Research | zak-ratajczak

Overview: resilience, tipping points, global change, and environmental diversity

“Tipping points” describe when the pressure on a system is simply too great, resulting in a breakdown of key stabilizing processes that can cause shifts in biodiversity and ecosystem services. Yet, leading up to a tipping point, the ecosystem might only show minor changes while these stabilizing processes remain in place. In other words, the ecosystem shows resilience. This is what makes tipping points and resilience so tricky to study--they might make sense in retrospect but they are usually difficult to foresee.

Tipping points have been identified in the human body, financial systems, and ecosystems. There is a growing concern that land management and climate change could reach a tipping point in some grasslands and forests, resulting in environmental transformations that prove difficult or impossible to reverse in our lifetime.

The aim of plant ecology and resilience lab is to increase or knowledge of plant ecology, in general, with an emphasis on grassland and forests responses to changes in land management, fire, and climate. Almost all work in the lab also assess resilience in some way, whether it’s the nitty-gritty of feedback mechanisms, broad vegetation patterns, or simulations to estimate future trends in vegetation. The following sections give brief overviews of our most active areas of current work.

Our work is focused on two sites with unique implications for conservation in North America. The first is Konza Prairie LTER, one of the largest remaining tracts of native tallgrass prairie. Tallgrass prairie, while once widespread in North America, has now been reduced to about 2% of its former extent, making it one of the most altered systems on the continent. Konza maintains a large-scale factorial experiment of fire manipulations and reintroduction/exclusion of bison that is an invaluable resource, just 20 minutes from campus. The second research site we work in is sub-alpine forests of Yellowstone National Park and surrounding areas Through simulation modelling, we are assessing whether forests will be resilient to a range of climate change scenarios.

Some core themes in the plant ecology and resilience lab are:

-Does biodiversity and spatial heterogeneity enhance resilience?

-How do species differ in their resilience to climate extremes?

-What types of feedbacks really underpin resilience?

-How do changes in multiple global change drivers affect resilience?

-How does the reintroduction of large native mammals--sometimes called 'rewilding'--affect the resilience to projected changes in climate?

A portion of the bison herd at Konza Prairie LTER. Bison have now been reintroduced for two decades, creating a great platform for understanding how these animals interact with plants, weather variation, and fire.

One year following stand-replacing fire in Grand Teton National Park--one of our study sites. The types of climate extremes that facilitate these types of fires is projected to be more common this century--will forests prove to be resilient?

Example publications:

Higuera P.E., Metcalf A.L., Miller C., Buma B., McWethy D.B., Metcalf E.C., Ratajczak Z., Nelson C.B., Chaffin B.C., Stedman R.C., McCaffrey S., Schoennagel T., Harvey B.J., Hood S.H., Schultz C.A., Black A.E., Campbell D., Haggerty J.H., Keane R.E., Krawchuk M.A., Kulig J.C., Rafferty R., and Virapongse A. (2019) Integrating subjective and objective dimensions of resilience in fire-prone landscapes. BioScience 68: 379-388.

McWethy D.B., Schoennagel T., Higuera P.E., Krawchuk M., Harvey B.J., Metcalf E.C., Schultz C., Miller C., Metcalf A.L., Buma B., Vorapongse A., Kulif J.C., Stedman R.C., Ratajczak Z., Nelson C.R., and Kolden C. (2019) Rethinking resilience to wildfire. Nature Sustainability 2: 797-804.

Seakell D. Carr J., Dell'Angelo J., Gephart J., Kummu M., Magliocca N., Porkka M., Prell P., Puma M.J., Ratajczak Z., Rulli M.C., Seekell D.A., Suweis S., Tavoni A., D'Odorico P. (2017) Conceptualizing and quantifying resilience in the global food system. Environmental Research Letters 12: 025010.

Ratajczak Z., Carpenter S., Ives A., Kucharik K., Ramiadantsoa T., Stegner A.M., Williams J., Zhang J., and Turner M.G. (2018) Abrupt change in ecological systems: diagnosis and inference. Trends in Ecology and Evolution 33: 513-526.

