Back in college, a classmate responded to the exam prompt “design an ecological experiment” with a proposal to study forest change…over a couple of centuries. Our prof wrote in the margin of the page, asking if the student expected a grade of “Incomplete” while we watched the experiment play out. Hah!
That story comes to mind as I consider my walk through the forest at Redtail Ridge Habitat Area in Cherokee County. But at Redtail, the experiment might need only a few decades, rather than centuries, to obtain the desired results. Such an “instant forest” is on display, and I thought about land management, environmental disturbance, and especially about succession, the ecological process of (more-or-less) orderly, predictable change in an ecological community. As I have described previously, the use of aerial photos is helpful to understand changes to an ecological community over time (historic photos are especially valuable; in Iowa, such photos are often available going back to the 1930s). But being able to “see the forest for the trees,” thinking about the forest as a community, is important as well. Let’s take a look.
I walked through a young forest on the floodplain of the Little Sioux River. The forest lies in lowlands west of the waterway, with the steep Redtail ridgeline along the east bank, and a parking area and marsh east of that ridge. This broad, flat floodplain features an extensive stand of Eastern Cottonwood (Populus deltoides) trees growing in a dense thicket. Almost all of the trees appeared to be of a similar height, and with similar-sized trunks. To add to the experience, a breeze made the leaves dance and shimmer above my head, and the seeds floated around me like an impromptu snowstorm.
It’s not unusual to find such a uniform stand of trees, but it is more often the result of standard forestry, a plantation or grove, the trees growing in straight rows with even spacing. Here, the trees appeared to have all appeared spontaneously, creating a dense, tangled thicket from bare ground.
And it seems to have happened in just that way! In an email, Laura, a naturalist at the County Conservation Board, confirmed that the site had been previously farmed; the forest self-seeded after farming ceased. I’ve often observed the collected “fluff” of cottonwood seeds, or a dense carpet of seedlings, appearing on bare mud of a newly-cleared site. It’s not hard to imagine many, many seeds germinating when the land was left fallow and then flooded.
These side-by-side photos show a startling fact about the Cottonwood: it not only finds the bare mud of a newly-available site in the floodplain, it grows REALLY FAST. In fact, Eastern cottonwood is the fastest growing native tree in North America. Compare the photo of the site on left with a recent photo on the right. Guess how many decades separate the photos.
Remarkably, this forest grew in about 30 years! The clear ground in the left photo was treeless in the 1980s. In the subsequent years, the trees have benefited from several conditions in floodplain wetlands fueling this remarkable rate of growth:
- Space. Few species can tolerate the frequent, sustained flooding at such a site. But Cottonwoods are well-adapted to flood conditions, and so have the ground more or less to themselves.
- Water. Of course, trees need water! Unlikely to be limiting; even in dry years, groundwater is almost certainly near the root zone and readily available.
- Nutrients. The river carries nitrogen, phosphorus, and other nutrients in its flow. Flooding delivers the nutrients to the trees in the floodplain.
Voilà! The trees are almost destined to grow at an accelerated rate, and so they have. Around the forest floor I found plenty of lush grass, no doubt able to take advatange of many of the conditions listed above; and the Cottonwoods have not (yet…) formed a shady “canopy” overhead. Such luxuriant plant growth must surely support a small mammal population. And…those small mammals will be excellent owl food.
Regular readers recall my fervent desire to find owls, or at least their pellets (“burps”), and I made a special search one winter evening at Wittenbreer Marsh in Howard County. I did hear the birds there, but never caught a glimpse.
Here at Redtail Ridge, I finally saw my owl! It was a sunny mid-afternoon, and the owl rose up out of the grass as I tromped through the thicket. The bird flew up to a low branch, and was immediately hounded (“mobbed”) by a blackbird. The small bird chased the large one off ahead of me, headed farther into the woods. It all happened so fast, I was unable to snap a photo, and didn’t have a chance to observe the owl for more than a moment. I am fairly confident it was a Great Horned Owl, (Bubo virginianus). Iowa is essentially in the center of the geographic range of the species, and preferred habitat includes “woods, particularly young woods interspersed with fields or other open areas.” Sounds about right!
And so we have “connected the dots,” with bare ground remaining after farming, cottony seeds blowing in the wind, special conditions in a floodplain, and a food chain (with some real-life drama, in broad daylight!). Perhaps you’ll be able to observe, and appreciate, the ecology of a wetland near you.