COS 179-5 - Interactive effects of drought and fire frequency on co-existing woody and herbaceous grassland communities

Background/Question/Methods

Woody encroachment drastically alters grassland ecosystem function. As the climate continues to warm, projected increases in frequency and intensity of drought events are likely to interact with land-management strategies (i.e., burn frequency) to impact the trajectory of woody encroachment in temperate grasslands. We currently lack necessary information to accurately predict grassland responses to drought-by-fire interactions in areas experiencing woody encroachment. In particular, information regarding the interactive effects of drought and fire-frequency on the physiology and productivity of coexisting grasses and shrubs would improve predictions of future changes to grassland ecosystem function. In this study, we constructed passive rainout shelters over mature Cornus drummondii shrubs and co-existing C₄ grasses (predominantly Andropogon gerardii) in two fire treatments (1-year and 4-year burn frequency) at the Konza Prairie Biological Station (north-eastern Kansas, USA). Rainout shelters were established in 2017 and exclude 50% of incoming precipitation. Measurements of leaf-level and whole-plant level responses to drought and fire frequency have been monitored from 2019 to 2021.

Results/Conclusions

For both species, photosynthetic rates and midday leaf water potentials (Ψ_leaf) were lower in drought treatments. This trend was more pronounced in the 4-year burn treatment. Turgor loss point (π_TLP) in C. drummondii was also lower under drought shelters, particularly at lower burn frequencies. π_TLP declined by >0.3 MPa in 2020, suggesting that these shrubs are able to physiologically adjust to water stress within one growing season. Source water-use by C. drummondii – measured using xylem and soil water stable isotopes (δ¹⁸O, δ²H) – also shifted from shallow to deeper water sources throughout the growing season, while A. gerardii consistently used water from the top 30 cm of soil regardless of surface soil moisture. These results indicate that both species are experiencing water stress, particularly in drought shelters that are burned less frequently. As these fire and drought treatments continue, we expect to see more severe water stress in both species, and potentially negative impacts of prolonged drought on the ability of C. drummondii to resprout following fire. Overall, results indicate that these clonal shrubs possess multiple strategies to withstand drought stress, and that multiple external pressures (drought and fire) are likely needed to reverse grassland-to-shrubland transitions in temperate grasslands.