Unraveling phenological responses to extreme drought and implications for water and carbon budgets

In recent years, extreme drought events in the United States have seen increases in frequency and severity underlining a need to improve our understanding of vegetation resilience and adaptation. Flash droughts are extreme events marked by rapid dry down of soils due to lack of precipitation, high temperatures, and dry air. These events are also associated with reduced preparation, response, and management time windows before and during drought which exacerbate their detrimental impacts on people and food systems. Improvements in actionable information for flash drought management are informed by atmospheric and land surface processes, including responses and feedbacks from vegetation. Phenologic state, or growth stage, is an important metric for modeling how vegetation interacts with the atmosphere. We investigate how uncertainty in vegetation phenology propagates through vegetation responses during drought and non-drought periods by coupling a land-surface hydrology model to a predictive phenology model. We identify plant processes that influence vegetation responses to drought and assess the role of vegetation in the partitioning of carbon, water, and energy fluxes. We selected study sites in Kansas, USA where extreme drought events have been observed, in particular the flash drought of 2012, and where AmeriFlux eddy covariance towers provide data which can be used to evaluate water movement between the land (surface and subsurface) and the atmosphere. We evaluate the evolution of plant phenology, water use, and productivity using different water stress events. Results show that phenological responses using model parameters generated from periods of average precipitation show slower responses to drought as compared to parameters generated to reflect isohydric or anisohydric tendencies. Evapotranspiration (ET) and gross primary productivity (GPP) show similarly timed responses to water stress. We find plants alter water use strategies under extreme drought, with plants nearly halting atmospheric water and carbon exchanges when under stress. Decreases in uncertainty from ensemble estimates of GPP and ET during the flash drought period reduce to winter levels implying variability in plant life stage and functionality during drought periods are similar to those of dormant months. These results have implications for improving predictions of drought impacts on vegetation.