Associated Forest Cover
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tarjonnut Silvics of North America
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.).
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Climate
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tarjonnut Silvics of North America
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).
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Damaging Agents
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tarjonnut Silvics of North America
Agents that cause periodic damage to individual
trees or stands of loblolly pine include wind, lightning, temperature
extremes, ice, drought, flooding, insects, and diseases. Voluminous
literature about the effects of these agents in loblolly pine stands on a
range of sites, soils, and stand conditions is available; a brief summary
follows.
Large dominant trees usually are more vulnerable to high winds than
smaller trees, and trees with large cankers caused by rust disease break
more readily than sound trees. In general, damage resulting from severe
winds associated with hurricanes or thunderstorms is caused primarily by
windthrow or blowdown. Windthrow is most common on shallow soils with
coarse-textured profiles. Wind damage is also more likely to occur in
recently thinned stands (37,105).
Direct losses to lightning are small, averaging only about 5 trees per
100 hectares (2/100 acres) per year. Large, dominant, open-grown trees are
generally the most vulnerable to lightning strikes. Probably more
important than the direct damage caused by lightning is the possibility
that a lightning-struck tree will become a center for insect infestation
(37).
Damage or seedling mortality caused by low or freezing temperatures
occurs primarily in the northern extremities of the loblolly pine range.
Older, vigorous trees can usually withstand occasional low temperatures
(37,79). Greater damage frequently occurs from ice or glaze storms. This
damage is normally associated with branch and stem breakage, severe
bending and, in some cases, uprooting. Ice damage is usually more severe
in recently thinned (particularly row thinned) plantations and in heavily
stocked stands made up of slender, small-crowned trees (37,91). Extremely
high summer temperatures and drought often cause mortality of seedlings
and, in some cases, of larger trees. Heat and drought more often cause
stress and a resultant loss of vigor and growth in larger trees, which can
lead to more serious problems with insect infestations.
Loblolly pine seedlings or saplings cannot withstand prolonged flooding.
Complete inundation for more than 2 weeks during the growing season often
results in significant mortality. Larger trees are classed as moderately
tolerant of flooding; typically they can survive one season but usually
succumb during the second growing season if continuously in 0.3 m (1 ft)
or more of water (37,113).
A comprehensive review of insects associated with loblolly pine is
provided by Baker (7). Loblolly pine serves as host to a multitude of
insect pests; however, insect outbreaks vary greatly in frequency, area,
and duration. The majority of outbreaks are small and short-lived and
usually consist of only one or a few spots in a stand, but some may expand
until they encompass hundreds of hectares and last for several years
before subsiding. With only a few exceptions, the majority of the insects
that attack loblolly pine are insignificant in terms of damage or
mortality.
The most serious insect pests to loblolly pine are bark beetles,
particularly the southern pine beetle (Dendroctonus frontalis), whose
attack may result in extensive mortality, and pine engraver beetles (Ips
spp.), that can cause death of isolated or small groups of trees; pine tip
moths (Rhyacionia spp.), that often infest young trees;
seedling debarking weevils (Hylobius spp.and Pachylobius
spp.), that sometimes result in girdling and death of young
seedlings up to 13 mm (0.5 in) in d.b.h.; and cone and seed feeders (Dioryctria
spp.and Leptoglossus spp.), that can
seriously reduce seed crops. Loblolly pine is generally the preferred host
of the southern pine beetle, which is the most destructive insect for this
species (102). Most infestations originate in stands that are under stress
because of poor site, adverse weather, overstocking, or overmaturity. Once
a buildup of southern pine beetle occurs, adjacent well-managed stands may
also be attacked. Preventive measures include avoidance of planting
offsite and maintenance of vigorous stands through silvicultural practices
such as controlling density through thinning and harvesting trees at or
before maturity (6,102).
A general account of diseases associated with loblolly pine is provided
by Hepting (54). The most common disease problems in loblolly pine are
related to seedling susceptibility to black root rot (Fusarium spp.,
Macrophomina spp., and possibly others) and fusiform rust (Cronartium
quercuum f. sp. fusiforme); sapling susceptibility to fusiform
rust; root rot by Heterobasidion annosum in thinned stands; and
heart rot in old stands with Phellinus pini in the bole and Phaeolus
schweinitzii primarily in the butt.
