Joseph P. McFadden

Subgrid-scale variability in the surface energy balance of arctic tundra

Surface fluxes of energy, water vapor, and CO2 over homogeneous areas of the major tundra vegetation types in arctic Alaska were measured using a mobile eddy covariance tower for 5-day periods in the middle of the 1994 growing season. In order to account for differences in weather and time of season, data were analyzed in comparison to a nearby, fixed tower that operated throughout the summer. Among the different vegetation types, evaporation ranged from 1.3 to 2.7 mm d−1. Net carbon uptake ranged from 0.5 to 2.4 g C m−2 d−1. Ground heat flux consumed 10–33% of midday net radiation. Typically, 38% of the net radiation was partitioned into latent heat flux, while the fraction of net radiation removed from the surface in sensible heat flux varied from 16 to 50% among vegetation types. The largest differences among vegetation types in surface energy partitioning were related to variations in soil moisture, with midday Bowen ratios ranging from 0.37 over wet sedge tundra to 2.25 over dry heath. Direct effects of vegetation on the driving gradients for energy and water vapor exchange were important in shrub tundra: shading of the moss layer by the canopy reduced ground heat flux and increased sensible heat flux, while latent heat flux was similar to other mesic vegetation types because the moss layer accounted for a larger portion of total evaporation than did evapotranspiration by shrubs. Scaling up from the vegetation types to the Alaskan arctic using an area-weighted average of the observed energy partitioning gave results similar to regional energy budgets measured over larger, more heterogeneous areas of tundra. An extrapolation based on the hypothesis that climate variability could cause a large fraction of the current tussock tundra vegetation to be converted to shrub tundra resulted in modest changes in the regional energy balance. However, nonlinear variations of surface evaporation with leaf area and uncertainties regarding changes in moss cover suggest that additional field experiments as well as modeling efforts will be required to predict the potential for changes in arctic tundra vegetation to feed back on regional climate.

Trees grow on money: Urban tree canopy cover and environmental justice

This study examines the distributional equity of urban tree canopy (UTC) cover for Baltimore, MD, Los Angeles, CA, New York, NY, Philadelphia, PA, Raleigh, NC, Sacramento, CA, and Washington, D.C. using high spatial resolution land cover data and census data. Data are analyzed at the Census Block Group levels using Spearman’s correlation, ordinary least squares regression (OLS), and a spatial autoregressive model (SAR). Across all cities there is a strong positive correlation between UTC cover and median household income. Negative correlations between race and UTC cover exist in bivariate models for some cities, but they are generally not observed using multivariate regressions that include additional variables on income, education, and housing age. SAR models result in higher r-square values compared to the OLS models across all cities, suggesting that spatial autocorrelation is an important feature of our data. Similarities among cities can be found based on shared characteristics of climate, race/ethnicity, and size. Our findings suggest that a suite of variables, including income, contribute to the distribution of UTC cover. These findings can help target simultaneous strategies for UTC goals and environmental justice concerns.

A comparative approach to regional variation in surface fluxes using mobile eddy correlation towers

We describe a comparative approach to micrometeorological measurements of surface energy, water vapor, and trace gas fluxes, inwhich mobile eddy correlation towers are moved among sites every 9–14days, allowing both direct and indirect comparison of fluxes amongecosystem types. Structurally distinct ecosystems in Alaskan arctic tundra differed in the relationships between net radiation and surface energyfluxes, whereas structurally similar ecosystems showed constantrelationships, even when they experienced quite different climate. An intercomparison of two towers simultaneously operated at the same location provided a reference for the systematic error of such comparisons.We suggest general criteria for comparing flux measurements made indifferent ecosystems.

