Plant community trends at Tallgrass Prairie National Preserve: 1998–2018

Abstract

The Heartland Inventory and Monitoring Network monitors plant communities at Tallgrass Prairie National Preserve and evaluates a variety of environmental variables that affect vegetation patterns, including climate and ecological disturbances such as fire and grazing. Here we report on 2002–2018 trends in management actions (fire and grazing) and key plant community indicators. Temperature has increased over the past 50 years in the region. Precipitation and a standardized precipitation-evapotranspiration index included a high degree of interannual variability and did not demonstrate directional change. We documented a decline in disturbance intensity (i.e., less frequent prescribed fire and lower stocking rates) since 2006. A preserve goal is to maintain 30 to 60% of the area as bare ground (soil and rock) for ideal greater prairie-chicken habitat. Bare areas have been in decline and minimally meet the goal preserve wide. Bare areas vary by pasture and year, with bare areas exceeding the threshold in earlier years and Big Pasture and Red House Pasture falling short in some recent years. Although the preserve-scale mean minimally met the objective, there was a great deal of heterogeneity across monitoring sites. Litter cover and depth were greater than ecological recommendations for the greater prairie-chicken, especially in 2018. Litter depth demonstrated a great deal of variability and included deep litter. Woody plants were targeted to remain below 5% cover. Preserve- and pasture-scale cover means were well below this threshold but are increasing. Species richness on a per site basis (alpha diversity) and preserve-wide richness (gamma diversity) showed no apparent directional change when corrected for differences in sample size. Comparison of native species composition between 2002 and 2018 revealed a 36.9% difference in the Sørensen Index, although observer error accounted for almost 2/3 of this apparent change. The preserve continues to have characteristic tallgrass prairie species, and nonnative species continue to be low. Similar to targeted invasive plant monitoring, we found the target species Kentucky bluegrass to be below park thresholds. Continued evaluation of fire frequency and grazing intensity will be critical to achieving ecological goals including conserving the greater prairie-chicken. Development of a grazing plan may assist with prescribing stocking rates that are consistent with the preserve’s ecological and cultural objectives and could include alternative herbivores, such as goats or expansion of bison.