COS 68-2 - Growing season precipitation and fire regime distinguish dominant C₄ grass lineages in the continental US

Background/Question/Methods

Warm season grasses are often grouped together because they share the C₄ photosynthetic trait which benefits from higher temperature environments compared to C₃ grasses. The C₄ pathway has multiple independent origins in the grass family, and these lineages have unique evolutionary histories that influence their ecological strategies, distributions, and important hydraulic and leaf economics traits. Warmer grasslands in North America are dominated by two C₄ grass lineages, the Andropogoneae and the Chloridoideae, which differ in ways that should influence fire regimes and water and carbon fluxes. We asked how well we can separate these two lineages climatically and mapped them spatially across the continental US (CONUS). We assembled vegetation surveys from grassy locations with many plots from across the landscape (e.g., NEON sites) and classified species by their C₄ lineage. We summed aerial cover values for these locations and calculated the proportion of each lineage in the landscape. We used Boosted Regression Trees (BRT) to model lineage proportion as a function of a wide range of environmental predictors. We used the BRT to project lineage distributions across a 5 arc-minute grid of C₄ grass distributions for CONUS and related these distributions to fire regimes.

Results/Conclusions

We found a threshold of growing season precipitation (GSP) that alone explained over 70% of cover between Chloridoideae-dominated and Andropogoneae-dominated grasslands in CONUS. Regions of Andropogoneae-dominance also had higher fire frequency. Chloridoideae-dominated regions had broader diurnal temperature ranges. Our maps show a strong gradient in lineage turnover with increasing Andropogoneae abundance from west to east. In the northern Great Plains, this pattern reveals that C₄ grasses in mesic regions are comprised mostly of Andropogoneae lineage species due to high GSP. C₄ grasses have higher water use efficiency than C₃ grasses, yet paradoxically, recent literature suggests that reduced water availability can favor C₃ in Andropogoneae-dominated grasslands. We suggest that differences in water use strategies between Andropogoneae and Chloridoideae can explain these observations. Reductions in GSP would decrease the Andropogoneae growth advantage in the warm season and favor C₃ grasses that are relatively more active in the wet and cool spring. Furthermore, lineage hydraulic differences add a new perspective to findings that shifts in C₄ fossil abundance are associated with changes in precipitation regime. Coupled with trait data, our maps of dominant C₄ lineages in CONUS could be used to better represent the ecological behavior of grasslands and savannas under climate change.