Browsing by Author "Heffernan, James B."

Now showing 1 - 9 of 9
  • Thumbnail Image
    Climate and lawn management interact to control C-4 plant distribution in residential lawns across seven US 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, J. Morgan; Hall, Sharon J.; Heffernan, James B.; Hobbie, Sarah E.; Larson, Kelli L.; Morse, Jennifer L.; Neill, Christopher; Nelson, Kristen C.; O'Neil-Dunne, Jarlath P.M.; Pearse, William D.; Chowdhury, Rinku Roy; Steele, Meredith K.; Wheeler, Megan M. (2019-06)
    In natural grasslands, C-4 plant dominance increases with growing season temperatures and reflects distinct differences in plant growth rates and water use efficiencies of C-3 vs. C-4 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 (delta C-13, index of C-3/C-4 relative abundance) and C-4 cover in residential lawns across seven U.S. cities to determine how climate, lawn plant management, or interactions between climate and plant management influenced C-4 lawn cover. We also calculated theoretical C-4 carbon gain predicted by a plant physiological model as an index of expected C-4 cover due to growing season climatic conditions in each city. Contrary to theoretical predictions, plant delta C-13 and C-4 cover in urban lawns were more strongly related to mean annual temperature than to growing season temperature. Wintertime temperatures influenced the distribution of C-4 lawn turf plants, contrary to natural ecosystems where growing season temperatures primarily drive C-4 distributions. C-4 cover in lawns was greatest in the three warmest cities, due to an interaction between climate and homeowner plant management (e.g., planting C-4 turf species) in these cities. The proportion of C-4 lawn species was similar to the proportion of C-4 species in the regional grass flora. However, the majority of C-4 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, C-4 plants were predominantly imported and cultivated. Elevated CO2 and temperature in cities can influence C-3/C-4 competitive outcomes; however, this study provides evidence that climate and plant management dynamics influence biogeography and ecology of C-3/C-4 plants in lawns. Their differing water and nutrient use efficiency may have substantial impacts on carbon, water, energy, and nutrient budgets across cities.
  • Thumbnail Image
    Continental-scale homogenization of residential lawn plant communities
    Wheeler, Megan M.; Neill, Christopher; Groffman, Peter M.; Avolio, Meghan L.; Bettez, Neil; Cavender-Bares, Jeannine; Chowdhury, Rinku Roy; Darling, Lindsay E.; Grove, J. Morgan; Hall, Sharon J.; Heffernan, James B.; Hobbie, Sarah E.; Larson, Kelli L.; Morse, Jennifer L.; Nelson, Kristen C.; Ogden, Laura A.; O'Neil-Dunne, Jarlath P.M.; Pataki, Diane E.; Polsky, Colin; Steele, Meredith K.; Trammell, Tara L. E. (2017-09)
    Residential 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.
  • Thumbnail Image
    Convergence of microclimate in residential landscapes across diverse cities in the United States
    Hall, Sharon J.; Learned, Jennifer; Ruddell, Benjamin L.; Larson, K. L.; Cavender-Bares, Jeannine; Bettez, Neil; Groffman, Peter M.; Grove, J. Morgan; Heffernan, James B.; Hobbie, Sarah E.; Morse, Jennifer L.; Neill, Christopher; Nelson, Kristen C.; O'Neil-Dunne, Jarlath P.M.; Ogden, Laura A.; Pataki, Diane E.; Pearse, William D.; Polsky, Colin; Chowdhury, Rinku Roy; Steele, Meredith K.; Trammell, Tara L. E. (2016-01)
    The urban heat island (UHI) is a well-documented pattern of warming in cities relative to rural areas. Most UHI research utilizes remote sensing methods at large scales, or climate sensors in single cities surrounded by standardized land cover. Relatively few studies have explored continental-scale climatic patterns within common urban microenvironments such as residential landscapes that may affect human comfort. We tested the urban homogenization hypothesis which states that structure and function in cities exhibit ecological "sameness" across diverse regions relative to the native ecosystems they replaced. We deployed portable micrometeorological sensors to compare air temperature and humidity in residential yards and native landscapes across six U.S. cities that span a range of climates (Phoenix, AZ; Los Angeles, CA; Minneapolis-St. Paul, MN; Boston, MA; Baltimore, MD; and Miami, FL). Microclimate in residential ecosystems was more similar among cities than among native ecosystems, particularly during the calm morning hours. Maximum regional actual evapotranspiration (AET) was related to the morning residential microclimate effect. Residential yards in cities with maximum AET < 50-65 cm/year (Phoenix and Los Angeles) were generally cooler and more humid than nearby native shrublands during summer mornings, while yards in cities above this threshold were generally warmer (Baltimore and Miami) and drier (Miami) than native forests. On average, temperature and absolute humidity were similar to 6 % less variable among residential ecosystems than among native ecosystems from diverse regions. These data suggest that common residential land cover and structural characteristics lead to microclimatic convergence across diverse regions at the continental scale.
