PS 6-58 - Developing spatial distribution of C₄ and C₃ grasses and modeling their responses to drought across the contiguous U.S.

Background/Question/Methods

C₄ plants occupy a substantial fraction of the global land surface, and typically have higher productivity and water use efficiency in warmer habitats compared to related C₃ plants. Thus, accurate representation of C₄ and C₃ biogeography, especially in herbaceous grasses where C₄ photosynthesis is most common, is important for Earth system models (ESMs) and macroecology and plant functional trait analyses. However, the spatiotemporal coverage of C₄ and C₃ grass cover prescribed or prognostically calculated in ESMs varies widely and generates significant uncertainties. Here, we developed annual 5-arcmin maps of C₄ and C₃ grass over the contiguous U.S. (CONUS) for the years 2000-2019. The maps were constructed by applying a physiologically based crossover temperature model to satellite-derived distributions of herbaceous vegetation cover. The developed C₄ grass maps are applied in the E3SM Land Model version 1 (ELMv1-ECA) to study the responses of C₄ and C₃ grasses to drought across CONUS.

Results/Conclusions

During 2000-2019, the estimated C₄ and C₃ grass cover areas averaged 54.2±8.5 and 137.9±8.6 Mha, respectively. Based on our approach, we documented small expansions in C₄ grass areas after 2009 but relatively constant C₃ grass areas. The predicted total area where C₄ grasses would be favored to have higher productivity than C₃ grasses varied annually, with the largest area in 2012, owing to nationwide warmer growing season temperatures, and the smallest area in 2009, due to a cooler summer. Furthermore, our grass layers highly improved the C₄ grass biogeography in the southeastern U.S. compared to the existing data product. The ELM simulation results show considerable declines in gross primary production of C₄ and C₃ grasses during the 2012 drought across the Great Plains.