Editor’s note: Landscape Confidential is a regular column by Audrey Clark, a curatorial assistant at the Pringle Herbarium at the University of Vermont.
This tree is a survivor. Two years ago, floodwaters rushed past, carrying debris that battered and scoured its trunk. The soil itself filled with water, choking out the oxygen the roots need to live. Soil was devoured and carried away, exposing and tearing roots. Leaves were submerged and coated with silt, which cut off the light that fuels life.
Yet the tree lives.
How is it that houses were battered and scoured, basements flooded and choked, foundations exposed and torn, roads submerged and cut off — and yet a tree, a simple tree, survived?
You can see the signs of flood even now. The bark is torn, scarred. Some branches are skeletal, dead. On the trunk you can see a line below which lichens and mosses refuse to grow.
You see signs that the tree was ready for the flood, too. The lower trunk is fluted, broad at the base, arcing out to shallow roots. Along the shore you can see roots trailing in the water, gleaning oxygen and nutrients from a difficult medium. If you dig a little, you’ll find spots and stripes of rust in the moist soil, where roots exuded oxygen. A neighboring tree was felled by the flood, and hundreds of seedlings now occupy the sunny gap in the forest.
This tree survives by a robust combination of resistance, resilience, opportunism, and blind luck.
It began life as a winged seed (a samara in botanical terms). The seed fluttered in the wind off the river, landing on moist soil in the river’s floodplain. That soil was well-drained and sandy, but contained a healthy helping of silt and clay, which hold onto nutrients. On top of that soil was a thick layer of decaying leaves, nutrient-rich humus that fed the soil that fed our germinating tree.
The seedling was occasionally inundated by floods, which stunted its growth, but these flooded were short and mild, allowing our seedling to recover. In a windstorm one day, a large, decaying cottonwood was torn apart and crashed to the ground. In the surprise of sunlight, our seedling took off, reaching pole size in a few years, then producing seeds of its own after just 15 years of life. Some combination of genes and circumstance allowed this particular tree to push past its fellows in the race to the sun.
Three years ago, our tree was a towering adult with a dignified demeanor. Hundreds of its offspring waved in the gentle breeze on the forest floor.
Then came 2011. The spring floods and the tropical storm pummeled the forest, uprooted trees, bashed bark, devoured soil. Because our tree was thick-barked, it withstood the pummeling. Because its roots spread far across the forest floor, it held firm when other trees toppled. Because the roots were shallow, they did not die when the soil was saturated, depriving deeper roots of oxygen.
Under water is not a friendly place for most plants. Oxygen is 30 times less abundant in water and diffuses through water 10,000 times slower than through air. Roots need oxygen to absorb nutrients and grow. When the soil is filled with water for long periods of time, most plants don’t last long.
Wetland plants deal with this in a slew of creative ways. Many have specialized tissue that allows oxygen to move into and through the plant more easily. This tissue, called aerenchyma, forms when a flood-adapted plant is immersed. Researchers believe this happens when certain cells die, make larger spaces for oxygen to move between cells.
Immersion can have other negative consequences for a plant as well. Microorganisms munch away at organic matter in the oxygen-deprived soil, producing organic acids that can harm plant cells. Waterlogging can lead to the accumulation of ethylene, a plant hormone that speeds flowering, fruiting, and leaf death.
Some trees flooded for prolonged periods of time will create a profusion of tiny pores along their trunks, enabling the intake of oxygen and the release of toxins, according to Theodore Kozlowski, a researcher at the University of California — Berkeley. Tree species that are better at surviving prolonged flooding often grow what are called adventitious roots, which allow for greater oxygen and nutrient uptake from the soil and water. These new roots are physiologically better at absorbing oxygen than roots grown in dry soil.
One way our tree can survive inundation is by oxidizing its rhizosphere. Oxidation, in its simplest form, means adding oxygen. “Rhizo,” means root — so a rhizosphere is the area around a root.
A tree oxidizes its rhizosphere by pulling oxygen in through the branches above water and transporting it down and out through the roots. Roots need oxygen around them to absorb nutrients, so by oxidizing its rhizosphere, the tree makes it possible for the root to absorb nutrients even in a flood. An added benefit is that the oxidation of the area around the root actually neutralizes some of the toxins, like ethylene and acids, that form in flooded soil.
Now get this. By adding more pores along the trunk and branches to take in more oxygen, using aerenchyma to transport oxygen more quickly, and making adventitious roots, flood-adapted trees can do an even better job oxidizing their rhizospheres, allowing them to take in even more nutrients and neutralize even more toxins.
When the next flood washes through the forest, our tree may look the same, but it will be undergoing a series of dramatic physiological changes to enable it to live in a different environment. And if it topples, it will make room for the next generation of seedlings to shoot to the sky before enduring — and changing with — the rushing waters.