Tupelo Leaves
October 9, 2014. Birdlife is slow this morning, but I do get good looks at a handsome male black-throated blue warbler foraging nearby in an ironwood. I wish all warblers were as deliberate and orderly in their movements as this species. He provides ample views as he methodically inspects leaves for caterpillars. I see him devour two in as many minutes before he moves out of view into thicker foliage. In contrast, a nearby magnolia warbler is acting very warbler-like with its frenetic, nervous movements. This bird doesn’t stay still long enough to get a good view, and you have to content yourself with glimpses.
A nice clump of fireweed (Erichtites hieracifolia) has poked up in a large canopy gap created by the February ice storm on the Sims Trail. This tall, omnipresent weed is abundant in the spring-burned woods along the drive to the visitor’s center, where it is easy to understand the origin of its common name.
In an open glade near the trail I see a new winter arrival to the park, an Eastern phoebe. I’m not sure where the lively little flycatcher spent the summer nesting season – perhaps not far enough away to even call him a migrant since their southerly-expanding breeding range now extends as far south as the Fall Line Sandhills just up the road. They are a common bird here in winter and even on the coldest days seem to find a few flying insects to feed on. Insect prey must be prevalent around shallow pools of water and the edges of waterways, as this seems to be preferred phoebe foraging winter habitat. When bugs are scarce, I sometimes see them feeding on poison ivy berries.
At 10:45, a cicada breaks the silence with a loud but brief “farewell to summer” call, then goes quiet.
A water tupelo leaf with multiple-teeth margins.
Returning via the low boardwalk, I notice a change in the canopies of the water tupelos. A number of leaves have already shriveled and turned brownish-gray while still on the tree. This time of year, they have an interesting mix of mottled, blotched coloring. Some are still mostly yellow (they are about the first tree in the swamp to turn color, a month or more before others), others a mix of green and yellow. The yellow color (known as xanthophylls) becomes visible in autumn leaves when photosynthesis shuts down, eliminating the green chlorophyll pigment. Many of the leaves have speckles of circular brown or gray blotches of dead leaf tissue, and for a good number, the entire leaf margin is brown.
Tupelo leaf with only a single tooth on the leaf margin.
I pick up a fallen water tupelo leaf and examine it closely. In the swamp, only the red mulberry leaf is bigger. It is attached to the twig by a long petiole (stalk) two-and-a-half inches in length. The leaf blade itself can measure up to nine inches long and four inches wide. Why would a tree have such large leaves? Some tree species have so-called “shade leaves,” larger-than-normal leaves found near the bottom of the tree, where there is not as much sunlight as in the canopy. Shade leaves are larger presumably because they have to increase their photosynthetic output to offset reduced sunlight. But most tupelo leaves occur in the canopy where they get plenty of sun. Perhaps the water tupelo’s cousin, swamp tupelo, Nyssa biflora, can shed some light on the question of large leaves, since it has much smaller ones. Swamp tupelo grows in boggy or muck soils whereas water tupelo grows in water, sometimes deep water, and for long periods of time. This has to be a stressful environment for a tree, so perhaps a large leaf is needed for this particular species to maintain an adequate photosynthetic output.
Tupelo leaf with smooth leaf margins.
Then there are those little pointed tips on the margins of the water tupelo leaf. Some leaves have only one; others two or three, while still others have a half dozen or more. And to further complicate things, some leaf margins have no tips at all. Since biologists believe that form has a function in nature, we wonder what purpose these little points could have.
I realize that I’m looking at perhaps the most marvelous of nature’s many exquisite creations, a photosynthetic factory that makes life on earth possible. Hardwood leaves are overwhelmingly the dominant visual object at Congaree for much of the year and give the park one of its most distinctive features. While we all admire and enjoy the beauty of leaves, especially in the fall, our admiration quickly turns to something else when they begin falling and collecting in our yards and gutters. But even after they die and fall to the ground, leaves continue to play a vital role in forest ecosystems.
By late November and early December the ground floor at Congaree will be thick with leaves and stay that way until winter floods carry them off elsewhere or deposit them on the bottoms of sloughs and waterways. If the flooding is frequent and heavy enough, the ground floor in places will be as clean and bare by spring as if an army of leaf blowers had been at work.
A bare Congaree ground floor with no leaves, courtesy of heavy flooding.
I close out the morning with a good find of salamanders from log-rolling – two southern duskies, one three-lined, and four marbled. And I also enjoy the petite blooms of ladies tresses orchids, Spiranthes, blooming near the low boardwalk. The small, urn-like white flowers really stand out amidst the leaf litter and dark soils of the “muck swamp.” 
