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  • 1
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    Burn severity influences postfire CO2 exchange in Arctic tundra (2011)
    Ecological Society of America
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    Publication Date: 2022-05-25
    Description: Author Posting. © Ecological Society of America, 2011. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Applications 21 (2011): 477–489, doi:10.1890/10-0255.1.
    Description: Burned landscapes present several challenges to quantifying landscape carbon balance. Fire scars are composed of a mosaic of patches that differ in burn severity, which may influence postfire carbon budgets through damage to vegetation and carbon stocks. We deployed three eddy covariance towers along a burn severity gradient (i.e., severely burned, moderately burned, and unburned tundra) to monitor postfire net ecosystem exchange of CO2 (NEE) within the large 2007 Anaktuvuk River fire scar in Alaska, USA, during the summer of 2008. Remote sensing data from the MODerate resolution Imaging Spectroradiometer (MODIS) was used to assess the spatial representativeness of the tower sites and parameterize a NEE model that was used to scale tower measurements to the landscape. The tower sites had similar vegetation and reflectance properties prior to the Anaktuvuk River fire and represented the range of surface conditions observed within the fire scar during the 2008 summer. Burn severity influenced a variety of surface properties, including residual organic matter, plant mortality, and vegetation recovery, which in turn determined postfire NEE. Carbon sequestration decreased with increased burn severity and was largely controlled by decreases in canopy photosynthesis. The MODIS two-band enhanced vegetation index (EVI2) monitored the seasonal course of surface greenness and explained 86% of the variability in NEE across the burn severity gradient. We demonstrate that understanding the relationship between burn severity, surface reflectance, and NEE is critical for estimating the overall postfire carbon balance of the Anaktuvuk River fire scar.
    Description: This work was supported by NSF grants #0632139 (OPP-AON), #0808789 (OPP-ARCSS SGER), #0829285 (DEB-NEON SGER), and #0423385 (DEBLTER) to the Marine Biological Laboratory.
    Keywords: Anaktuvuk River fire ; Alaska, USA ; Burn severity ; EVI2 (MODIS two-band enhanced vegetation index) ; NBR (normalized burn ratio) ; NEE (net ecosystem exchange of CO2) ; Tundra ; Upscaling
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Past, present, and future roles of long-term experiments in the LTER Network (2012)
    American Institute of Biological Sciences
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Institute of Biological Sciences, 2012. This article is posted here by permission of American Institute of Biological Sciences for personal use, not for redistribution. The definitive version was published in BioScience 62 (2012): 377-389, doi:10.1525/bio.2012.62.4.9.
    Description: The US National Science Foundation—funded Long Term Ecological Research (LTER) Network supports a large (around 240) and diverse portfolio of long-term ecological experiments. Collectively, these long-term experiments have (a) provided unique insights into ecological patterns and processes, although such insight often became apparent only after many years of study; (b) influenced management and policy decisions; and (c) evolved into research platforms supporting studies and involving investigators who were not part of the original design. Furthermore, this suite of long-term experiments addresses, at the site level, all of the US National Research Council's Grand Challenges in Environmental Sciences. Despite these contributions, we argue that the scale and scope of global environmental change requires a more-coordinated multisite approach to long-term experiments. Ideally, such an approach would include a network of spatially extensive multifactor experiments, designed in collaboration with ecological modelers that would build on and extend the unique context provided by the LTER Network.
    Description: 2012-10-01
    Keywords: Climate change ; Global change ; Long-term research ; LTER Network ; Multifactor experiments
    Repository Name: Woods Hole Open Access Server
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  • 3
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    Response of dark respiration to temperature in Eriophorum vaginatum from a 30-year-old transplant experiment in Alaska (2013)
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Taylor & Francis for personal use, not for redistribution. The definitive version was published in Plant Ecology & Diversity 6 (2013): 377-381, doi:10.1080/17550874.2012.729618.
    Description: Background: In the Arctic region, temperature increases are expected to be greater under anticipated climate change than the global average. Understanding how dark respiration (Rd) of common Arctic plant species acclimates to changes in the environment is therefore important for predicting changes to the Arctic carbon balance. Aims: The aim of this study is to investigate the influence of genotype and growing environment on Rd, the temperature response (Q10) of Rd, and foliar N (Nleaf) of the Arctic sedge Eriophorum vaginatum. Methods: We measured Rd, and determined its Q10 and Nleaf of E. vaginatum populations that were reciprocally transplanted 30 years previously along a latitudinal transect of 370 km in northern Alaska. Results: Rd and Q10 did not differ among populations (ecotypes) of E. vaginatum, but the local environment had a significant effect on both variables. Rd as well as Nleaf was higher in northern, colder sites, while Q10 was lower there. Conclusions: Rd in the different populations of E. vaginatum is a very plastic trait and controlled by growing environment, as is Nleaf. The lower Q10 values in the northern sites were most likely a consequence of substrate inhibition of Rd at higher temperatures.
    Description: Funding for this research was provided by National Science Foundation grant ARC-0908936, with additional support from NSF-OPP 0807639.
