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 of millet, giant cut grass, sedges and rushes on Monopoly Marsh, providing more nesting and feeding areas for songbirds, shorebirds and herons. The improved plant diversity has even drawn least bitterns to the marsh.

“The drawdown promotes seed germination, and it helps the water become cleaner and clearer,” Wood said.

Suspended sediment becomes soil after a drawdown, and stays down even after water levels rise again. Freshly germinated marsh plants bloom quickly in the new soil and generate an abundance of seeds. These provide a feast for all swamp creatures, including invertebrates, which in turn become nutritious food for waterfowl.

Wood, who constantly tracks species diversity on Monopoly Marsh, noticed that American featherfoil showed up in the marsh following Monopoly’s drawdown in 2000. Featherfoil favors clear, cool water in swamps and marshes. Conditions must be right at Monopoly, for the dainty swamp plant’s numbers have been increasing for the past three years.

Bottoming Out

Flooded timber in October can make for a hunter’s delight in November. Hunting flooded timber is not a common opportunity in Missouri, but it can be habit-forming when you see the waterfowl banking through the trees toward well-placed decoys.

Waterfowl hunters have good memories of hunting the flooded timber of Duck Creek Conservation Area’s pools 2 and 3 and Mingo’s Pool 8, but the hunting is no longer as good as it was. During what were considered the “golden years” of the ’60s, ’70s and ’80s, heavy pin and willow oak acorn crops made for fabulous mallard hunting in the green tree reservoirs.

What attracts ducks to flooded timber are the small acorns produced by pin and willow oaks. They are the right size for ducks to swallow and are loaded with carbohydrates and fat, allowing waterfowl to replenish their energy stores during their southward migration.

To attract and hold ducks, and to increase hunting opportunities, these areas usually were flooded in mid-October. This was earlier than they would flood naturally, but early flooding ensured that these wetlands would be in optimal condition for ducks and hunters.

The strategy seemed to work well. The areas drew in lots of ducks, and waterfowlers had great hunting. In the 1990s, however, the larger oaks on the wetter sites began declining. Many large pin oaks died while others were unhealthy and produced few acorns.

Mike Anderson, a Conservation Department forester and an avid waterfowl hunter, said the problem arose because repetitive early flooding stressed the oaks and made them unhealthy.

He explained that trees can survive winter flooding because, during dormancy, their cells cease activity and require no oxygen. Flooding before trees go dormant, however, cuts back on the amount of time that tree roots have to store carbohydrates. Trees can survive occasional flooding, but 50 years of regular early flooding took its toll on the oaks.

Why were just the oaks affected? Anderson explained that “dormancy doesn’t just happen when the leaves change color, its later than we once thought.” Most bottomland oaks don’t go dormant until near the end of November, while maples, ash, elms, overcup oaks and tupelo are dormant by the end of October. This means that mid-October flooding negatively impacts the oaks that make the pools most attractive to waterfowl.

Looking to the Future

Repeated poorly timed flooding also reduced the survival of seedling oaks. Older, vigorous, middle-aged trees can tolerate some flooding outside of dormancy. Early fall floods, however, can kill oak seedlings and saplings, preventing them from replacing the aging trees that spawned them.

Cold weather floods are an intrinsic part of the life cycle of these green tree reservoirs. Flooding deposits fresh sediment that mixes with decomposing matter to enrich the soil. After water levels drop, spring plants flourish on the abundant nutrients.

We know that flooding an oak bottomland forest early in the fall is good for ducks and hunters, at least in the short term, but the long-term effect deserves some scrutiny. We are seeing changes in these bottomland pools, as maples and ashes replace oaks in the canopy.

Ash and maple seeds drop early and aren’t as readily available to ducks during their migration, nor are the seeds as nutritious as acorns. On the other hand, the leaf litter produced by these species encourages a proliferation of invertebrates, which fuel waterfowl on their early spring migration to northern breeding grounds.

The best bottomland forest, therefore, should contain a mix of oaks, maples, ash and other trees. Achieving this mix might require a trade-off or balance in the timing of the flooding to ensure that this unique hunting opportunity can exist for future generations.

Varying the timing and duration of floods in these forests mimics the natural cycle. Hydrology studies of southeast Missouri have shown that only one in seven years tends to be really wet, while five in seven years tend to have average rainfall, and one in seven years is really dry.

Managers can fluctuate years of drying with years of early flooding, based on water availability and rainfall patterns over several years. Natural rainfall still contributes to flooding, but many wetlands no longer flood naturally because drainage canals have lowered the water table and changed the abundance of surface water. That’s why it is critical to have a management plan that adjusts water levels to encourage maximum diversity in, and longevity of, a wetland.

Delaying flooding in most years until after oaks go dormant might be required to maintain canopies of oaks in our bottomland forests. Otherwise, we risk letting species that can tolerate early flooding take over.

“We have the responsibility of trying to find a balance between sound ecosystem management and providing recreation opportunities,” Anderson said.

The problem is particularly pressing because more than 90 percent of Missouri’s wetlands have been lost. Of these wetlands, bottomland forests are one of the most rare types. There are not enough of these precious resources left to experiment with various strategies, and so the management choices we make now are crucial to maintaining them.

Anderson hopes that the management strategies being employed at Otter Slough, Monopoly Marsh and Duck Creek will teach us how best to preserve the integrity of wetlands throughout the state.

So, as I walked through the dry swamp, I thought that it’s not so bad if a swamp dries out now and then, if it means that the swamps will be there for our grandchildren. I saw a formation of ducks soaring above the horizon, and I imagined that they also would approve of the work we are doing.

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