Indiana Woodland Steward - The EQIP program promotes native plant diversity

The EQIP program promotes native plant diversity in Indiana forests

Aubrey Franks

Invasion by non-native shrubs has negatively affected forests throughout the eastern United States. The onslaught of invasive shrubs, including bush honeysuckle, multiflora rose, privet, and autumn olive, has created dense forest thickets. These dense thickets are novel; historically, most of our forests lacked very dense shrub layers, except for localized patches of spicebush or gaps with thick blackberry growth. This green wall is not just challenging to traverse; it can decrease the native plant diversity of the forest understory by reducing the resources (light, growing space, nutrients, water) available to understory plants. This can be detrimental to understory plants and can reduce the number of native species within our forests.

Traditionally, many foresters viewed herbaceous species as “stepovers” on the way to the trees. However, herbaceous species are more than just pretty flowers, they are crucial to our forest ecosystems. Herbaceous species are critical components of the nutrient cycle within forests. Research has shown that they comprise 12-16% of the total forest litter and contain higher nitrogen, phosphorus, potassium, and other nutrients than tree leaves. Spring ephemerals have also been found to reduce nutrient loss after snow melts before trees break dormancy. Besides their ecosystem functions, herbaceous plants give life to a forest. Observing a diverse herbaceous layer in bloom is a joy that can’t be replicated. The lack of wildflowers under dense thickets of invasive shrubs has led many forest owners to endure the time and expense of controlling invasive shrubs. An essential question after these treatments is whether the herbaceous layer can recover.

Great white trillium, a forest herbaceous species of high conservation value. Photo credits: Aubrey Franks

Because invasive plant management can be very daunting, forest owners often don’t know where to start. That is why the Environmental Quality Incentives Program (EQIP) was introduced with the Farm Bill of 1996. EQIP provides financial and technical assistance with conservation practices for private landowners. One of EQIPs most funded practices is brush management, a practice dedicated to removing invasive shrubs. Between 2005 and 2022, approximately 80 million acres across the United States have been covered by brush management EQIP contracts. Even though EQIP has become a critical conservation assistance program in the United States, no one has studied whether this program has helped the herbaceous layer of treated forests recover.

In 2022 and 2023, a research team from Purdue assessed forest recovery in privately owned forests where invasive plants were treated through EQIP contracts. Forty-five sites were chosen across Indiana; 35 were treated under EQIP contacts, and 10 were reference sites where invasive plants were not treated. We collected data from each site's herbaceous, shrub, and overstory layers.

A field crew from Purdue University is collecting field data at an EQIP contract site. Photo credits: Aubrey Franks

From these data, we calculated several metrics that capture the diversity of plant species: species richness (S), which is the number of native plant species at a site; species evenness, which measures how equal the covers of different plant species are, diversity, which combines richness and evenness in a single value, floristic quality index (FQI), which captures the conservation value of the species at a site, native plant species cover, and non-native plant species cover.

Our study found 235 plant species across all sites. Two hundred-eight species were native, and 27 were non-native. The most common native species in order of cover are the Virginia creeper, clustered black snakeroot, white snakeroot, and sedges. The most common non-native species were multiflora rose, garlic mustard, Amur honeysuckle, and winter creeper.

Our study found that EQIP treatments promoted native plant species and reduced the cover of non-native species. EQIP sites had significantly greater species richness, diversity, and FQI than untreated reference sites. Meanwhile, our reference sites had substantially greater non-native cover than EQIP sites.

Violin plots showing the diversity values measured at our EQIP and reference plots. If there are two different letters above the two plot types (EQIP and reference), then they are significantly different.

Our study also looked at the treatment history of EQIP sites. We found that sites that were treated with herbicides (foliar spray, basal bark, and girdling with herbicide) and sites that were treated with a combination of mechanical (cut stump with herbicide, cut stump without herbicide,, and mechanical) and herbicides had greater FQI than mechanical treatments along Sites treated for more than one type of non-native species (shrubs, vines, trees, or herbs) had greater species richness and diversity than sites where a single species was treated. Our results found that using various methods and treating more than one non-native species can aid the recovery of native herbs within our forests.

We conclude that treatment through the EQIP contract has promoted native plant species compared to untreated reference sites. However, many of the same native plant species were found on both EQIP and reference sites, and species associated with high-quality undisturbed forests were lacking across our study sites. This similarity between EQIP and reference sites could result from shared site histories. Many forests in Indiana regrew on abandoned agricultural sites. These sites have lost their biological legacies (native seed bank, soil chemistry, rhizomes, buds) due to years of plowing and other agricultural practices. Furthermore, many of our forests in Indiana are small woodlots isolated from different forests. This can disrupt the dispersal of seeds and prevent native plants from expanding into recovering forests. Lastly, high deer populations have reduced many native plant populations. The two most common species at both EQIP and reference sites were Virginia creeper and black clustered snakeroot, two generalist species resistant to disturbance. After initial efforts to control invasive plants, what follow-up actions can forest owners take to promote native plant diversity?

First, continued invasive species management is critical. Conducting invasive species management once produces short-lived gains. Many invasive species have long-lived seed banks, so as soon as you remove them, they will pop right back up. For shrub species, it is best to cut down the larger plants and apply herbicide on the outer ring of the stump. Then as the growing season continues, use a foliar spray to kill all the resprouts around the plant. If you are worried about accidentally spraying native species, you can treat invasives later in the growing season. Bush honeysuckle and other non-native plants keep their leaves longer than their native counterparts, so herbicides can be applied when native species are dormant.

Adaptive management is critical! Management should be adjusted based on the invasive plants that need treatment yearly. After treatment, another invasive species may replace the treated species. For example, garlic mustard is often established after you remove invasive shrubs. However, garlic mustard has shallow roots and is easy to hand-pull. Once you pull it, the plant can also be used in cooking; garlic mustard is an excellent substitute for basil in pesto!

After invasive species management, you may consider doing native plantings to boost the herbaceous plant diversity. Doing some research can produce better results. Many retail wildflower mixes do not contain native species from your local area. Some may include invasive species, which will only create new problems. The Indiana Native Plant Society is an excellent resource for native plants. With over 1,000 members, the Society advocates for native plant species and seeks to educate the public about plant natives. Another great resource is the Native Wildlife Federation’s native plant finder. This tool lets you enter your zip code and see the native plants within your area. This can give you a great idea of what to plant.

The Midwest has high deer populations that significantly impact the herbaceous layer within our forests by browsing on plants. This heavy browse not only decreases the number of native plants but also increases the amount of space within a forest, making it easier for invasive plants to facilitate. Suppose you are planting trees or shrubs, and it's best to put netting on them for protection. Eight-foot-high fencing can also prevent deer from eating any recovering or newly planted species. If possible, managing deer populations through hunting can also reduce the browsing pressure on the herbaceous layer.

Our study found that the EQIP program positively affects the diversity of native plants in Indiana. However, many forest owners may not be aware that this program is available to help them control invasive plants. The Indiana Woodland Steward recently published an article by Brian Kruse that goes into more detail about EQIP (https://www.inwoodlands.org/eqip-financial-assistance/). You can also contact your local NRCS office to learn about EQIP and its practices.