Mowing through challenges: cutting-edge ways to improve grassland restoration efforts

Featured image caption: Invasive grasses are actually the enemies of many native grassland species. Too much non-native grass growing in a single area can cause an abundance of dead plant material, or thatch, to build up and block growth of other small flowering plants like the California golden violets and common yarrow shown here. This study proposes solutions for restoring grasslands to their blooming best while keeping weeds at bay. (Image source: ‘California Golden Violet’ by Lauren Glevanik, CC-BY-NC 4.0, via iNaturalist)

Source article: Nguyen M.A., Kimball S., Burger J.C., et al. “Applying community assembly theory to restoration: overcoming dispersal and abiotic filters is key to diversifying California grassland.” Restoration Ecology. (2024). https://doi.org/10.1111/rec.14018

Underneath the superbloom’s surface, native grassland species are fighting intense battles with invasive species that threaten to take over the landscape. Grassland ecosystems are home to more plant species than just grasses; they also host a diversity of small, leafy, non-grass flowering plants collectively called forbs. These are things like clovers, violets, daisies, or any other little plant that might be called a “wildflower.”

Due to continued urban development, lack of land protections, and human activities that have introduced non-native species, invasive weeds are spreading and making their homes in the remnants of existing grasslands. These plants can outcompete the native plants for resources like water, nutrients and space, making it difficult for our beloved wildflowers to keep coming back year after year. In degraded habitats where invasive plants are common, there is still hope for our favorite flowers in the form of restoration efforts.

In annual grasslands where plants have short life cycles, dead plant material called thatch can build up and block sunlight from reaching the ground where wildflower seedlings desperately need it. Some strategies like controlled burns, mowing, and animal grazing are used in grasslands to help reduce thatch, keep invasive species at bay, and promote native species growth. These actions can be effective when used together in some combination, but they don’t always work perfectly. One of the major hurdles in restoring grasslands is targeting individual species to keep or remove. How would you get rid of thousands of weedy plants without also taking all of the other flowers, too?

Picking Flowers: How Nature Chooses Species through Filtering

The authors of a new study published in Restoration Ecology approach this question using a principle from ecology known as “filtering.” Filters are essentially environmental conditions that limit what can grow in an area. For example, if an environment is a dry desert, it will “filter” for species that can tolerate very little water. Only species that can succeed under those environmental conditions meet the criteria to live in that spot. 

There are three main types of filters: abiotic (non-living environment), biotic (interactions with other living things), and dispersal. Abiotic filters are part of the environment like water, temperature, soil nutrition, or light availability – a plant must be able to tolerate the physical spot in which it grows. Biotic filters are determined by a species’ interactions with its neighbors. If one plant is very aggressive in drawing up water in an area, it may outcompete others and filter them out of the area. Having lots of very competitive plants in a small area can filter “weaker” competitors out of the neighborhood. Dispersal filters are the last major type of filtering. If seeds can’t physically reach a location, it can’t be part of the community, even if it could technically survive the other abiotic and biotic filters. 

How do these filters help us figure out how to get the right grassland species growing again? One proposed strategy is to use filters to our advantage and find a set of conditions that allows native species to thrive, but filters out non-natives. Essentially, what methods of management creates a good environment for our favorite forbs and a bad environment for invasive species at the same time?

This research focused on a few common restoration techniques and evaluated how well they filtered for the desired species. They tested adding seeds in different orders and different ways of removing unwanted grasses through mowing and herbicide application. The ultimate goal here was to determine what methods can easily be applied across grasslands to favor native species and make them more resilient to invasion by non-natives. 

Seeding success in grassland restoration

Researchers conducted their study at two grassland sites in southern California, which were initially over 95% covered by non-native species. Both sites were prepared for restoration over a 2-year period where they were mowed and treated with an herbicide to cut back the invasive species and expose the soil. The first site tested how adding seeds at different times affected restoration outcomes. At this site, a native bunchgrass was planted and then native wildflower seeds were (1) not added, (2) added at the same time, (3) added 3 months later, and (4) added 1 year later. The second site tested how two methods of controlling unwanted grass species could be effective. They mowed and raked plots to remove thatch, and applied an herbicide on top of some of the mowed plots to stop non-native grass seedlings from growing. 

The sites were surveyed over three years to track which species successfully established under each treatment. In the first treatment where the timing of adding seeds was manipulated, this study found that staggering when seeds were sown did not significantly change the success of native forb species. This was an unexpected result, since it contradicted previous studies, but it could simplify future restoration efforts if continually adding seeds doesn’t have a huge effect on the plant community. 

Restoration revelations: Finding strategies that work

The biggest finding was how to deal with the buildup of dead thatch: removing dead plant material from the ground increased native forb establishment. Thatch blocks light from reaching germinating seedlings, so physically removing it through mowing or raking helps native plants shine through. Researchers discussed how light availability is an important environmental filter in grasslands that could be limiting native species’s growth and filtering them out of the landscape. While mowing is a clear winning strategy here, adding herbicide had a more subtle effect. The plots with extra herbicide added to them didn’t show any better forb cover, but they did tend to have less non-native grass cover. While it’s not the most effective way to control grass alone, it could be part of an effective solution in combination with physically removing dead plant material.

Tracking the plants at each site also allowed the researchers to observe how invasive species behaved with each of the restoration methods. Over the three years of the experiment, non-native grasses weren’t affected by the timing of sowing native seeds – they steadily encroached on the plots and increased in number every year in each treatment. The best strategy there is to manage weeds during the first few years of the restoration project to give the up-and-coming native plants a chance to grow larger and become established before seriously competing with non-native grasses. In the sites that were mowed, however, non-native grasses were reduced below normal levels of grassland without any restoration treatments. This is further support that physical removal of thatch and disruption of non-native plant growth really helps out our native forb friends!

Clearing the path to better grasslands

Overall, applying ecological theories to restoration efforts provides a sound scientific basis for designing and implementing restoration projects that prioritize native species diversity, ecosystem resilience, and long-term ecological health. These results show how understanding ecological filtering can inspire better practical strategies for restoration efforts. Professional restoration ecologists and land managers rely on studies like these that provide evidence of successful restoration and offer insights on how best to bring a diversity of plants back into a degraded landscape. 

Studies on grassland restoration lay the groundwork for how we revitalize and sustain healthy ecosystems, and this research matters for lots of reasons. Restoring and diversifying grassland ecosystems can have positive impacts on biodiversity, soil health, water quality, and overall ecosystem resilience. Healthy ecosystems provide essential services such as carbon sequestration and support for pollinator populations, which are crucial for human well-being. Increasing native forb species also boosts biodiversity within the plant community, which increases the adaptability and resilience of the community to climate change. Protecting plants creates habitat for everything else that calls the grassland home, including countless birds, reptiles, mammals, insects, and fungi. 

Understanding how to better set up our restoration projects for success and maintain them so they’re effective ensures that the time, care, and money we put into keeping our planet healthy is actually working. Testing restoration methods in long-term experiments ensures we’re on the right track and doing our best to protect and nurture our planet. 

Reviewed by: