A Visual of Tillage Legacy

If you are someone who is interested in native prairie, you probably know that the conversion rate of grassland to cropland has spiked over the past few years, coinciding with a spike in grain prices. The relationship between farm profits and conversion rates has a historical precedence. Over the decades, the profitability of farming has oscillated due to market prices, technological advances and government policies. During times when farming profits were low, marginal farmland was planted back to grassland so that it could be grazed by cattle or harvested as hay.

There have been three to four periods since 1900 when marginal farmland was planted back to grass or left for nature to reclaim on its own. These grasslands are still around today and are commonly referred to as "go back" grasslands. However, not all grasslands are created equal. The legacy of past tillage is evident in the plant species composition of “go back” grasslands versus grasslands that grow on native sod.

Sometimes “go back” prairie is nestled in a native sod prairie. These cases provide us with an excellent opportunity to understand the value of native sod. If a person knows what to look for, you can often identify areas that were once cropped just by walking through them and taking note of the plant species. Although there are exceptions, “go back” grasslands usually have lower species diversity and more invasive species than native sod grasslands that have been managed properly.

Cropping can leave an extreme legacy. The photo below was taken in a large prairie that had been under the same management for many years. When I walked through the prairie for the first time, I came across a stark transition in plant species composition in the middle of the grassland. There were no fences delineating the transition and no change of management, but it almost seemed like there was an invisible barrier. On one side of the barrier was a grassland completely dominated by the non-native species smooth brome and alfalfa, and on the other side was a diverse native prairie with approximately 60 species/100 m². This photo below was taken at the transition point.

Because the transition was much clearer in person, I have pointed out the approximate transition line in the photo below. On the left was the smooth brome grassland and on the right was the native grassland. Notice the wildflowers on the right side of the photo, showcasing the species diversity.

Another way to distinguish the differences in species in the photo is at the horizon line of the hill. On the left side you can see the multiple brown seeds from the smooth brome dominated pasture. On the right you do not see a solid brown line of smooth brome seed heads, but instead you see individual grass seed heads and wild flowers.  

When each side is blown up, it is even easier to see the difference.

I had access to a database that had record of all land registered in a farm program since the 1950’s, which confirmed my suspicions that the smooth brome grassland had been cropped in the past. After being cropped, it was likely seeded into non-native smooth brome. The smooth brome that was planted then, still stands today in a solid monoculture, whereas the native sod grassland has a high diversity of species. Even though the two prairies are not separated by any kind of visible barrier, the native prairie species have not spread to the formerly tilled grassland.

What this image sums up so succinctly is something that is rarely taken into account when discussing the value of native prairies. Once native prairie is tilled, the native sod is altered, and once altered, the native prairie species do not always grow back. The stand of smooth brome remains in this prairie as a legacy of human change on the landscape that is not quickly or easily reversed.