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  • 11
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    Shrub encroachment in Arctic tundra : Betula nana effects on above- and belowground litter decomposition (2017)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Ecological Society of America, 2017. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecology 98 (2017): 1361–1376, doi:10.1002/ecy.1790.
    Description: Rapid arctic vegetation change as a result of global warming includes an increase in the cover and biomass of deciduous shrubs. Increases in shrub abundance will result in a proportional increase of shrub litter in the litter community, potentially affecting carbon turnover rates in arctic ecosystems. We investigated the effects of leaf and root litter of a deciduous shrub, Betula nana, on decomposition, by examining species-specific decomposition patterns, as well as effects of Betula litter on the decomposition of other species. We conducted a 2-yr decomposition experiment in moist acidic tundra in northern Alaska, where we decomposed three tundra species (Vaccinium vitis-idaea, Rhododendron palustre, and Eriophorum vaginatum) alone and in combination with Betula litter. Decomposition patterns for leaf and root litter were determined using three different measures of decomposition (mass loss, respiration, extracellular enzyme activity). We report faster decomposition of Betula leaf litter compared to other species, with support for species differences coming from all three measures of decomposition. Mixing effects were less consistent among the measures, with negative mixing effects shown only for mass loss. In contrast, there were few species differences or mixing effects for root decomposition. Overall, we attribute longer-term litter mass loss patterns to patterns created by early decomposition processes in the first winter. We note numerous differences for species patterns between leaf and root decomposition, indicating that conclusions from leaf litter experiments should not be extrapolated to below-ground decomposition. The high decomposition rates of Betula leaf litter aboveground, and relatively similar decomposition rates of multiple species below, suggest a potential for increases in turnover in the fast-decomposing carbon pool of leaves and fine roots as the dominance of deciduous shrubs in the Arctic increases, but this outcome may be tempered by negative litter mixing effects during the early stages of encroachment.
    Description: National Science Foundation Grant Numbers: OPP-0909507, OPP-0807639, ARC-0806451; Arctic LTER Project. Grant Number: DEB-1026843
    Keywords: Arctic shrub encroachment ; Exoenzyme activity ; Litter decomposition ; Microbial respiration ; Mixing effects ; Moist acidic tundra ; Root decomposition ; Winter decomposition
    Repository Name: Woods Hole Open Access Server
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  • 12
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    Ecosystem feedbacks and cascade processes : understanding their role in the responses of Arctic and alpine ecosystems to environmental change (2009)
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © The Authors, 2009. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Global Change Biology 15 (2009): 1153-1172, doi:10.1111/j.1365-2486.2008.01801.x.
    Description: Global environmental change, related to climate change and the deposition of airborne N-containing contaminants, has already resulted in shifts in plant community composition among plant functional types in arctic and temperate alpine regions. In this paper, we review how key ecosystem processes will be altered by these transformations, the complex biological cascades and feedbacks that may result, and some of the potential broader consequences for the earth system. Firstly, we consider how patterns of growth and allocation, and nutrient uptake, will be altered by the shifts in plant dominance. The ways in which these changes may disproportionately affect the consumer communities, and rates of decomposition, are then discussed. We show that the occurrence of a broad spectrum of plant growth forms in these regions (from cryptogams to deciduous and evergreen dwarf shrubs, graminoids and forbs), together with hypothesized low functional redundancy, will mean that shifts in plant dominance result in a complex series of biotic cascades, couplings and feedbacks which are supplemental to the direct responses of ecosystem components to the primary global change drivers. The nature of these complex interactions is highlighted using the example of the climate-driven increase in shrub cover in low arctic tundra, and the contrasting transformations in plant functional composition in mid-latitude alpine systems. Finally, the potential effects of the transformations on ecosystem properties and processes which link with the earth system are reviewed. We conclude that the effects of global change on these ecosystems, and potential climate-change feedbacks, can not be predicted from simple empirical relationships between processes and driving variables. Rather, the effects of changes in species distributions and dominances on key ecosystem processes and properties must also be considered, based upon best estimates of the trajectories of key transformations, their magnitude and rates of change.
    Description: We thank the International Arctic Science Committee (IASC) for their support for this Mini- Review as part of the IASC Circum-Arctic Terrestrial Biodiversity initiative (CAT-B) and as part of International Polar Year 2007/2008.
