Response of vegetation phenology to urbanization in the conterminous United States

The influence of urbanization on vegetation phenology is gaining considerable attention due to its implications for human health, cycling of carbon and other nutrients in Earth system. In this study, we examined the relationship between change in vegetation phenology and urban size, an indicator of urbanization, for the conterminous United States. We studied more than 4500 urban clusters of varying size to determine the impact of urbanization on plant phenology, with the aids of remotely sensed observations since 2003–2012. We found that phenology cycle (changes in vegetation greenness) in urban areas starts earlier (start of season, SOS) and ends later (end of season, EOS), resulting in a longer growing season length (GSL), when compared to the respective surrounding urban areas. The average difference of GSL between urban and rural areas over all vegetation types, considered in this study, is about 9 days. Also, the extended GSL in urban area is consistent among different climate zones in the United States, whereas their magnitudes are varying across regions. We found that a tenfold increase in urban size could result in an earlier SOS of about 1.3 days and a later EOS of around 2.4 days. As a result, the GSL could be extended by approximately 3.6 days with a range of 1.6–6.5 days for 25th ~ 75th quantiles, with a median value of about 2.1 days. For different vegetation types, the phenology response to urbanization, as defined by GSL, ranges from 1 to 4 days. The quantitative relationship between phenology and urbanization is of great use for developing improved models of vegetation phenology dynamics under future urbanization, and for developing change indicators to assess the impacts of urbanization on vegetation phenology.     

Carbon stored in human settlements: the conterminous United States

Urban areas are home to more than half of the world's people, responsible for >70% of anthropogenic release of carbon dioxide and 76% of wood used for industrial purposes. By 2050 the proportion of the urban population is expected to increase to 70% worldwide. Despite fast rates of change and potential value for mitigation of carbon dioxide emissions, the organic carbon storage in human settlements has not been well quantified. Here, we show that human settlements can store as much carbon per unit area (23–42 kg C m⁻² urban areas and 7–16 kg C m⁻²exurban areas) as tropical forests, which have the highest carbon density of natural ecosystems (4–25 kg C m⁻²). By the year 2000 carbon storage attributed to human settlements of the conterminous United States was 18 Pg of carbon or 10% of its total land carbon storage. Sixty-four percent of this carbon was attributed to soil, 20% to vegetation, 11% to landfills, and 5% to buildings. To offset rising urban emissions of carbon, regional and national governments should consider how to protect or even to increase carbon storage of human-dominated landscapes. Rigorous studies addressing carbon budgets of human settlements and vulnerability of their carbon storage are needed.     

Mapping the potential mycorrhizal associations of the conterminous United States of America

Mycorrhizal associations are recognized as key symbioses in a changing world, yet our understanding of their geographic distribution and temporal dynamics remains limited. We combined data on mycorrhizal associations and historical dominant vegetation to map the pre-European Settlement mycorrhizal associations of the conterminous United States of America (USA). As a demonstration of the map's utility, we estimated changes in mycorrhizal associations due to urbanization, agriculture and the establishment of non-native species in two regions. We found that the conterminous USA was dominated by vegetation associated with arbuscular mycorrhizas, but that ∼40% of vegetation types included multiple mycorrhizal associations. Shifting land use to agriculture and the introduction of non-native species has disproportionately affected ectomycorrhizas, as did urbanization. These preliminary results set a baseline for mycorrhizal biogeography of the USA and illustrate how synthesis of available data can help us understand the impact of anthropogenic changes on an important mutualism.     

The Holdridge life zones of the conterminous United States in relation to ecosystem mapping

