Pinus taeda L.

Loblolly Pine

Pinaceae -- Pine family

James B. Baker and 0. Gordon Langdon

Loblolly pine (Pinus taeda), also called Arkansas pine, North Carolina pine, and oldfield pine, is the most commercially important forest species in the southern United States, where it is dominant on about 11.7 million ha (29 million acres) and makes up over one-half of the standing pine volume. It is a medium-lived, intolerant to moderately tolerant tree with rapid juvenile growth. The species responds well to silvicultural treatments and can be managed as either even-aged or uneven-aged natural stands, or can be regenerated artificially and managed in plantations.

Habitat

Native Range

The native range of loblolly pine extends through 14 States from southern New Jersey south to central Florida and west to eastern Texas. It includes the Atlantic Plain, the Piedmont Plateau, and the southern extremities of the Cumberland Plateau, the Highland Rim, and the Valley and Ridge Provinces of the Appalachian Highlands. Loblolly pine does not grow naturally in the Mississippi River flood plain and is scarce in the deep, coarse sands of the lower Atlantic Plain and sandhills of North and South Carolina; it is important only in localized areas in southeastern Georgia and northern Florida (37,55,69).

Loblolly pine is an adaptable species that has been successfully planted along the periphery of its natural range and has been introduced on other continents with varying degrees of success.

{The native range of Pinus taeda}
- The native range of loblolly pine.

Climate

The climate over most of the loblolly pine range is humid, warm-temperate with long, hot summers and mild winters. Average annual rainfall varies from 1020 to 1520 mm (40 to 60 in). The frost-free period varies from 5 months in the northern part of the range to 10 months along the southern coastal States. Mean annual temperatures range from 13° to 24° C (55° to 75° F); average July temperature is 27° C (80° F) and frequently exceeds 38° C (100° F). January temperature averages 4° to 16° C (40° to 60° F) and occasionally drops to -23° C (-10° F) in the northern and western parts of the range (69).

During both winter and summer, weather within the range of loblolly pine differs from that immediately outside the range. There are a greater number of days with rain, a greater frequency of effective amounts of rain, that is, more than 13 mm (0.5 in), and higher average winter temperatures. In spring and autumn, the weather within and outside the range is more nearly the same (37).

The main factor limiting northern extension of the species is probably low winter temperature with associated damage from ice, snow, and sleet and cold damage during flowering. Lack of adequate growing-season precipitation probably limits western extension of loblolly pine in Oklahoma and Texas (37).

Soils and Topography

Soils within the native range of loblolly pine are predominantly Ultisols. Small areas of Entisols and Spodosols are found in the Southeastern States and there are some Alfisols throughout the region. Loblolly pine grows on a wide variety of these soils, ranging from the flat, poorly drained Aquults and Aquods of the coastal portion of the Atlantic Plain to the relatively dry Psamments, Udults, and Udalfs of the inland portion of the Atlantic Plain, Piedmont, and upland Provinces (107). Best growth is on moderately acid soils with imperfect to poor surface drainage, a thick medium-textured surface layer, and a fine-textured subsoil. These soils are common in the uplands of the Atlantic Plain and on the flood plains and terraces of rivers and streams. Poorest performance is on shallow soils, eroded soils, and very wet or waterlogged sites (37).

Some typical examples of Ultisols on which loblolly pine grows include the Coxville, Bladen, Beauregard, Wahee, Dunbar, Ruston, Norfolk, Orangeburg, and Smithdale series found in the Atlantic Plain; the Cecil, Davidson, and Appling series in the Piedmont; and the Hartsells and Linker series in the upland Provinces. Ultisols have a site index measured at base age 50 years for loblolly pine of 23 to 30 m (75 to 100 ft) in the Coastal Plain, 20 to 29 m (65 to 95 ft) in the Piedmont, and 18 to 24 m (60 to 80 ft) in the upland Provinces. Typical Entisols on which loblolly pine is found include deep sands (Chipley, Eustis, and Lakeland series) and alluvial soils (Alpin and Osier series), with a site index ranging from 20 to 30 m (65 to 100 ft). Representative Spodosols include the Leon and Lynn Haven series, with a site index of 18 to 26 m (60 to 85 ft). Within the Atlantic Plain but confined to a strip on each side of the Mississippi River are loessial soils represented by the Memphis, Grenada, Providence, Calhoun, and Henry series. These loessial soils, as well as Caddo, Wrightsville, Meggett, and Bude series, all having a site index ranging from 23 to 34 m (75 to 110 ft), are some representative Alfisols on which loblolly pine grows.

