by
Bruce J. Zobel1 and Keith W. Dorman2
GENERAL
Loblolly pine (Pinus taeda L.) is widely planted in the United States in regions of the South and Southeast. It is a species with great adaptability, good wood qualities and it is easy to manage. Because of its extensive native range it is complex genetically and consists of a number of geographic races. It hybridizes freely with several other species to the extent that it is difficult to say what is pure loblolly. However, it always seems to be the “dominant” species, and areas of mixture tend toward loblolly pine.
Because of its wide geographic variability, loblolly pine has not been as successful or widely used as an exotic as it should be. Many plantings have been failures because the person making the test has obtained seeds of unknown origin and planted them in environments to which they were not adapted. Because of this there has been an unfortunate opinion in many subtropical areas that the more widely planted slash pine (P. elliottii) is better and loblolly is inferior.
In the past it has not always been easy to obtain proper sources of loblolly, but the recently organized Seed Center of the United States Forest Service has been most helpful 3. As genetically improved stock becomes more available, and as the geographic diversity is recognized and used, the value of loblolly pine as an exotic will continue to increase. This paper outlines (1) some major characteristics of loblolly pine, and (2) some considerations when using it as an exotic.
1 Professor of Forest Genetics, North Carolina State University, Raleigh, N.C.
3 For further information see page 48.
LOBLOLLY PINE RANGE
The range of loblolly pine is roughly rectangular from east to west in the southeastern United States. But there are extensions northeastward along the Atlantic Coast and south into Florida (Fig. 1). The species does not occur in the Mississippi River area. Locations of seed sources with distinctive performance are shown and these as well as others are discussed later. Also, physiographic regions such as the Coastal Plain, Fall Line or Sandhills, and the Piedmont, important to our discussion, are shown in Figure 1.
Climographs which can be used with latitude and longitude to help estimate homoclimes when loblolly pine is used as an exotic are also shown (Fig. 2). In Florida, rainfall is high during periods of high temperature, but in Texas rainfall is low when temperatures are high. The letters J, A and O identify the months of January, August and October in each graph. The data are from detailed climatological summaries given in a separate paper (Hocker, 1955).
Figure 1: Botanical range of loblolly pine and major physiographic regions
KEY TO SEED SOURCES
1-MARYLAND
2-DAIRE COUNTY, N.CAROLINA
3-ONSLOW COUNTY, N CAROLINA
4-GEORGETOWN COUNTY, S. CAROLINA
5-MARION COUNTY, FLORIDA
6-LIVINGSTON PARISH, LOUISIANA
7-“LOST PINES”, TEXAS
Figure 2: Range in climatic types throughout the loblolly pine region (after Wahlenberg, W.G. and Ostrom, C.E., 1956 “Geographic variation in climate in the loblolly pine region”. Res. Note S.E. For. Exp. Sta. No. 94).
CHARACTERISTICS OF LOBLOLLY PINE
The amount and kind of variation in loblolly pine is of sufficient importance to play an important part in any tree improvement programme involving introduction of seed, racial selection, plus-tree selection, or hybridization. More has been learned about this species than for any other southern pine, but the information is more complete for kinds of variation than the precise amount for each trait. The discussion to follow contains only a few selected references for each trait or subject, but additional information is available in a longer paper, “The Genetics of Loblolly Pine” (Dorman and Zobel, 1973).
Variation and Inheritance
Growth rate
At ages 5, 10 and 15 years the range-wide seed source study sponsored by the Forest Tree Improvement Committee showed strong clinal variation in growth, with trees from coastal areas generally growing faster than those from inland areas in all but the coldest planting locations (Wakeley, 1961; Wells and Wakeley, 1966; Wells, 1969). When trees were moved from one extremity of the range to another, they produced less than one-half of the volume of trees grown from local seed, because of a combination of poor survival and slow growth; but when trees of coastal origin were moved inland as far as 225 miles in the southern part of the range, they grew faster than trees that originated in inland areas.
Four of the 15 sources of loblolly pine in this range-wide study had considerably less fusiform rust than the others. These relatively resistant trees were those originating west of the Mississippi River, in Livingston Parish, Louisiana, and on the eastern shore of Maryland. Another range-wide study of 36 seed sources grown at Crossett (Ashley County), Arkansas, also demonstrates the fast growth potential of loblolly pines from coastal areas, especially from trees from the Carolinas (Grigsby, 1973).