Ratajczak Z., Nippert J.B., and Collins S.L. (2012) Woody encroachment decreases diversity across North American grasslands and savannas. Ecology 93: 697-703.

Grassland burned every twenty years (on the left) vs every two years (on the right)

Land management and vegetation shifts

The arrival of Europeans continues to transform land-use management in North America. One the greatest impacts on land management has been a change in grazers, from native ungulates to cattle, and changes in fire, often in the form of fire suppression. For instance, large extents of grasslands in the Central U.S. are rarely burned, even though many of these locations were probably burned every two to ten years before Europeans arrived. We seek to understand if these land use changes can result in abrupt ecological changes in plant communities. For an example, see the above photo from Konza Prairie: left of the road has been burned every twenty years versus the right side of the road, which has been burned every two years. 30+ years into this experiment the differences are stark--the area burned every two years is still dominated by grasses and other prairie plants, whereas the area burned less is dominated by trees and shrubs.

Most of our work in on management is centered around transitions from tallgrass prairie to shrublands and woodlands. As part of the Konza Prairie LTER, we use long term experiments that continue fire suppression, coupled with experiments that reintroduce fire to encroached areas. We are now looking to better understand the role that grazers and climate play in this widespread ecological transition to woody plants.

Land management is not a major part of our research in Yellowstone, but students interested in forest management are very welcome to apply!

A map of fire frequency in Flint Hills grasslands from 2000 to 2011. Areas not shaded blue could see transitions due to shrub- or woodland in coming decades (see Ratajczak et al. 2016 for more details).

Example publications:

Ning C., Yu K., and Ratajczak Z. (2019) A dryland re-vegetation in northern China: Success or failure? Quick transitions or long lags? Ecosphere 10: e02678.

Ratajczak Z., D’Odorico P.D., Nippert J.B., Collins S.L., Brunsell N. and Ravi S. (2017) Changes in spatial variance during a grassland to shrubland state transition. Journal Ecology 105: 750-760.

Ratajczak Z., Briggs J.M., Goodin D.G., Luo L., Mohler R.L., Nippert J.B. and Obermeyer B. (2016) Assessing the potential for transitions from tallgrass prairie to woodlands: are we operating beyond critical fire thresholds?. Journal of Rangeland Ecology and Management 69: 280-287.

Ratajczak Z., Blair J.B., Nippert J.B. and Briggs J.M. (2014) Fire dynamics distinguish grasslands, shrublands, and woodlands as alternative attractors in the Central Plains. Journal of Ecology 102: 1374-1385.

Climate change and climate extremes

Growing evidence suggests that climate is becoming more extreme in many grasslands, savannas, and forests. Longer droughts, more intense heatwaves, and periods of anomalously high rain are increasingly common. Will grasslands and forests be resilient to these climate extremes? We use observational data and next generation models of forests to answer this question and develop tools that might identify when ecosystems are nearing collapse. For example, with collaborators at UW-Madison (the Turner lab) and BOKU Vienna (the Seidl lab), we are currently using the model iLand—an individual based model of trees—to simulate forest responses to changes in climate and in particular, wildfire activity. The figure to the right shows one of our simulations in Bridger-Teton National Forest, south of Yellowstone National Park.

One of our next priorities is to establish climate extreme experiments in areas with native bison and tree encroachment to see how these key land use changes affect the response to climate extremes. The goal of this work will be to project changes in plant diversity, animal nutrition, and wildfire intensity.

A quick video of one of our forest simulations: dark green denotes dense forest, light green is sparse forest, and tan ecosystems are non-forest, mostly due to recent wildfires.

A big chunk of the Northern Rockies modelling team, from left to right: Tyler Hoecker (UW-Madison), Werner Rammer (BOKU), Monica Turner (UW-Madison), Rupert Seidl (BOKU), Winslow Hansen (Columbia), Kristin Braziunas (UW-Madison), and Zak Ratajczak (UW-Madison/Kansas State Univ). (Photo-credit: M.G. Turner)

Example publications:

Turner, M.G., Calder W.J., Cumming G.S., Hughes T.P., Jentsch A., LaDeau S.L., Lenton T.M., Shuman B.N., Turetsky M.R., Ratajczak Z., Williams J.W., Williams A.P., and Carpenter S.C. (In Press) Climate Change, Ecosystems, and Abrupt Change: A Science Agenda. Proceedings of the Royal Society B Biological Sciences.