  • Thumbnail Image
    Ecological homogenization of soil properties in the American residential macrosystem
    Ryan, Christopher D.; Groffman, Peter M.; Grove, J. Morgan; Hall, Sharon J.; Heffernan, James B.; Hobbie, Sarah E.; Locke, Dexter H.; Morse, Jennifer L.; Neill, Christopher; Nelson, Kristen C.; O'Neil-Dunne, Jarlath; Chowdhury, Rinku Roy; Steele, Meredith K.; Trammell, Tara L. E. (Wiley, 2022-09)
    The conversion of native ecosystems to residential ecosystems dominated by lawns has been a prevailing land-use change in the United States over the past 70 years. Similar development patterns and management of residential ecosystems cause many characteristics of residential ecosystems to be more similar to each other across broad continental gradients than that of former native ecosystems. For instance, similar lawn management by irrigation and fertilizer applications has the potential to influence soil carbon (C) and nitrogen (N) pools and processes. We evaluated the mean and variability of total soil C and N stocks, potential net N mineralization and nitrification, soil nitrite (NO2-)/nitrate (NO3-) and ammonium (NH4+) pools, microbial biomass C and N content, microbial respiration, bulk density, soil pH, and moisture content in residential lawns and native ecosystems in six metropolitan areas across a broad climatic gradient in the United States: Baltimore, MD (BAL); Boston, MA (BOS); Los Angeles, CA (LAX); Miami, FL (MIA); Minneapolis-St. Paul, MN (MSP); and Phoenix, AZ (PHX). We observed evidence of higher N cycling in lawn soils, including significant increases in soil NO2-/NO3-, microbial N pools, and potential net nitrification, and significant decreases in NH4+ pools. Self-reported yard fertilizer application in the previous year was linked with increased NO2-/ NO3- content and decreases in total soil N and C content. Self-reported irrigation in the previous year was associated with decreases in potential net mineralization and potential net nitrification and with increases in bulk density and pH. Residential topsoil had higher total soil C than native topsoil, and microbial biomass C was markedly higher in residential topsoil in the two driest cities (LAX and PHX). Coefficients of variation for most biogeochemical metrics were higher in native soils than in residential soils across all cities, suggesting that residential development homogenizes soil properties and processes at the continental scale.
  • Thumbnail Image
    Ecosystem services in managing residential landscapes: priorities, value dimensions, and cross-regional patterns
    Larson, K. L.; Nelson, Kristen C.; Samples, S. R.; Hall, Sharon J.; Bettez, Neil; Cavender-Bares, Jeannine; Groffman, Peter M.; Grove, J. Morgan; Heffernan, James B.; Hobbie, Sarah E.; Learned, Jennifer; Morse, Jennifer L.; Neill, Christopher; Ogden, Laura A.; O'Neil-Dunne, Jarlath P.M.; Pataki, Diane E.; Polsky, Colin; Chowdhury, Rinku Roy; Steele, Meredith K.; Trammell, Tara L. E. (2016-03)
    Although ecosystem services have been intensively examined in certain domains (e.g., forests and wetlands), little research has assessed ecosystem services for the most dominant landscape type in urban ecosystems-namely, residential yards. In this paper, we report findings of a cross-site survey of homeowners in six U.S. cities to 1) examine how residents subjectively value various ecosystem services, 2) explore distinctive dimensions of those values, and 3) test the urban homogenization hypothesis. This hypothesis posits that urbanization leads to similarities in the social-ecological dynamics across cities in diverse biomes. By extension, the thesis suggests that residents' ecosystem service priorities for residential landscapes will be similar regardless of whether residents live in the humid East or the arid West, or the warm South or the cold North. Results underscored that cultural services were of utmost importance, particularly anthropocentric values including aesthetics, low-maintenance, and personal enjoyment. Using factor analyses, distinctive dimensions of residents' values were found to partially align with the Millennium Ecosystem Assessment's categories (provisioning, regulating, supporting, and cultural). Finally, residents' ecosystem service priorities exhibited significant homogenization across regions. In particular, the traditional lawn aesthetic (neat, green, weed-free yards) was similarly important across residents of diverse U.S. cities. Only a few exceptions were found across different environmental and social contexts; for example, cooling effects were more important in the warm South, where residents also valued aesthetics more than those in the North, where low-maintenance yards were a greater priority.
