My local hardwood (deciduous) trees are undergoing fall color change. Local trees are glowing gold, ablaze in red, and even a few have a purple color. It’s strikingly beautiful. Since I teach a bit of botany, I feel compelled to talk about this in class, and I also encourage students to keep track of the color change and the dropping of the leaves. And today, I ask you: why do the trees do this? And, would wetland trees differ from the rest? Read on!
The basic idea is explained well enough on-line: trees drop leaves in preparation for winter, when weather renders those leaves a dangerous liability. Some pages describe the chemistry of the colors and even show photomicrographs of leaf tissues. The traditional understanding is that leaves contain chlorophyll, a green pigment which is most visible to us most of the time; but chlorophyll fades, revealing other colors from “accessory pigments.” But about a decade ago, another part of the story was described: certain red or purple colors—anthocyanins—are specifically produced this time of year as a sunscreen, protecting leaves from UV light damage; this helps trees hold onto the leaves longer. Synthesizing anthocyanin is a chore, but is worth it, because doing so allows the tree to recover nutrients from the leaf before leaf they drop.
Of course it’s only “worth it” to synthesize anthocyanins and recover nutrients, when nutrients are scarce. If nutrients are plentiful, why bother? And that study compared floodplain trees (where nutrients are plentiful) with nearby uplands (where nutrients may be limiting to tree growth). And sure enough, floodplain trees were less colorful (specifically, levels of anthocyanins). So, we might expect other floodplain wetlands to be less-colorful (at least in certain colors).
I thought about this when I recently visited the Nolan Addition (Fuller Addition??) in Keokuk County (near the borders with Iowa and Washington counties). The site was beginning to show autumn color, and I had a pleasant walk, despite high water making it tough going down on the floodplain. I did see signs of fall color, but…the colors were a bit lacking, frankly. To what extent was I ahead of the fall color peak? Maybe a bit, although I did see color in uplands nearby. I’d guess that, as predicted, this floodplain wetland was less colorful, because the trees had plenty of nutrients available.
Colorful or not, I enjoyed my walk…seeing a story written in the scene around me. The recent agricultural past was still present in signs of water control (drainage, levees). The flooding of the nearby English River was very recent, and much of the low area was wet (inundated)—a floodplain wetland doing its job! Good thing, too-lots of rivers running high all around at this time. Anyway, the smell was musty, vaguely fishy; the vegetation was covered in some spots by deposits of silt left behind when the water was even higher.
I enjoyed a little color here and there, even if some of it was on ground just a little higher than the floodplain; for example, Sumac (Rhus) produces a blazing scarlet in Fall, and always catches my eye. The low ground had plenty of Birch (Betula) and Cottonwood (Populus) another deciduous tree.
A floodplain wetland receiving nutrients from its stream is natural and normal, and although it may dim the colors this time of year, it is something really important to us all. Nutrients deposited and retained in a wetland, are not flowing downstream to cause water quality problems. Watch for future posts about this topic, oh-so-crucial to Iowa. In the meantime, I encourage you to get out and see those lovely Fall colors. Enjoy!