Shortleaf Pine

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There is no typical site for shortleaf pine. Like most forest trees, it grows best on moist but well-drained, or mesic, sites. While occurring with many other species, shortleaf pine also grows in pure stands from the West Gulf Coastal Plain to the northeastern extremity of the region.

Often, this pine is the initial tree to seed-in on old fields freshly abandoned from agriculture. Favorable weather must accompany good seed crops for this to occur. Oaks, as a rule, enter old-field stands when the pines are about 20 years old. By that time, enough litter has collected to protect the soil from desiccation and to improve the porosity of the surface horizon. The importance of soil moisture has been shown by artificially inducing drought in a 35-year-old shortleaf pine stand in a Piedmont sandy loam soil in an effort to mimic littleleaf disease. Branches died from the bottom upward, needle length was greatly reduced, needle retention time was shortened, and growth practically ceased after 5 years of such stress.14

Height growth of shortleaf pine following germination depends on the depth of the surface soil and consistence and texture of the subsoil. Fastest growth takes place on the deepest surface soils, the most friable subsoils, and where the subsoil has less than 20 percent clay. The degree of slope also influences rates of growth: It is best on level land.

While available water is the most important variable influencing both growth and survival, the species' drought resistance has been attributed to the capacity to absorb water when moisture is limited, to hold water in its needles during drought, and to maintain high solute concentrations in the foliage when recovering from drought. Perhaps optimal growth for shortleaf pine is in the south-central (Coastal Plain) area of Arkansas.

Littleleaf Disease—The study noted above relating to drought was installed to learn the effect xeric soils could have on a malady first reported in 1934, that by the 1950s caused appreciable reduction in growth of shortleaf pines, especially in the Piedmont of South Carolina. Why were needles abnormally short, sparse, chlorotic, and hanging as tufts at the ends of twigs? Drought alone does not cause this abnormal phenomenon called littleleaf disease. While the amount of nitrogen was normal in drought-induced needles, the nutrient was deficient in foliage characterized by the disease-shortened leaves.15

The malady occurs only where severely eroded or abandoned farm lands have been reforested. Originally, on such sites, a deep, friable and fertile surface soil bore hardwood climax forests. The surface zone, in which roots of trees are most abundant, was deep and well-drained. With cultivation of these rich-appearing virgin soils, however, the surface layer washed away, leaving a thin mantle that forced roots to grow into the impervious, poorly aerated subsoil which, because of erosion, now lies near, or at, the surface.16

Figure 2.10 Shortleaf pine growth and soil. Site index on Coastal Plain soil in Arkansas depends on depth of surface soil, clay content of subsoil, and steepness of the slope. Steep slopes lose top soil to erosion, deep surface soils lose water to underground aquifers, while tight subsoils hold water perched over shallower surface horizons. Too much clay binds water so that it is not available for trees; too little allows water to be lost to drainage and evaporation. (after USDA Forest Service chart by R. Zahner, 1953)

Figure 2.10 Shortleaf pine growth and soil. Site index on Coastal Plain soil in Arkansas depends on depth of surface soil, clay content of subsoil, and steepness of the slope. Steep slopes lose top soil to erosion, deep surface soils lose water to underground aquifers, while tight subsoils hold water perched over shallower surface horizons. Too much clay binds water so that it is not available for trees; too little allows water to be lost to drainage and evaporation. (after USDA Forest Service chart by R. Zahner, 1953)

On lands like this, the shortleaf pines introduced following agricultural abandonment exhibited the disease. While the deficiency turned out to be the nitrogen nutrient, the relationship is not direct. The minute nitrogen-absorbing pine roots were gradually killed by a parasitic soil fungus. Phytophthora cinnamomi interferes with nutrient absorption, especially that of nitrogen, by injuriously infecting the root ends. Spores of the fungi occur widely. In well-drained soils in which sufficient moisture is periodically present for swarm spore production and root infection, spores spread during rainfall to infect other sites. Later, as the soil provides optimum conditions for root growth, new roots form and effectively replace those lost by the infection. In poorly drained soils, the period of high moisture is extended, and root recovery consequently retarded. Applications of nitrogen fertilizer delay death of the trees (see Chapter 6). The disease may also be related to manganese toxicity.17

Why is the problem not so prevalent with loblolly pine under similar conditions? Shortleaf pine feeding roots are most abundant in the upper few inches of the soil; while loblolly pine roots, though fewer, are larger, more deeply distributed, and show greater ability to penetrate heavy soils. Loblolly pines are also more tolerant of poor soil aeration. Perhaps, too, in the Piedmont province the longer-needle species is inherently more vigorous than is shortleaf pine.

Prolific Sprouter—Shortleaf pine is one of the most prolific sprout producers among the southern pines, especially after fire. New trees emerge from damaged seedlings at a "collar" at the juncture of the hypocotyl and the primary root. This is the zone where the root meets the stem in larger trees. As sprouts are traced to the axils of primary needles at that position, new stems, therefore, do not form from adventitious buds, as often thought. Nor do shoots arise from roots.18

For young seedlings, the stem lies horizontally prostrate as shoot growth begins. The stem then turns upward from a point just above the cotyledons on seed leaves. These bends form the crook that typifies shortleaf pine seedlings. (Careful lifting of seedlings of any age enables observation of the crook, a sure way to distinguish between shortleaf and loblolly pines when foliage appears similar.) Then, primary needles and their auxiliary buds form a cluster just below the second bend of the crook. Further thickening engulfs the short horizontal portion. Rootlets develop from the uppermost root tissue close to the bud cluster that is often covered by soil.

Figure 2.11 Bud steles in a shortleaf pine cross-section. A 3.5-inch diameter section basal view shows branching of the steles and the origin at the pith. (authors' collection photo by Earl Stone)

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