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Grasslands are globally important ecosystems, covering more than 40% of Earth’s ice-free land area and housing a wide variety of plant and animal life. In addition to their natural beauty, grasslands provide forage for livestock, store carbon belowground, and provide habitat for many species. Our lab has many ongoing projects aimed at understanding the dynamics of the North American tallgrass prairie ecosystem, including (1) the natural responses of this system to its primary drivers of fire, climate, and grazing (Herbivore Removals, conSME, Ghost Fire), (2) how these drivers and grassland responses are being altered by human-caused global change factors such as nutrient pollution (Nutrient Network, Phosphorus Plots, ChANGE), and (3) sustainable management practices to promote the health of this important ecosystem for cattle grazing (Patch Burn Grazing) and the prevention of shrub encroachment (Shrub Removals).

Herbivore Removals

Over ten years, we have found that the removal of invertebrate herbivores from a tallgrass prairie ecosystem resulted in large shifts in plant community composition, from grass-dominated communities to forb-dominated communities. In particular, we found that insects increase forb diversity (number of species), but depress forb biomass and responses to nutrient addition (La Pierre et al 2015). In 2018, after ten years of treatment applications, we stopped our experimental treatments and are currently following recovery to determine how lasting these changes are. This experiment provided the inspiration for the conSME experiment.

consme:
​ consumer size manipulation Experiment

Herbivores of varying size classes exist within the grassland biome (large mammals, small mammals, insects), however, their independent and interactive effects on grassland plant species composition and function are understudied. With this project, we aim to tease apart the effects of three size classes of herbivores within tallgrass prairie, and whether these effects vary across fire regimes. We have been monitoring changes in the plant community composition, plant biomass production, and demography of the dominant plant species in response to the removal of bison, small mammals, and/or insects since 2018.

Ghost Fire

Frequent burning is a common land practice in many grasslands worldwide, and this land use strategy has large impacts on a wide variety of ecosystem functions and services. Fire in tallgrass prairie alters plant community composition, decreases richness, and increases plant production. Proposed mechanisms for the changes in community composition and function are that fire decreases N availability (through volatilization) and removes litter (thereby increasing light availability and decreasing soil moisture). However, few experiments explicitly test these mechanisms, and those that do monitor short-term effects. Yet, the strength of these mechanisms likely intensifies over longer time scales, as other ecosystem attributes (e.g., plant community composition) change through time.
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With this project, we aim to determine the mechanisms behind community and ecosystem differences between annually burned grassland and unburned grassland by experimentally manipulating N availability and litter. We impose litter and nitrogen conditions found in unburned grassland onto annually burned grassland, and litter and nitrogen conditions typically found in annually burned grassland onto unburned grassland. Importantly, we are monitoring both below- and above-ground plant community and ecosystem dynamics as well other dimensions of the ecosystem including soil microbial communities and insect community composition and biomass.
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Litter addition treatment in freshly burned prairie.

Patch Burn Grazing

Frequently, cattle production comes at the expense of plant and wildlife biodiversity, with cattle management goals at odds with environmental health and conservation. Patch burn grazing is a new management strategy that promises land owners the ability to prioritize cattle weight gain and promote conservation and sustainability practices simultaneously. Patch burn grazing employs fire and grazing interactions to create a “shifting mosaic” of forage across a landscape through time. In an unfenced management unit, fire is used to lure grazing animals to a portion (patch) of the unit that has recently burned, while unburned patches experience reduced levels of grazing. The patchiness of disturbances across a patch burn grazing landscape leads to heterogeneity of vegetation, which is predicted to be important for ecosystem health. Importantly, in addition to its purported benefits for wildlife and natural resources, patch burn grazing still provides equivalent cattle production to more traditional management practices, such as annual burning. Yet evidence for many of the potential benefits of patch burn grazing for bird, small mammal, insect, and plant communities is limited or equivocal, and patch burn grazing’s belowground effects (e.g., soil health, carbon sequestration) remain completely unexplored. With this project, we aim to examine the effects of patch burn grazing on the health and sustainability of a grazing ecosystem.
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Nutrient addition treatment.

Nutrient Network
(NUTNET)

Nutrient pollution and reductions in the abundance and identities of consumers are two important problems facing grasslands worldwide. We are contributors to the Nutrient Network (NutNet), which aims to examine the effects of these drivers through a coordinated research effort across more than 130 sites around the world. Altogether, NutNet research has shown strong effects of increased nutrient availability and removal of vertebrate herbivores on plant community composition (Harpole et al 2016), primary productivity (Borer et al 2014 ,  La Pierre et al 2016 ), insect communities (La Pierre et al 2016), microbial communities (Leff et al 2015), and biogeochemical processes.

Phosphorous Plots
(PPlots)

While North American grasslands are typically considered nitrogen-limited ecosystems, the interactive effects of altered nitrogen and phosphorus pollution are relatively understudied. Here we are investigating the combined effects of increased nitrogen and phosphorus availability in an experiment that has been ongoing for nearly two decades (since 2003). Through time, we have found substantial changes in the species present in the plant community despite no net species loss (Avolio et al 2014), with large consequences for the stability of biomass production under high nitrogen and phosphorus additions (Koerner et al 2016).
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Sampling PPlots in a high Conyza production year.

ChANGE
(Chronic Addition of Nitrogen Gradient Experiment)

Anthropogenic changes in nutrients are occurring globally, with human activities greatly increasing nitrogen deposition and nitrogen fertilization running off into natural ecosystems. These changes in nitrogen availability create novel conditions that differentially favor some species at the expense of others, leading to declines in biodiversity and altered plant communities. In 2013, we established ChANGE to assess the sensitivity of tallgrass prairie plant community and ecosystem function to a gradient of chronic N additions, from projected low levels of N deposition, to levels used in other experiments (e.g., PPlots, NutNet), up to more extreme levels reflecting agricultural inputs.

Shrub Removals

Frequent natural fires and grazing are two drivers that keep grassland ecosystems in a grassy state. However, encroachment of woody plant species has become an increasing threat to the tallgrass prairie ecosystem, due in part to less frequent burning. Returning the system to its natural fire regime is often not enough to restore the system to a grassland state. We aim to test whether cutting back shrubs and burning the ecosystem in one or more intensive fires could be used as a restoration technique to remove shrubs.