    Keywords: Arctic ; Alpine ; Carbon ; Ecosystem ; Energy ; Global change ; Feedback ; Nitrogen ; Herbivory ; Plant functional type
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 13
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    Differential physiological responses to environmental change promote woody shrub expansion (2013)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecology and Evolution 3 (2013): 1149–1162, doi:10.1002/ece3.525.
    Description: Direct and indirect effects of warming are increasingly modifying the carbon-rich vegetation and soils of the Arctic tundra, with important implications for the terrestrial carbon cycle. Understanding the biological and environmental influences on the processes that regulate foliar carbon cycling in tundra species is essential for predicting the future terrestrial carbon balance in this region. To determine the effect of climate change impacts on gas exchange in tundra, we quantified foliar photosynthesis (Anet), respiration in the dark and light (RD and RL, determined using the Kok method), photorespiration (PR), carbon gain efficiency (CGE, the ratio of photosynthetic CO2 uptake to total CO2 exchange of photosynthesis, PR, and respiration), and leaf traits of three dominant species – Betula nana, a woody shrub; Eriophorum vaginatum, a graminoid; and Rubus chamaemorus, a forb – grown under long-term warming and fertilization treatments since 1989 at Toolik Lake, Alaska. Under warming, B. nana exhibited the highest rates of Anet and strongest light inhibition of respiration, increasing CGE nearly 50% compared with leaves grown in ambient conditions, which corresponded to a 52% increase in relative abundance. Gas exchange did not shift under fertilization in B. nana despite increases in leaf N and P and near-complete dominance at the community scale, suggesting a morphological rather than physiological response. Rubus chamaemorus, exhibited minimal shifts in foliar gas exchange, and responded similarly to B. nana under treatment conditions. By contrast, E. vaginatum, did not significantly alter its gas exchange physiology under treatments and exhibited dramatic decreases in relative cover (warming: −19.7%; fertilization: −79.7%; warming with fertilization: −91.1%). Our findings suggest a foliar physiological advantage in the woody shrub B. nana that is further mediated by warming and increased soil nutrient availability, which may facilitate shrub expansion and in turn alter the terrestrial carbon cycle in future tundra environments.
    Description: This study was supported by the National Science Foundation #0732664; Australian Research Council DP0986823; and Marsden Fund of the Royal Society of New Zealand.
    Keywords: Betula nana nana ; Carbon gain efficiency ; Eriophorum vaginatum ; Kok effect ; Photosynthesis ; Respiration ; Rubus chamaemorus ; Tundra shrub encroachment
    Repository Name: Woods Hole Open Access Server
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  • 14
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    Shallow soils are warmer under trees and tall shrubs across arctic and boreal ecosystems (2021)
    IOP Publishing
    Details
    Publication Date: 2022-10-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Kropp, H., Loranty, M. M., Natali, S. M., Kholodov, A. L., Rocha, A., V., Myers-Smith, I., Abbot, B. W., Abermann, J., Blanc-Betes, E., Blok, D., Blume-Werry, G., Boike, J., Breen, A. L., Cahoon, S. M. P., Christiansen, C. T., Douglas, T. A., Epstein, H. E., Frost, G., V., Goeckede, M., Hoye, T. T., Mamet, S. D., O'Donnell, J. A., Olefeldt, D., Phoenix, G. K., Salmon, V. G., Sannel, A. B. K., Smith, S. L., Sonnentag, O., Vaughn, L. S., Williams, M., Elberling, B., Gough, L., Hjort, J., Lafleur, P. M., Euskirchen, E. S., Heijmans, M. M. P. D., Humphreys, E. R., Iwata, H., Jones, B. M., Jorgenson, M. T., Gruenberg, I., Kim, Y., Laundre, J., Mauritz, M., Michelsen, A., Schaepman-Strub, G., Tape, K. D., Ueyama, M., Lee, B., Langley, K., & Lund, M. Shallow soils are warmer under trees and tall shrubs across arctic and boreal ecosystems. Environmental Research Letters, 16(1), (2021): 015001. doi:10.1088/1748-9326/abc994.