Carbon, nitrogen, and phosphorus fluxes in household ecosystems in the Minneapolis‐Saint Paul, Minnesota, urban region

Rapid worldwide urbanization calls for a better understanding of the biogeochemical cycling of those macroelements that have large environmental impacts in cities. This study, part of the Twin Cities Household Ecosystem Project, quantified fluxes of carbon (C), nitrogen (N), and phosphorus (P) at the scale of individual households in the Minneapolis-Saint Paul metropolitan area in Minnesota, USA. We estimated input and output fluxes associated with several components of household activities including air and motor vehicle travel, food consumption, home energy use, landscape, pets, and paper and plastic use for 360 owner-occupied, stand-alone households. A few component fluxes dominated total input fluxes of elements. For instance, air and motor vehicle transportation, together with home energy use, accounted for 85% of total C consumption and emissions. All total and component fluxes were skewed to varying degrees, suggesting that policies targeting disproportionately high fluxes could be an effective and efficient way to reduce pollution. For example, 20% of households contributed 75% of air travel emissions and 40% of motor vehicle emissions. Home energy use was more nearly normally distributed. Nitrogen fluxes were dominated by human diet and lawn fertilizer applications, which together accounted for 65% of total household N inputs. The majority of P inputs were associated with human diet, use of detergents, and pet food. A large portion of the variation among household fluxes of C, N, and P was related to a few biophysical variables. A better understanding of the biophysical, demographic, and behavioral drivers of household activities that contribute to C, N, and P fluxes is pivotal for developing accurate urban biogeochemical models and for informing policies aimed at reducing sources of pollution in urban ecosystems.

Phylogenetic and functional characteristics of household yard floras and their changes along an urbanization gradient

Urban areas are among the most heavily managed landscapes in the world, yet they harbor a remarkable richness of species. Private yards are common habitats in urban areas and are places where cultivated species manage to escape cultivation and become part of the spontaneous species pool. Yards are novel ecosystems where community assembly is driven by both natural and anthropogenic processes. Phylogenetic diversity and functional traits are increasingly recognized as critical to understanding processes of community assembly. Recent evidence indicates that urban areas may select more closely related plant species from the pool of regionally occurring species than do nonurban areas, and that exotic species are phylogenetically clustered within communities. We tested whether phylogenetic diversity and functional trait composition in privately managed yards change along a gradient of housing density in the Minneapolis–Saint Paul metropolis, Minnesota, USA, in accordance with these predictions. We also identif...

Seasonal contributions of vegetation types to suburban evapotranspiration

[1]xa0Evapotranspiration is an important term of energy and water budgets in urban areas and is responsible for multiple ecosystem services provided by urban vegetation. The spatial heterogeneity of urban surface types with different seasonal water use patterns (e.g., trees and turfgrass lawns) complicates efforts to predict and manage urban evapotranspiration rates, necessitating a surface type, or component-based, approach. In a suburban neighborhood of Minneapolis–Saint Paul, Minnesota, United States, we simultaneously measured ecosystem evapotranspiration and its main component fluxes using eddy covariance and heat dissipation sap flux techniques to assess the relative contribution of plant functional types (evergreen needleleaf tree, deciduous broadleaf tree, cool season turfgrass) to seasonal and spatial variations in evapotranspiration. Component-based evapotranspiration estimates agreed well with measured water vapor fluxes, although the imbalance between methods varied seasonally from a 20% overestimate in spring to an 11% underestimate in summer. Turfgrasses represented the largest contribution to annual evapotranspiration in recreational and residential land use types (87% and 64%, respectively), followed by trees (10% and 31%, respectively), with the relative contribution of plant functional types dependent on their fractional cover and daily water use. Recreational areas had higher annual evapotranspiration than residential areas (467 versus 324 mm yr−1, respectively) and altered seasonal patterns of evapotranspiration due to greater turfgrass cover (74% versus 34%, respectively). Our results suggest that plant functional types capture much of the variability required to predict the seasonal patterns of evapotranspiration among cities, as well as differences in evapotranspiration that could result from changes in climate, land use, or vegetation composition.