Nursery seedlings are subject to root rot in soils with pH above 6.0
under moist conditions; however, root rot becomes severe only if soil
temperatures remain above 32° C (90° F) for long periods.
Fusiform rust is also a major nursery disease in many parts of the South,
requiring rigid spray programs to keep infections low.
The most serious stem disease is fusiform rust, which kills and
disfigures young trees from Virginia to Texas. Saplings and older trees,
especially if planted, are subject to attacks by Heterobasidion
annosum in stands where some cutting has taken place. It is considered
a disease problem in plantation management second only to fusiform rust.
Losses in natural stands or in the absence of some cutting are generally
negligible.
Phaeolus schweinitzii causes a root and butt rot, usually after
basal or root injuries, and in the Deep South it has caused more loss in
some areas than Heterobasidion annosum. Red heart (Phellinus
pini), entering almost entirely through dead branch stubs, is rarely a
factor under the age of 60 years. However, when large branches that have
heartwood begin to die, red heart can set in and destroy much of a tree.
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Flowering and Fruiting
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tarjonnut Silvics of North America
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.
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Growth and Yield
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tarjonnut Silvics of North America
Growth of loblolly pine stands is inherently
good when compared to most hardwood competitors and on many sites doubles
or triples the production of common associates (108). Growth is influenced
by the physical and chemical properties of soils (texture, compaction,
aeration, moisture, pH, nutrients), light, temperature, photoperiod,
allelopathy, precipitation and its seasonal pattern, and intra- and
inter-species competition for space and essential elements. Because many
of these factors interact, it is difficult to specify the most limiting
one. Consequently, these biotic and environmental effects are commonly
expressed as the average height of dominant trees at age 50 years, that
is, site index.
Yield estimates for natural, even-aged loblolly pine in fully stocked
stands were first made more than 50 years ago (3,106). Additional
estimates have been made in more recent years for stands of various
stocking levels (18,81,90,99).
Normal yields of natural, even-aged loblolly pine stands on average
sites, such as those with a site index of 27 m (90 ft), have ranged from
133.0 m³/ha (1,900 ft³/acre) in trees 9 cm (3.6 in) and larger
in d.b.h. including 29.4 m³/ha (2,100 fbm/acre) in trees 24 cm (9.6
in) and larger d.b.h. at age 20 to 427.7 m³/ha (6,110 ft³ or
40,000 fbm/acre) at age 60 (all board-foot volumes reported in
International quarter-inch rule). Mean annual cubic volume growth
generally culminates at about age 40 on these sites with approximately 8.0
m³/ha (115 ft³/acre). As a result of larger sawtimber
merchantability limits, mean annual board-foot growth culminates at about
age 50 at a rate of 9.5 m³/ha (680 fbm/acre).
Growth of loblolly pine may be affected adversely by drought, excess
moisture (flooding), and nutrient deficiencies. Growth of this species is
highly correlated with departure from the normal rainfall of April through
October. Extreme negative and positive departures (-117 vs. 229 mm or -4.6
vs. 9.0 in) in seasonal rainfall over 21 years resulted in differences of
nearly 2.1 m³ (74 ft³) of annual growth (12,39,65). Drainage
(including bedding) and fertilization have been shown to increase dominant
height and basal-area growth, resulting in dramatic increases in volume
growth (45,63,76,101).
Growth of planted loblolly pine is affected by the same factors
affecting natural stands. Sites are usually prepared before planting on
cutover lands, and some are fertilized to correct nutrient deficiencies.
Such practices are applied to control competition and to supply nutrients
at optimum levels to establish vigorous, uniform stands at spacings that
will fully utilize site potentials.
Yields of planted loblolly pine vary with plantation age, site quality,
number of trees planted, and interactions of these variables. Yields
generally increase with increasing age and site quality. Yields also
increase with higher planting density or closer spacing; however, on some
sites, moderately wide spacing of 2.4 by 2.4 m (8 by 8 ft) or 3.0 by 3.0 m
(10 by 10 ft) outproduce both wider and closer spacing. Mean annual
increment culminates at younger ages on better sites than on poorer ones.