In the Atlantic Plain, the productivity of mineral soils generally decreases with improvement in surface drainage. Productivity is sensitive to soil fertility, however, and if fertility is low on poorly drained sites, productivity decreases (63). The presence of a spodic horizon within the rooting zone, as in the Leon series, frequently is associated with low productivity. Deep, excessively drained sands are also very low in site quality unless a water table or a clay lens which holds moisture lies within reach of the tree roots (37).

In the Piedmont Plateau, where surface drainage is well developed, physical characteristics of the soil, rather than surface drainage, determine the availability of moisture, nutrients, and aeration. Here uneroded soils with a thick surface layer and a friable subsoil have a site index of 24 to 27 m (80 to 90 ft). Common series in this category are Appling, Durham, Davidson, Georgeville, and Cecil. The least productive sites are eroded soils with a very plastic subsoil such as the Orange and Iredell series. When the A horizon is gone, site index is less than 12 m (40 ft) (37).

In the Ridge and Valley Provinces loblolly pine site index of 18 to 26 m (60 to 85 ft) generally increases from ridge tops to bottoms. This variation is related to landform, slope position and aspect, and geology. Soil features that determine site quality, such as soil temperatures, surface soil thickness, subsoil consistency, and soil moisture, are correlated with topography. However, past land use, differences in soil parent material, and other factors also affect soil profile development and cause variations in site quality independent of topography (92).

Perhaps as significant as the soils on which loblolly pine grows are those soils in the region where loblolly pine does not grow. These are principally Mollisols of the Blackbelt, Entisols of calcareous river bottoms and terraces (that is, soils in the Louisa, Miller, and Precris series characterized by high base saturation and high pH) and Alfisols of the Coastal Prairie of Louisiana and Texas with moderately high base saturation. These soils may also have other unidentified properties which exclude pine (72).

The topography throughout the loblolly pine range varies from flat near the coast to mountainous in the interior highlands. The topography can best be related to the physiographic regions within the loblolly pine range.

The Atlantic Plain is generally flat near the coast but becomes rolling and hilly inland with elevations ranging up to 150 m (500 ft). The Piedmont Plateau is more rolling, with highly developed drainage patterns and generally finer textured soils. Elevations range up to 305 m (1,000 ft) in Georgia. The Ridge and Valley Province is about 64 km (40 mi) wide and extends into the loblolly pine range from southeastern Tennessee into northern Georgia and Alabama. The topography is characterized by a group of valley floors separated by long, narrow, zigzagging ridges; elevations range from about 185 m (600 ft) to about 365 m (1,200 ft). The Cumberland Plateau, which lies just west of the Ridge and Valley Province, is underlaid by massive sandstone and its topography is characterized by winding narrow-crested ridges and narrow valleys. In some places the sandstone has given rise to local upland flats and mesa-like forms or knobs. Elevations range from 150 m (500 ft) in the southern part of the region and in the valley floors to 305 m (1,000 ft) at the northern end of the region and on ridge tops. The topography of the Highland Rim that extends into south-central Tennessee and northern Alabama is undulating with depressions and low domes where elevations range from 150 to 245 m (500 to 800 ft).

Associated Forest Cover

Loblolly pine is found in pure stands and in mixtures with other pines or hardwoods, and in association with a great variety of lesser vegetation. When loblolly pine predominates, it forms the forest cover type Loblolly Pine (Society of American Foresters Type 81) (31). Within their natural ranges, longleaf, shortleaf, and Virginia pine (Pinus palustris, P. echinata, and P. virginiana), southern red, white, post, and blackjack oak (Quercus falcata, Q. alba, Q. stellata, and Q. marilandica), sassafras (Sassafras albidum), and persimmon (Diospyros virginiana) are frequent associates on well-drained sites. Pond pine (Pinus serotina), spruce pine (P. glabra), blackgum (Nyssa sylvatica), red maple (Acer rubrum), and water oak (Quercus nigra), willow oak (Q. phellos), and cherrybark oak (Q. falcata var. pagodifolia) are common associates on moderately to poorly drained sites. In the southern part of its range, loblolly frequently is found with slash pine (Pinus elliottii) and laurel oak (Quercus laurifolia).

In east Texas, southern Arkansas, Louisiana, and the lower Piedmont, loblolly and shortleaf pine are often found in mixed stands. In Loblolly Pine-Shortleaf Pine (Type 80), loblolly predominates except on drier sites and at higher elevations. When shortleaf pine predominates, the mixture forms Shortleaf Pine (Type 75).

