When Wetlands Aren’t Wet

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Published on: Aug. 2, 2008

Last revision: Dec. 8, 2010


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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