Repeated extreme droughts decrease root production, but the not potential for post-drought recovery of root production, in a mesic grassland

Authors: SLETTE, INGRID - Colorado State University; Hoover, David; SMITH, MELINDA - Colorad0 State University; KNAPP, ALAN - Colorado State University

Submitted to: Oikos
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2022
Publication Date: N/A
Citation: N/A

Interpretive Summary: Global climate change is expected to cause more frequent extreme droughts in many parts of the world. As return intervals between droughts decrease, responses to drought will likely reflect both past and current drought conditions. The potential consequences of repeated droughts, ranging from increased acclimation to decreased resistance, remain unresolved. This is especially true belowground, despite the crucial role of roots in water acquisition and soil organic matter formation. We conducted a 6-yr experiment, imposing extreme drought (66% reduction in growing season precipitation) on mesic grassland plots during either the first 2-yrs, the last 2-yrs, or both the first and last 2-yrs. Control plots received average ambient precipitation. We quantified fine root production during year six and one year later, after all treatments had ended. Repeated drought decreased root mass production more than twice as much as a single drought (-63% vs. -27%, respectively, relative to controls). Thus, previous drought exposure decreased resistance to a subsequent drought. The decrease was driven not by the dominant C4 grass species, Andropogon gerardii, but by the subdominant species in the community. Root traits (diameter, tissue density, and specific root length) did not differ among treatments, but A. gerardii roots were thicker, denser, and deeper than those of the subdominant species across treatments. In year 6, root production had not recovered from the drought during the first 2-yrs (-21% relative to controls), but when precipitation was above average in the post-treatment year, root production recovered in all former drought treatments. Our results highlight the complexity of predicting belowground responses to climate change. Repeated droughts can have increasingly large negative effects on root production, but this does not preclude recovery in wet years. Thus, in an increasingly variable climate, the sequence of extremes will determine root dynamics, with important implications for ecosystem carbon cycling.

Technical Abstract: As the frequency of droughts increase with climate change, the responses of plant communities to drought will likely reflect both current and past drought conditions, yet the ecological impacts are largely unknown. In this study, we created an experiment in a native tallgrass prairie to examine how plant roots respond to repeated drought. We found that root production during drought was reduced by half if the plant community had been exposed to a prior drought. However, wet conditions allowed for roots to fully recover after the drought ended.