Better sites can carry more stocking than poor sites; consequently,
initial spacing can be closer (9,77,93).
Closer spacing tends to produce higher total cubic volumes at younger
ages than does wider spacing; however, average tree sizes are larger on
wider spacings than on closer ones. If sawtimber is a primary management
objective, then wider spacing or lower density would be advantageous.
Although thinning seldom increases cubic volume yield of loblolly pine,
light thinnings that salvage suppressed and moribund trees have increased
net yields by as much as 20 percent in 50 years. Thinnings usually result
in increased diameter growth of residual trees and allow the growth to be
put on the better trees in the stand. Another benefit is that thinnings
provide intermediate returns on investment (2,17).
Average total solid-wood yields of unthinned loblolly pine planted at
1,730 seedlings per hectare (700/acre) on non-old-field sites at various
locations within its range were predicted to increase from approximately
155 m³/ha (2,200 ft³/acre) at age 15 to 300 m³/ha (4,200 ft³/acre)
at age 30. Mean annual increment at age 30 was about 10 m³/ha (145 ft³/acre)
(1,4,27,33,67). Estimates are also available for a variety of site and
stand conditions and geographic areas (8,21,22,23,25,44,68,71).
Growth and yield in natural uneven-aged loblolly pine stands is
dependent on stand structure, stocking, and site quality. To optimize
average annual growth on average sites with a site index of 27 m (90 ft),
stand structure should be manipulated so that approximately 70 percent of
the merchantable cubic volume is in the saw-log portion of the stand, that
is, trees 25 cm (10 in) in d.b.h. and larger. On average sites, stands
with approximately 17 m²/ha (75 ft²/acre) of basal area, or 140
m³/ha (2,000 ft³/acre) total merchantable volume, or 10,000 fbm
saw-log volume at the end of the cutting cycle would be considered well
stocked (5,84,86).
On good sites in southern Arkansas, with a site index of 27 m (90 ft)
managed uneven-aged loblolly pine stands that are well stocked have
averaged 0.7 m²/ha (3 ft²/acre) of basal-area growth, 5.6 m³/ha
(80 ft³/acre) of merchantable volume growth, or 432 fbm/acre of
saw-log volume growth per year for a 29-year period. On somewhat poorer
sites in the Georgia Piedmont with a site index of 23 m (75 ft), annual
growth has averaged 5.3 m³/ha (76 ft³/acre) or 319 fbm/acre over
a 21-year period (5,17,82,85,86).
In sapling stands, differences in growth rate of individual loblolly
pines are evident at early ages when competition between trees begins. The
growth differentiation process begins at earlier ages on better sites or
at higher levels of stocking; it begins later on poor sites or at low
levels of stocking (51). The result is separation of trees into crown
classes. Growth in height is a critical factor in the occupation of
available space. Loblolly pine is a species in which individual trees tend
to express dominance at an early age, and the most vigorous individuals
that are best adapted to the microsite environment become dominants as the
stand ages.
Faster growing trees develop larger live-crown ratios than do slower
growing trees. Diameter growth of individual trees generally increases as
crown surface area and crown ratio increase, with optimal diameter growth
occurring when trees have at least a 40 percent live-crown ratio. Diameter
increment does not occur uniformly on portions of the bole. Annual
diameter growth is greatest within the crown and decreases with increased
distance below the crown. This phenomenon causes the bole of loblolly pine
trees to become cylindrical with increasing age. Height growth is not as
sensitive as diameter growth to differences in crown size. Height growth
of codominants is significantly less, however, in dense stands of trees
with small crowns than in low-density stands of trees with larger crowns
(37,38,51).
Loblolly pine is a medium-lived tree. Maximum recorded age of one tree
in a small stand of 20 trees in North Carolina was 245 years, with the
group averaging 240 years. The largest tree in this stand was 135 cm (53
in) in d.b.h. and 45.7 m (150 ft) tall. Currently, the champion for the
species in the "National Register of Big Trees" is located near
Urania, LA, and is 143 cm (56.3 in) in d.b.h. and 49.7 m (163 ft) tall
(52).