Aim Our main goals were to develop a map of the life zones for the conterminous United States, based on the Holdridge Life Zone system, as a tool for ecosystem mapping, and to compare the map of Holdridge life zones with other global vegetation classification and mapping efforts. Location The area of interest is the forty-eight contiguous states of the United States. Methods We wrote a PERL program for determining life zones from climatic data and linked it to the image processing workbench (IPW). The inputs were annual precipitation (Pann), biotemperature (T_bio), sea-level biotemperature (T₀bio), and the frost line. The spatial resolution chosen for this study (2.5 arc-minute for classification, 4-km for mapping) was driven by the availability of current state-of-the-art, accurate and reliable precipitation data. We used the Precipitation-elevation Regressions on Independent Slopes Model, or PRISM, output for the contiguous United States downloaded from the Internet. The accepted standard data for air temperature surfaces were obtained from the Vegetation/Ecosystem Modelling and Analysis Project (VEMAP). This data set along with station data obtained from the National Climatic Data Center for the US, were used to develop all temperature surfaces at the same resolution as the Pann. Results The US contains thirty-eight life zones (34% of the world's life zones and 85% of the temperate ones) including one boreal, twelve cool temperate, twenty warm temperate, four subtropical, and one tropical. Seventy-four percent of the US falls in the ‘basal belt’, 18% is montane, 8% is subalpine, 1% is alpine, and < 0.1% is nival. The US ranges from superarid to superhumid, and the humid province is the largest (45% of the US). The most extensive life zone is the warm temperate moist forest, which covers 23% of the country. We compared the Holdridge life zone map with output from the BIOME model, Bailey's ecoregions, Küchler potential vegetation, and land cover, all aggregated to four cover classes. Despite differences in the goals and methods for all these classification systems, there was a very good to excellent agreement among them for forests but poor for grasslands, shrublands, and nonvegetated lands. Main conclusions We consider the life zone approach to have many strengths for ecosystem mapping because it is based on climatic driving factors of ecosystem processes and recognizes ecophysiological responses of plants; it is hierarchical and allows for the use of other mapping criteria at the association and successional levels of analysis; it can be expanded or contracted without losing functional continuity among levels of ecological complexity; it is a relatively simple system based on few empirical data; and it uses objective mapping criteria.     

Environmental limitation mapping of potential biomass resources across the conterminous United States

Several crops have recently been identified as potential dedicated bioenergy feedstocks for the production of power, fuels, and bioproducts. Despite being identified as early as the 1980s, no systematic work has been undertaken to characterize the spatial distribution of their long‐term production potentials in the United states. Such information is a starting point for planners and economic modelers, and there is a need for this spatial information to be developed in a consistent manner for a variety of crops, so that their production potentials can be intercompared to support crop selection decisions. As part of the Sun Grant Regional Feedstock Partnership (RFP), an approach to mapping these potential biomass resources was developed to take advantage of the informational synergy realized when bringing together coordinated field trials, close interaction with expert agronomists, and spatial modeling into a single, collaborative effort. A modeling and mapping system called PRISM‐ELM was designed to answer a basic question: How do climate and soil characteristics affect the spatial distribution and long‐term production patterns of a given crop? This empirical/mechanistic/biogeographical hybrid model employs a limiting factor approach, where productivity is determined by the most limiting of the factors addressed in submodels that simulate water balance, winter low‐temperature response, summer high‐temperature response, and soil pH, salinity, and drainage. Yield maps are developed through linear regressions relating soil and climate attributes to reported yield data. The model was parameterized and validated using grain yield data for winter wheat and maize, which served as benchmarks for parameterizing the model for upland and lowland switchgrass, CRP grasses, Miscanthus, biomass sorghum, energycane, willow, and poplar. The resulting maps served as potential production inputs to analyses comparing the viability of biomass crops under various economic scenarios. The modeling and parameterization framework can be expanded to include other biomass crops.     

Butterfly community structure and landscape composition in agricultural landscapes of the central United States

Agricultural landscapes worldwide are under increased pressure to provide food, feed, fiber, and fuel for a growing human population. These demands are leading to changes in agricultural landscapes and subsequent declines in biodiversity. We used citizen science data from the North American Butterfly Association and remotely-sensed land cover data from the US Department of Agriculture to study relationships between agricultural landscape composition and butterfly community structure in the Midwestern US. Landscape-level butterfly species richness (based on rarefaction estimates) was highest in agricultural landscapes with relatively low amounts of cropland, relatively high amounts of woodland, and intermediate amounts of grassland and wetland. Rarefied richness generally declined with the dominance of any of these land cover types. Unlike other land cover types, urban development had a consistent negative effect on rarefied richness. Butterfly community structure (based on relative abundance) was also significantly related to the amount of cropland, woodland, grassland, and wetland in the landscape. The rarest butterfly species were associated with woodland-, grassland-, and wetland-dominated landscapes, likely due to their association with plant species occurring in savannahs, prairies, and marshes, respectively. Assuming that variation across space reflects changes over time, our results support conclusions from previous studies that removal of natural and seminatural habitats alters butterfly community structure and decreases species diversity in agricultural landscapes.     