The results of the range-wide studies have been corroborated and expanded by studies in which intensive samples were made of loblolly pines throughout Georgia (Kraus, 1967), Alabama (Goggans, Lynch and Garin, 1972), Mississippi (Wells and Switzer, 1971), and North Carolina (Lantz and Hofmann, 1969). A study of several populations of loblolly pine from Louisiana that were planted elsewhere in the same state revealed that those from Livingston Parish and western Louisiana were resistant to rust, but differences in height between northern and southern Louisiana sources, while suggestive, were not statistically significant (Crow, 1964; Merrifield, 1965).
The within-state studies have established that, in addition to the patterns of variation established by the range-wide studies, loblolly pine varies clinally in survival, potential growth to some extent, and resistance to fusiform rust. Loblolly pines from the Gulf and the South Atlantic Coasts survive planting slightly less well and have less inherent resistance to rust than do those from inland areas.
Inherent differences in growth among individual parent trees are large in addition to that among geographic sources. In Texas, growth of progenies of loblolly pine parents in seed production areas has been, on the average, about the same as that of the controls, but these progenies have been more uniform and there have been wide differences among individual families (van Buijtenen, 1969). The ten highest-producing families yielded 1.82 cords per acre per year, the controls yielded 1.54 cords, and the ten lowest-producing families yielded 1.14 cords. The lowest growth of an individual family was 0.90 cord per acre per year, and the highest growth of an individual family was 2.05 cords. Large differences in yield of dry wood per acre occured in open-pollinated families of certain parental trees selected for industrial seed orchards (Zobel, Kellison and Matthias, 1969). Yield varied from 1.8 to 2.7 tons of dry wood per acre per year among 7-year-old families growing on an excellent site. In general, growth of progenies from rogued clonal seed orchards is expected to be about 10 to 20 percent higher than that of commercial planting stock. When progeny of 11 trees selected in southeast, central, and northwest Georgia were planted in central Georgia, the fastest-growing family from each of the three areas had a height superiority over the slowest-growing family of 17 to 31 percent after five growing seasons (Barber, 1966). The slowest-growing family of the slowest growing seed source averaged 8.4 feet, and the fastest-growing family of the fastest-growing seed source averaged 12.0 feet — or nearly 43 percent greater height — indicating that the effects of racial selection and plus-tree selections may be cumulative.
Crown and stem form
Loblolly pine is a highly vigorous species with relatively poor form; consequently, certain trees may have long and heavy branches, poor pruning, large branch angle with the horizontal, forked stems, or crooked stems. However, trees with a combination of good crown and stem traits do occur and selection should concentrate on them.
Large differences in stem and crown form occur in progeny tests. Heritability of crown width in 7-year-old open-pollinated progeny of trees with well-formed crowns and of trees with poorly formed crowns was 34 percent (Trousdell, Dorman and Squillace, 1963). In Texas, the ratio of mean limb length to tree height was significantly lower in progeny of three out of five trees selected for good quality than in check trees, but there were no significant differences in mean limb angles (Goddard, Brown and Campbell, 1959).
Controlled pollinations were made among crooked loblolly pine trees in Florida and the progeny were compared with those of straight trees (Perry, 1960). At two years of age, progenies from crosses between straight trees contained four times as many straight seedlings as did progeny from crosses among crooked parents. At age 7, about the same proportion of crooked and straight trees existed (Goddard and Strickland, 1964). It was concluded that there were reasonably strong general and specific combining abilities for stem straightness.
Wood properties
The heavy and strong wood of loblolly pine is used for a wide range of wood products and types of paper. Studies of variation in wood and fiber characteristics have been done because of the need to maintain high and uniform quality while improving growth and yield and, if feasible, special characteristics for various products. As a basis for determining inherent differences, nondestructive techniques were developed for sampling wood characteristics in relation to tree size, age and environmental factors. The results of these studies in techniques have been summarized (Zobel, 1961, 1964) and need not be repeated. Geographic variation in wood quality has been extensively investigated because of its importance in pulp yield and quality as well as in lumber grade. Studies conducted throughout the Southeast by the USDA Forest Products Laboratory (1965), industries and universities indicated that wood specific gravity decreases from southeast to northeast, from coast to Piedmont. Differences were found to be rather large and consistent; they have a major effect on yield and pulp quality and have been important in forestry activities and land ownership patterns in this region.