  • Thumbnail Image
    Homogenization of plant diversity, composition, and structure in North American urban yards
    Pearse, William D.; Cavender-Bares, Jeannine; Hobbie, Sarah E.; Avolio, Meghan L.; Bettez, Neil; Chowdhury, Rinku Roy; Darling, Lindsay E.; Groffman, Peter M.; Grove, J. Morgan; Hall, Sharon J.; Heffernan, James B.; Learned, Jennifer; Neill, Christopher; Nelson, Kristen C.; Pataki, Diane E.; Ruddell, Benjamin L.; Steele, Meredith K.; Trammell, Tara L. E. (Ecological Society of America, 2018-02)
    Urban ecosystems are widely hypothesized to be more ecologically homogeneous than natural ecosystems. We argue that urban plant communities assemble from a complex mix of horticultural and regional species pools, and evaluate the homogenization hypothesis by comparing cultivated and spontaneously occurring urban vegetation to natural area vegetation across seven major U.S. cities. There was limited support for homogenization of urban diversity, as the cultivated and spontaneous yard flora had greater numbers of species than natural areas, and cultivated phylogenetic diversity was also greater. However, urban yards showed evidence of homogenization of composition and structure. Yards were compositionally more similar across regions than were natural areas, and tree density was less variable in yards than in comparable natural areas. This homogenization of biodiversity likely reflects similar horticultural source pools, homeowner preferences, and management practices across U.S. cities.
  • Thumbnail Image
    Respiration regimes in rivers: Partitioning source-specific respiration from metabolism time series
    Bertuzzo, Enrico; Hotchkiss, Erin R.; Argerich, Alba; Kominoski, John S.; Oviedo-Vargas, Diana; Savoy, Philip; Scarlett, Rachel; von Schiller, Daniel; Heffernan, James B. (Wiley, 2022-09)
    Respiration in streams is controlled by the timing, magnitude, and quality of organic matter (OM) inputs from internal primary production and external fluxes. Here, we estimated the contribution of different OM sources to seasonal, annual, and event-driven characteristics of whole-stream ecosystem respiration (ER) using an inverse modeling framework that accounts for possible time-lags between OM inputs and respiration. We modeled site-specific, dynamic OM stocks contributing to ER: autochthonous OM from gross primary production (GPP); allochthonous OM delivered during flow events; and seasonal pulses of leaf litter. OM stored in the sediment and dissolved organic matter (DOM) transported during baseflow were modeled as a stable stock contributing to baseline respiration. We applied this modeling framework to five streams with different catchment size, climate, and canopy cover, where multi-year time series of ER and environmental variables were available. Overall, the model explained between 53% and 74% of observed ER dynamics. Respiration of autochthonous OM tracked seasonal peaks in GPP in spring or summer. Increases in ER were often associated with high-flow events. Respiration associated with litter inputs was larger in smaller streams. Time lags between leaf inputs and respiration were longer than for other OM sources, likely due to lower biological reactivity. Model estimates of source-specific ER and OM stocks compared well with existing measures of OM stocks, inputs, and respiration or decomposition. Our modeling approach has the potential to expand the scale of comparative analyses of OM dynamics within and among freshwater ecosystems.
