Publications by authors named "Sarah E Hobbie"

83 Publications

Low-intensity frequent fires in coniferous forests transform soil organic matter in ways that may offset ecosystem carbon losses.

Glob Chang Biol 2021 Apr 21. Epub 2021 Apr 21.

Department of Earth System Science, Stanford University, Stanford, CA, USA.

The impact of shifting disturbance regimes on soil carbon (C) storage is a key uncertainty in global change research. Wildfires in coniferous forests are becoming more frequent in many regions, potentially causing large C emissions. Repeated low-intensity prescribed fires can mitigate wildfire severity, but repeated combustion may decrease soil C unless compensatory responses stabilize soil organic matter. Here, we tested how 30 years of decadal prescribed burning affected C and nitrogen (N) in plants, detritus, and soils in coniferous forests in the Sierra Nevada mountains, USA. Tree basal area and litter stocks were resilient to fire, but fire reduced forest floor C by 77% (-36.4 Mg C/ha). In mineral soils, fire reduced C that was free from minerals by 41% (-4.4 Mg C/ha) but not C associated with minerals, and only in depths ≤ 5 cm. Fire also transformed the properties of remaining mineral soil organic matter by increasing the proportion of C in a pyrogenic form (from 3.2% to 7.5%) and associated with minerals (from 46% to 58%), suggesting the remaining soil C is more resistant to decomposition. Laboratory assays illustrated that fire reduced microbial CO respiration rates by 55% and the activity of eight extracellular enzymes that degrade cellulosic and aromatic compounds by 40-66%. Lower decomposition was correlated with lower inorganic N (-49%), especially ammonium, suggesting N availability is coupled with decomposition. The relative increase in forms of soil organic matter that are resistant to decay or stabilized onto mineral surfaces, and the associated decline in decomposition suggest that low-intensity fires may promote mineral soil C storage in pools with long mean residence times in coniferous forests.
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http://dx.doi.org/10.1111/gcb.15648DOI Listing
April 2021

Sensitivity of grassland carbon pools to plant diversity, elevated CO, and soil nitrogen addition over 19 years.

Proc Natl Acad Sci U S A 2021 Apr;118(17)

Department of Forest Resources, University of Minnesota, St. Paul, MN 55108.

Whether the terrestrial biosphere will continue to act as a net carbon (C) sink in the face of multiple global changes is questionable. A key uncertainty is whether increases in plant C fixation under elevated carbon dioxide (CO) will translate into decades-long C storage and whether this depends on other concurrently changing factors. We investigated how manipulations of CO, soil nitrogen (N) supply, and plant species richness influenced total ecosystem (plant + soil to 60 cm) C storage over 19 y in a free-air CO enrichment grassland experiment (BioCON) in Minnesota. On average, after 19 y of treatments, increasing species richness from 1 to 4, 9, or 16 enhanced total ecosystem C storage by 22 to 32%, whereas N addition of 4 g N m ⋅ y and elevated CO of +180 ppm had only modest effects (increasing C stores by less than 5%). While all treatments increased net primary productivity, only increasing species richness enhanced net primary productivity sufficiently to more than offset enhanced C losses and substantially increase ecosystem C pools. Effects of the three global change treatments were generally additive, and we did not observe any interactions between CO and N. Overall, our results call into question whether elevated CO will increase the soil C sink in grassland ecosystems, helping to slow climate change, and suggest that losses of biodiversity may influence C storage as much as or more than increasing CO or high rates of N deposition in perennial grassland systems.
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http://dx.doi.org/10.1073/pnas.2016965118DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8092561PMC
April 2021

Disease and fire interact to influence transitions between savanna-forest ecosystems over a multi-decadal experiment.

Ecol Lett 2021 May 10;24(5):1007-1017. Epub 2021 Mar 10.

Department of Forest Resources, University of Minnesota, St. Paul, MI, 55108, USA.

Global change is shifting disturbance regimes that may rapidly change ecosystems, sometimes causing ecosystems to shift between states. Interactions between disturbances such as fire and disease could have especially severe effects, but experimental tests of multi-decadal changes in disturbance regimes are rare. Here, we surveyed vegetation for 35 years in a 54-year fire frequency experiment in a temperate oak savanna-forest ecotone that experienced a recent outbreak of oak wilt. Different fire regimes determined whether plots were savanna or forest by regulating tree abundance (r  = 0.70), but disease rapidly reversed the effect of fire exclusion, increasing mortality by 765% in unburned forests, but causing relatively minor changes in frequently burned savannas. Model simulations demonstrated that disease caused unburned forests to transition towards a unique woodland that was prone to transition to savanna if fire was reintroduced. Consequently, disease-fire interactions could shift ecosystem resilience and biome boundaries as pathogen distributions change.
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http://dx.doi.org/10.1111/ele.13719DOI Listing
May 2021

Decadal changes in fire frequencies shift tree communities and functional traits.

Nat Ecol Evol 2021 04 25;5(4):504-512. Epub 2021 Feb 25.

Department of Earth System Science, Stanford University, Stanford, CA, USA.