Accounting for ecosystem alteration doubles estimates of conservation risk in the conterminous United States

Previous national and global conservation assessments have relied on habitat conversion data to quantify conservation risk. However, in addition to habitat conversion to crop production or urban uses, ecosystem alteration (e.g., from logging, conversion to plantations, biological invasion, or fire suppression) is a large source of conservation risk. We add data quantifying ecosystem alteration on unconverted lands to arrive at a more accurate depiction of conservation risk for the conterminous United States. We quantify ecosystem alteration using a recent national assessment based on remote sensing of current vegetation compared with modeled reference natural vegetation conditions. Highly altered (but not converted) ecosystems comprise 23% of the conterminous United States, such that the number of critically endangered ecoregions in the United States is 156% higher than when calculated using habitat conversion data alone. Increased attention to natural resource management will be essential to address widespread ecosystem alteration and reduce conservation risk.     

Nutrient uptake in streams draining agricultural catchments of the midwestern United States

1. Agriculture is a major contributor of non‐point source pollution to surface waters in the midwestern United States, resulting in eutrophication of freshwater aquatic ecosystems and development of hypoxia in the Gulf of Mexico. Agriculturally influenced streams are diverse in morphology and have variable nutrient concentrations. Understanding how nutrients are transformed and retained within agricultural streams may aid in mitigating increased nutrient export to downstream ecosystems. 2. We studied six agriculturally influenced streams in Indiana and Michigan to develop a more comprehensive understanding of the factors controlling nutrient retention and export in agricultural streams using nutrient addition and isotopic tracer studies. 3. Metrics of nutrient uptake indicated that nitrate uptake was saturated in these streams whereas ammonium and phosphorus uptake increased with higher concentrations. Phosphorus uptake was likely approaching saturation as evidenced by decreasing uptake velocities with concentration; ammonium uptake velocity also declined with concentration, though not significantly. 4. Higher whole‐stream uptake rates of phosphorus and ammonium were associated with the observed presence of stream autotrophs (e.g. algae and macrophytes). However, there was no significant relationship between measures of nutrient uptake and stream metabolism. Water‐column nutrient concentrations were positively correlated with gross primary production but not community respiration. 5. Overall, nutrient uptake and metabolism were affected by nutrient concentrations in these agriculturally influenced streams. Biological uptake of ammonium and phosphorus was not saturated, although nitrate uptake did appear to be saturated in these ecosystems. Biological activity in agriculturally influenced streams is higher relative to more pristine streams and this increased biological activity likely influences nutrient retention and transport to downstream ecosystems.     

The influence of biomanipulation on fish community development in a southeastern United States cooling reservoir

L Lake, a reactor cooling reservoir in South Carolina, USA was managed after filling to promote the development of healthy ecological communities similar to those in mature regional cooling reservoirs. Two types of biomanipulation were undertaken to achieve this goal, the introduction of typical southeastern US reservoir fishes (bluegill and largemouth bass) and artificial planting of native aquatic macrophytes. Fish assemblages were monitored by electrofishing from reservoir filling in 1986 until 1998. Multivariate analysis divided the fish samples into five sequential periods resulting from species replacements and additions. Small species that colonized L Lake from a feeder stream predominated in the first period but were mostly eliminated, as bluegill, largemouth bass, and other lentic species increased in the second period. A rapid increase in threadfin shad abundance characterized the third period, and small littoral zone and phytophilous fishes increased during the fourth and fifth periods coincident with the proliferation of aquatic macrophytes. Analysis of Bray-Curtis similarities and the species accumulation rate indicated that the rate of fish community change decreased with time and that fish community structure changed little during the last several years of the study. By the end of the study, community structure was similar to that in a nearby cooling reservoir that supported diverse and resilient biota. Biomanipulation contributed to the rapid establishment of lentic species and later increases in small littoral and phytophilous species suggesting that biomanipulation may be useful in accelerating fish community development in new cooling reservoirs.     

Largely underestimated carbon emission from land use and land cover change in the conterminous United States

Carbon (C) emission and uptake due to land use and land cover change (LULCC) are the most uncertain term in the global carbon budget primarily due to limited LULCC data and inadequate model capability (e.g., underrepresented agricultural managements). We take the commonly used FAOSTAT‐based global Land Use Harmonization data (LUH2) and a new high‐resolution multisource harmonized national LULCC database (YLmap) to drive a land ecosystem model (DLEM) in the conterminous United States. We found that recent cropland abandonment and forest recovery may have been overestimated in the LUH2 data derived from national statistics, causing previously reported C emissions from land use have been underestimated due to the definition of cropland and aggregated LULCC signals at coarse resolution. This overestimation leads to a strong C sink (30.3 ± 2.5 Tg C/year) in model simulations driven by LUH2 in the United States during the 1980–2016 period, while we find a moderate C source (13.6 ± 3.5 Tg C/year) when using YLmap. This divergence implies that previous C budget analyses based on the global LUH2 dataset have underestimated C emission in the United States owing to the delineation of suitable cropland and aggregated land conversion signals at coarse resolution which YLmap overcomes. Thus, to obtain more accurate quantification of LULCC‐induced C emission and better serve global C budget accounting, it is urgently needed to develop fine‐scale country‐specific LULCC data to characterize the details of land conversion.     

