Essay: The All-Aged Forest
On occasion I hear that some first-time visitors are disappointed after seeing the park. They’ve heard about the big trees, the champion trees, the so-called “redwoods of the east,” a term that came into use during the citizens campaign to save the swamp in the 1970s. They have come expecting to see solid stands of big trees, much like the redwoods, sequoias, and other old-growth coniferous forests of California and the Pacific Northwest. Comparing old coniferous forests with old-growth hardwood forests is like comparing, well, apple trees and orange trees. The two are entirely different. Stands of coniferous trees are typically of the same size and age and in fact are referred to as “even-aged forests.” They owe their condition to the fact that young conifers need lots of sunlight to get started and that this sunlight is often provided by some catastrophic event such as a wildfire that burns and destroys large areas at a time. Fire also burns the duff – built-up ground litter and decomposed leaves – that expose the soil and allow the conifer seeds to germinate and get started free of competing vegetation.
Hardwoods forests offer a far different picture. Many are not conducive to fire – they are too wet, or don’t provide the type of fuel to carry a fire. For the most part they are created and maintained by episodic wind events such as hurricanes, tornadoes, and severe wind storms that rather than destroy the entire forest like a wildfire, create a patchwork of tree destruction ranging from small openings of downed trees to larger areas with intermittent tree loss. In between is a continuum ranging from light to heavy damage. The end result is a forest of variable tree ages and sizes, an “all-aged forest.” It can be visualized as a pyramid, consisting of a few very large trees at the top, followed by many medium-sized ones in the middle, and even more smaller, younger trees at the base.
Forest ecologists sometimes refer to this type tree pyramid of size and number as the “reverse J-shaped diameter distribution.” When plotted on a graph with the number of trees on the vertical axis and their diameters on the horizontal axis, the shape of the plotted line is like a reverse or backwards J. This again applies primarily to old hardwood forests since a plotted even-aged old coniferous forest would appear somewhat like a J in its correct position, or an upside-down pyramid.
The famous Singer Tract in Northeastern Louisiana was, at 80,000 acres, the largest remaining old-growth bottomland hardwood forest in the South until it was logged just before and during World War II. Here, young graduate student James Tanner studied the last surviving population of ivory-billed woodpeckers. Part of Tanner’s research consisted of measuring tree sizes of the various species, and his results are informative: 75% of the trees in his Singer Tract samples were a foot in diameter or less, while only slightly more than 1% of the trees were larger, i.e. three feet in diameter or greater. This is the same pattern that fits the Congaree. I’d say on average there is about one three-foot diameter or greater tree per acre, followed by a fair number of twenty-four to thirty-five inchers, and then by lots of trees less than two feet in diameter. The really big boys – four feet in diameter or greater – may be spread over several acres, but in some instances, as befits wind storms of varying intensities, there might be several very large ones (typically sweetgum) growing in a small area and with an approximation of the “redwoods look.”
Public perception of the size of large hardwoods also comes into play. At least in the temperate zone, hardwoods simply don’t have the capacity or life span to attain the sizes of redwoods and sequoias. For the latter, a twelve-foot-diameter tree is not that unusual, whereas a five-foot- diameter hardwood is. Even a three-foot-diameter hardwood is outside of the norm for most of the Eastern deciduous forest, which has been cut over numerous times the past three hundred years. For the Congaree, a four-foot-diameter sweetgum, a rare tree just about anywhere else, is so common that people routinely walk by them without a second look. And we shouldn’t forget that most Congaree hardwoods put most of their growth into height in order to reach the sun for photosynthesis. Because the forest is so dense with trees, they basically grow upward rather than outward, resulting in one of the tallest hardwood forests in the temperate world.
Although Congaree’s flat floodplain at first glance appears to be all the same and even monotonous, the terrain is really quite varied with a wide range of growth sites, all shaped by differing soils and hydrology. Some areas are more conducive for large-tree growth than others. Many of the flats and low places, for example, have clayey soils that could be so poorly drained as to be water-logged or somehow compacted to the point of being anaerobic. The tree roots may be oxygen-starved and are just able to maintain the life of the tree with minimal growth. On the other hand, many of the really large hardwoods are found growing on fertile, well-drained ridges and the high banks at the edges of sloughs and guts.
The banks of the Congaree River, and to some degree Cedar Creek, both experience frequent, high-energy flooding and are another area where large, super-canopy trees are scarce. Although river- and creek-bank soils are some of the most productive within the floodplain, shifting channels and undercut banks from regular, heavy flooding prevent most trees from getting too large before they fall over.
Finally, it needs to be remembered that nearly a third of Congaree’s original 15,000-acre old-growth was logged just prior to it becoming a national monument (now national park) in 1976 (current park acreage is about 27,000 acres, of which 11,000 acres is considered old-growth). A good deal of this cutting took place in the western and central portions of the park, both of which contained nice stands of many large, super-canopy trees.
In my old age, my attention has been drawn more and more from Congaree’s champion trees to the incredibly diverse and complex forest itself. The big trees have served as a type of portal to draw me deeper into the forests’ mysteries and secrets. People who focus only on the park’s large trees are losing sight of the big picture and literally can’t see the forest because of the trees.