In fertile, well-drained coves and along stream bottoms, especially in the eastern part of the range, yellow-poplar (Liriodendron tulipifera), American beech (Fagus grandifolia), and white and Carolina ash (Fraxinus americana and F. caroliniana) are often found in the Loblolly Pine-Shortleaf Pine cover type.

Loblolly pine also grows in mixture with hardwoods throughout its range in Loblolly Pine-Hardwood (Type 82). On moist to wet sites this type often contains such broadleaf evergreens as sweetbay (Magnolia virginiana), southern magnolia (M. grandiflora), and redbay (Persea borbonia), along with swamp tupelo (Nyssa aquatica), red maple, sweetgum, water oak, cherrybark oak, swamp chestnut oak (Quercus michauxii), white ash, American elm (Ulmus americana), and water hickory (Carya aquatica). Occasionally, slash, pond, and spruce pine are present.

In the Piedmont and in the Atlantic Plain of northern Virginia and Maryland, loblolly pine grows with Virginia Pine (Type 79). In northern Mississippi, Alabama, and in Tennessee it is a minor associate in the eastern redcedar-hardwood variant of Eastern Redcedar (Type 46). On moist lower Atlantic Plain sites loblolly pine is found in Longleaf Pine (Type 70), Longleaf Pine-Slash Pine (Type 83), and Slash Pine-Hardwood (Type 85).

In the flood plains and on terraces of major rivers (except the Mississippi River) loblolly pine is a minor associate in Swamp Chestnut Oak-Cherrybark Oak (Type 91). On moist, lower slopes in the Atlantic Plain it is an important component in the Sweetgum-Yellow Poplar (Type 87). In bays, ponds, swamps, and marshes of the Atlantic Plain it is a common associate in Pond Pine (Type 98), the cabbage palmetto-slash pine variant of Cabbage Palmetto (Type 74), and Sweetbay-Swamp Tupelo-Red Bay (Type 104).

There is a great variety of lesser vegetation found in association with loblolly pine. Some common understory trees and shrubs include flowering dogwood (Cornus florida), American holly (Ilex opaca), inkberry (I. glabra), yaupon (I. vomitoria), hawthorn (Crataegus spp.), southern bayberry (Myrica cerifera), pepperbush (Clethra spp.), sumac (Rhus spp.), and a number of ericaceous shrubs. Some common herbaceous species include bluestems (Andropogon spp.), panicums (Panicum spp.), sedges (Carex spp. and Cyperus spp.), and fennels (Eupatorium spp.).

Life History

Reproduction and Early Growth

Flowering and Fruiting- Loblolly pine is monoecious; male flowers form in clusters at the tip of the preceding year's growth and female flowers form on the new year's growth. The pollen-bearing staminate flowers are catkin-like in appearance; they range from 2.5 to 3.8 cm (1.0 to 1.5 in) in length and vary from light green to red and yellow depending on stage of development. The pistillate flowers are generally ovoid and range from 1.0 to 1.5 cm (0.4 to 0.6 in) in length. They vary from light green through shades of pink to red depending on stage of development.

Flowering of loblolly pine is initiated in July and August in a quiescent bud that is set from middle June to early July. The male strobili form in this bud in late July and the female in August, but they are not differentiated into recognizable structures until late September or October. In October the staminate buds develop at the base of a vegetative bud and the pistillate buds develop at the apex of a vegetative bud a few weeks later; both remain dormant until early February (37,41). The date of peak pollen shed depends on the accumulation of 353° C (636° F) day-heat units above 13° C (55° F) after February 1 (16). Flowering is also related to latitude, beginning earlier at lower latitudes than at higher ones, and it can occur between February 15 and April 10. Staminate flowers on a given tree tend to mature before the pistillate flowers, which helps to reduce self-pollination. Fertilization of the pistillate strobili takes place in the spring of the following year (37).

Loblolly pine does not normally flower at an early age, although flowering has been induced on young grafts with scion age of only 3 years. The phenomenon of inducing such early flowering in seedlings is dependent on reducing vegetative shoot growth so that quiescent buds are formed in the latter part of the growing season to allow for the initiation and differentiation of reproductive structures. The formation of quiescent buds in seedlings and saplings does not usually occur during that period because four to five growth flushes are common for trees of this age. As a loblolly pine tree ages, the number of growth flushes decreases, which accounts in part for increased flowering of trees at older ages. Flowering is also genetically controlled and is influenced by moisture (May-July rainfall) and nutrient stresses.