  • Thumbnail Image
    Sediment chemistry of urban stormwater ponds and controls on denitrification
    Blaszczak, Joanna R.; Steele, Meredith K.; Badgley, Brian D.; Heffernan, James B.; Hobbie, Sarah E.; Morse, Jennifer L.; Rivers, Erin N.; Hall, Sharon J.; Neill, Christopher; Pataki, Diane E.; Groffman, Peter M.; Bernhardt, Emily S. (Ecological Society of America, 2018-06)
    Stormwater ponds and retention basins are ubiquitous features throughout urban landscapes. These ponds are potentially important control points for nitrogen (N) removal from surface water bodies via denitrification. However, there are possible trade-offs to this water quality benefit if high N and contaminant concentrations in stormwater pond sediments decrease the complete reduction of nitrous oxide (N2O), a potent greenhouse gas, to dinitrogen (N-2) during denitrification. This may occur through decreasing the abundance or efficiency of denitrifiers capable of producing the N2O reductase enzyme. We predicted that ponds draining increasingly urbanized landscapes would have higher N and metal concentrations in their sediments, and thereby greater N2O yields. We measured potential denitrification rates, N2O reductase (nosZ) gene frequencies, as well as sediment and pore water chemistry in 64 ponds distributed across eight U.S. cities. We found almost no correlation between the proportion of urban land cover surrounding ponds and the nutrient and contaminant concentrations in the stormwater pond sediments within or across all cities. Regression analysis revealed that the proportion of potential N-2 and N2O production that could be explained was under different environmental controls. Our survey raises many new questions about why N fluxes and transformations vary so widely both within and across urban environments, but also allays the concern that elevated metal concentrations in urban stormwater ponds will increase N2O emissions. Urban stormwater ponds are unlikely to be a problematic source of N2O to the atmosphere, no matter their denitrification potential.
  • Thumbnail Image
    Urban soil carbon and nitrogen converge at a continental scale
    Trammell, Tara L. E.; Pataki, Diane E.; Pouyat, Richard, V; Groffman, Peter M.; Rosier, Carl; Bettez, Neil; Cavender-Bares, Jeannine; Grove, J. Morgan; Hall, Sharon J.; Heffernan, James B.; Hobbie, Sarah E.; Morse, Jennifer L.; Neill, Christopher; Steele, Meredith K. (2020-05)
    In urban areas, anthropogenic drivers of ecosystem structure and function are thought to predominate over larger-scale biophysical drivers. Residential yards are influenced by individual homeowner preferences and actions, and these factors are hypothesized to converge yard structure across broad scales. We examined soil total C and total delta C-13, organic C and organic delta C-13, total N, and delta N-15 in residential yards and corresponding reference ecosystems in six cities across the United States that span major climates and ecological biomes (Baltimore, Maryland; Boston, Massachusetts; Los Angeles, California; Miami, Florida; Minneapolis-St. Paul, Minnesota; and Phoenix, Arizona). Across the cities, we found soil C and N concentrations and soil delta N-15 were less variable in residential yards compared to reference sites supporting the hypothesis that soil C, N, and delta N-15 converge across these cities. Increases in organic soil C, soil N, and soil delta N-15 across urban, suburban, and rural residential yards in several cities supported the hypothesis that soils responded similarly to altered resource inputs across cities, contributing to convergence of soil C and N in yards compared to natural systems. Soil C and N dynamics in residential yards showed evidence of increasing C and N inputs to urban soils or dampened decomposition rates over time that are influenced by climate and/or housing age across the cities. In the warmest cities (Los Angeles, Miami, Phoenix), greater organic soil C and higher soil delta C-13 in yards compared to reference sites reflected the greater proportion of C-4 plants in these yards. In the two warm arid cities (Los Angeles, Phoenix), total soil delta C-13 increased and organic soil delta C-13 decreased with increasing home age indicating greater inorganic C in the yards around newer homes. In general, soil organic C and delta C-13, soil N, and soil delta N-15 increased with increasing home age suggesting increased soil C and N cycling rates and associated C-12 and N-14 losses over time control yard soil C and N dynamics. This study provides evidence that conversion of native reference ecosystems to residential areas results in convergence of soil C and N at a continental scale. The mechanisms underlying these effects are complex and vary spatially and temporally.