Loblolly Pine

Four distinctly different climatic conditions occur within the range of loblolly pine: east coast, central Florida (though this species is not abundant there), mid-continent of Arkansas and East Texas, and the Lost Pines of the Bastrop area of central Texas. The Lost Pines have inherited characteristics of drought hardiness: the annual rainfall within their range is about one-half that of the Atlantic Coast. Regeneration is, conversely, reliably obtained in the high summer rainfall zones of the Carolina and Georgia coasts.

Figure 2.9 Four internodes of 1 year's height growth. The first internode is always the longest, the last the shortest. Six or seven flushes of growth, resulting in as many internodes, may occur in the southern extremity of the range of loblolly pine. (USDA Forest Service photo)

Of the climatic factors that determine this species range, probably the most important are the number of summer days each year with precipitation of 1/4 inch or more and the number of days each year with rain of at least 1/4 inch. Both of these relate to the replenishing of moisture lost by évapotranspiration—the combined losses of moisture from the soil by evaporation and from plants by the transpiration of minute droplets of water from the small stoma openings on the surfaces of foliage.

Winter temperatures affect the range of loblolly pine by controlling availability of soil water. The warmer the soil, the more readily the soil's moisture is absorbed by roots. In addition, roots may continue to grow all winter where soil temperatures are high. This, in turn, improves tree vigor and the chance for survival.

Differences between day and night temperatures affect growth of this species. For example, optimum growth occurs along the Atlantic Coast at the Virginia-North Carolina line. There, night and day temperatures differ more than elsewhere in the species' range.

High temperatures late in the summer may also reduce growth. Apparently, oxygen deficits develop in buds, resulting in anaerobic respiration. Growing-tip respiration with inadequate oxygen may cause formation of a hormonal inhibitor that induces dormancy earlier in the autumn than otherwise expected.

Photoperiod, the botanist's term for day length, is the cause of inception and termination of dormancy in trees in the fall and spring. When red light is introduced at night, for example, steady increases in height growth follow. This species grows all winter in cold climates where the day length is artificially increased by providing supplemental light. However, in the warmer climate of the southeastern coast, loblolly pine trees grow all year under irrigated and fertilized conditions.11

The effect of soil properties on the growth of loblolly pine has been demonstrated, most thoroughly perhaps in the Piedmont of South Carolina. There, site index decreases as much as 50 percent with sheet erosion. Where most of the surface soil has been washed away during cultivation, the index may be 50 units less than that of an adjacent virgin soil. Site index also diminishes with diminishing depth to subsoil and decreases with decreasing subsoil friability, most easily recognized by the amount of sand in the soil. Very plastic, heavy clay subsoils at least 18 inches below the surface may have a site index as low as 66. These are very friable.12

In the Atlantic and Gulf coastal plains, depth to subsoil (the zone that contains the greatest amount of clay and is the least permeable to water), subsoil texture, and surface drainage interact to determine site index. As surface soils in coastal flatwoods are quite shallow (usually less than 12 inches) and poorly drained, soil texture becomes the principal criterion for measuring land productivity; the more fine silt and clay in the subsoil, the better the growth of trees. Well-drained soils of the Coastal Plain usually have subsoils permeable to water and roots. However, where permeability appears poor, as evidenced by water standing on the land, root extension seems not to be greatly retarded.

The western Gulf Coastal Plain differs from that of the Atlantic, as the land surface for the former developed during periods of submergence following the Cretaceous period when material was carried down streams and rivers from the older main land to the north. These sediments were deposited as horizontal beds of sands and clays in shallow coastal waters, and later uplifted. Here, growth predictions for loblolly pine are based on texture, surface drainage, and depth to subsoil. Good surface drainage is associated with hilly and rolling uplands, while poor passage of water occurs in the flatlands. Hence, sandy soils of good drainage with less than 10 inches of surface soil are the poorest: Clay sites with poor drainage and more than 20 inches of surface soil are preferred.

Depth to subsoil is important because the thickness of the surface soil reflects the volume of soil available for root penetration without restriction. Site quality also improves as soil texture becomes finer until a certain point is reached. It then falls off due to poor aeration, with further reduction in soil particle size. Such soil relationships to loblolly pine growth have been ascertained throughout the region.

Periodic droughts are perennial obstacles to loblolly pine growth, even where annual rainfall exceeds 40 inches. The effect of drought on growth was calculated for several soil types in the lower Piedmont of Georgia where 6-week periods without rain occur in spring and summer an average of once every 25 years. Four-week droughts take place every 5 years and dry spells of at least 2 weeks virtually every year. Where severe erosion occurs, 2 weeks without rain result in drought conditions, especially in islands of shallow soil interspersed among the deeper, more fertile soil types. In one dry year, for instance, diameter growth was one-half of normal. While the width of the previous year's earlywood ring, when rainfall was normal, equaled the width of the late wood, earlywood in a droughty year was three-fourths of the total annual ring. This suggests that drought losses are largely in wood laid down during the latter part of the growing season.

Depth of soil, next to rainfall, is the most important criterion controlling mortality under drought conditions. Mortality was greatest during the worst drought on record where depth to a plastic, compact subsoil or to bedrock was less than 20 inches. Whole stands were killed where soil was less than 16 inches deep. Stand density was not a factor.

How loblolly pines adapt to excess atmospheric sulfur caused by ore smelting, coal burning, and other industrial processes, has been studied using radioactive sulfur. Translocation of the radiolabeled element throughout the tree's tissues, rather than leaching of the element from the foliage, appears to be the mechanism by which forest trees adapt physiologically to this environmental stress.13

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