<ir="ltr">Over the past week, southern Minnesota has received up to 13 inches of rain, resulting in mild to severe flooding, and many a bewildered student and Arb critter wondering just where is this water going to go?
The answer is: anywhere it can. Presently, the water has crept up to the 900 ft. topographic line, spreading throughout the low-lying, riverside terrain in the Cowling Arboretum. Turtle and Oxbow ponds have been consumed, both sides of the millpond dike are flooded, and water laps gently at the foot of Hillside Prairie. Many of the organisms living in these sections of the Lower Arb have the luxury of running for the hills, quite literally. However, what about those rooted to the ground? What physiological adaptations allow floodplain forest trees to survive these periods of intense and prolonged immersion?
Floodplain trees—hackberry, cottonwood, silver maple, green ash and willow—face several challenges throughout the course of a flood, including inundation, deposition of foreign sediment, erosion, and chemical stresses under hypoxic soils. So, just how do they survive?
Flooding events expose mineral substrates and nutrients in the soil that the trees use to colonize ground formerly occupied by understory plants. This promotes rapid regeneration of lost biomass. In periods of high water, many flood tolerant trees are able to prevail over the oxygen deprivation caused by root submersion with lenticels (small slits in the trunk). These allow oxygen to diffuse into a spongy tissue called aerenchyma, which in turn directs it to the roots. Other trees sprout what are called “adventitious roots” from mature bark. These tendrils float freely and take advantage of aerobic floodwater. These also contain aerenchyma. This entire root structure desiccates and falls off in the days following the receding of the floodwater.
Some trees outlast floods by entering a state of dormancy. A Minnesota Department of Natural Resources (DNR) study showed this behavior to dramatically increase a tree’s chance of survival. However, this applies primarily to spring floods induced by meltwater. This same study also stated that middle-aged trees are more resistant than young or mature individuals of the same species. For more information on these phenomena, check out the MNDNR website