Global change has resulted in chronic shifts in fire regimes. Variability in the sensitivity of tree communities to multi-decadal changes in fire regimes is critical to anticipating shifts in ecosystem structure and function, yet remains poorly understood. Here, we address the overall effects of fire on tree communities and the factors controlling their sensitivity in 29 sites that experienced multi-decadal alterations in fire frequencies in savanna and forest ecosystems across tropical and temperate regions. Fire had a strong overall effect on tree communities, with an average fire frequency (one fire every three years) reducing stem density by 48% and basal area by 53% after 50 years, relative to unburned plots. The largest changes occurred in savanna ecosystems and in sites with strong wet seasons or strong dry seasons, pointing to fire characteristics and species composition as important. Analyses of functional traits highlighted the impact of fire-driven changes in soil nutrients because frequent burning favoured trees with low biomass nitrogen and phosphorus content, and with more efficient nitrogen acquisition through ectomycorrhizal symbioses. Taken together, the response of trees to altered fire frequencies depends both on climatic and vegetation determinants of fire behaviour and tree growth, and the coupling between fire-driven nutrient losses and plant traits.
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http://dx.doi.org/10.1038/s41559-021-01401-7DOI Listing
April 2021

Experimental nitrogen fertilisation globally accelerates, then slows decomposition of leaf litter.

Ecol Lett 2021 Apr 14;24(4):802-811. Epub 2021 Feb 14.

Department of Ecology, Evolution, Behavior, University of Minnesota, Saint Paul, MN, 55108, USA.

Plant litter decomposition is a central process in the carbon (C) cycle and sensitive to ongoing anthropogenic nitrogen (N) fertilisation. Previous syntheses evaluating the effect of N fertilisation on litter decomposition relied largely on models that define a constant rate of mass loss throughout decomposition, which may mask hypothesised shifts in the effect of N fertilisation on litter decomposition dynamics. In this meta-analysis, we compared the performance of four empirical decomposition models and showed that N fertilisation consistently accelerates early-stage but slows late-stage decomposition when the model structure allows for flexibility in decomposition rates through time. Within a particular substrate, early-stage N-stimulation of decomposition was associated with reduced rates of late-stage decay. Because the products of early- vs. late-stage decomposition are stabilised in soils through distinct chemical and physical mechanisms, N-induced changes in the litter decomposition process may influence the formation and cycling of soil C, the largest terrestrial C pool.
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http://dx.doi.org/10.1111/ele.13700DOI Listing
April 2021

Stimulation of soil respiration by elevated CO is enhanced under nitrogen limitation in a decade-long grassland study.

Proc Natl Acad Sci U S A 2020 12 14;117(52):33317-33324. Epub 2020 Dec 14.

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, 100084 Beijing, China;

Whether and how CO and nitrogen (N) availability interact to influence carbon (C) cycling processes such as soil respiration remains a question of considerable uncertainty in projecting future C-climate feedbacks, which are strongly influenced by multiple global change drivers, including elevated atmospheric CO concentrations (eCO) and increased N deposition. However, because decades of research on the responses of ecosystems to eCO and N enrichment have been done largely independently, their interactive effects on soil respiratory CO efflux remain unresolved. Here, we show that in a multifactor free-air CO enrichment experiment, BioCON (Biodiversity, CO, and N deposition) in Minnesota, the positive response of soil respiration to eCO gradually strengthened at ambient (low) N supply but not enriched (high) N supply for the 12-y experimental period from 1998 to 2009. In contrast to earlier years, eCO stimulated soil respiration twice as much at low than at high N supply from 2006 to 2009. In parallel, microbial C degradation genes were significantly boosted by eCO at low but not high N supply. Incorporating those functional genes into a coupled C-N ecosystem model reduced model parameter uncertainty and improved the projections of the effects of different CO and N levels on soil respiration. If our observed results generalize to other ecosystems, they imply widely positive effects of eCO on soil respiration even in infertile systems.
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http://dx.doi.org/10.1073/pnas.2002780117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7777058PMC
December 2020

Root traits as drivers of plant and ecosystem functioning: current understanding, pitfalls and future research needs.

New Phytol 2020 Nov 7. Epub 2020 Nov 7.

Geobotany, Faculty of Biology, University of Freiburg, Schänzlestr. 1, Freiburg, 79104, Germany.

The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis-based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.
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http://dx.doi.org/10.1111/nph.17072DOI Listing
November 2020

Increasing effects of chronic nutrient enrichment on plant diversity loss and ecosystem productivity over time.

Ecology 2021 02 18;102(2):e03218. Epub 2021 Jan 18.

Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, Minnesota, 55108, USA.

Human activities are enriching many of Earth's ecosystems with biologically limiting mineral nutrients such as nitrogen (N) and phosphorus (P). In grasslands, this enrichment generally reduces plant diversity and increases productivity. The widely demonstrated positive effect of diversity on productivity suggests a potential negative feedback, whereby nutrient-induced declines in diversity reduce the initial gains in productivity arising from nutrient enrichment. In addition, plant productivity and diversity can be inhibited by accumulations of dead biomass, which may be altered by nutrient enrichment. Over longer time frames, nutrient addition may increase soil fertility by increasing soil organic matter and nutrient pools. We examined the effects of 5-11 yr of nutrient addition at 47 grasslands in 12 countries. Nutrient enrichment increased aboveground live biomass and reduced plant diversity at nearly all sites, and these effects became stronger over time. We did not find evidence that nutrient-induced losses of diversity reduced the positive effects of nutrients on biomass; however, nutrient effects on live biomass increased more slowly at sites where litter was also increasing, regardless of plant diversity. This work suggests that short-term experiments may underestimate the long-term nutrient enrichment effects on global grassland ecosystems.
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http://dx.doi.org/10.1002/ecy.3218DOI Listing
February 2021

Municipal regulation of residential landscapes across US cities: Patterns and implications for landscape sustainability.

J Environ Manage 2020 Dec 29;275:111132. Epub 2020 Sep 29.

Department of Plant and Soil Sciences, University of Delaware, Newark, DE, 19716, USA. Electronic address:

Local regulations on residential landscapes (yards and gardens) can facilitate or constrain ecosystem services and disservices in cities. To our knowledge, no studies have undertaken a comprehensive look at how municipalities regulate residential landscapes to achieve particular goals and to control management practices. Across six U.S. cities, we analyzed 156 municipal ordinances to examine regional patterns in local landscape regulations and their implications for sustainability. Specifically, we conducted content analysis to capture regulations aimed at: 1) goals pertaining to conservation and environmental management, aesthetics and nuisance avoidance, and health and wellbeing, and 2) management actions including vegetation maintenance, water and waste management, food production, and chemical inputs. Our results reveal significant variation in local and regional regulations. While regulatory goals stress stormwater management and nuisance avoidance, relatively few municipalities explicitly regulate residential yards to maintain property values, mitigate heat, or avoid allergens. Meanwhile, biological conservation and water quality protection are common goals, yet regulations on yard management practices (e.g., non-native plants or chemical inputs) sometimes contradict these purposes. In addition, regulations emphasizing aesthetics and the maintenance of vegetation, mowing of grass and weeds, as well as the removal of dead wood, may inhibit wildlife-friendly yards. As a whole, landscaping ordinances largely ignore tradeoffs between interacting goals and outcomes, thereby limiting their potential to support landscape sustainability. Recommendations therefore include coordinated, multiobjective planning through partnerships among planners, developers, researchers, and non-government entities at multiple scales.
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http://dx.doi.org/10.1016/j.jenvman.2020.111132DOI Listing
December 2020

Diversity-dependent soil acidification under nitrogen enrichment constrains biomass productivity.

Glob Chang Biol 2020 Nov 18;26(11):6594-6603. Epub 2020 Sep 18.

Department of Forest Resources, University of Minnesota, Minneapolis, MN, USA.

In most plant communities, the net effect of nitrogen enrichment is an increase in plant productivity. However, nitrogen enrichment also has been shown to decrease species richness and to acidify soils, each of which may diminish the long-term impact of nutrient enrichment on productivity. Here we use a long-term (20 year) grassland plant diversity by nitrogen enrichment experiment in Minnesota, United States (a subexperiment within the BioCON experiment) to quantify the net impacts of nitrogen enrichment on productivity, including its potential indirect effects on productivity via changes in species richness and soil pH over an experimental diversity gradient. Overall, we found that nitrogen enrichment led to an immediate positive increment in productivity, but that this effect became nonsignificant over later years of the experiment, with the difference in productivity between fertilized and unfertilized plots decreasing in proportion to nitrogen addition-dependent declines in soil pH and losses of plant diversity. The net effect of nitrogen enrichment on productivity could have been 14.5% more on average over 20 years in monocultures if not for nitrogen-induced decreases in pH and about 28.5% more on average over 20 years in 16 species communities if not for nitrogen-induced species richness losses. Together, these results suggest that the positive effects of nutrient enrichment on biomass production can diminish in their magnitude over time, especially because of soil acidification in low diversity communities and especially because of plant diversity loss in initially high diversity communities.
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http://dx.doi.org/10.1111/gcb.15329DOI Listing
November 2020

Microbial functional genes commonly respond to elevated carbon dioxide.

Environ Int 2020 11 29;144:106068. Epub 2020 Aug 29.

Institute for Environmental Genomics, The University of Oklahoma, Norman, OK 73019, United States; Department of Microbiology and Plant Biology, The University of Oklahoma, Norman, OK 73019, United States; Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States; State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China. Electronic address:

Atmospheric CO concentration is increasing, largely due to anthropogenic activities. Previous studies of individual free-air CO enrichment (FACE) experimental sites have shown significant impacts of elevated CO (eCO) on soil microbial communities; however, no common microbial response patterns have yet emerged, challenging our ability to predict ecosystem functioning and sustainability in the future eCO environment. Here we analyzed 66 soil microbial communities from five FACE sites, and showed common microbial response patterns to eCO, especially for key functional genes involved in carbon and nitrogen fixation (e.g., pcc/acc for carbon fixation, nifH for nitrogen fixation), carbon decomposition (e.g., amyA and pulA for labile carbon decomposition, mnp and lcc for recalcitrant carbon decomposition), and greenhouse gas emissions (e.g., mcrA for methane production, norB for nitrous oxide production) across five FACE sites. Also, the relative abundance of those key genes was generally increased and directionally associated with increased biomass, soil carbon decomposition, and soil moisture. In addition, a further literature survey of more disparate FACE experimental sites indicated increased biomass, soil carbon decay, nitrogen fixation, methane and nitrous oxide emissions, plant and soil carbon and nitrogen under eCO. A conceptual framework was developed to link commonly responsive functional genes with ecosystem processes, such as pcc/acc vs. soil carbon storage, amyA/pulA/mnp/lcc vs. soil carbon decomposition, and nifH vs. nitrogen availability, suggesting that such common responses of microbial functional genes may have the potential to predict ecosystem functioning and sustainability in the future eCO environment.
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http://dx.doi.org/10.1016/j.envint.2020.106068DOI Listing
November 2020

Microbial processing of plant remains is co-limited by multiple nutrients in global grasslands.