In the Carolinas, wood specific gravity, percentage of summerwood, alcohol-benzene extractives, pulp yield and zero-span tensile strenth were found to be higher for trees in the Coastal Plain than for those in the Piedmont (Einspahr, Peckham and Mathes, 1964). In Maryland and Delaware, specific gravity and tracheid length were found to decrease from south to north (Whitesell, Zobel and Roberds, 1966). In the southeastern United States, differences in wood specific gravity and various types of cellulose were found, but the differences were smaller among geographic locations than among individual trees (Zobel and McElwee, 1958; Zobel, Thorbjornsen and Henson, 1960).
Specific gravity and moisture content among trees in a given area usually are closely (and inversely) related. Therefore, patterns of specific gravity closely parallel patterns of moisture content. This relationship does not always hold for geographic distributions, however: moisture contents of the loblolly pines with lower specific gravity in the mountainous area of Tennessee were found to be high as expected, but in certain areas of the Georgia Coastal Plain, where specific gravity is high, moisture content was found to be higher than for trees in the adjoining Piedmont (Zobel, Matthias, Roberds and Kellison, 1967).
Tree-to-tree variation in wood specific gravity, tracheid length and other wood morphological traits has been intensively studied in relation to growth. The first important paper of this type, by Zobel and Rhodes (1955), showed that wide variation in specific gravity occurs among trees as well as within and between geographic areas, but there was a low correlation between diameter growth and specific gravity. Many additional papers have been published with similar findings. Although these cannot be cited here they are illustrated by such papers as van Buijtenen (1963), Einspahr, van Buijtenen and Peckham (1969), and Zobel et al. (1969). Findings from the bulk of the papers can be summarized as follows:
For all wood characteristics studied, large tree-to-tree variation is found among trees of the same age on the same site. Much of the difference has been found to be under genetic control, with narrow-sense heritabilities usually about 0.5 to 0.6.
Many important wood characteristics (e.g., specific gravity and tracheid length) are not highly correlated; therefore, it is possible to breed for them separately.
Age of tree has a major effect on wood properties, with younger trees having lower wood density, shorter tracheids and thinner cell walls.
Wood qualities are affected by different environments, including fertilizer. Nitrogen fertilizers appear to have the most effect on wood of loblolly pine.
On older trees, growth rate and specific gravity are not highly correlated. This lack of correlation allows the development of fast-growing strains of trees either with high or low specific gravities.
Trees in areas with high summer rainfall and warmer climates tend to have higher wood density than those in droughty areas or cooler climates. This trend is well documented for specific gravity and other morphological characteristics of wood.
The pattern of wood variation, particularly the tree-to-tree differences, is such that good progress can be obtained by breeding for wood quality.
Resistance to drought and cold
At certain locations in the western part of the loblolly pine range, rainfall is low during periods of high temperature, and drought resistance has been included among desired traits in breeding programmes. Droughts also occur in the deep sands in areas of heavier rainfall. Selection for increased drought resistance has been successful, as indicated by the fact that progenies of selected trees had higher survival than the controls (van Buijtenen, 1966; Gilmore, 1957) and no loss in height growth (Goddard and Brown, 1959; Banks, 1966).
Resistance to pests
Susceptibility to damage from insects and diseases is important in breeding loblolly pine because of the effects on seed production, growth of individual trees and yield and quality of wood products per acre. Important racial and tree-to-tree differences in susceptibility to one disease has been found and studies of other pests are being made.
The most important pest of loblolly pine is fusiform rust (Cronartium fusiforme Hedgc. & Hunt ex Cumm.). It is highly destructive on many hundreds of thousands of acres, some of which are over 90 percent infected, in a broad belt from South Carolina and Georgia west to Louisiana (Roth & McComb, 1971).