Ratajczak Z., Churchill A., Ladwig L, Taylor J.H. and Collins S.L. (2019) The combined effects of an extreme heatwave and wildfire on tallgrass prairie vegetation. Journal of Vegetation Science 30: 687-697.

Ratajczak Z. and Ladwig L. (2019) Will climate change push grasslands past critical thresholds? Book chapter for “Grasslands and Climate Change” Gibson D. and Newman J. (eds) British Ecological Society and Cambridge University Press, Cambridge UK. Pages 98-114.

Marchand P., Carr J., Dell'Angelo J., Gephart J., Kummu M., Magliocca N., Porkka M., Prell P., Puma M.J., Ratajczak Z., Rulli M.C., Seekell D.A., Suweis S., Tavoni A., D'Odorico P. (2016) Reserves and trade jointly determine exposure to food supply shocks. Environmental Research Letters 11: 095009.

Ladwig L.M., Ratajczak Z., Ocheltree T.W., Hafich K., Churchill A.C., Frey S.J.K., Fuss C.B., Kazanski C.E., Muñoz J.D., Petrie M., Reinmann A.B. and Smith J.G. (2016) Beyond arctic and alpine: the influence of winter climate on temperate ecosystems. Ecology 92: 372-382.

Testing and advancing ecological theory

Theory is a powerful tool for distilling our ecology knowledge and is a key part of generating hypotheses. Theory is especially useful for generating "blue sky hypotheses"--how we think a system behave under ideal circumstances and if our intuition is right.

We use theory to guide our empirical work, comparing empirical patterns to expectation derived from theory. Theory is also invaluable in our age of environmental whack-a-mole: with pressing environmental issues arising in so many ecosystems, it is impossible for the finite number of ecologists to thoroughly study every ecosystem. Theory can help fill in some baseline assumptions of how systems might respond to global change pressures.

The relationship between theoretical and empirical ecology is reciprocal. When we observe interesting empirical patterns that are inconsistent with theory, we have modified models and simulations to see if there are qualitative changes in theoretical predictions. Students in the lab are strongly encouraged to work with simulation models to extend the reach of their work. Theoretical work to date has included work on savanna tree coexistence, forest responses to disturbances, and how networks respond to disturbances.

Nate Brunsell (left, Kansas University) and Paolo D'Odorico (center, UC Berkely) visiting us at Konza Prairie for a workshop on theory related to tipping points.

Example publications:

Stegner, A.M., Ratajczak Z., Carpenter S., and Williams J, (2019) Inferring critical transitions in paleoecological time series with irregular sampling and variable time-averaging. Quaternary Science Reviews 207, 49-63.

Brunsell N., Van Vleck E.S., Nosshi M., Ratajczak Z., Nippert J.B. (2017) Assessing the roles of fire frequency and precipitation in determining woody plant expansion in central U.S. grassland. Journal of Geophysical Research Biogeosciences 112: https://doi.org/10.1002/2017JG004046.

Ratajczak Z., D’Odorico P. and Yu K. (2017) The enemy of my enemy: why coexisting with grasses might be an adaptive strategy for savanna trees. Ecosystems 20: 1278-1295.

Ratajczak Z., D’Odorico P., Collins S.L, Bestelmeyer B., Isbell F. and Nippert J.B. (2017) The interactive effects of press/pulse intensity and duration on regime shifts at multiple scales. Ecological Monographs 87: 198-218.

Other

Some other areas we work on are:

Methods for determining the cause of abrupt changes
Early warning signs of impending abrupt shifts
Synthesis across grassland ecosystems
Resilience of social-ecological systems
Networks and how the properties influence sensitivity and resilience

In fact, a lot of these areas are woven into much of our research.