ridge and Valley section (McNab and Avers 1994). However, there are still typically more than 200 frost-free days per year in the Piedmont, (Hunt 1967), with a growing season typically lasting from 205-235 days (Bailey 1995). Annual precipitation ranges from 112 to 140 cm (44 to 55 inches) in the Piedmont section and 90-140 cm (35 to 55 inches) in the Southern Ridge and Valley section (McNab and Avers 1994). The frequency of thunderstorms generally decreases along a gradient from coastal to inland areas, declining to as low as 40-60 days annually in Kentucky and other interior regions (White et al. 1998). Although inland forests are often impacted by hurricanes, wind speeds are typically lower and disturbance severity is generally less than in coastal areas.

Terrain

Loblolly pine occurs over a broad elevation gradient ranging from sea level to nearly 900 m (2,953 feet) in northern Alabama and Georgia. Across this range, loblolly pine is found in a variety of landforms. Approximately 36% of its range is lowlands or smooth plains, 49% irregular or rolling plains and 15% is hilly or mountainous land (Schultz 1997).

The Coastal Plains are predominantly comprised of flat, weakly dissected alluvial plains. Elevations range from sea level to 150 m (492 feet), with local relief usually less than 35 m (115 feet) on the Gulf Coastal Plain and 15 m (49 feet) on the Atlantic Coastal plain (Ware et al. 1993). Despite the low relief of the Coastal Plain, relatively small changes in elevation can result in large shifts in soil moisture and associated vegetation.

The Piedmont is characterized by rolling hills, with elevations ranging from 100 m (328 feet) along the fall line bordering the Coastal Plain to 550 m (1804 feet) in the Appalachian foothills (McNab and Avers 1994). Terrain is moderately dissected and characterized by narrow to broad upland ridges, short hillslopes, and narrow valley floors. Local relief typically varies from 30 to 90 m (98 to 295 feet). Slope position and exposure to solar radiation influence soil moisture and the distribution of major tree species (Cowell 1995).

The Southern Cumberland Plateau and Southern Ridge and Valley are highly dissected landscapes, occurring at elevations of 100 to 400 m (328 to 1,312 feet) (McNab and Avers 1994). Both consist primarily of plains with hills or open high hills. Local relief ranges from 90 to 150 m (295 to 492 feet) in both sections. Because of the greater variability in terrain, topographic constraints have an even greater influence on local environment and forest community composition than in the Piedmont (Bolstad et al. 1998).

Soils

Settlers originally applied the term “loblolly” to moist depressions and swamps, and loblolly pines were historically associated with these mesic soils (Wahlenberg 1960). However, loblolly pine can grow successfully on a much wider range of sites in the absence of fire and competition from other species. In the modern landscape, loblolly pine can be found on a variety of soils ranging from flat, poorly drained Spodosols in the lower Coastal Plain to the well-drained Ultisols in the rolling Piedmont and Plateau.

Most soils of the Atlantic Coastal Plain and the Piedmont are Ultisols, which underlie about three-quarters of the Southeast (Skeen et al. 1993). Ultisols are mineral soils derived from a variety of parent materials and are characterized by sandy or loamy surface horizons and loamy or clayey subsurface horizons. Udults and Aqults are the principal suborders. Udults are well-drained, humus-poor and are yellowish brown or red in color. Aqults are saturated at some point during the year and typically have gray or olive colored subsoils. The remaining soils are Alfisols (11%), Entisols (7%) and Spodosols (3%) with about 1% each in the remaining soil orders (Schultz 1997). Most of the well drained Ultisols and Alfisols are now in agricultural use and the wetter areas of these soils are being drained for crop production (Skeen et al. 1993).

Major Species

Loblolly Pine

Loblolly pine is a medium to large tree averaging 27 to 34 m (89 to 112 feet) in height and 61 to 76 cm (24 to 30 inches) in diameter when it is between 80 and 100 years old on average sites (Baker and Langdon 1990). When fully developed, loblolly is the tallest of the southern pines, reaching a record height of 55.5 m (182 feet). Although individual trees may survive for up to 300 years, most of the original cohort from an even-aged pine stand will have been replaced by hardwoods within 175 years (Schultz 1997).

Juvenile loblolly pine is moderately shade tolerant, but trees become more shade intolerant with age. Because of their shade-tolerance, seedlings and saplings can successfully compete with herbaceous annuals, shrubs, and early-successional hardwoods (Bormann 1953), Mature trees typically achieve maximum growth when 50% or more of the live crown is exposed to direct sunlight (Schultz 1997). Shade tolerance of loblolly pine is similar to that of shortleaf and Virginia pine, lower than that of most hardwoods and greater than slash and longleaf pine (Wahlenberg 1960, Eyre 1980).

Prior to European settlement and the accompanying land use changes, loblolly was associated with wet areas that were not subjected to long periods of flooding or frequent fire. These sites were typically found in major river bottoms, along stream margins, or at wetland edges (Schultz 1997, Hardin et al. 2001). With the exclusion of fire from much of the Southeast, loblolly has proven very successful on an enormous variety of site conditions. Loblolly pines can develop a taproot up to 3 meters (9.8 feet) long in sandy soils, whereas taproots are much shorter on moist sites or in heavy clay soils (Peterson 2002). Sensitivity to moisture stress varies regionally, with individuals in the western portion of the range more drought resistant than those in the east (Fowells 1965). Best growth is on moderately acidic soils with moderate to poor surface drainage, a thick medium textured surface layer and fine textured subsoil. These soil types are commonly found in the uplands of the Atlantic coastal plain and on flood plains of rivers and streams. Poor performance is on shallow or eroded soils or very wet sites. Deep, excessively drained sands are low in quality unless a water table is within reach of the potential growth of roots (Baker and Langdon 1990).

