Wheeler Megan M.
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ArticleClimate and lawn management interact to control C4 plant distribution in residential lawns across seven U.S. cities.(Ecological Society of America, 2019-04-01) Trammell, Tara ; Pataki, Diane E. ; Still, Christopher J. ; Ehleringer, James R. ; Avolio, Meghan L. ; Bettez, Neil D. ; Cavender-Bares, Jeannine ; Groffman, Peter M. ; Grove, J. Morgan ; Hall, Sharon J. ; Heffernan, James B. ; Hobbie, Sarah E. ; Larson, Kelli L. ; Morse, Jennifer L. ; Neill, Christopher ; Nelson, Kristen ; O'Neil-Dunne, Jarlath ; Pearse, William D. ; Roy Chowdhury, Rinku ; Steele, Meredith K. ; Wheeler, Megan M.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.
ArticleContinental-scale homogenization of residential lawn plant communities(Elsevier, 2017-05-20) Wheeler, Megan M. ; Neill, Christopher ; Groffman, Peter M. ; Avolio, Meghan L. ; Bettez, Neil D. ; Cavender-Bares, Jeannine ; Roy Chowdhury, Rinku ; Darling, Lindsay ; Grove, J. Morgan ; Hall, Sharon J. ; Heffernan, James B. ; Hobbie, Sarah E. ; Larson, Kelli L. ; Morse, Jennifer L. ; Nelson, Kristen ; Ogden, Laura ; O'Neil-Dunne, Jarlath ; Pataki, Diane E. ; Trammell, TaraResidential lawns are highly managed ecosystems that occur in urbanized landscapes across the United States. Because they are ubiquitous, lawns are good systems in which to study the potential homogenizing effects of urban land use and management together with the continental-scale effects of climate on ecosystem structure and functioning. We hypothesized that similar homeowner preferences and management in residential areas across the United States would lead to low plant species diversity in lawns and relatively homogeneous vegetation across broad geographical regions. We also hypothesized that lawn plant species richness would increase with regional temperature and precipitation due to the presence of spontaneous, weedy vegetation, but would decrease with household income and fertilizer use. To test these predictions, we compared plant species composition and richness in residential lawns in seven U.S. metropolitan regions. We also compared species composition in lawns with understory vegetation in minimally-managed reference areas in each city. As expected, the composition of cultivated turfgrasses was more similar among lawns than among reference areas, but this pattern also held among spontaneous species. Plant species richness and diversity varied more among lawns than among reference areas, and more diverse lawns occurred in metropolitan areas with higher precipitation. Native forb diversity increased with precipitation and decreased with income, driving overall lawn diversity trends with these predictors as well. Our results showed that both management and regional climate shaped lawn species composition, but the overall homogeneity of species regardless of regional context strongly suggested that management was a more important driver.