The combined effects of an extreme heatwave and wildfire on tallgrass prairie vegetation

This article was published in the Journal of Vegetation Science March 22, 2019.

https://doi.org/10.1111/jvs.12750

Abstract

Questions

Climate extremes are predicted to become more common in many ecosystems. Climate extremes can promote and interact with disturbances, but the combined effects of climate extremes and disturbances have not been quantified in many ecosystems. In this study, we ask whether the dual impact of a climate extreme and concomitant disturbance (wildfire) has a greater affect than a climate extreme alone.

Location

Tallgrass prairie in the Konza Prairie Biological Station, northeastern Kansas, USA.

Methods

We quantified the response of a tallgrass prairie plant community to a 2-year climate extreme of low growing-season precipitation and high temperatures. In the first year of the climate extreme, a subset of plots was burned by a growing-season wildfire. This natural experiment allowed us to compare community responses to a climate extreme with and without wildfire.

Results

In plots exposed to the climate extreme but not wildfire, community structure, diversity, and composition showed minor to insignificant changes, such as a 20% reduction in grass cover and a slight increase in species diversity. Plots exposed to both the climate extreme and wildfire underwent larger changes, including an 80% reduction in grass cover, 50% increase in forb cover, and increased plant diversity. Two years after the climate extreme, structural shifts in burned plots showed little sign of recovery, indicating a potentially lasting shift in plant community structure.

Conclusions

Our results suggest that community responses to climate extremes need to account for climate-related disturbances — in this case, high temperatures, drought and wildfire. This response diverged from our expectation that heat, drought, and an additional fire would favor grasses. Although growing-season wildfires in tallgrass prairie have been rare in recent decades, they will likely become more common with climate change, potentially leading to changes in grassland structure.

Citation

Ratajczak, Zak, Amber C. Churchill, Laura M. Ladwig, Jeff H. Taylor, and Scott L. Collins. "The combined effects of an extreme heatwave and wildfire on tallgrass prairie vegetation." Journal of Vegetation Science 30, no. 4 (2019): 687-697.

“What is an anomaly?”*

TPOS note:

New research on the 2016 drought in the southern Appalachians demonstrates this event was an anomaly given recent regional climate trends. Since the 1950s, average fall weather has seen higher than average precipitation and lower than average daily high temperatures. The drought of 2016 (sometimes called a "flash drought") was notable for the "...intensity and rapid onset... and its destructive impact on wildfire and human water resources..."

However, the authors also reviewed longer-term climate data and concluded, "...droughts as strong as the 2016 event are more likely than indicated from a shorter 60 year perspective."

The infamous Chimney Tops 2 Fire occured during this flash drought. Following ignition by an arsonist, this fire burned from Great Smokey Mountains National Park to Gatlinburg, Tennessee. With severe drought, strong winds, steep terrain, and high fuel loads (including recent leaf fall and dry heavy fuels) the fire covered 14,000 acres, destroyed structures, and resulted in 14 fatalities.

Here are two questions we hope will lead to further discussion on this blog or elsewhere:

• do these events suggest wildfire risk assessments for the Midwest should be reviewed to determine if models include parameters adequate to modeling wildfire spread in worst case (so-called "unthinkable") scenarios?
• what questions do you have for the authors about this research?

Read the abstract below or check out the article "The 2016 southeastern U.S.drought: An extreme departure from centennial wetting and cooling" at http://onlinelibrary.wiley.com/doi/10.1002/2017JD027523/full

* thanks to Megan Sebasky, Northeast Region LANDFIRE coordinator, for helpful comments on this post and suggesting this title (read LANDFIRE's interview with Megan). Thanks also to Jed Meunier, Wisconsin DNR fire ecologist, for reviewing and sharing comments on this post.

