Pond Pine

Pond pine is closely related botanically to pitch pine. The similarity is limited to the flower parts and arrangement, the basis for Linnaeus' original classification of the serotinous trees. Pond pine occupies unique sites along the Atlantic Coastal Plain. Areas too poorly drained even for loblolly pine, but in which this scrubby tree does well, include Carolina Bays and pocosins. The Bays, characterized by shallow elliptical depressions with well-defined sandy rims, often appear as peat swamps. Some counts tally as many as a half-million such concave features of various sizes, occurring from New Jersey to northeastern Florida. Pocosins, in contrast, found closer to the ocean shore, may each cover a few to several thousand acres. Prevailing winds probably encouraged these lower terraces to fill with soft black muck or brown fibrous peat. In these swamps-on-a-hill (slight bowl-shape rises in a nearly level terrain), loblolly pines commingle with the pond pines.

Stands of the species also occur along stream banks. In rather isolated tracts in southern Alabama, pond pines hugs the coast. There longleaf and slash pines intermingle with the serotinous species.

Pond pine forests are probably climax in bays and ponds, maintained there in pure stands in the absence of fire because of their ability to survive long seasons of flooding, and regenerated as a consequence of fire. Thus, stands sometimes are uneven-aged, the 15- to 20-year intervals in which the organic sites are swept by wildfires dividing the age classes.

Needles of pond pine are arranged on twigs in a manner that provides for mutual shading. This diminishes photosynthesis, in turn slightly increasing shade tolerance. Thus, these trees respond favorably to release from overtopping shade.

Unusual for species growing in wet organic soils, pond pines withstand high winds. The root system, penetrating to great depths through perched water tables to mineral soils, provides stability.

Drainage encourages oxidation of the peat or muck layer, sometimes completely eliminating it. Such destruction of the organic matter reduces the elevation of the land. Hence, water tables may be nearer the ground surface after drainage than they were before canal and ditch construction.33 Pond pine, however, responds to drainage with improved growth.

Growth is slower on wet organic soils than on mineral matter. That is because inundation prevents oxygen penetration into the soil, which, in turn, retards decay of organic matter. High acidity follows, and with it reduced nitrogen fixation to a form of the element the trees can use.

When growing on mineral soils, height growth appears unaffected by soil moisture or stand density. Even where burned over, these poorly drained sites produce trees only about 30 feet tall in their first 20 years. At age 50, heights are about 50 feet, boles having no more than two merchantable logs. From then on, growth rate declines.

Figure 2.19 Cones of pond pine. Open cones of the serotinous species release seeds following fires that dissolve the resinous seals of the cone scales and burn the organic pocosin soils, the usual habitat of the species. (USDA Forest Service photo)

Trees bear seeds when young; cones are borne on stems only 10 years old. For these young ages, cone production and tree age are directly related and remain so until about 30 years old. By then, a single tree is expected to produce 200 cones with 75 seeds in each, half of which will be viable. Some pond pine cones open during periods of seasonally warm temperature, winter or summer.

Sprouting following fire is common, many stems arising from dormant buds formed in the axils of primary needles, which are later protected by bark. Injury triggers the mechanism that causes the buds to break dormancy.

Fire—While pond pine regenerates after fire, fire is also its chief enemy. Normally waterlogged, these forests are highly flammable when the organic matter that makes up the soil dries out. A layer of the organic surface soil as much as 18 inches thick is consumed in a single severe burn, leaving a stratum of mineral soil at the surface. This mineral zone may be a hardpan in which seeds do not readily germinate. A deficiency of nutrients then also occurs, though base elements like calcium and potassium are released from the organic matter in its oxidation by fire.

Switch cane abundance provides a clue to the fire history of a pocosin site. The reed-like grass, growing to a height of 15 feet, sprouts after burning. Where protected from fire, evergreen shrubs soon replace the switch cane.

Figure 2.20 A 160-year-old stand of pond pine in a North Carolina pocosin. Note the poor form, in part due to frequent fires running through the organic soils. (authors' collection)

Nutrients and Toxic Chemicals—As inundation prevents oxygen penetration into the soil which, in turn, retards decay of organic matter, growth is slower on organic soils than on mineral sands. The higher acidity in the swamps also results in lessening the fixation of atmospheric nitrogen to a form of the nutrient element that trees can use.

Nutritional relationships for trees in organic soils are complex, especially for nitrogen. The low level of the nitrate form of the element is due to the high water table, which prohibits oxygen from penetrating the soil. Nitrogen is thus retained in a chemically reduced state. Even when oxidized nitrate forms of commercial nitrogen fertilizer are added, seedling needles exhibit nitrogen-deficient chlorosis.

Calcium often tests surprisingly high in these acidic soils. The pH, controlling the availability of phosphorus and iron, is in turn influenced by nitrogen in the organic matter. The relative acidity or alkalinity also controls the occurrence in these soils of fungi, other flora, fauna, and bacteria.

Salt contamination in coastal soils seems to have little influence on pond pine growth. Even though storms send ocean-water salt inland with seawater and air currents, the soils analyze low in sodium salts.

Some lesser tree-form vegetation disappears as pond pines grow toward maturity; holly, waxmyrtle, and swamp ironwood diminish. Subsequent expansion of grass and sedge bogs, reducing the area covered with pond pine, is attributed to a toxic substance in the soil not unlike that in English heaths. There, gliotoxin, produced by a mycorrhizal fungus of the genus Penicillium growing on roots, inhibits growth of some woody species. It is a naturally selective herbicide, encouraging grasses and sedges to invade and shrubby broadleaf trees to pass from the biome. (This phenomenon closely resembles allelopathy, the exuding from certain plant roots and foliage toxic chemicals that injure plants of other species growing on the same site.)

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