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.