Glob Chang Biol 2020 08 10;26(8):4572-4582. Epub 2020 Jun 10.

German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Leipzig, Germany.

Microbial processing of aggregate-unprotected organic matter inputs is key for soil fertility, long-term ecosystem carbon and nutrient sequestration and sustainable agriculture. We investigated the effects of adding multiple nutrients (nitrogen, phosphorus and potassium plus nine essential macro- and micro-nutrients) on decomposition and biochemical transformation of standard plant materials buried in 21 grasslands from four continents. Addition of multiple nutrients weakly but consistently increased decomposition and biochemical transformation of plant remains during the peak-season, concurrent with changes in microbial exoenzymatic activity. Higher mean annual precipitation and lower mean annual temperature were the main climatic drivers of higher decomposition rates, while biochemical transformation of plant remains was negatively related to temperature of the wettest quarter. Nutrients enhanced decomposition most at cool, high rainfall sites, indicating that in a warmer and drier future fertilized grassland soils will have an even more limited potential for microbial processing of plant remains.
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http://dx.doi.org/10.1111/gcb.15146DOI Listing
August 2020

Strong mineralogic control of soil organic matter composition in response to nutrient addition across diverse grassland sites.

Sci Total Environ 2020 Sep 6;736:137839. Epub 2020 Apr 6.

Earth and Biological Sciences Directorate, Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99354, USA; Department of Ecology, Evolution & Organismal Biology, Iowa State University, 251 Bessey Hall, Ames, IA 50011, USA. Electronic address:

Soil organic matter (SOM) dynamics are central to soil biogeochemistry and fertility. The retention of SOM is governed initially by interactions with minerals, which mediate the sorption of chemically diverse organic matter (OM) molecules via distinct surface areas and chemical functional group availabilities. Unifying principles of mineral-OM interactions remain elusive because of the multi-layered nature of biochemical-mineral interactions that contribute to soil aggregate formation and the heterogeneous nature of soils among ecosystems. This study sought to understand how soil mineralogy as well as nitrogen (N) enrichment regulate OM composition in grassland soils. Using a multi-site grassland experiment, we demonstrate that the composition of mineral-associated OM depended on the clay content and specific mineral composition in soils across the sites. With increasing abundance of ferrihydrite (Fh) across six different grassland locations, OM in the hydrophobic zone became more enriched in lipid- and protein-like compounds, whereas the kinetic zone OM became more enriched in lignin-like molecules. These relationships suggest that the persistence of various classes of OM in soils may depend on soil iron mineralogy and provide experimental evidence to support conceptual models of zonal mineral-OM associations. Experimental N addition disrupted the accumulation of protein-like molecules in the hydrophobic zone and the positive correlation of lignin-like molecules in the kinetic zone with Fh content, compared to unfertilized soils. These data suggest that mineralogy and clay content together influence the chemical composition not only of mineral-associated OM, but also of soluble compounds within the soil matrix. If these relationships are prevalent over larger spatial and temporal scales, they provide a foundation for understanding SOM cycling and persistence under a variety of environmental contexts.
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http://dx.doi.org/10.1016/j.scitotenv.2020.137839DOI Listing
September 2020

Warming and disturbance alter soil microbiome diversity and function in a northern forest ecotone.

FEMS Microbiol Ecol 2020 07;96(7)

Department of Biology, Stanford University, Stanford, CA 94305 USA.

The response to global change by soil microbes is set to affect important ecosystem processes. These impacts could be most immediate in transitional zones, such as the temperate-boreal forest ecotone, yet previous work in these forests has primarily focused on specific subsets of microbial taxa. Here, we examined how bacterial and fungal communities respond to simulated above- and below-ground warming under realistic field conditions in closed and open canopy treatments in Minnesota, USA. Our results show that warming and canopy disturbance shifted bacterial and fungal community structure as dominant bacterial and fungal groups differed in the direction and intensity of their responses. Ectomycorrhizal and saprotrophic fungal communities with greater connectivity (higher prevalence of strongly interconnected taxa based on pairwise co-occurrence relationships) were more resistant to compositional change. Warming effects on soil enzymes involved in the hydrolytic and oxidative liberation of carbon from plant cell walls and nutrients from organic matter were most strongly linked to fungal community responses, although community structure-function relationships differed between fungal guilds. Collectively, these findings indicate that warming and disturbance will influence the composition and function of microbial communities in the temperate-boreal ecotone, and fungal responses are particularly important to understand for predicting future ecosystem functioning.
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http://dx.doi.org/10.1093/femsec/fiaa108DOI Listing
July 2020

Interactive effects of elevated CO , warming, reduced rainfall, and nitrogen on leaf gas exchange in five perennial grassland species.

