BELOWGROUND PLANT RESPONSES TO COMPOUND CLIMATE EXTREMES IN THE SHORTGRASS STEPPE

Abstract

Climate change is increasing the frequency and intensity of extreme precipitation events, resulting in greater variability in water availability across terrestrial ecosystems. Semi-arid grasslands are particularly sensitive to these changes because productivity and carbon cycling are strongly regulated by precipitation. While the impacts of drought on aboveground processes have been extensively studied, much less is known about how belowground productivity and root functional strategies respond to compound climate extremes, such as drought followed by extreme rainfall events. Understanding these responses is critical because roots represent a major pathway for carbon inputs into soils and strongly influence ecosystem resilience.This study examined how prolonged drought and an extreme precipitation event (deluge) interact to influence belowground net primary productivity (BNPP) and root functional traits in the shortgrass steppe of northeastern Colorado. Using a manipulative field experiment at the Central Plains Experimental Range, we evaluated responses to ambient conditions, drought, deluge, and drought followed by deluge. Root ingrowth cores were used to quantify BNPP across soil depths (0–10, 10–20, and 20–30 cm), and root morphological traits, including specific root length (SRL) and root tissue density (RTD), were measured to assess shifts in belowground resource acquisition strategies. Drought strongly reduced BNPP across the soil profile and shifted root traits toward a more conservative strategy characterized by lower SRL and higher RTD. The addition of a deluge during drought substantially increased root production but did not fully restore BNPP to ambient levels, indicating persistent drought legacy effects. Deluge responses varied with soil depth, with increased root production occurring throughout the measured soil profile during drought and particularly strong responses in deeper soil layers during the post-deluge period. Additionally, deluge during drought reversed drought-induced trait shifts, increasing SRL and decreasing RTD, suggesting a transition toward a more acquisitive root strategy following increased water availability. These results demonstrate that compound drought–deluge events influence not only the magnitude of belowground productivity but also the spatial distribution and functional strategies of root systems. While extreme rainfall events may partially offset drought impacts by stimulating root production, shifts toward acquisitive traits may increase root turnover and alter the fate of belowground carbon inputs. Incorporating root trait responses and drought legacy effects into ecosystem models will improve predictions of grassland carbon dynamics under increasingly variable precipitation regimes.