Importance of biophysical effects on climate warming mitigation potential of biofuel crops over the conterminous United States

Current quantification of climate warming mitigation potential (CWMP) of biomass‐derived energy has focused primarily on its biogeochemical effects. This study used site‐level observations of carbon, water, and energy fluxes of biofuel crops to parameterize and evaluate the community land model (CLM) and estimate CO₂ fluxes, surface energy balance, soil carbon dynamics of corn (Zea mays), switchgrass (Panicum virgatum), and miscanthus (Miscanthus × giganteus) ecosystems across the conterminous United States considering different agricultural management practices and land‐use scenarios. We find that neglecting biophysical effects underestimates the CWMP of transitioning from croplands and marginal lands to energy crops. Biogeochemical effects alone result in changes in carbon storage of ?1.9, 49.1, and 69.3 g C m^?2 y^?1 compared to 20.5, 78.5, and 96.2 g C m^?2 y^?1 when considering both biophysical and biogeochemical effects for corn, switchgrass, and miscanthus, respectively. The biophysical contribution to CWMP is dominated by changes in latent heat fluxes. Using the model to optimize growth conditions through fertilization and irrigation increases the CWMP further to 79.6, 98.3, and 118.8 g C m^?2 y^?1, respectively, representing the upper threshold for CWMP. Results also show that the CWMP over marginal lands is lower than that over croplands. This study highlights that neglecting the biophysical effects of altered surface energy and water balance underestimates the CWMP of transitioning to bioenergy crops at regional scales.     

Soil nutrient budgets following projected corn stover harvest for biofuel production in the conterminous United States

Increasing demand for food and biofuel feedstocks may substantially affect soil nutrient budgets, especially in the United States where there is great potential for corn (Zea mays L) stover as a biofuel feedstock. This study was designed to evaluate impacts of projected stover harvest scenarios on budgets of soil nitrogen (N), phosphorus (P), and potassium (K) currently and in the future across the conterminous United States. The required and removed N, P, and K amounts under each scenario were estimated on the basis of both their average contents in grain and stover and from an empirical model. Our analyses indicate a small depletion of soil N (?4 ± 35 kg ha^?1) and K (?6 ± 36 kg ha^?1) and a moderate surplus of P (37 ± 21 kg ha^?1) currently on the national average, but with a noticeable variation from state to state. After harvesting both grain and projected stover, the deficits of soil N, P, and K were estimated at 114–127, 26–27, and 36–53 kg ha^?1 yr^?1, respectively, in 2006–2010; 131–173, 29–32, and 41–96 kg ha^?1 yr^?1, respectively, in 2020; and 161–207, 35–39, and 51–111 kg ha^?1 yr^?1, respectively, in 2050. This study indicates that the harvestable stover amount derived from the minimum stover requirement for maintaining soil organic carbon level scenarios under current fertilization rates can be sustainable for soil nutrient supply and corn production at present, but the deficit of P and K at the national scale would become larger in the future.     

Effects of landscape disturbance on mosquito community composition in tropical Australia

Emerging infectious diseases are considered to be a growing threat to human and wildlife health. Such diseases might be facilitated by anthropogenic land-use changes that cause novel juxtapositions of different habitats and species and result in new interchanges of vectors, diseases, and hosts. To search for such effects in tropical Australia, we sampled mosquito populations across anthropogenic disturbance gradients of grassland, artificial rainforest edge, and rainforest interior. From >15,000 captured mosquitoes, we identified 26 species and eight genera. Surprisingly, there was no significant difference in community composition or species richness between forest edges and grasslands, but both differed significantly from rainforest interiors. Mosquito species richness was elevated in grasslands relative to the rainforest habitats. Seven species were unique to grasslands and edges, with another 13 found across all habitats. Among the three most abundant species, Culex annulirostris occurred in all habitat types, whereas Verrallina lineata and Cx. pullus were more abundant in forest interiors. Our findings suggest that the creation of anthropogenic grasslands adjacent to rainforests may increase the susceptibility of species in both habitats to transmission of novel diseases via observable changes and mixing of the vector community on rainforest edges.     

