Tue, Aug 16, 2022: 10:15 AM-10:30 AM
514B
Background/Question/MethodsWarm 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/ConclusionsWe 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.
Results/ConclusionsWe 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.