Associated pines

Shortleaf pine (Pinus echinata) is a frequent associate of loblolly pine, but tends to be more competitive on drier sites and at higher elevations. It has a more northern and western range than loblolly pine and hence increases in importance on the upper Piedmont (Baker and Langdon 1990). Slash pine (Pinus elliottii) occurs on much of the coastal plain and was historically confined to mesic sites similar to those occupied by loblolly pine. Like loblolly pine, slash pine has expanded its range in the modern landscape though invasion of cutover areas and abandoned agricultural lands. Pond pine (Pinus serotina), pitch pine (Pinus rigida) and Virginia pine (Pinus virginiana) are also common associates where their ranges overlap that of loblolly pine (Wade et al. 2000).

Major hardwoods

Throughout its range, loblolly pine is often found in mixed stands with a variety of hardwood species. Even when pines comprise the majority of stand basal area, the structure and species composition of the hardwood community may have a major influence on the understory environment, forest succession, and wildlife habitat quality. Common associates found throughout most of the range of loblolly pine include southern red oak (Quercus falcata), northern red oak (Quercus rubra), white oak (Quercus alba), post oak (Quercus stellata), water oak (Q. nigra), pignut hickory (Carya glabra), mockernut hickory (Carya tomentosa), red maple (Acer rubrum), blackgum (Nyssa sylvatica), sassafrass (Sassafras albidum), sweetgum (Liquidambar styraciflua), and persimmon (Diospyros virginiana) (Quarterman and Keever 1962, Cowell 1993, Bragg 2004). In addition, laurel oak (Quercus hemisphaerica), American beech (Fagus grandifolia), southern magnolia (Magnolia grandiflora), and sweetbay (Magnolia virginiana) are often found in association with loblolly pine in the Coastal Plain (Quarterman and Keever 1962, Delcourt and Delcourt 1974, Marks and Harcombe 1981).

Understory vegetation

Although understory vegetation accounts for only a small portion of total forest biomass, it is central to the fire ecology and management of loblolly pine forests. Grasses, forbs, and shrubs provide much of the fuel that affects the behavior and severity of surface fires. Furthermore, the application of prescribed fire is often aimed at manipulating understory vegetation to reduce wildfire risk, enhance wildlife habitat, or reduce competition with crop trees.

Pioneering grasses and forbs establish along with loblolly pine seedlings following agricultural abandonment or timber harvest. Typical early-successional herbs include broomsedge (Andropogon virginicus), ragweed (Ambrosia artemisifolia), crabgrass (Digitaria sanguinalis), and heath aster (Aster ericoides) (Schultz 1997). Composites, grasses, and other pioneer species are also found in mature pine forests that have been recently burned (Cain et al. 1998).

Common understory trees and shrubs occurring throughout the range of loblolly pine include flowering dogwood (Cornus florida), American holly (Ilex opaca), hawthorn (Crataegeus spp.), blueberry (Vaccinium spp.), beautyberry (Callicarpa americana), and viburnum (Viburnum spp.). Pawpaw (Asimina triloba), Wax myrtle (Myrica cerifera), inkberry (Ilex glabra), and yaupon (Ilex vomitoria) are also important in the Coastal Plain. Common species of woody vines include greenbriers (Smilax spp.), Japanese honeysuckle (Lonicera japonica), Carolina Jessamine (Gelsemium sempervirens), Virginia creeper (Parthenocissus quinquefolia), poison ivy (Rhus toxicodendron) and muscadine grape (Vitis rotundifolia) (Hodgkins 1958).

In the Piedmont, understory herbs are usually sparse and often include bluestems (Andropogon spp.), hairy bedstraw (Galium pilosum), striped wintergreen (Chimaphila maculata), creeping lespedeza (Lespedeza repens), and flowering spurge (Euphorbia corollata) (Wade et al. 2000). The density and diversity of understory herbs is typically greater in Coastal Plain forests, particularly on sites that have never been cultivated. Typical herbs include bluestems, panic grasses (Panicum spp.), spike uniola (Chasmanthium laxum), dogfennel (Eupatorium capillifolium) and lespedeza (Lespedeza spp.) (Lewis and Harshbarger 1976). In the Coastal Plain of Georgia, forests on old field sites were characterized by Canada goldenrod (Solidago canadensis), Japanese honeysuckle, and Canadian horseweed (Conyza canadensis), whereas forests on cutover sites were characterized by twinflower (Dyschoriste oblongifolia), anisescented goldenrod (Solidago odora), wiregrass (Aristida stricta), little bluestem (Schizachyrium scoparium), bracken fern (Pteridium aquilinum), scaleleaf aster (Aster adnatus), wild petunia (Ruellia caroliniensis), and goat’s rue (Tephrosia virginiana) (Hedman et al. 2000). In a unharvested, mature mixed loblolly-shortleaf forest in Texas herbaceous species included panic grasses, chasmanthium (Chasmanthium sessiliflorum), sedges (Carex spp.), downy milkpea (Galactia volubilis), elephant’s foot (Elephantopus tomentosus), goldenrod (Solidago spp), copperleaf (Acalypha spp.), twin-eyed berry (Mitchella repens) and noseburn (Tragia urens) (Stransky et al. 1986).