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Reaction to Competition
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tarjonnut Silvics of North America
Loblolly pine is moderately tolerant
when young but becomes intolerant of shade with age. Its shade tolerance
is similar to that of shortleaf and Virginia pines, less than that of most
hardwoods, and more than that of slash and longleaf pines (31,37,108).
Loblolly pine is most accurately classed as intolerant of shade.
Succession in loblolly pine stands that originate in old fields and
cutover lands exhibit a rather predictable pattern. The more tolerant
hardwoods (including various species of oaks and hickories, sweetgum,
blackgum, beech, magnolia, holly, and dogwood) invade the understory of
loblolly pine stands and, with time, gradually increase in numbers and in
basal area. The hardwoods finally share dominance with each other and with
loblolly pine (37,83,100).
The climax forest for the loblolly pine type has been described as
oak-hickory, beech-maple, magnolia-beech, and oak-hickory-pine in various
parts of its range (28,37). Others view the climax forest as several
possible combinations of hardwood species and loblolly pine. There is
evidence that within the range of loblolly pine several different tree
species could potentially occupy a given area for an indefinite period of
time and that disturbance is a naturally occurring phenomenon. If this is
so, then the climax for this southern forest might best be termed the
southern mixed hardwood-pine forest (83).
Competition affects the growth of loblolly pine in varying degrees
depending on the site, the amount and size of competing vegetation, and
age of the loblolly pine stand. Across the southern region, average loss
of volume production resulting from hardwood competition has been
estimated at 25 percent in natural stands and 14 percent in plantations
(35). In a North Carolina study, residual hardwoods after logging reduced
cubic-volume growth of a new stand of loblolly pine by 50 percent at 20
years, and where additional small hardwoods of sprout and seedling origin
were present, growth was reduced by another 20 percent by age 20 (10,64).
Similar growth responses in young seedling and sapling stands have been
observed in Arkansas, Louisiana, and Texas (24,26,39). Although several
short-term studies (5 years or less) of the effects of understory
hardwoods on growth of older loblolly pine did not show measurable effects
(58), a long-term study (11 to 14 years) showed growth increases of 20 to
43 percent in cubic volume and 21 to 54 percent in board-foot volume after
removal of understory vegetation (39). Control of both residual overstory
and understory hardwoods is a financially attractive silvicultural
treatment for loblolly pine management (10).
Silvicultural practices such as prescribed burns, the use of herbicides,
and mechanical treatments arrest natural succession in loblolly pine
stands by retarding the growth and development of hardwood understories.
Prescribed fire is effective for manipulating understory vegetation,
reducing excessive fuel (hazard reduction), disposing of logging slash,
preparing planting sites and seedbeds, and improving wildlife habitat.
Responses of the understory to prescribed fire varies with frequency and
season of burning. Periodic winter burns keep hardwood understories in
check, while a series of annual summer burns usually reduces vigor and
increases mortality of hardwood rootstocks (62). In the Atlantic Coastal
Plain, a series of prescribed bums, such as a winter bum followed by three
annual summer bums before a harvest cut, has been more effective than
disking for control of competing hardwood vegetation and improvement of
pine seedling growth after establishment of natural regeneration
(103,104).
Loblolly pine expresses dominance early, and various crown classes
develop rapidly under competition on good sites; but in dense stands on
poor sites, expression of dominance and crown differentiation are slower
(37).
Dense natural stands of loblolly pine usually respond well to
precommercial thinning. To ensure, the best volume gains, stocking should
be reduced to 1,235 to 1,730 stems per hectare (500 to 700/acre) by age 5.
When managing for sawtimber, thinnings increase diameter growth of
residual trees and allow growth to be put on the better trees in the
stand, thus maximizing saw-log volume growth and profitability (56,78).
Loblolly pines that have developed in a suppressed condition respond in
varying degrees to release. Increases in diameter growth after release are
related to live-crown ratio and crown growing space, but trees of large
diameter generally respond less than trees of small diameter. Trees with
well-developed crowns usually respond best to release. Trees long
suppressed may also grow much faster in both height and diameter after
release but may never attain the growth rate of trees that were never
suppressed (37,75).
