Assistant Professor, Kansas State University

Can grazed tall grass prairie remain resilient to woody encroachment?

We report on a range of long-term large-scale experiments at Konza Prairie Biological Station and other sites in the Central Great Plains that manipulate fire and grazing. Our observations derive primarily from remote sensing, including airborne lasers. In ungrazed areas, one and two year fire return intervals (FRI) have largely prevented encroachment, but a small shift to a three to four FRI results in extensive encroachment, covering about 40% of the landscape. In areas grazed by bison, a one year FRI results in ~8% landscape encroachment, a two year FRI results in ~18% encroachment, and a three to four year FRI results in ~40% encroachment. Areas grazed by cattle see similar patterns, although average shrub height is potentially lower. Before European colonization, the estimated average FRI in the Central Great Plains was around three to four years, but this rate of burning no longer maintains resilience against encroachment. We have explored more extreme measures to prevent or reverse encroachment, such as reintroducing annual fires to encroached areas, combining annual fires with mechanical shrub removal, combining long-term drought and fire, and extreme fires that combine long-term rest from grazing and hot windy conditions during fires. None of these approaches have fully reversed encroachment. These observations suggest that large-scale factors are decreasing grassland resilience to encroachment. Primary suspects are rising atmospheric CO2 (a stimulant for woody plant growth). Woody vegetation is increasing regionally, which increases woody plant seed rain. For instance, in the northern Flint Hills, around 33% of the landscape is now woody vegetation.

In the 21st century, preventing or reversing encroachment might require new measures, such as reintroducing browsers (e.g. elk or goats), more artificial inputs such as chemical applications, or reduced grazer stocking rates, which would reduce ranching incomes. The fieldtrip associated with this symposium will show the results from most of these experiments. We are keenly interested in hearing approaches other land stewards have taken to prevent or reverse encroachment—this is a difficult problem that will require crowd-sourcing to find viable solutions.