When Wetlands Aren’t Wet

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Published on: Aug. 2, 2008

Last revision: Dec. 8, 2010

My heart was thumping to the beat of teal wings in flight as I approached one of my favorite playgrounds: the swamp at Otter Slough Conservation Area. I was excited because of what was new at the swamp—it was dry!

I used to think wetlands had to be wet at all times. So did most other wetland managers. We have since learned that it helps swamps to occasionally dry out. This is a new concept, and I was anxious to see firsthand how lowering the water level for a short time would affect the swamp.

Drying Out Otter Slough

Conservation Department Biologist Tommy Marshall works at Otter Slough and is carrying out and monitoring a management plan that involves temporarily draining the swamp to let parts of it dry out.

According to Marshall, the tupelo trees that have long stood like quiet sentries watching over the swamp immediately benefited from the dry conditions. Tupelos, like many swamp trees, can tolerate flooding and the resultant low oxygen levels near their roots, but they don’t thrive in permanently flooded conditions. After numerous years of growing in water, the tupelos at Otter Slough had yellowish green leaves, extremely swollen trunks, and only about 2 inches of annual growth.

This year, however, Marshall found 12 inches of limb growth, and the trees have dark green leaves and vigorous stems. He said cypress trees and buttonbush seedlings also benefited. Roots of all tree species need oxygen from the soil, especially when the trees are young. It was as if each tree and plant took a deep breath and showed what it could really do.

Providing a dry period for wetlands helps regenerate all types of vegetation. Seeds germinate when the soil and air are allowed to mix. Cover the land with water, and oxygen drops from 20 percent to less than 1 percent. Little oxygen equals little germination.

Down on Monopoly

Monopoly Marsh on Mingo National Wildlife Refuge is surrounded by swamp and bottomland trees. The open marsh has been dominated by spatterdock and lotus for many years because these species can germinate underwater. Although these plants are native to Missouri and provide places for immature ducks to feed, their seeds are not highly nutritious and the vegetation makes poor nesting cover.

Daniel Wood of the U.S. Fish and Wildlife Service at Mingo National Wildlife Refuge has been adjusting water levels to improve plant diversity at the marsh.

Occasional drawdowns in summer have increased the occurrence

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