Researcher finds grasses seeded in rainy years are less successful because they have more weeds and fewer native plants
Before there were farms, there were prairies. When the early settlers arrived on the Great Plains, they were greeted by a sea of grasses rich in a biodiversity that supported everything from butterflies to bison and rejuvenated seasonally by prairie wildfires.
In time, those rich soils were plowed under to support agriculture. Now, these vast expanses of prairie grasses have all but disappeared.
Today, there are landowners who have been trying to return some of their land to its roots and convert fallow fields or depleted and abandoned areas to native prairie. But it hasn’t been as simple as they originally thought. Scientists researching the problem at Michigan State University have found out why.
“Variations in success can relate to weed management, investment in diverse seed mixes containing many native prairie plant species, appropriate use of prescribed fire, and other factors,” said Lars Brudvig, associate professor of plant biology.
“Our paper also shows that weather conditions during the year a prairie was planted affect success. Prairies planted during what turn out to be rainy years are less successful because they have more weeds and somewhat fewer native prairie plants living within them.”
Brudvig worked with former MSU graduate student Anna Funk to study data going back 20 years. They looked at a wide swath of restored prairies, both good and bad. Each of the 83 sites investigated had been started roughly the same with similar management strategies — the use of herbicides, regular mowing, and controlled burning — and the plants were native prairie species. The researchers needed to identify any patterns in the fields planted in a rainy year or in a hot, dry year.
Landowners were interviewed to pinpoint planting dates, the number of prairie species planted and other specific details. Researchers then examined each site, documented the abundance and diversity of prairie plants and weeds and took soil samples to test productivity.
“They were pure prairie plant seeds, including grasses, wildflowers (forbs), and sedges,” said Brudvig. “Each site we worked at was started by spreading seeds of different species and different numbers of species, from eight to 200 different plant species.”
It would seem logical that, with soil conditions being critically important to prairie growth, the success of prairie restoration would depend on the quality of the soil. Yet the study showed that more productive soils had greater weed cover so that, by soil health alone, productive soils were actually less successful. Instead of looking at the ground, the researchers looked at the sky.
“We looked at weather from a few different perspectives, including rainfall, temperature and the amount of drought each prairie experienced during the year it was planted,” he said. “Of these, rainfall was the most important for how restorations turned out. Prairies planted in rainier years had a greater number of weed species and greater cover of weeds than those planted during dry years.”
That fact became critical to their key findings. Other work has shown that when water is added early in a restoration program, plants actually perform better. But it became apparent that prairies planted in rainy years had growth suppressed by weeds.
“There was a bit of tantalizing evidence from just a couple of studies that suggested the weather you get during a prairie restoration can actually have a long-lasting effect on the success of the project,” said Funk in the news release. “It was an idea that land managers were familiar with anecdotally, but it hadn’t been carefully studied.”
She said that, when they did a big analysis of all the data, they expected certain factors to be really important, such as the frequency the site had been managed with prescribed fire.
“I was very surprised to find not only an effect of weather but that sometimes planting-year weather conditions had the biggest effect of all.”
Native prairie plants have a long lifespan, which will prove critical to seeing them survive despite the first year’s weather leaving its mark on the grasses for a long time.
“Many prairie plant species can live a long time, perhaps a century or more in some cases,” said Brudvig. “What this means is that when the weather favours some species over others early on in restoration, those plants then form the prairie that we see for years or even decades. We did expect, however, that the signature of the weather in the planting year would become less apparent as restored prairies became older. Amazingly, we didn’t see this. Prairies planted even two decades previously still bore the signature of the weather they were planted under.”
A critical factor is the influence of climate change. Over the past century, weather patterns have changed and the global temperature has warmed by more than a degree.
“We anticipate more springtime rainfall in our region in the future and this may be challenging for prairie restoration because many weeds can respond strongly to elevated springtime rain,” said Brudvig. “We also know that the vegetation has changed substantially during the last one or two centuries with declines to native vegetation (since early settlement).”
Brudvig said prioritizing prairie restoration plantings during anticipated dry years would help facilitate success in the future. Long range and seasonal weather forecasting is increasingly more efficient, making planting decisions easier.
“Other possible ways to mitigate planting-year weather effects might be increased weed control (like mowing) during wet planting years or repeating seed spreading in later years, following wet planting years,” said Brudvig.
The research was published recently in Scientific Reports.