Of great value to applied breeding projects, progeny tests of seed orchard clones show that wide variation in rust resistance occurs among families (Woessner, 1965). In a large heritability study with parental stock from a wild population, Kinloch and Stonecypher (1969) found heritabilities of rust resistance of 0.65 to 0.85, variation from 17.3 to 100 percent for infection among families and positive correlation for resistance among parents and offspring. Variation in the pathogen with latitude and longitude has been shown (Snow, Kais and Dinus, 1972), and albino forms have been described (Kais, 1966). The picture as to the importance of races of rust is far from clear but studies now underway will help.
A good example of geographic variation is in susceptibility to fusiform rust. In loblolly pine, incidence of infection by fusiform rust is attributable to both environmental and genetic factors (Wells and Wakeley, 1966). Rust infection in loblolly pines growing in eastern North Carolina is low, but the infection rate is high in North Carolina trees planted in middle Georgia, an area of high rust incidence. Incidence of rust infection is also low in east Texas loblolly pines but remains low when Texas trees are planted in middle Georgia. Perhaps the most important finding of the genetic effort in the southern pine region has been the degree of resistance to this disease by some parents. Special disease resistance seed orchards have been established to take advantage of the early findings (Blair, 1970; Zobel, Blair and Zoerb, 1971).
The Nantucket pine tip moth (Rhyacionia frustrana Comst.) is a common pest but apparently little if any intraspecific variation occurs in resistance to attack in loblolly pine. Resistance to bark beetles is being studied.
Association among traits
The kind and amount of variation among individual loblolly pine trees is wide but high statistical correlations among traits are few. Thus, selection of plus-trees should be on the basis of many traits, such as by an index system, to insure a good combination of silviculturally important traits. Selection for multiple traits will help avoid the opportunity for one bad trait to offset several good ones, as might occur if the tandem selection method is used. It is true, however, that broad relationships occur in loblolly pine, as in other southern pines, such as an increase in stem diameter, branch length and crown length with increased growing space. But exceptions occur, and trees with which the tree breeder can work are to be found that have high growth rate and many other desirable traits of crown and stem form, seed production, resistance to pests and wood quality.
Natural and Artificial Hybrids
The major southern pines and certain minor species hybridize naturally with loblolly pine. Hybridization contributes to variation among geographic areas and trees within stands. The number of traits and extent to which they are influenced by hybridization are economically important. Thus, hybridization is an important factor in selection of geographic races or individual trees for planting or use in breeding projects.
Evidence of at least three generations of hybridization of longleaf and loblolly pine with intermediate forms was found in a natural stand in Louisiana (Namkoong, 1966). The existence of trees intermediate between loblolly and shortleaf (Pinus echinata Mill.) pines in morphological traits (Zobel, 1953) and electrophoretic patterns of seed proteins (Hare and Switzer, 1969) indicate that this hybrid occurs in Texas where infection by fusiform rust is low. In Mississippi, trees similar to artificial hybrids of loblolly and shortleaf pines were found at two locations (Mergen, Stairs and Snyder, 1965). In the northeastern part of the range, hybrids of loblolly pine with pond (P. serotina Michx.) and pitch (P. rigida Mill.) pines occur (Little, Little and Doolittle, 1967; Smouse, 1970).
In artificial hybrids involving loblolly pine, many morphological traits are intermediate (Little and Righter, 1965; Little et al., 1967; Schmitt, 1968) but seed yield per cone may be extremely low (Snyder and Squillace, 1966) and resistance to fusiform rust high. Hybrids with pitch pine are commercially important in Korea because the form is better than that of pitch and the cold resistance better than that of loblolly (Hyun, 1970). Loblolly pine pollen of northern sources is preferred in Korea.
Mutations
Loblolly pine is diploid normally but mixoploid and polyploid seedlings occur infrequently in nursery seed beds. Aberrant plants are low in vigour and usually do not mature so they are rare in stands. Twenty-two phenotypically dissimilar mutant forms were found in 30 out of a total of 119 selfed loblolly pine families (Franklin, 1969). Generally, mutants are not sufficiently desirable to be a goal or prevalent enough to be avoided in selection programmes.
