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.