Biological and environmental controls on tree transpiration in a suburban landscape

[1]xa0Tree transpiration provides a variety of ecosystem services in urban areas, including amelioration of urban heat island effects and storm water management. Tree species vary in the magnitude and seasonality of transpiration owing to differences in physiology, response to climate, and biophysical characteristics, thereby complicating efforts to manage evapotranspiration at city scales. We report sap flux measurements during the 2007 and 2008 growing seasons for dominant tree species in a suburban neighborhood of Minneapolis–Saint Paul, Minnesota, USA. Evergreen needleleaf trees had significantly higher growing season means and annual transpiration per unit canopy area (1.90 kg H2O m−2 d−1 and 307 kg H2O m−2 yr−1, respectively) than deciduous broadleaf trees (1.11 kg H2O m−2 d−1 and 153 kg H2O m−2 yr−1, respectively) because of a smaller projected canopy area (31.1 and 73.6 m2, respectively), a higher leaf area index (8.8 and 5.5 m2 m−2, respectively), and a longer growth season (8 and 4 months, respectively). Measurements also showed patterns consistent with the species differences in xylem anatomy (conifer, ring porous, and diffuse porous). As the growing season progressed, conifer and diffuse porous genera had increased stomatal regulation to high vapor pressure deficit, while ring porous genera maintained greater and more constant stomatal regulation. These results suggest that evaporative responses to climate change in urban ecosystems will depend in part on species composition. Overall, plant functional type differences in canopy structure and growing season length were most important in explaining species differences in midsummer and annual transpiration, offering an approach to predicting the evapotranspiration component of urban water budgets.

The residential landscape: fluxes of elements and the role of household decisions

We assessed biogeochemical cycling of elements through residential household landscapes to evaluate the importance of annual to decadal household-level decisions for element fluxes that contribute to urban and regional pollution. We combined a mailed survey, vegetation measurements, and allometric and biogeochemical models to estimate fluxes and accumulation of carbon (C), nitrogen (N), and phosphorus (P) in landscapes of 360 single-family homes in the Minneapolis-Saint Paul, Minnesota metropolitan area. Carbon inputs and accumulation were strongly influenced by the presence of trees on the property. Nitrogen inputs to the landscape exceeded estimated ecosystem demand for N on average by 51% and were dominated by N fertilizer application. Because Minnesota state law restricts the use of P fertilizer, pet waste was responsible for 84% of P inputs to the landscape. The results have implications for understanding sources of urban pollution and the potential flexibility (i.e., the potential for change) in household behaviors such as tree planting, fertilization, and pet waste management that contribute to such pollution.

Remotely sensed heat anomalies linked with Amazonian forest biomass declines

The occurrence of two major Amazonian droughts in 2005 and 2010 underscores the need for improved understanding of how drought affects tropical forest. During both droughts, MODIS land surface temperature data detected anomalously high daytime and nighttime canopy temperatures throughout drought-affected regions. Daytime thermal anomalies explained 38.6% of the variability in the reduction of aboveground living biomass (p < 0.01; n = 17) in drought-affected forest sites. Multivariate linear models of heat and moisture stress explained a greater proportion of the variability, at 65.1% (p < 0.01; n = 17), providing substantively greater explanatory power than precipitation-only models. Our results suggest that heat stress played an important role in the two droughts and that models should incorporate both heat and moisture stress to predict drought effects on tropical forests.

Influence of seasonality and vegetation type on suburban microclimates

Urbanization is responsible for some of the fastest rates of land-use change around the world, with important consequences for local, regional, and global climate. Vegetation, which represents a significant proportion of many urban and suburban landscapes, can modify climate by altering local exchanges of heat, water vapor, and CO2. To determine how distinct urban forest communities vary in their microclimate effects over time, we measured stand-level leaf area index, soil temperature, infrared surface temperature, and soil water content over a complete growing season at 29 sites representing the five most common vegetation types in a suburban neighborhood of Minneapolis–Saint Paul, Minnesota. We found that seasonal patterns of soil and surface temperatures were controlled more by differences in stand-level leaf area index and tree cover than by plant functional type. Across the growing season, sites with high leaf area index had soil temperatures that were 7°C lower and surface temperatures that were 6°C lower than sites with low leaf area index. Site differences in mid-season soil temperature and turfgrass ground cover were best explained by leaf area index, whereas differences in mid-season surface temperature were best explained by percent tree cover. The significant cooling effects of urban tree canopies on soil temperature imply that seasonal changes in leaf area index may also modulate CO2 efflux from urban soils, a highly temperature-dependent process, and that this should be considered in calculations of total CO2 efflux for urban carbon budgets. Field-based estimates of percent tree cover were found to better predict mid-season leaf area index than satellite-derived estimates and consequently offer an approach to scale up urban biophysical properties.