Seed Production and Dissemination- Seed production of loblolly pine varies according to physiographic region, climatic factors, and tree or stand condition. In the southern coastal portions of the Atlantic Plain, loblolly is generally a prolific and consistent seed producer, but in some of the inland portions of the Atlantic Plain, the Piedmont, and in the western extremities of its range, seed production is often lower and more erratic. Year-to-year variations in seed crops can range from failure to bumper crops. For example, in 27 years of seedfall records in the Atlantic Plain of South Carolina, there was one seed-crop failure but there were three seed crops of more than 2.5 million sound seeds per hectare (1 million/acre) with the other crops falling between these extremes. At most locations where seed-crop records have been kept, however, such wide annual variations have not been observed.

Despite fluctuations in seed production, loblolly usually produces some seeds every year and good seed crops normally occur at intervals of 3 to 6 years. More than 198,000 sound seeds per hectare (80,000/acre) is considered a good seed crop; 74,000 to 198,000/ha (30,000 to 80,000/acre) is an average crop, and less than 74,000/ha (30,000/acre) is considered marginal, depending on seedbed characteristics and weather conditions.

Throughout the range of loblolly pine, usually cones mature and seeds ripen by the second October after flowering or about 26 months after the strobili are initiated. The mature cones are light reddish brown and range from 7.5 to 15.0 cm (3 to 6 in) in length. They are narrowly conical to ovoid-cylindrical. Each cone scale is tipped with a stout triangular spine. Mature cones have a specific gravity of 0.89 or less (they float in SAE 20 oil). Individual cones may contain from less than 20 to more than 200 seeds, and the percentage of sound seeds may vary from about 15 percent to nearly 100 percent. Loblolly seeds vary in size from 27,100/kg (12,300/lb) to 58,200/kg (26,400/lb) and average 40,100/kg (18,200/lb) (37,88).

Seed production of individual trees increases with tree age, size, and freedom from crown competition. By age 25, enough seeds may be produced in widely spaced trees to regenerate a stand; however, trees at 40 years generally produce three to five times more. Rotations shorter than 30 years usually do not lend themselves to natural regeneration.

In well-stocked and overstocked stands, cone production of loblolly pine can be stimulated threefold to tenfold by releasing the seed trees from competitors at least three growing seasons before the seed is needed. If seed-tree release is delayed later than May 1, seed-crop stimulation will be delayed 1 year. In overstocked stands, if seed trees are not released before a harvest cutting, then seed-crop stimulation will be delayed 2 or 3 years, depending on the season of the harvest cut (37,61,95).

Seedfall usually begins in October, and the bulk of the seeds are released in November and early December. Seedfall is hastened by dry, warm, windy weather and retarded by cool, wet weather. Seed dispersal in or adjacent to a stand varies with height and stocking level of the seed-source trees, magnitude of the seed crop, terrain, and weather conditions at the time of seedfall. The effective seeding distance ranges from 61 to 91 m (200 to 300 ft) in a downwind direction from the seed source and 23 to 30 m (75 to 100 ft) in other directions. Viability of seeds varies with seed-crop size and the month that the seed is dispersed. Seed viability is often lower in years of poor seed crops and in seeds dispersed late in the season (37).

Loblolly pine seeds generally go through a stage of dormancy after seedfall, which lasts longer than that of any other southern pine. Seed dormancy is related to the impermeable properties of the seedcoat that constrain water imbibition and oxygen uptake; chemical germination inhibitors do not play a significant role (11,73). Dormancy is broken naturally as the seeds overwinter on the forest floor. Germination is epigeal (88). Natural seed germination usually begins in March when daytime temperatures range between 18° and 27° C (65° and 80° F). Few seeds remain viable (not more than 0.1 percent) on the forest floor for germination in the second year after seedfall (70). Secondary seed dormancy can be induced during seed handling procedures. Cold, moist stratification of the seed for 30 to 90 days at temperatures 3° to 5° C (37° to 41° F) are generally recommended to artificially break dormancy for direct seeding or for nursery sowing (74).

Seedling Development- Moisture is a critical factor in seed germination and seedling establishment; the amount of rainfall in the spring is related directly to seedling catches. Scarifying the seedbed exposes mineral soil and increases contact of the seeds with moist soil surfaces. Failure of the root radicle to penetrate compacted or puddled soil surfaces reduces seedling establishment, especially on major skid trails and log decks. Soil compaction and puddling also reduce root growth, seedling survival, and shoot growth (36,37,40,61).