LOBLOLLY PINE AS AN EXOTIC
Despite its past “bad name” from poor performance when the incorrect seed source has been used, loblolly pine is currently becoming widely used as an exotic in a number of countries. Its importance has increased in suitable areas such as the Transvaal in South Africa, New South Wales (Australia), or the higher elevations or latitudes of Brazil and adjacent areas of Argentina.
If loblolly pine is to be successful as an exotic, the seed source must be very carefully chosen. For example, in the warmer, essentially frost-free regions to which loblolly pine is adapted, the Florida source grows best, followed closely by the Livingston Parish, Louisiana source and perhaps loblolly pine from South Georgia. Differences in growth rate between the Florida and the more northerly Piedmont sources in areas of low frost incidence can be as great as two to one; in the cooler regions, sources from the Atlantic Coast region such as Onslow County, North Carolina will do well, while in really cold climates the higher elevation sources in Tennessee or the high latitude sources from Maryland will do well. In general, the more southerly sources of loblolly pine are best in frost-free, or nearly frost-free, areas and they grow more rapidly than the northern or Piedmont sources. Care must be used in differentiating between the more moist Coastal Plain sources and the loblolly growing naturally inland in the Piedmont and sandhills regions. Loblolly pine is occasionally found in northern Georgia and Alabama at elevations nearly to 3,000 feet, but it occurs primarily below 1,000 feet elevation. The higher elevation and northern latitude sources are much more cold-hardy than the southern sources and require cool nights to grow properly. Some of the western sources, such as those from the Lost Pines Region of Texas are more drought-hardy than many of the more eastern sources; the Texas source is also more disease-resistant.
Useful Subdivisions
Piedmont and Coastal Plain source loblolly have many differences. The Piedmont zone occupies the foothills inland from the flat plain along the coast; it has lower temperatures, usually lower rainfall with a spring drought, and clay loam and clay soils, while the Coastal Plain often has sands over the clays. There is a narrow zone between the two referred to as the sandhills with deep, droughty soils to which loblolly pine is not well adapted. Subdivisions are often made between the northern Coastal Plain and the southern Coastal Plain along the Atlantic Coast. In tree improvement, separate seed orchards are established for the Piedmont, the northern Coastal Plain and the southern Coastal Plain regions.
Similar divisions are employed farther west along the Gulf Coast where the divisions are referred to as the Lower Coastal Plain, the Upper Coastal Plain, and sometimes a Piedmont or foothill zone. A sandhill or Fall Line region also occurs in this area. Some clear differences occur between the Piedmont (and Upper Coastal Plain) and the Coastal Plain provenances. Loblolly from the Coastal Plain provenance grows faster, both on Coastal Plain and Piedmont sites, but in abnormally dry years or when sudden, severe freezes occur it will succumb when the Piedmont provenance survives. Studies on bark thickness have shown that the bark in young trees of the Coastal Plain provenance is thinner than that of the Piedmont provenance (25 percent compared with 33 percent in ten-year-old trees). It remains to be seen whether this difference is maintained in the mature trees, for which a rough average figure for bark is 12 to 16 percent. The Piedmont provenance produces seed earlier and more abundantly than the Coastal Plain provenance, even when both have been planted on the same site. In some instances a seed orchard from a Piedmont provenance has produced as much as eight or ten times the quantity of seed as a Coastal Plain provenance of the same age handled in an identical manner. Knowledge of such a difference in seed production is most important for planning the correct size of a seed orchard.
A broad working definition of useful provenance within loblolly pine would include:
North Central Florida. This includes the limestone marl near the coast referred to as Gulf Hammock and the deeper soils in Marion County. This source is suitable for subtropical conditions; it is very fast-growing, with wood of high specific gravity. It is very susceptible to cold damage and is useful only in frost-free areas or where cold is not severe; it also succumbs to drought rather readily. The north Florida source is the fastest-growing provenance of loblolly pine under trial in several areas such as Queensland, parts of Brazil and South Africa. This source tends to be more crooked and larger-limbed than the more northern sources. In its native range it is a poor seed producer, while seed production on some clones in seed orchards is moderately good but delayed in starting. It appears that under conditions in the southeastern United States heavy seed production of the Florida loblolly will be a problem.