Plant Cell Environ 2020 08 14;43(8):1862-1878. Epub 2020 Jun 14.

Department of Forest Resources, University of Minnesota, St. Paul, Minnesota, USA.

Global changes can interact to affect photosynthesis and thus ecosystem carbon capture, yet few multi-factor field studies exist to examine such interactions. Here, we evaluate leaf gas exchange responses of five perennial grassland species from four functional groups to individual and interactive global changes in an open-air experiment in Minnesota, USA, including elevated CO (eCO ), warming, reduced rainfall and increased soil nitrogen supply. All four factors influenced leaf net photosynthesis and/or stomatal conductance, but almost all effects were context-dependent, i.e. they differed among species, varied with levels of other treatments and/or depended on environmental conditions. Firstly, the response of photosynthesis to eCO depended on species and nitrogen, became more positive as vapour pressure deficit increased and, for a C grass and a legume, was more positive under reduced rainfall. Secondly, reduced rainfall increased photosynthesis in three functionally distinct species, potentially via acclimation to low soil moisture. Thirdly, warming had positive, neutral or negative effects on photosynthesis depending on species and rainfall. Overall, our results show that interactions among global changes and environmental conditions may complicate predictions based on simple theoretical expectations of main effects, and that the factors and interactions influencing photosynthesis vary among herbaceous species.
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http://dx.doi.org/10.1111/pce.13783DOI Listing
August 2020

Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands.

Glob Chang Biol 2020 Feb 3. Epub 2020 Feb 3.

Department of Terrestrial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, The Netherlands.

Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature - herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local-scale herbivory, and its interaction with nutrient enrichment and climate, within global-scale models to better predict land-atmosphere interactions under future climate change.
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http://dx.doi.org/10.1111/gcb.15023DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7155038PMC
February 2020

Nature-based approaches to managing climate change impacts in cities.

Philos Trans R Soc Lond B Biol Sci 2020 03 27;375(1794):20190124. Epub 2020 Jan 27.

School of Life Sciences, Arizona State University, Tempe, AZ 85282, USA.

Managing and adapting to climate change in urban areas will become increasingly important as urban populations grow, especially because unique features of cities amplify climate change impacts. High impervious cover exacerbates impacts of climate warming through urban heat island effects and of heavy rainfall by magnifying runoff and flooding. Concentration of human settlements along rivers and coastal zones increases exposure of people and infrastructure to climate change hazards, often disproportionately affecting those who are least prepared. Nature-based strategies (NBS), which use living organisms, soils and sediments, and/or landscape features to reduce climate change hazards, hold promise as being more flexible, multi-functional and adaptable to an uncertain and non-stationary climate future than traditional approaches. Nevertheless, future research should address the effectiveness of NBS for reducing climate change impacts and whether they can be implemented at scales appropriate to climate change hazards and impacts. Further, there is a need for accurate and comprehensive cost-benefit analyses that consider disservices and co-benefits, relative to grey alternatives, and how costs and benefits are distributed across different communities. NBS are most likely to be effective and fair when they match the scale of the challenge, are implemented with input from diverse voices and are appropriate to specific social, cultural, ecological and technological contexts. This article is part of the theme issue 'Climate change and ecosystems: threats, opportunities and solutions'.
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http://dx.doi.org/10.1098/rstb.2019.0124DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7017761PMC
March 2020

Horticultural availability and homeowner preferences drive plant diversity and composition in urban yards.

Ecol Appl 2020 06 28;30(4):e02082. Epub 2020 Feb 28.

Department of Forest Resources and Department of Fisheries, Wildlife and Conservation Biology, University of Minnesota, Saint Paul, Minnesota, 55108, USA.

Understanding the factors that influence biodiversity in urban areas is important for informing management efforts aimed at enhancing the ecosystem services in urban settings and curbing the spread of invasive introduced species. We determined the ecological and socioeconomic factors that influence patterns of plant richness, phylogenetic diversity, and composition in 133 private household yards in the Minneapolis-Saint Paul Metropolitan area, Minnesota, USA. We compared the composition of spontaneously occurring plant species and those planted by homeowners with composition in natural areas (at the Cedar Creek Ecosystem Science Reserve) and in the horticulture pool of species available from commercial growers. Yard area and fertilizer frequency influenced species richness of the spontaneous species but expressed homeowner values did not. In contrast, the criteria that homeowners articulated as important in their management decisions, including aesthetics, wildlife, neatness and food provision, significantly predicted cultivated species richness. Strikingly, the composition of plant species that people cultivated in their yards resembled the taxonomic and phylogenetic composition of species available commercially. In contrast, the taxonomic and phylogenetic composition of spontaneous species showed high similarity to natural areas. The large fraction of introduced species that homeowners planted was a likely consequence of what was available for them to purchase. The study links the composition and diversity of yard flora to their natural and anthropogenic sources and sheds light on the human factors and values that influence the plant diversity in residential areas of a major urban system. Enhanced understanding of the influences of the sources of plants, both native and introduced, that enter urban systems and the human factors and values that influence their diversity is critical to identifying the levers to manage urban biodiversity and ecosystem services.
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http://dx.doi.org/10.1002/eap.2082DOI Listing
June 2020

Residential household yard care practices along urban-exurban gradients in six climatically-diverse U.S. metropolitan areas.