Seedbed preparation by scarification or burning greatly increases seed germination and seedling survival, which reduces the number of seeds required to produce one seedling. For example, undisturbed seedbeds with a litter depth of 8 to 10 cm. (3 to 4 in) require 5 to 6 times more seeds to produce the number of seedlings produced in disturbed seedbeds.

Seed germination decreases with age of seedbed and increases with clay content of the soil. Two-year-old seedbeds require 3 to 4 times more seed for successful establishment than do 1-year-old seedbeds, and 3-year-old seedbeds require 9 to 14 times more seed than is needed in the first year. Thus, favorable seedbeds usually exist for only 1 year after disturbance, after which they rapidly deteriorate. Heavier textured soils provide better seedbeds which results in higher seedling survival than do lighter textured soils (37,104).

Drought is a major cause of mortality for planted loblolly pine seedlings, especially in areas with low rainfall during the growing season. Improper care, handling, and planting of nursery stock and inadequate site preparation for control of competing vegetation also contribute to poor survival by indirectly increasing moisture stress (34,57).

Height growth of loblolly pine seedlings occurs annually in a series of two to five growth flushes and is dependent on variables such as temperature, day length, soil moisture, nutrients, competition, and genetics. Temperature has a dominant influence on the initiation of height growth in the spring. High day temperatures increase height growth, but high night temperatures decrease it. When day and night temperatures differ by 12° to 13° C (54° to 55° F), the best height growth occurs (15,43).

Soil moisture influences growth of loblolly pine by its effect on internal water relations and vital physiological processes. Growth is reduced with increasing water deficits. For example, at a soil moisture tension of 1520 mm of mercury (2 atm), height growth of loblolly pine seedlings is greatly reduced and at 2660 mm of mercury (3.5 atm), height growth ceases. Height and diameter growth are significantly reduced by a late spring and summer drought, which also reduces early height growth the following year (37,98,116).

Growth of loblolly pine seedlings in a natural stand is inversely related to overstory stocking of pine and hardwoods. As the proportion of hardwoods increases for a given pine stocking, loblolly pine seedling growth decreases. Size and shape of openings affect seedling growth up to 9 m (30 ft) from edges of openings. Seedlings growing beneath overstory hardwoods are not likely to survive more than a few years and if they do survive their growth will be slow. Growth and survival of loblolly pine seedlings during the first 7 years after a stand is regenerated may be reduced by 80 percent because of the faster growth of competing hardwood sprouts and shrubs. Pine seedlings not overtopped by hardwoods at age 3 or older have an excellent chance to outgrow the hardwood competition (37).

Photosynthesis in loblolly pine seedlings is related to light and soil moisture conditions, which in turn are affected by competing hardwoods. Photosynthetic rates of many hardwoods are inherently higher than those of loblolly pine at relatively low light intensities and with low soil moisture (37).

Fertilization often increases seedling growth in waterlogged soils. In some instances where specific nutrients are limiting growth, fertilization results in growth equal to or greater than that with drainage. Loblolly pine grows well on wet, fertile sites because of the effects of moisture on nutrient availability (63,101).

Vegetative Reproduction- Young loblolly pine seedlings up to 3 years of age may sprout from buds in axils of primary needles if tops are clipped off, but older trees will not produce basal sprouts at root collars if stems are cut or top-killed by fire, nor do they produce root sprouts. Rooting is related to tree age and is more successful with cuttings from younger trees. Techniques and materials used to root cuttings are of critical importance. For example, a fine mist over the rooting bench is better than a heavy mist, and Hare's powder is a better compound to use than indolebutyric acid when rooting loblolly pine cuttings. Although needle bundles and buds of loblolly pine have been rooted, the success rate has been low. Air layering, a modification of rooting cuttings, has been the more successful method of the two. Success rates have been high for young trees but older trees are more difficult to air layer (29,42,48,110).

Grafting is the most common method of vegetative propagation used to produce genetically uniform trees, especially in seed orchards. Grafting success is usually high but varies with scion material because problems may develop from incompatibility of scion and root stock (29,37,66).

Producing genetically uniform plantlets from tissue cultures is a promising technique, and research is underway to develop procedures for the commercial production of loblolly pine clones (19,94).

Sapling and Pole Stages to Maturity

Growth and Yield- Growth of loblolly pine stands is inherent