    Description: Soils are warming as air temperatures rise across the Arctic and Boreal region concurrent with the expansion of tall-statured shrubs and trees in the tundra. Changes in vegetation structure and function are expected to alter soil thermal regimes, thereby modifying climate feedbacks related to permafrost thaw and carbon cycling. However, current understanding of vegetation impacts on soil temperature is limited to local or regional scales and lacks the generality necessary to predict soil warming and permafrost stability on a pan-Arctic scale. Here we synthesize shallow soil and air temperature observations with broad spatial and temporal coverage collected across 106 sites representing nine different vegetation types in the permafrost region. We showed ecosystems with tall-statured shrubs and trees (〉40 cm) have warmer shallow soils than those with short-statured tundra vegetation when normalized to a constant air temperature. In tree and tall shrub vegetation types, cooler temperatures in the warm season do not lead to cooler mean annual soil temperature indicating that ground thermal regimes in the cold-season rather than the warm-season are most critical for predicting soil warming in ecosystems underlain by permafrost. Our results suggest that the expansion of tall shrubs and trees into tundra regions can amplify shallow soil warming, and could increase the potential for increased seasonal thaw depth and increase soil carbon cycling rates and lead to increased carbon dioxide loss and further permafrost thaw.
    Description: We thank G Peter Kershaw, LeeAnn Fishback, Cathy Wilson, and Coleen Iversen for assistance in collection of data. We thank the Permafrost Carbon Network for support and organization of the data synthesis. We thank Vladimir Romanovsky for his feedback and contribution of publicly available data. This project was supported by the National Science Foundation (Grant No. 1417745 to M L, Grant No. 1417700 to S M N, Grant No. 1417908 to A K, Grant No. 1556772 to A R, Grant No. 1637459 to L G, Grant No. 1636476 and Grant No. 1503912 to E S E, Grant No. 1806213 to B M J, Grant No. 1833056 to K D T), UK Natural Environment Research Council (Grant No. NE/M016323/1 to I H M S, Grant No. NE/K00025X/1 to G K P, Grant No. NE/K000292/1 to M W), Natural Sciences and Engineering Research (to P L, I H M S, Grant No. RGPIN-2016-04688 to D O), Council of Canada, Canadian Graduate Scholarship to (I H M -S), Greenland Ecosystem Monitoring Programme: ClimateBasis (to J A and K A), The Next-Generation Ecosystem Experiments (NGEE Arctic) project is supported by the Office of Biological and Environmental Research in the DOE Office of Science (to A L B), Engineer Research and Development Center Army Direct (6.1) Research Program and the Strategic Environmental Research and Development Program (projects RC-2110 and 18-1170 to T A D), United States Geological Survey (to E E S), Arctic Challenge for Sustainability (ArCS; Grant No. JPMXD1300000000) and ArCS II (Grant No. JPMXD1420318865) (to M U and H I), the Danish National Research Foundation (Grant No. CENPERM DNRF100 to B E), the Academy of Finland (Grant No. 315519), the National Research Foundation of Korea (Grant Nos. NRF-2016M1A5A1901769; KOPRI-PN20081 to K Y and B Y L), Research Network for Geosciences in Berlin and Potsdam (to I G), the Swiss National Science Foundation (Grant No. 140631 to G S S), the URPP Global Change and Biodiversity, University of Zurich (to G S S), the University of Alberta Northern Research Awards (to D O), and the Northern Scientific Training Program (to D O), and UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE) Office of Science, Biological and Environmental Research (to V G S). S M has been supported by grants and/or in-kind from Natural Sciences and Engineering Research Council of Canada, AMAX Northwest Mining, Co. (North American Tungsten Corp., Ltd), Imperial Oil, Ltd, University of Alberta, Earthwatch International (EI), The Garfield Weston Foundation, Wapusk National Park, Churchill Northern Studies Centre, and the Northern Scientific Training Program. All code for this project are archived (DOI: 10.5281/zenodo.4041165). The data that support the findings of this study are openly available through the Arctic Data Center (Heather Kropp, Michael Loranty, Britta Sannel, Jonathan O'Donnell, Elena Blanc-Betes, et al 2020. Synthesis of soil-air temperature and vegetation measurements in the pan-Arctic. 1990-2016. Arctic Data Center. doi:10.18739/A2736M31X).