Loblolly pine can be regenerated and managed with any of the four
recognized reproduction cutting methods and silvicultural systems.
Even-aged management is most commonly used on large acreages; however,
uneven-aged management with selection cutting has proved to be a
successful alternative.
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Rooting Habit
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tarjonnut Silvics of North America
The rooting habit of loblolly pine is strongly
influenced by tree age, soil, and the soil environment. A young tree
develops a short taproot but in most cases it ceases growth in favor of an
extensive lateral-root system. A taproot 1.5 to 2.0 m (5.0 to 6.5 ft) long
is often produced on deep, sandy or loamy soils. On heavy clay soils, the
taproot tends to be stout and short. Taproots of loblolly pines are much
smaller and shorter than those of shortleaf and longleaf pines. On
excessively wet sites or when a water table or an impenetrable hardpan
confines the roots to surface layers of soil, lateral roots are prominent
in a superficial system (3,50,108).
In a 6-year-old loblolly pine plantation in southeast Louisiana, 83
percent of total root weight was in the upper 46 cm (18 in) of soil. In a
31-year-old natural stand in North Carolina, the majority of the feeder
roots less than 2.5 mm (0.1 in) in diameter were concentrated in the 15-cm
(6-in) deep A horizon; practically no lateral roots were found below the
15- to 53-cm (6- to 21-in) depth of the B horizon (14,59).
Roots of loblolly generally spread laterally farther than their crowns.
As a result, root grafting is a common occurrence both in natural stands
and closely spaced plantations. Roots grow at all times of the year, but
most root growth occurs in April and May, and in late summer and early
fall (37,80,89,108).
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Seed Production and Dissemination
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tarjonnut Silvics of North America
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).
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Seedling Development
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tarjonnut Silvics of North America
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).
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Soils and Topography
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tarjonnut Silvics of North America
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).
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Special Uses
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tarjonnut Silvics of North America
Natural loblolly pine stands as well as intensively managed plantations
provide habitat for a variety of game and nongame wildlife species. The
primary game species that inhabit pine and pine-hardwood forests include
white-tailed deer, gray and fox squirrel, bobwhite quail, wild turkey,
mourning doves, and rabbits (94). Some of these species utilize the
habitat through all stages of stand development, while others are
attracted for only a short time during a particular stage of development.
For example, a loblolly pine plantation can provide forage for deer only
from the time of planting to crown closure. Without modifying management
practices, this usually occurs in 8 to 10 years (13). Bobwhite tend to use
the plantation until a decline in favored food species occurs (20). As the
habitat deteriorates, deer and quail usually move to mature pine or
pine-hardwood forests (47) or to other newly established plantations.
Management modifications such as wider planting spacing and early and
frequent thinnings will delay crown closure, and periodic prescribed bums
will stimulate wildlife food production.
Wild turkeys inhabit upland pine and pine-hardwood forests and do
particularly well on large tracts of mature timber with frequent openings
and where prescribed burning is conducted (96,97).
Pine lands are the chief habitat for some birds such as the pine
warbler, brown-headed nuthatch, and Bachman's warbler. Old-growth stands
are very important to the existence of the red-cockaded woodpecker. Large
loblolly pine trees are favorite roosting places for many birds and
provide an important nesting site for ospreys and the bald eagle (46).
In urban forestry, loblolly pines often are used as shade trees and for
wind and noise barriers throughout the South. They also have been used
extensively for soil stabilization and control of areas subject to severe
surface erosion and gullying. Loblolly pine provides rapid growth and site
occupancy and good litter production for these purposes (114,115).
Biomass for energy is currently being obtained from precommercial
thinnings and from logging residue in loblolly pine stands. Utilization of
these energy sources will undoubtedly increase, and loblolly pine energy
plantations may become a reality.
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Vegetative Reproduction
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tarjonnut Silvics of North America
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).
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Distribution
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tarjonnut Silvics of North America
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 loblolly pine.
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Brief Summary
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tarjonnut Silvics of North America
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.
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