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Climate and lawn management interact to control C4 plant distribution in residential lawns across seven U.S. cities

Trammell, Tara L. E., Pataki, Diane E., Still, Christopher J., Ehleringer, James R., Avolio, Meghan L., Bettez, Neil, Cavender‐Bares, Jeannine, Groffman, Peter M., Grove, Morgan, Hall, Sharon J., Heffernan, James, Hobbie, Sarah E., Larson, Kelli L., Morse, Jennifer L., Neill, Christopher, Nelson, Kristen C., O'Neil‐Dunne, Jarlath, Pearse, William D., Chowdhury, Rinku Roy, Steele, Meredith, Wheeler, Megan M.
Ecological applications 2019 v.29 no.4 pp. e01884
C3 photosynthesis, C3 plants, C4 photosynthesis, C4 plants, biocenosis, biogeography, carbon, carbon dioxide, carbon dioxide enrichment, cities, climatic factors, ecosystems, energy, flora, grasslands, growing season, herbaceous plants, humans, lawns and turf, models, nutrient use efficiency, plant growth, planting, population distribution, prediction, species diversity, temperature, turf grasses, uncertainty, water use efficiency, weeds, winter, United States
In natural grasslands, C₄ plant dominance increases with growing season temperatures and reflects distinct differences in plant growth rates and water use efficiencies of C₃ vs. C₄ photosynthetic pathways. However, in lawns, management decisions influence interactions between planted turfgrass and weed species, leading to some uncertainty about the degree of human vs. climatic controls on lawn species distributions. We measured herbaceous plant carbon isotope ratios (δ¹³C, index of C₃/C₄ relative abundance) and C₄ cover in residential lawns across seven U.S. cities to determine how climate, lawn plant management, or interactions between climate and plant management influenced C₄ lawn cover. We also calculated theoretical C₄ carbon gain predicted by a plant physiological model as an index of expected C₄ cover due to growing season climatic conditions in each city. Contrary to theoretical predictions, plant δ¹³C and C₄ cover in urban lawns were more strongly related to mean annual temperature than to growing season temperature. Wintertime temperatures influenced the distribution of C₄ lawn turf plants, contrary to natural ecosystems where growing season temperatures primarily drive C₄ distributions. C₄ cover in lawns was greatest in the three warmest cities, due to an interaction between climate and homeowner plant management (e.g., planting C₄ turf species) in these cities. The proportion of C₄ lawn species was similar to the proportion of C₄ species in the regional grass flora. However, the majority of C₄ species were nonnative turf grasses, and not of regional origin. While temperature was a strong control on lawn species composition across the United States, cities differed as to whether these patterns were driven by cultivated lawn grasses vs. weedy species. In some cities, biotic interactions with weedy plants appeared to dominate, while in other cities, C₄ plants were predominantly imported and cultivated. Elevated CO₂ and temperature in cities can influence C₃/C₄ competitive outcomes; however, this study provides evidence that climate and plant management dynamics influence biogeography and ecology of C₃/C₄ plants in lawns. Their differing water and nutrient use efficiency may have substantial impacts on carbon, water, energy, and nutrient budgets across cities.