PLoS One 2019 13;14(11):e0222630. Epub 2019 Nov 13.

University of Vermont, Spatial Analysis Lab, Rubenstein School of Environment and Natural Resources, Aiken Center, Burlington, VT, United States of America.

Residential land is expanding in the United States, and lawn now covers more area than the country's leading irrigated crop by area. Given that lawns are widespread across diverse climatic regions and there is rising concern about the environmental impacts associated with their management, there is a clear need to understand the geographic variation, drivers, and outcomes of common yard care practices. We hypothesized that 1) income, age, and the number of neighbors known by name will be positively associated with the odds of having irrigated, fertilized, or applied pesticides in the last year, 2) irrigation, fertilization, and pesticide application will vary quadratically with population density, with the highest odds in suburban areas, and 3) the odds of irrigating will vary by climate, but fertilization and pesticide application will not. We used multi-level models to systematically address nested spatial scales within and across six U.S. metropolitan areas-Boston, Baltimore, Miami, Minneapolis-St. Paul, Phoenix, and Los Angeles. We found significant variation in yard care practices at the household (the relationship with income was positive), urban-exurban gradient (the relationship with population density was an inverted U), and regional scales (city-to-city variation). A multi-level modeling framework was useful for discerning these scale-dependent outcomes because this approach controls for autocorrelation at multiple spatial scales. Our findings may guide policies or programs seeking to mitigate the potentially deleterious outcomes associated with water use and chemical application, by identifying the subpopulations most likely to irrigate, fertilize, and/or apply pesticides.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0222630PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6853287PMC
March 2020

Non-symbiotic soil microbes are more strongly influenced by altered tree biodiversity than arbuscular mycorrhizal fungi during initial forest establishment.

FEMS Microbiol Ecol 2019 10;95(10)

Department of Plant and Microbial Biology, University of Minnesota -- Twin Cities, 1475 Gortner Ave., St. Paul, MN 55108, USA.

While the relationship between plant and microbial diversity has been well studied in grasslands, less is known about similar relationships in forests, especially for obligately symbiotic arbuscular mycorrhizal (AM) fungi. To assess the effect of varying tree diversity on microbial alpha- and beta-diversity, we sampled soil from plots in a high-density tree diversity experiment in Minnesota, USA, 3 years after establishment. About 3 of 12 tree species are AM hosts; the other 9 primarily associate with ectomycorrhizal fungi. We used phospho- and neutral lipid fatty acid analysis to characterize the biomass and functional identity of the whole soil bacterial and fungal community and high throughput sequencing to identify the species-level richness and composition of the AM fungal community. We found that plots of differing tree composition had different bacterial and fungal communities; plots with conifers, and especially Juniperus virginiana, had lower densities of several bacterial groups. In contrast, plots with a higher density or diversity of AM hosts showed no sign of greater AM fungal abundance or diversity. Our results indicate that early responses to plant diversity vary considerably across microbial groups, with AM fungal communities potentially requiring longer timescales to respond to changes in host tree diversity.
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http://dx.doi.org/10.1093/femsec/fiz134DOI Listing
October 2019

Strong photosynthetic acclimation and enhanced water-use efficiency in grassland functional groups persist over 21 years of CO enrichment, independent of nitrogen supply.

Glob Chang Biol 2019 09 24;25(9):3031-3044. Epub 2019 Jun 24.

Department of Forest Resources, University of Minnesota, St. Paul, Minnesota.

Uncertainty about long-term leaf-level responses to atmospheric CO rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO (eCO ) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long-lived perennials. Here, we report the effects of eCO on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free-Air CO Enrichment experiment, BioCON. Monocultures of species belonging to C grasses, C grasses, forbs, and legumes were exposed to two levels of CO and nitrogen supply in factorial combinations over 21 years. eCO increased photosynthesis by 12.9% on average in C species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C grasses the least as expected, but did not further diverge over time. Leaf-level water-use efficiency increased by 50% under eCO primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO in nitrogen-limited systems, and that significant and consistent declines in stomatal conductance and increases in water-use efficiency under eCO may allow plants to better withstand drought.
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http://dx.doi.org/10.1111/gcb.14714DOI Listing
September 2019

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

Ecol Appl 2019 06 1;29(4):e01884. Epub 2019 Apr 1.

School of Life Sciences, Arizona State University, Tempe, Arizona, 85287, USA.

