Near the mouth of the Sabine River lies a micro-wilderness called Blue Elbow Swamp. This primeval jungle of modest proportions—four square miles—lies at the edge of industrialization. Had John Bunyan journeyed here, the sight could have inspired his description of the Slough of Despond, the almost impenetrable terrain through which his pilgrim had to travel en route to the Celestial City. Here a stand of baldcypress and a mixture of deciduous broadleaf trees, once of commercial value, occur.
One observer described the locale this way: "Tie up to a cypress root and walk a hundred yards inland (from the river) along a piney ridge, and the human signs are gone. Deeper still, except for an occasional fisherman along the lush canals (cut and dug by early day loggers), the swamp seems as elemental as it must have been when the first logger set his saw against his chosen tree.''21
The words ring true, but there is more. Here is a place where sea water intrudes into the fresh, where salt spray during Gulf coastal storms alters vegetation composition, and where new land forms at the urging of the river. Here, too, although the odors of the manufacturing of paper, petroleum products, and cement may permeate the air, one witnesses primitive wilderness. Man intrudes, but, with few exceptions, only to the swamp's edge.
Blue Elbow, the wet morass, may have gotten its name by fishermen and loggers in the area 100 years past, the color alluding to the black water, laden with lignin from decaying forest vegetation, that accumulates in the pond as overflow from the usually quiet river near where it empties into the Gulf of Mexico. The elbow, the sharp bend in the river, reminds one of the crook in an arm.
Soils of the Blue Elbow were delivered in water suspensions of the sluggish river, coming from faraway fertile calcareous blacklands and picking up silt and clay particles along the way. The fibrous organic soils in the swamp smell sour, indicative of high acidity and low pH. Spanish moss, the epiphytic member of the pineapple family, drapes gracefully from the arching branches of the cypress trees, especially those standing in deep water. The deciduous conifers, on which the weeping tufts of grayish-green strands root and from which they gain nutrients and water, grow well.22
In the flatwoods and savannas of peninsular Florida, "domes" of baldcypress and pondcypress are formed by confluent tops of crowded trees which, beginning with short ones at the outer edge, grow progressively taller toward the center of the stand. Pronounced domes occur up to 25 miles inland and only at elevations no more than 100 feet above sea level. They may be from 1 to 100 acres in size. In this area, baldcypress appears to be limited to the domes, other species being prevalent beyond the peripheries of the structures. Pondcypress domes are encountered in depressions or sluggish creeks of the flatwoods and in ponds intermingled with the higher-elevation sandy soils of Florida.
Perhaps trees at edges are dwarfed by unknown soil conditions that become so toxic as to finally exclude baldcypress beyond the dome. Perhaps seedlings invade as the site becomes more favorable for bald- and pondcypress inward from the edge. This could be due to fire prevention and cattle exclusion or to gradual deepening of water in the basin. Or, perhaps, outer trees are younger than inner ones, a situation attributed to earlier mortality of edge stems caused by fire, grazing, or drainage. New shoots or seedlings replacing dead trees are evidence that mortality increases as stand edges are approached. Damage to existing trees attributed to natural causes is greatest at a dome's edge.
When surface fires in a dome damage trees at their bases, the trees sprout, but the sprouts are often killed by the next fire. Again sprouts develop. Crown fires, conversely, kill terminal shoots, allowing low lateral branches to become dominant. As fire crosses the grass prairies surrounding the wetter domes, trees at the periphery, where water is shallowest, are more susceptible to injury than stems further inward, or may reflect fire influence on baldcypress height growth.
There is also the possibility that cattle brushing flies by rubbing against trees, or deer rubbing their antlers, damage stems at the edge of a forest, thereby reducing the vigor and rate of growth of trees near dome edges. None of this fully explains the gradual nature of the decline in the height of trees outward from the center of the dome.
Engineers have used cypress domes for tertiary treatment of sewage effluent. However, surface water quality degrades, as evidenced by low levels of dissolved oxygen and high nutrient levels. Underlying organic soil and strata of sandy clay impede percolation of chemical elements to the shallow aquifer below. The quality of standing water within the domes does not return to its uncontaminated purity for almost two years after effluent dispersal ceases.23
Trunk buttresses and cypress knees are an enigma, their role and cause of origin a matter for speculation. Buttresses, extreme taper at tree bases, extend below the surface of the ground. In deep-water swamps, buttresses terminate at a level below which water seldom recedes. From the water surface downward to the soil surface, trunks are narrow, possibly because the section of the trunk under water is saturated, but without adequate air. If that is the case, the buttress is a response to the aeration of tissues associated with rising and lowering water levels. The boles of unbuttressed trunks contain no excess water; they do hold an abundance of air. Only slight swelling occurs in swamps not subject to high floodwaters. The fluted bases of pondcypress trees are rounded in contrast to the sharp ridges of baldcypress.
Knees, peculiar conical structures, arise from shallow widespread root systems. As little gas exchange occurs between roots and knees, the latter seem not to be essential for root aeration. Respiring knees have been shown to reduce oxygen content in closed air-tight containers surrounding the protuberances, but most of the oxygen absorbed is used by the large amount of cambial tissue in these organs. This is particularly evident since knee growth is more rapid than root growth and, therefore, more demanding of oxygen. Furthermore, trees grow well in deep water in the absence of knees; and their removal seems to have no effect on bole growth and survival.
Knees may be important for basal support in organic soils described as "trembling earth." That such support is necessary is indicated by the inability of a 10-foot engineers' pole to remain erect for long on typical baldcypress sites.
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