Climate change and the death of the Big Woods forest

Photos forest canopy decline at Nerstrand Big Woods State Park


Nerstrand Big Woods State Park is now much wetter than 100 years ago. Rainfalls in the summer are more frequent and heavier. Places in the park that have flat and poorly draining soil now hold water too long for most of the tree species to survive.

About 12% of the park is experiencing forest death due to lack of oxygen around flooded tree roots. In these areas ash trees tolerant of wet soil are replacing the Big Woods. The Minnesota Department of Natural Resources (MNDNR) is determining what management options exist for these areas.

Warmer air can hold more water vapor, the fuel for rain and snow. To help minimize the effects of climate change, the MNDNR is committed to reducing its emissions of climate warming gases. Help us conserve Minnesota’s resources by reducing your energy use when possible.

In Nerstrand Big Woods State Park some areas of forest are dying due to a changing climate.

About 200 acres of Big Woods forest within the park is dead or showing signs of dying due to changes in rainfall patterns. The main area where this is happening is mostly south of County Road 29. This is about 10-12% of the total size of the park. The kinds of trees that are dying include many different species such as maples, oaks, basswood and others. This is a kind of tree community known as Big Woods.

Rain is falling differently than in the past.

Nerstrand Big Woods State Park is a lot wetter in summer than it was 100 years ago. Heavy rains are now more common and more intense than at any time on record. Long-term observation sites have seen dramatic increases in 1-inch, 2-inch, and 3-inch rains, along with the average size of the heaviest rainfall of the year. Although the area will see occasional dry episodes and even drought, climate projections indicate that it will be wetter in the decades ahead than it is now, and that heavy rains will continue increasing into the future.

Climate and soil define what type of forest can grow on a landscape.

Big Woods have grown in the affected area for hundreds of years. Big Woods forests need enough time between rainfalls to let the soil around the roots dry out. The recorded climate history of the area includes soaking rains with regular dry periods. Increases in rainfall over last 30 years are eliminating the chance that some soils can dry out. The affected areas are very flat and poorly draining. Water can’t flow away from anything so it must flow downward. A layer of thick clay an arm’s reach below the ground stops water from draining downward. This creates pools of water around the roots of trees. There must be enough time between rainfalls for this type of soil to dry or it just stays wet. Most species of trees that grow in the Big Woods can’t survive long periods of water pooled water around the roots. These trees eventually die from lack of oxygen.

Ash trees tolerate wet soil and this has its own problems.

Ash trees can tolerate very wet soil. We are observing that green ash trees are almost the only tree coming back in the affected areas. Green ash is very susceptible to death by damage from Emerald Ash Borer beetles, a non-native invasive species. So a very diverse forest is being replaced by a nearly solid stand of ash that is at risk from a nearby Emerald Ash-Borer infestation.

The Minnesota Department of Natural Resources is taking action and deciding next steps.

We will be learning more about how the affected forest is changing and determining what forest management actions to take over the next year. In the meantime, the Minnesota Department of Natural Resources is committed to reducing its emissions of climate warming gases. You can help and save money by conserving energy when possible.

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