Naturalist's Corner - Slipping into Senescence

It's always a pleasure to welcome back Linda Haugen of Iowisota, a nature-themed retreat and education center located in the Driftless Area of Northeast Iowa. Linda and her husband Dennis have created a marvelous learning center along the Mississippi River. They offer programs and retreat space year round centered on the nature and arts that surround them. In her blog, Linda often reflects deeply on some aspect of nature, bringing her curiosity, expertise in science and US Forest Service background to the forefront as she writes the Iowisota blog. Her latest post is a deep dive into the natural cycle of living oraganisms' lives as fall descends. The following blog post is reprinted with permission from the September 22, 2026 Iowasota blog

Senescence

By Linda Haugen, Iowisota blog 

Senescence seemed an appropriate topic to delve into as the leaves begin to change color, so I dug out my old plant physiology text to refresh my memory. I was reminded that senescence is “the processes that accompany aging and that lead to death of an organ or organism”. I was also reminded that college textbooks have a magic ability to induce drowsiness! Although the science is still valid, my textbook is from 1985, so the probability of inducing a nap correlates with “the processes that accompany aging” in my own body! I’ll try to lighten up this discussion of PLANT senescence enough to keep you awake!

In herbaceous annual plants and grasses (like flowers and crops), senescence begins when the plants flower and continues as the plant redirects its energy to the production of seeds. Eventually the leaves die, and the seeds are left to restart the cycle next year. Perennial plants are similar, with energy being redirected to roots, tubers, and other storage organs that will overwinter.  Trees are a bit different, since the whole plant does not die every year. But the leaves die, and the sugars and nutrients that were previously in the leaves are conserved over the winter in the twig, stem and root tissues of the tree. Plant growth regulators (like cytokinin, gibberellins, and auxins) are chemicals that signal the plant to keep energy in the leaves for growth or redirect it to other plant parts.

In trees, when the leaves start to senesce, they often change color. Sometimes from green to strikingly beautiful colors. Most of the colors are due to the balance of chlorophyll (green), carotenoids (yellow, orange and brown), and anthocyanins (red). The chemicals, and thus the colors, are there all through the season; as the chlorophyll is broken down, the other colors show up. Different tree species have signature patterns of color change, including timing and color. The sumac shrub is always one of the first brilliant changes that we see in northeast Iowa, with thickets of sumac forming bright red patches of vegetation before they fall off. Right now, I am seeing some yellows in the leaves of the walnuts and basswood. We have not yet had a frost, so the triggers for this are probably day length and leaf diseases.  As the season progresses, we will see bright golden, orange, and red patches of maples. The reds, browns and russets we will see on the hillsides later in the season are often the oaks. Iowa DNR maintains a Fall Color Report with updates of the development of color across the State (starting September 30) and information on the common colors of different tree species. Wisconsin and Minnesota DNRs also have fall color pages.

Earlier in the summer, I’ve shared pictures of the verdant green colonies of lotus in the backwaters of the Mississippi River. The lotus is done blooming now, and the energy from all of that chlorophyll-filled photosynthesis has been packed into the seedheads and tubers. The leaves are senescing and fading away.