Essay: Secrets of the Sweetgums
To the average person a sweetgum is one of those weedy trees with the pesky, prickly balls that cover a lawn when they start falling. And they are correct. Sweetgums are often weedy in nature, and proliferate in hedgerows, abandoned fields, and so-called “waste places.” And stepping on a sweetgum ball barefooted is no fun. It therefore comes as a surprise for many to see the Congaree and other bottomland forests throughout the South dominated by the weedy sweetgum. It is here in these fertile, moist, clayey soils that the lowly sweetgum reaches its full glory. And only in the Congaree does this tree attain sizes it was meant to have. In fact the Congaree, except for the sloughs and ponds, is a sweetgum-dominated forest where about one in every five overstory trees is a sweetgum.
Sweetgums have a magnificent growth form when they get large, rising out of the forest floor like gray columns, free of limbs for seventy-five feet. A twelve-foot-circumference sweetgum, nearly four feet in diameter, is hardly worth noticing in the Congaree. Many years ago I tried to record all of the park’s sweetgums that were twelve feet or greater in circumference but finally gave up – they were just too many of them.
The author poses with Congaree’s former national champion sweetgum.
It should probably come as no surprise that for several years the national champion sweetgum, meaning the biggest of its kind anywhere, was found in the Congaree. It measured nearly seventeen feet in circumference, almost six feet across, and was 150 feet tall. The size of this stupendous tree, experience has taught me, is about their maximum in a floodplain environment. They can’t get much bigger because their shallow root system cannot support their heavy weight in high winds, or they become more susceptible to insects and disease in old age, or die from a lightning strike from any of the numerous electrical storms that pass over the swamp in summer (the next time you’re in the visitor center, check out the excellent model of a downed sweetgum in front of the Blue Sky mural; the craftsmen that constructed this model put a gash from a lightning hit on the trees’ bole which killed it and which in turn resulted in the wind pushing it over).
This particular champion, located along the banks of lower Cedar Creek, managed to survive the ninety-mile-per-hour sustained winds of Hurricane Hugo in 1989, losing only one large canopy limb. But it finally lost its canopy and a fourth of its top to a windstorm some twenty years later. I would love to know the age of this swamp patriarch (it is still alive as of 2019 but looks quite forlorn without a top).
You can get so fixated on the large gums that it’s easy to overlook the fact that Congaree sweetgums, like loblolly pines, don’t regenerate under a dense canopy. In other words, there are few young sweetgum seedlings and saplings growing in the understory to replace the big ones when they eventually die. It gives one pause to look up at the sweetgum giants in winter and see a canopy full of sweetgum balls, all of which rain down thousands of seeds year after year, and except for feeding birds and squirrels, all to no avail. How, then, did they get here in such dominant numbers in the first place?
Hurricane Hugo was an eye opener in a lot of ways. This storm caused immense damage and untold amounts of human suffering when it struck the South Carolina coast on the night of September 21, 1989. Its scale was unprecedented in modern times and lulled us out of our false sense of security of the past thirty-five years, a period of hurricane quiescence (and accordingly a period of exponential coastal growth in the Carolinas). The storm also taught us a lot about the complexities of forest regeneration and recovery at Congaree and other bottomland forests.
Many hardwood forests, including Congaree, regenerate themselves primarily through the effects of wind storms, which periodically knock over the large trees and create a “light-gap,” or hole in the canopy, which allows enough sunlight to reach the ground floor to stimulate growth for the younger trees and saplings. You can imagine the size of a canopy gap in the Congaree when a 140-foot tall tree falls, usually taking several smaller ones with it. These light gaps quickly fill up with pawpaw, laurel oak, red maple, green ash, sugarberry, American elm, and others, but strangely few, if any, sweetgums.
Hurricane Hugo provided us a rare opportunity to see how sweetgums have come to dominate the Congaree forest. The typical canopy gap, even those created from very large trees, does not seem to provide enough daylight for sweetgums to get started. Instead, it takes a larger canopy gap, created by several big canopy giants falling together.