In natural grasslands, C plant dominance increases with growing season temperatures and reflects distinct differences in plant growth rates and water use efficiencies of C vs. C 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 (δ C, index of C /C relative abundance) and C 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 lawn cover. We also calculated theoretical C carbon gain predicted by a plant physiological model as an index of expected C cover due to growing season climatic conditions in each city. Contrary to theoretical predictions, plant δ C and C cover in urban lawns were more strongly related to mean annual temperature than to growing season temperature. Wintertime temperatures influenced the distribution of C lawn turf plants, contrary to natural ecosystems where growing season temperatures primarily drive C distributions. C cover in lawns was greatest in the three warmest cities, due to an interaction between climate and homeowner plant management (e.g., planting C turf species) in these cities. The proportion of C lawn species was similar to the proportion of C species in the regional grass flora. However, the majority of C 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 plants were predominantly imported and cultivated. Elevated CO and temperature in cities can influence C /C competitive outcomes; however, this study provides evidence that climate and plant management dynamics influence biogeography and ecology of C /C plants in lawns. Their differing water and nutrient use efficiency may have substantial impacts on carbon, water, energy, and nutrient budgets across cities.
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http://dx.doi.org/10.1002/eap.1884DOI Listing
June 2019

Legumes regulate grassland soil N cycling and its response to variation in species diversity and N supply but not CO.

Glob Chang Biol 2019 07 29;25(7):2396-2409. Epub 2019 Apr 29.

Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, Minnesota.

Legumes are an important component of plant diversity that modulate nitrogen (N) cycling in many terrestrial ecosystems. Limited knowledge of legume effects on soil N cycling and its response to global change factors and plant diversity hinders a general understanding of whether and how legumes broadly regulate the response of soil N availability to those factors. In a 17-year study of perennial grassland species grown under ambient and elevated (+180 ppm) CO and ambient and enriched (+4 g N m  year ) N environments, we compared pure legume plots with plots dominated by or including other herbaceous functional groups (and containing one or four species) to assess the effect of legumes on N cycling (net N mineralization rate and inorganic N pools). We also examined the effects of numbers of legume species (from zero to four) in four-species mixed plots on soil N cycling. We hypothesized that legumes would increase N mineralization rates most in those treatments with the greatest diversity and the greatest relative limitation by and competition for N. Results partially supported these hypotheses. Plots with greater dominance by legumes had greater soil nitrate concentrations and mineralization rates. Higher species richness significantly increased the impact of legumes on soil N metrics, with 349% and 505% higher mineralization rates and nitrate concentrations in four-species plots containing legumes compared to legume-free four-species plots, in contrast to 185% and 129% greater values, respectively, in pure legume than nonlegume monoculture plots. N-fertilized plots had greater legume effects on soil nitrate, but lower legume effects on net N mineralization. In contrast, neither elevated CO nor its interaction with legumes affected net N mineralization. These results indicate that legumes markedly influence the response of soil N cycling to some, but not all, global change drivers.
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http://dx.doi.org/10.1111/gcb.14636DOI Listing
July 2019

Global patterns in fine root decomposition: climate, chemistry, mycorrhizal association and woodiness.

Ecol Lett 2019 Jun 19;22(6):946-953. Epub 2019 Mar 19.

Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN, 55108, USA.

Fine root decomposition constitutes a critical yet poorly understood flux of carbon and nutrients in terrestrial ecosystems. Here, we present the first large-scale synthesis of species trait effects on the early stages of fine root decomposition at both global and local scales. Based on decomposition rates for 279 plant species across 105 studies and 176 sites, we found that mycorrhizal association and woodiness are the best categorical traits for predicting rates of fine root decomposition. Consistent positive effects of nitrogen and phosphorus concentrations and negative effects of lignin concentration emerged on decomposition rates within sites. Similar relationships were present across sites, along with positive effects of temperature and moisture. Calcium was not consistently related to decomposition rate at either scale. While the chemical drivers of fine root decomposition parallel those of leaf decomposition, our results indicate that the best plant functional groups for predicting fine root decomposition differ from those predicting leaf decomposition.
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http://dx.doi.org/10.1111/ele.13248DOI Listing
June 2019

Soil organic carbon stability in forests: Distinct effects of tree species identity and traits.

Glob Chang Biol 2018 Dec 16. Epub 2018 Dec 16.

Chair of Soil Science, Technical University Munich (TUM), Freising, Germany.

Rising atmospheric CO concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40 year old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover, microbial community descriptors, and soil physicochemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate occluded particulate organic matter (POM) and mineral associated SOM, and bulk SOM δ N and ∆ C. The stability of SOM varied substantially among tree species, and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral associated SOM and bulk soil ∆ C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change.
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http://dx.doi.org/10.1111/gcb.14548DOI Listing
December 2018

Allometry of fine roots in forest ecosystems.

Ecol Lett 2019 Feb 28;22(2):322-331. Epub 2018 Nov 28.

Institute of Biological & Environmental Sciences, School of Biological Sciences, University of Aberdeen, Aberdeen, AB24 3UU, UK.

Theoretical predictions regarding fine root production are needed in many ecosystem models but are lacking. Here, we expand the classic pipe model to fine roots and predict isometric scaling relationships between leaf and fine root biomass and among all major biomass production components of individual trees. We also predict that fine root production scales more slowly against increases in leaf production across global forest ecosystems at the stand level. Using meta-analysis, we show fine root biomass scales isometrically against leaf biomass both at the individual tree and stand level. However, despite isometric scaling between stem and coarse root production, fine root production scales against leaf production with a slope of about 0.8 at the stand level, which probably results from more rapid increase of turnover rate in leaves than in fine roots. These analyses help to improve our understandings of allometric theory and controls of belowground C processes.
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http://dx.doi.org/10.1111/ele.13193DOI Listing
February 2019

Plant diversity maintains multiple soil functions in future environments.