    Keywords: Arctic ; Boreal forest ; Soil temperature ; Vegetation change ; Permafrost
    Repository Name: Woods Hole Open Access Server
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  • 15
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    Model responses to CO(2) and warming are underestimated without explicit representation of Arctic small-mammal grazing (2021)
    Ecological Society of America
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    Publication Date: 2022-10-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Rastetter, E. B., Griffin, K. L., Rowe, R. J., Gough, L., McLaren, J. R., & Boelman, N. T. Model responses to CO(2) and warming are underestimated without explicit representation of Arctic small-mammal grazing. Ecological Applications, (2021): e02478, https://doi.org/10.1002/eap.2478.
    Description: We use a simple model of coupled carbon and nitrogen cycles in terrestrial ecosystems to examine how “explicitly representing grazers” vs. “having grazer effects implicitly aggregated in with other biogeochemical processes in the model” alters predicted responses to elevated carbon dioxide and warming. The aggregated approach can affect model predictions because grazer-mediated processes can respond differently to changes in climate compared with the processes with which they are typically aggregated. We use small-mammal grazers in a tundra as an example and find that the typical three-to-four-year cycling frequency is too fast for the effects of cycle peaks and troughs to be fully manifested in the ecosystem biogeochemistry. We conclude that implicitly aggregating the effects of small-mammal grazers with other processes results in an underestimation of ecosystem response to climate change, relative to estimations in which the grazer effects are explicitly represented. The magnitude of this underestimation increases with grazer density. We therefore recommend that grazing effects be incorporated explicitly when applying models of ecosystem response to global change.
    Description: This work was supported in part by the National Science Foundation under NSF grants 1651722, 1637459, 1603560, 1556772, 1841608 to E.B.R.; 1603777 to N.T.B. and K.L.G.; 1603654 to R.J.R.; 1603760 to L.G.; and 1603677 to J.R.M.
    Keywords: Arctic tundra ; Biogeochemistry ; Carbon cycling ; Carbon-nitrogen ecosystem model ; Climate change ; Nitrogen cycling ; Population cycles ; Small-mammal herbivores
    Repository Name: Woods Hole Open Access Server
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  • 16
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    Time lags: insights from the U.S. Long Term Ecological Research Network (2021)
    Ecological Society of America
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    Publication Date: 2022-10-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Rastetter, E. B., Ohman, M. D., Elliott, K. J., Rehage, J. S., Rivera-Monroy, V. H., Boucek, R. E., Castaneda-Moya, E., Danielson, T. M., Gough, L., Groffman, P. M., Jackson, C. R., Miniat, C. F., & Shaver, G. R. Time lags: insights from the U.S. Long Term Ecological Research Network. Ecosphere, 12(5), (2021): e03431, https://doi.org/10.1002/ecs2.3431.
    Description: Ecosystems across the United States are changing in complex ways that are difficult to predict. Coordinated long-term research and analysis are required to assess how these changes will affect a diverse array of ecosystem services. This paper is part of a series that is a product of a synthesis effort of the U.S. National Science Foundation’s Long Term Ecological Research (LTER) network. This effort revealed that each LTER site had at least one compelling scientific case study about “what their site would look like” in 50 or 100 yr. As the site results were prepared, themes emerged, and the case studies were grouped into separate papers along five themes: state change, connectivity, resilience, time lags, and cascading effects and compiled into this special issue. This paper addresses the time lags theme with five examples from diverse biomes including tundra (Arctic), coastal upwelling (California Current Ecosystem), montane forests (Coweeta), and Everglades freshwater and coastal wetlands (Florida Coastal Everglades) LTER sites. Its objective is to demonstrate the importance of different types of time lags, in different kinds of ecosystems, as drivers of ecosystem structure and function and how these can effectively be addressed with long-term studies. The concept that slow, interactive, compounded changes can have dramatic effects on ecosystem structure, function, services, and future scenarios is apparent in many systems, but they are difficult to quantify and predict. The case studies presented here illustrate the expanding scope of thinking about time lags within the LTER network and beyond. Specifically, they examine what variables are best indicators of lagged changes in arctic tundra, how progressive ocean warming can have profound effects on zooplankton and phytoplankton in waters off the California coast, how a series of species changes over many decades can affect Eastern deciduous forests, and how infrequent, extreme cold spells and storms can have enduring effects on fish populations and wetland vegetation along the Southeast coast and the Gulf of Mexico. The case studies highlight the need for a diverse set of LTER (and other research networks) sites to sort out the multiple components of time lag effects in ecosystems.