Hugo was a Category 4, almost a 5, hurricane when it hit the coast with 135-mile-per-hour winds and a twenty-foot surge of water. The monster storm raced quickly inland, the eye passing about fifteen miles east of the park, and brought with it sustained winds of 90 mph, a figure that had rarely been seen this far inland.
It was a few weeks after Hugo that I was first able to get down to the park. In some areas it appeared as if a logging operation had been underway, and the normally closed, dense canopy was open everywhere with sunlight streaming in to the ground floor. A few years earlier, in 1980, I had initiated a Breeding Bird Census on a twenty-acre plot of representative old-growth in the park. Besides conducting a bird census, I did a vegetation inventory of the site. I remember the shock I felt when I first arrived at the plot several weeks after the storm. It was a shambles – trees, limbs, and splintered wood on the ground everywhere, and bright sunlight overhead as if it were January instead of early October. Over the next two days, I did a post-storm tree inventory on the plot and found that Hugo, in one night, had knocked over sixty-eight trees that were one foot in diameter or greater. Perhaps more significantly, the storm felled twelve large canopy trees three feet in diameter or greater. Before the storm I was losing on the plot about one large canopy tree three feet or greater per year to wind or other natural causes.
A rare sight before Hurricane Hugo – a clump of young sweetgum saplings.
The secret of the sweetgums revealed itself only a couple of years after Hugo when, for the first time, I started seeing sweetgum seedlings in the large canopy gaps created by the storm. This was an epiphany, and I started looking at the large gums no longer with a sense of unknowing, but with the knowledge that they were living evidence of past hurricanes and, in a few cases, tornadoes. A history of hurricanes in South Carolina shows that a number of them have reached the Midlands with enough wind force to blow down lots of shallow-rooted hardwood trees growing in soft soil. These storms may be fifty or one-hundred-year events, but they happen with enough regularity to create a perpetual, sweetgum-dominated forest at Congaree.
Forest ecologist Robert Jones did a wonderful study some years ago on the champion trees of the park. He referred to the Congaree as an “early- to mid-successional” forest, which sounds like a contradiction in terms with the old-growth forest that it is. His use of the term was due, in part, to the dominance of sweetgums in the canopy, a tree normally considered a pioneer, early successional tree, and one of the first to invade old fields and clearings. Sweetgums at Congaree can be thought of as a hardwood version of the loblolly pine: they grow fast, straight, and tall, and attain large size relatively quickly, and in the process overshadow and out compete other hardwoods.
Under the normal sequence of bottomland forest succession, the sweetgums would eventually be replaced by late-successional hardwoods such as beech, sugarberry, elm, and others. However, this assumes there would be no disturbance to the stand, and we know now that the Congaree, like nearly all southern bottomland hardwood forests, is subject to periodic wind storms from strong hurricanes and which allows sweetgums to perpetuate themselves. I now like to think of the Congaree as a hurricane-maintained forest and the sweetgum as a hurricane tree.
Twenty-five years after Hugo, I still see signs of ongoing sweetgum regeneration in the park. They are one of the few hardwoods I’m familiar with that sprout from roots, and I see a good bit of such root sprouting in the park. These young root sprouts are often found growing under a partially-shaded canopy, places that you wouldn’t normally think would provide enough sunlight for a sweetgum. Even stranger are small patches of nothing but pure sweetgum saplings growing in swales with no parent tree nearby. In some cases these patches consist of more than a hundred saplings eight- to fifteen-feet tall growing within inches of one another. A possible origin is from a dense concentration of seeds washed in from a flood, under conditions favorable for viable seedling establishment.
Graham Norman points out young sweetgum trees that will one day become forest giants.
I sense now that the silent sweetgums are patiently biding their time, waiting for the next big storm or hurricane to blow through the Midlands. And with an average life span of about 250-300 years, the big gums will have more than enough opportunities to reproduce and continue to dominate the Congaree floodplain.