Livingston Parish, Louisiana and Vicinity. Loblolly pine from this area appears to be an outstanding source which has performed very well in the southwide seed source study, as reported by O.O. Wells (1969). Growth rate is good, form is generally good and it appears to be adequate in flower production. Although few direct comparisons have yet matured to the desired age, it is our feeling that the Livingston Parish source is not as fast growing as the Florida loblolly. It appears to be unique in its ability to withstand fusiform rust, even when planted into the most badly infected areas. It is now under test in a number of areas in the southeastern region of the United States, but the trials are not yet old enough to determine its reaction to cold, to drought, or to the droughty clay Piedmont soils inland from the coastal area. We feel that this is certainly a mandatory source to test for subtropical areas or regions of light frost.
Other Coastal Plain Sources. Trees from the southern Georgia or southern South Carolina Coastal Plain grow very well and in many instances may be nearly as good as (a) and (b) above. Individual parents and stands must be carefully chosen if maximum gains are to be obtained. Tests of trees from Onslow County, North Carolina have shown good results; it is our experience that good trees from good stands from coastal Carolinas and Georgia grow well under subtropical conditions. Although not as fast-growing as the Florida source, they are much more adaptable and hardy.
Piedmont and Upper Coastal Plain Loblolly Pine (Inland Sources). This group covers a large area inland to both the Gulf and Atlantic Coastal Plains (Fig. 1). Trees from these inland areas should be tested where severe climates occur, such as droughts or freezes. Although slower-growing than some coastal source loblolly, the inland sources should be used when attempts are made to extend the range in which loblolly pine is to be planted. Although the Piedmont and Upper Coastal Plain sources are grouped together, differences do occur and decisions as to whether the Virginia, North Carolina, South Carolina, Georgia, Alabama or more western material is to be used depends upon the severity of the environments to be encountered. Considerable genetically improved seed from seed orchards is available from this region.
Eastern Shore Source. In Virginia and Delaware, loblolly pine grows on a lowlying peninsula, on sandy and sometimes wet clay soils. This Eastern Shore source has not been tested but it appears to grow well under somewhat saline conditions. Natural stands are well formed and produce heavy volumes per acre.
Northern or High-Elevation Sources. Loblolly usually does not grow above 1,000 feet elevation but stands sometimes occur at elevations of nearly 3,000 feet. These trees have been tested and are more resistant to ice and snow than normal loblolly, although they are slower-growing than coastal sources. This highelevation loblolly would be suitable for regions with considerable temperature extremes in uplands of the subtropical areas where subtropical pines do not grow well. Sources of loblolly pine from the more northern latitudes, such as Maryland and Delaware, also are quite hardy to cold but their growth has been disappointing.
Excessively wet or dry sites. Loblolly pine has apparently developed tolerance to extremes of moisture. For example, the drought-resistant pines from the western areas such as the Lost Pine region of Texas (Bastrop County) have been widely tested. They are quite hardy to extreme droughts, and grow, as well as survive, under hot and dry conditions. Choice of parents is very important because some tests have produced trees excessively limby or crooked. The droughthardy source has been found resistant to fusiform rust. Use of drought-resistant loblolly can considerably extend the range where this species may be of value.
Certain populations of loblolly pine are growing in deep peat, sometimes as much as 20 feet of peat before mineral soil is found. Because of a unique root system they stand up and grow well under such an environment. They have been tested and appear to have promise under excessively wet conditions if the water is not stagnant. Orchards of such “water-tolerant” loblolly have been established, primarily for coastal regions of North or South Carolina.
SUMMARY
Loblolly pine is a remarkable species that grows in many environments and has wide adaptability. It is very variable and hybridizes naturally with a number of other pine species. It responds well to genetic manipulation and great strides have been made in improving loblolly pine.
It has not had proper use as an exotic. Because of its wide adaptability, the correct geographic source is basic to success; many early tests ignored this fact and the resultant poor performance has caused some foresters to brand it as an undesirable species. It is generally a rapid-growing tree with good wood but rather poor form. This makes parental selection mandatory.
The southern sources are particularly valuable and fast-growing in the subtropical climates. Wood of loblolly pine is generally of satisfactory quality irrespective of the environment under which it grows. It is generally hardy to diseases and insects as an exotic. Loblolly pine will become more widely used, especially now that improved genetic stock is becoming available.
LITERATURE CITED
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