Professor Kansas State University Division of Biology, Kansas, United States
Background/Question/Methods Woody encroachment, the increasing abundance and density of woody plants in herbaceous ecosystems, is occurring globally with substantial impacts on the structure and functioning of grasslands and savannas. Woody encroachment is generally driven by a combination of reduced fire frequency, climate change, and the ability of woody plants to use deep soil water. Thus, a comprehensive understanding of the hydraulic traits that characterize water use in woody plants will provide greater insight into the mechanisms of woody encroachment, particularly in a changing climate. Here, we assessed how experimental changes in water availability and fire frequency impact root hydraulic traits in Cornus drummondii, the primary encroaching shrub within North American tallgrass prairies, and Andropogon gerardii, a dominant C4 grass. Shelters that reduced precipitation by 50% (drought) and 0% (control) were built over shrubs and grasses growing in watersheds that were burned at 4-year and 1-year frequencies. We assessed the water transport capability of shrubs and grasses by measuring the maximum hydraulic conductance (Kmax) of entire root systems across two growing seasons. We also assessed the vulnerability of shrub root segments to loss of hydraulic function by measuring the pressure at which 50% of the maximum hydraulic conductivity is lost (P50).
Results/Conclusions Shrub and grass roots showed different responses to drought. Shrub root Kmax did not differ significantly between treatments, while grasses growing in drought plots had lower root Kmax than control grasses. Additionally, shrub roots grown under drought were less vulnerable to water stress than control roots (P50=-1.5 and -0.20 MPa, respectively). These results suggest that the ability of grass roots to use water declined with drought, while the ability of shrub roots to resist water stress increased with drought. Thus, woody encroachment of tallgrass prairie will likely continue under warmer, drier conditions. This conclusion is supported by other recent work illustrating that C. drummondii roots exhibit unique combinations of anatomical traits that may be advantageous in a changing climate. Future work that continues to investigate root hydraulic traits will provide greater mechanistic insight regarding functional strategies facilitating woody encroachment in tallgrass prairie.