Abstract

The fall 2016 drought in the southeastern United States (SE U.S.) appeared exceptional based on its widespread impacts, but the current monitoring framework that only extends from 1979 to present does not readily facilitate evaluation of soil-moisture anomalies in a centennial context. A new method to extend monthly gridded soil-moisture estimates back to 1895 is developed, indicating that since 1895, October–November 2016 soil moisture (0–200 cm) in the SE U.S. was likely the second lowest on record, behind 1954. This severe drought developed rapidly and was brought on by low September–November precipitation and record-high September–November daily maximum temperatures (Tmax). Record-high Tmax drove record-high atmospheric moisture demand, accounting for 28% of the October–November 2016 soil-moisture anomaly. Drought and heat in fall 2016 contrasted with 20th century wetting and cooling in the region but resembled conditions more common from 1895–1956. Dynamically, the exceptional drying in fall 2016 was driven by anomalous ridging over the central United States that reduced south-southwesterly moisture transports into the SE U.S. by approximately 75%. These circulation anomalies were partly promoted by a moderate La Niña and warmth in the tropical Atlantic, but these processes accounted for very little of the SE U.S. drying in fall 2016, implying a large role for internal atmospheric variability. The extended analysis back to 1895 indicates that SE U.S. droughts as strong as the 2016 event are more likely than indicated from a shorter 60 year perspective and continued multidecadal swings in precipitation may combine with future warming to further enhance the likelihood of such events.

Citation

Park Williams, A., Cook, B. I., Smerdon, J. E., Bishop, D. A., Seager, R., & Mankin, J. S. (2017). The 2016 southeastern U.S.drought: An extreme departure from centennial wetting and cooling. Journal of Geophysical Research: Atmospheres, 122. https://doi.org/10.1002/2017JD027523

Corresponding author: A. Park Williams, williams "at" ldeo.columbia.edu

"Developing a Conceptual Framework of Landscape and Hydrology on Tallgrass Prairie: A Critical Zone Approach"

TPOS note:

New interdisciplinary research published in the Vadose Zone Journal integrates ecology, climate, hydrology, and soils at Konza Prairie, a reference site for the tallgrass prairie ecosystem. While this paper may lack immediate applications, it provides a long view on grassland management.

Check out the abstract below or follow this link to the article (this article is not open access): https://dl.sciencesocieties.org/publications/vzj/abstracts/0/0/vzj2017.03.0069

Core Ideas: 

• Konza Prairie is a reference ecosystem for evaluation of current and former grasslands.
• Hydrologic change includes climatic and geologic factors such as karstification.
• Future research is needed to expand vadose zone knowledge.

Abstract

Agricultural intensification and urbanization have greatly reduced the extent of tallgrass prairie across North America. To evaluate the impact of these changes, a reference ecosystem of unperturbed prairie is required. The Konza Prairie Biological Station in northeastern Kansas is a long-term research site at which a critical zone approach has been implemented. Integration of climatic, ecologic, and hydropedologic research to facilitate a comprehensive understanding of the complex environment provides the basis for predicting future aquifer and landscape evolution. We present a conceptual framework of the hydrology underpinning the area that integrates the extensive current and past research and provides a synthesis of the literature to date. The key factors in the hydrologic behavior of Konza Prairie are climate, ecology, vadose zone characteristics and management, and groundwater and bedrock. Significant interactions among these factors include bedrock dissolution driven by cool-season precipitation and hence a climatic control on the rate of karstification. Soil moisture dynamics are influenced at various timescales due to the short- and long-term effects of prescribed burning on vegetation and on soil physical characteristics. The frequency of burning regimes strongly influences the expansion of woody species in competition with native tallgrasses, with consequent effects on C and N dynamics within the vadose zone. Knowledge gaps exist pertaining to the future of Konza Prairie (a model for US tallgrass prairie)—whether continued karstification will lead to increasingly flashy and dynamic hydrology and whether compositional changes in the vegetation will affect long-term changes in water balances.

Citation:

Vero, S.E., G.L. Macpherson, P.L. Sullivan, A.E. Brookfield, J.B. Nippert, M.F. Kirk, S. Datta, and P. Kempton. 2017. Developing a Conceptual Framework of Landscape and Hydrology on Tallgrass Prairie: A Critical Zone Approach. Vadose Zone J. 0. doi:10.2136/vzj2017.03.0069