Factors Affecting the Expansion of Restored Remnant Prairies

Many remnant prairies that survive today are gradually being overtaken by trees and brush. Friends of the Blufflands, in collaboration with the Coulee Region Chapter of The Prairie Enthusiasts, has led the effort to restore several of these prairies in and around Hixon Forest. This has included hiring contractors and leading work parties to clear brush and trees from the remnant and clearing an area around the remnant into which it can expand. This cleared area is referred to as the “first buffer”. When time and volunteers allow, a “second buffer” is sometimes created to keep the larger and especially berry-producing invasive brush even further away.

Each year, seeds are collected from the central prairie and dispersed into the first buffer in hopes that prairie plants will eventually grow. Initially, these newly cleared areas can look like a disaster area, with few visible prairie plants but with an explosion of weeds. It may take years before the prairie plants appear, and some highly conservative species may not emerge for a long time, if at all. What factors are at play that hinder or help expansion? 

First, newly opened ground provides ideal conditions for weedy “colonizer” species. These fast-growing weeds can quickly occupy areas with bare soil now exposed to the sun and can hinder the number of prairie seeds that germinate while smothering the tiny seedlings that do. This can significantly decrease the success of expanding the prairie. Many of these colonizers are transient such as burnweed, Queen Anne's lace, common mullein, and giant foxtail which will eventually be outcompeted by the native prairie plants. Most restorationists just ignore these transient intruders. Other species can be more of a problem, such as sweet clover and nonnative biennial and perennial thistles. Many go after these species, but not everyone. The current chatter seems to favor leaving the thistle alone, but going after sweet clover. There is broader agreement that species like reed canary grass and crown vetch should be actively eradicated.

Second, certain chemicals in the soil—known as allelopathic compounds—can suppress prairie growth. Juglone from walnut trees is a familiar example. Another is emodin, produced by buckthorn. Emodin enters the soil through roots and decomposing leaves, acting somewhat like a preemergent herbicide by inhibiting seed germination and root development of some plants. It may also disrupt beneficial mycorrhizal fungi. Although emodin is estimated to break down in about three years, this varies widely. Other invasive plants, including honeysuckle, barberry, and bittersweet, also produce allelochemicals, though their impact is not well understood.

Third, the soil in newly cleared areas often contains a substantial seed bank of buckthorn and other undesirable species. Some of these seeds can remain viable for years, continuing to compete with prairie expansion. While additional off site invasive seeds may be introduced into the buffer such as by birds or other animals, this becomes less problematic once mature prairie sod establishes and regular fire is implemented.

Fourth, excess nitrogen from decades of accumulation of decaying plant matter and more recently from the decomposition of buckthorn leaves enriches the soil in ways that favor weeds and shrubs over native prairie plants. This nutrient imbalance may also alter soil pH and microbial communities unfavorable for prairie plants. Reducing nitrogen to prairie-friendly levels can take considerable time, even with prescribed fire which is known to remove some nitrogen with each burn.

However, fire—the fifth factor—can be difficult to implement in the early years due to insufficient fuel in newly cleared areas. This delay limits fire’s initial benefits. Beyond reducing nitrogen and suppressing brush seedlings, fire may also stimulate the germination of some seeds of native plants through compounds in the smoke called karrikins. For this reason, some practitioners have started experimenting with “smoke water” to encourage the establishment of new prairie plant species.

Sixth, herbicides used during site preparation may leave residual chemicals, particularly if applied as foliar sprays. Their persistence depends on many factors such as sunlight, soil type, pH, temperature, moisture, and microbial activity. Much remains unknown about the lingering effects of these chemicals, including on the soil fungi community.

Seventh, seeds dispersed on opened areas of steep bluff prairies can be washed downslope if an ill-timed heavy rain occurs. This will obviously hinder the estabishment of plants on the higher slopes.

Finally, the soil fungal community plays a significant role. Most prairie plants form symbiotic relationships with arbuscular mycorrhizal fungi. In this mutual partnership, plants supply the fungi with sugars while the fungi improve nutrient and water uptake by plants. These relationships are essential for most plants with many associating with specific fungal partners. If those particular fungi are absent from a newly opened area, plant establishment can be difficult. At the same time, fungi depend on their host plants, creating a gradual, interdependent expansion—a kind of “chicken-and-egg” dynamic. Some restorationists have attempted to jump start this process by transferring soil from intact prairies to expanded areas to introduce beneficial fungi, though this is obviously not a viable option in most cases because of the damage that would be done to the intact prairie. Others have experimented with laboratory-prepared fungal inoculants, but most experts believe commercially available inoculants are not yet reliable for use on prairies.

This last issue is one that Friends of the Blufflands hopes to untangled a little through research by University of Wisconsin La Crosse student Lane Hansen. A senior planning to continue into graduate studies at the University, she is interested in fungal networks in remnant prairies. We hope that her work will help shed light on plant–fungal interactions and their dynamics during prairie expansion.

Stry Prairie in February 2024 With a Lot of Open Space Into Which to Expand