Microbial ecoenzyme stoichiometry,nutrient limitation,and organic matter decomposition in wetlands of the conterminous United States

Microbial respiration (R_m) and ecoenzyme activities (EEA) related to microbial carbon, nitrogen, and phosphorus acquisition were measured in 792 freshwater and estuarine wetlands (representing a cumulative area of 217,480 km²) across the continental United States as part of the US EPA’s 2011 National Wetland Condition Assessment. EEA stoichiometry was used to construct models for and assess nutrient limitation, carbon use efficiency (CUE), and organic matter decomposition (? k). The wetlands were classified into ten groups based on aggregated ecoregion and wetland type. The wetlands were also assigned to least, intermediate, and most disturbed classes, based on the extent of human influences. Ecoenzyme activity related to C, N and P acquisition, R_m, CUE, and ? k differed among ecoregion–wetland types and, with the exception of C acquisition and ? k, among disturbance classes. R_m and EEA were positively correlated with soil C, N and P content (r = 0.15–0.64) and stoichiometry (r = 0.15–0.48), and negatively correlated with an index of carbon quality (r = ? 0.22 to ? 0.39). EEA stoichiometry revealed that wetlands were more often P- than N-limited, and that P-limitation increases with increasing disturbance. Our enzyme-based approach for modeling C, N, and P acquisition, and organic matter decomposition, all rooted in stoichiometric theory, provides a mechanism for modeling resource limitations of microbial metabolism and biogeochemical cycling in wetlands. Given the ease of collecting and analyzing soil EEA and their response to wetland disturbance gradients, enzyme stoichiometry models are a cost-effective tool for monitoring ecosystem responses to resource availability and the environmental drivers of microbial metabolism, including those related to global climate changes.     

Bioenergy production and forest landscape change in the southeastern United States

Production of woody biomass for bioenergy, whether wood pellets or liquid biofuels, has the potential to cause substantial landscape change and concomitant effects on forest ecosystems, but the landscape effects of alternative production scenarios have not been fully assessed. We simulated landscape change from 2010 to 2050 under five scenarios of woody biomass production for wood pellets and liquid biofuels in North Carolina, in the southeastern United States, a region that is a substantial producer of wood biomass for bioenergy and contains high biodiversity. Modeled scenarios varied biomass feedstocks, incorporating harvest of ‘conventional’ forests, which include naturally regenerating as well as planted forests that exist on the landscape even without bioenergy production, as well as purpose‐grown woody crops grown on marginal lands. Results reveal trade‐offs among scenarios in terms of overall forest area and the characteristics of the remaining forest in 2050. Meeting demand for biomass from conventional forests resulted in more total forest land compared with a baseline, business‐as‐usual scenario. However, the remaining forest was composed of more intensively managed forest and less of the bottomland hardwood and longleaf pine habitats that support biodiversity. Converting marginal forest to purpose‐grown crops reduced forest area, but the remaining forest contained more of the critical habitats for biodiversity. Conversion of marginal agricultural lands to purpose‐grown crops resulted in smaller differences from the baseline scenario in terms of forest area and the characteristics of remaining forest habitats. Each scenario affected the dominant type of land‐use change in some regions, especially in the coastal plain that harbors high levels of biodiversity. Our results demonstrate the complex landscape effects of alternative bioenergy scenarios, highlight that the regions most likely to be affected by bioenergy production are also critical for biodiversity, and point to the challenges associated with evaluating bioenergy sustainability.     

Relating fish biomass to habitat and chemistry in headwater streams of the northeastern United States

Stream pH and stream habitat have both been identified as important environmental features influencing total fish biomass in streams, but few studies have evaluated the relative influence of habitat and pH together. We measured total fish biomass, stream habitat, and stream pH in sixteen sites from three tributary systems in the northeastern United States. The habitat metrics included total pool area, a cover score, large wood frequency, and stream temperature. We created and compared nine linear models relating total fish biomass in summer to stream pH and stream habitat using Akaike’s Information Criterion (AIC) analysis. The best (most parsimonious) models included pool area and stream pH. These results and a separate comparison of three regressions (low-flow pH, pool area, and these two metrics together versus total fish biomass) suggest that both habitat and stream buffering capacity affect the total biomass of fish in northeastern US headwater streams. When stream pH is adequate (low-flow pH greater than at least 5.7), physical habitat is likely to be more important, but under lower pH conditions, habitat is likely to be less effective in accounting for the total biomass of fish in these streams. This work demonstrates the continued effects of stream acidification in the northeastern US and more generally, it illustrates the importance of considering both physical and chemical conditions of a stream when evaluating the factors influencing fish communities.