Elife 2018 11 28;7. Epub 2018 Nov 28.

Department of Forest Resources, University of Minnesota, St Paul, United States.

Biodiversity increases ecosystem functions underpinning a suite of services valued by society, including services provided by soils. To test whether, and how, future environments alter the relationship between biodiversity and multiple ecosystem functions, we measured grassland plant diversity effects on single soil functions and ecosystem multifunctionality, and compared relationships in four environments: ambient conditions, elevated atmospheric CO, enriched N supply, and elevated CO and N in combination. Our results showed that plant diversity increased three out of four soil functions and, consequently, ecosystem multifunctionality. Remarkably, biodiversity-ecosystem function relationships were similarly significant under current and future environmental conditions, yet weaker with enriched N supply. Structural equation models revealed that plant diversity enhanced ecosystem multifunctionality by increasing plant community functional diversity, and the even provision of multiple functions. Conserving local plant diversity is therefore a robust strategy to maintain multiple valuable ecosystem services in both present and future environmental conditions.
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http://dx.doi.org/10.7554/eLife.41228DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6296783PMC
November 2018

Effects of climate warming on photosynthesis in boreal tree species depend on soil moisture.

Nature 2018 10 3;562(7726):263-267. Epub 2018 Oct 3.

Department of Forest Resources, University of Minnesota, St. Paul, MN, USA.

Climate warming will influence photosynthesis via thermal effects and by altering soil moisture. Both effects may be important for the vast areas of global forests that fluctuate between periods when cool temperatures limit photosynthesis and periods when soil moisture may be limiting to carbon gain. Here we show that the effects of climate warming flip from positive to negative as southern boreal forests transition from rainy to modestly dry periods during the growing season. In a three-year open-air warming experiment with juveniles of 11 temperate and boreal tree species, an increase of 3.4 °C in temperature increased light-saturated net photosynthesis and leaf diffusive conductance on average on the one-third of days with the wettest soils. In all 11 species, leaf diffusive conductance and, as a result, light-saturated net photosynthesis decreased during dry spells, and did so more sharply in warmed plants than in plants at ambient temperatures. Consequently, across the 11 species, warming reduced light-saturated net photosynthesis on the two-thirds of days with driest soils. Thus, low soil moisture may reduce, or even reverse, the potential benefits of climate warming on photosynthesis in mesic, seasonally cold environments, both during drought and in regularly occurring, modestly dry periods during the growing season.
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http://dx.doi.org/10.1038/s41586-018-0582-4DOI Listing
October 2018

Contrasting dynamics and trait controls in first-order root compared with leaf litter decomposition.

Proc Natl Acad Sci U S A 2018 10 25;115(41):10392-10397. Epub 2018 Sep 25.

Centre d'Ecologie Fonctionnelle et Evolutive, UMR 5175, CNRS-Université de Montpellier-Université Paul-Valéry Montpellier-Ecole Pratique des Hautes Etudes, 34293 Montpellier, France.

Decomposition is a key component of the global carbon (C) cycle, yet current ecosystem C models do not adequately represent the contributions of plant roots and their mycorrhizae to this process. The understanding of decomposition dynamics and their control by traits is particularly limited for the most distal first-order roots. Here we followed decomposition of first-order roots and leaf litter from 35 woody plant species differing in mycorrhizal type over 6 years in a Chinese temperate forest. First-order roots decomposed more slowly ( = 0.11 ± 0.01 years) than did leaf litter (0.35 ± 0.02 years), losing only 35% of initial mass on average after 6 years of exposure in the field. In contrast to leaf litter, nonlignin root C chemistry (nonstructural carbohydrates, polyphenols) accounted for 82% of the large interspecific variation in first-order root decomposition. Leaf litter from ectomycorrhizal (EM) species decomposed more slowly than that from arbuscular mycorrhizal (AM) species, whereas first-order roots of EM species switched, after 2 years, from having slower to faster decomposition compared with those from AM species. The fundamentally different dynamics and control mechanisms of first-order root decomposition compared with those of leaf litter challenge current ecosystem C models, the recently suggested dichotomy between EM and AM plants, and the idea that common traits can predict decomposition across roots and leaves. Aspects of C chemistry unrelated to lignin or nitrogen, and not presently considered in decomposition models, controlled first-order root decomposition; thus, current paradigms of ecosystem C dynamics and model parameterization require revision.
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http://dx.doi.org/10.1073/pnas.1716595115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6187159PMC
October 2018

Response to Comment on "Unexpected reversal of C versus C grass response to elevated CO during a 20-year field experiment".

Science 2018 09;361(6407)

Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN 55108, USA.

Nie and colleagues suggest a key role for interannual climate variation as an explanation for the temporal dynamics of an unexpected 20-year reversal of biomass responses of C-C grasses to elevated CO However, we had already identified some climate-dependent differences in C and C responses to eCO and shown that these could not fully explain the temporal dynamics we observed.
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http://dx.doi.org/10.1126/science.aau8982DOI Listing
September 2018