    Description: This research was supported by the National Science Foundation Long Term Ecological Research program grants to the Arctic (Grants DEB-1637459 and 1026843), California Current (Grants OCE-1637632 and 1026607), Coweeta (Grants DEB-1637522, 1440485, 0823293, 9632854, and 0218001), and Florida Coastal Everglades (Grants DEB-9910514 and 1237517 and DBI-0620409) sites. We also acknowledge the sustained efforts of the CalCOFI program, present and previous staff of the SIO Pelagic Invertebrate Collection, and the late Ed Brinton for his pioneering insights in euphausiid ecology. The Coweeta research and synthesis were also supported by the USDA Forest Service, Southern Research Station, Coweeta Hydrologic Laboratory. Partial funding to VHRM was provided by the U.S. Department of the Interior South-Central Climate Science Center through Cooperative Agreement # G12AC00002.
    Keywords: Climate change ; Climate change detection ; Climate signal filtering ; Ecosystem response ; Special Feature: Forecasting Earth's Ecosystems with Long-Term Ecological Research
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  • 17
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    Herbivore absence can shift dry heath tundra from carbon source to sink during peak growing season (2021)
    IOP Publishing
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    Publication Date: 2022-05-26
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Min, E., Wilcots, M. E., Naeem, S., Gough, L., McLaren, J. R., Rowe, R. J., Rastetter, E. B., Boelman, N. T., & Griffin, K. L. Herbivore absence can shift dry heath tundra from carbon source to sink during peak growing season. Environmental Research Letters, 16(2), (2021): 024027, https://doi.org/10.1088/1748-9326/abd3d0.
    Description: In arctic tundra, large and small mammalian herbivores have substantial impacts on the vegetation community and consequently can affect the magnitude of carbon cycling. However, herbivores are often absent from modern carbon cycle models, partly because relatively few field studies focus on herbivore impacts on carbon cycling. Our objectives were to quantify the impact of 21 years of large herbivore and large and small herbivore exclusion on carbon cycling during peak growing season in a dry heath tundra community. When herbivores were excluded, we observed a significantly greater leaf area index as well as greater vascular plant abundance. While we did not observe significant differences in deciduous dwarf shrub abundance across treatments, evergreen dwarf shrub abundance was greater where large and small herbivores were excluded. Both foliose and fruticose lichen abundance were higher in the large herbivore, but not the small and large herbivore exclosures. Net ecosystem exchange (NEE) likewise indicated the highest carbon uptake in the exclosure treatments and lowest uptake in the control (CT), suggesting that herbivory decreased the capacity of dry heath tundra to take up carbon. Moreover, our calculated NEE for average light and temperature conditions for July 2017, when our measurements were taken, indicated that the tundra was a carbon source in CT, but was a carbon sink in both exclosure treatments, indicating removal of grazing pressure can change the carbon balance of dry heath tundra. Collectively, these findings suggest that herbivore absence can lead to changes in plant community structure of dry heath tundra that in turn can increase its capacity to take up carbon.
    Description: The authors would like to thank Jess Steketee, Austin Roy, Matthew Suchocki, Ruby An, Cody Lane and the Arctic LTER (NSF Grant No. 1637459) for maintaining the long-term herbivore exclosure experiment. This work was supported by funding from the NSF (Grant Nos. OPP-1603677 to J R M, OPP-1603760 to L G, OPP-1603654 to R J R, OPP-1603560 to E R, OPP-1603777 to N B and K L G). We also acknowledge financial support for Megan Wilcots from the Department of Ecology, Evolution, and Environmental Biology at Columbia University.
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  • 18
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    Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire : an expert assessment (2016)
    IOPScience
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    Publication Date: 2022-05-26
    Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Environmental Research Letters 11 (2016): 034014, doi:10.1088/1748-9326/11/3/034014.
    Description: As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%–85% of permafrost carbon release can still be avoided if human emissions are actively reduced.
    Description: This work was supported by the National Science Foundation ARCSS program and Vulnerability of Permafrost Carbon Research Coordination Network (grants OPP-0806465, OPP-0806394, and 955713) with additional funding from SITES (Swedish Science Foundation), Future Forest (Mistra), and a Marie Curie International Reintegration Grant (TOMCAR-Permafrost #277059) within the 7th European Community Framework Programme.
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