    Keywords: Arctic ; Common garden experiment ; Eriophorum vaginatum ; Leaf respiration ; Leaf nitrogen ; Q10 ; Reciprocal transplant
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Contrasting effects of long term versus short-term nitrogen addition on photosynthesis and respiration in the Arctic (2013)
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    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Plant Ecology 214 (2013): 1273-1286, doi:10.1007/s11258-013-0250-6.
    Description: We examined the effects of short (〈1 to 4 years) and long-term (22 years) nitrogen (N) and/or phosphorus (P) addition on the foliar CO2 exchange parameters of the arctic species Betula nana and Eriophorum vaginatum in northern Alaska. Measured variables included: the carboxylation efficiency of Rubisco (Vcmax), electron transport capacity (Jmax), dark respiration (Rd), chlorophyll a and b content (Chl), and total foliar N (N). For both B. nana and E. vaginatum, foliar N increased by 20-50% as a consequence of 1 to 22 years of fertilisation, respectively, and for B. nana foliar Nincrease was consistent throughout the whole canopy. However, despite this large increase in foliar N, no significant changes in Vcmax and Jmax were observed. In contrast, Rd was significantly higher (〉25%) in both species after 22 years of N addition, but not in the shorter-term treatments. Surprisingly, Chl only increased in both species the first year of fertilisation (i.e. the first season of nutrients applied), but not in the longer-term treatments. These results imply that: 1) Under current (low) N availability, these Arctic species either already optimize their photosynthetic capacity per leaf area, or are limited by other nutrients; 2) Observed increases in Arctic NEE and GPP with increased nutrient availability are caused by structural changes like increased leaf area index, rather than increased foliar photosynthetic capacity and 3) Short-term effects (1-4 years) of nutrient addition cannot always be extrapolated to a larger time scale, which emphasizes the importance of long-term ecological experiments.
    Description: This work was funded by NSF grants from the division of Environmental Biology (Arctic LTER Project) and from the office of Polar Programs (Arctic Natural Sciences, Arctic Systems Science).
    Keywords: Nitrogen use efficiency ; Fertilisation ; LTER ; Alaska ; Chlorophyll ; Canopy ; Leaf mass per area
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Home site advantage in two long-lived arctic plant species : results from two 30-year reciprocal transplant studies (2012)
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    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2012. 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 Journal of Ecology 100 (2012): 841-851, doi:10.1111/j.1365-2745.2012.01984.x.
    Description: Reciprocal transplant experiments designed to quantify genetic and environmental effects on phenotype are powerful tools for the study of local adaptation. For long-lived species, especially those in habitats with short growing seasons, however, the cumulative effects of many years in novel environments may be required for fitness differences and phenotypic changes to accrue. We returned to two separate reciprocal transplant experiments thirty years after their initial establishment in interior Alaska to ask whether patterns of differentiation observed in the years immediately following transplant have persisted. We also asked whether earlier hypotheses about the role of plasticity in buffering against the effects of selection on foreign genotypes were supported. We censused survival and flowering in three transplant gardens created along a snowbank gradient for a dwarf shrub (Dryas octopetala) and six gardens created along a latitudinal gradient for a tussock-forming sedge (Eriophorum vaginatum). For both species, we used an analysis of variance to detect fitness advantages for plants transplanted back into their home site relative to those transplanted into foreign sites. For D. octopetala, the original patterns of local adaptation observed in the decade following transplant appeared even stronger after three decades, with the complete elimination of foreign ecotypes in both fellfield and snowbed environments. For E. vaginatum, differential survival of populations was not evident 13 years after transplant, but was clearly evident 17 years later. There was no evidence that plasticity was associated with increased survival of foreign populations in novel sites for either D. octopetala or E. vaginatum. Synthesis. We conclude that local adaptation can be strong, but nevertheless remain undetected or underestimated in short-term experiments. Such genetically-based population differences limit the ability of plant populations to respond to a changing climate.
    Description: Funding for this research was provided by National Science Foundation grant ARC-0908936 with additional support from NSF-BSR-9024188.
    Keywords: Adaptation ; Dryas octopetala ; Ecological genetics and ecogenomics ; Eriophorum vaginatum ; Genetic differentiation ; Phenotypic plasticity ; Tussock tundra
    Repository Name: Woods Hole Open Access Server
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  • 6
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    The response of Arctic vegetation and soils following an unusually severe tundra fire (2013)
    The Royal Society
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    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 Philosophical Transactions of the Royal Society B Biological Sciences 368 (2013): 20120490, doi:10.1098/rstb.2012.0490.
    Description: Fire causes dramatic short-term changes in vegetation and ecosystem function, and may promote rapid vegetation change by creating recruitment opportunities. Climate warming likely will increase the frequency of wildfire in the Arctic, where it is not common now. In 2007, the unusually severe Anaktuvuk River fire burned 1039 km2 of tundra on Alaska's North Slope. Four years later, we harvested plant biomass and soils across a gradient of burn severity, to assess recovery. In burned areas, above-ground net primary productivity of vascular plants equalled that in unburned areas, though total live biomass was less. Graminoid biomass had recovered to unburned levels, but shrubs had not. Virtually all vascular plant biomass had resprouted from surviving underground parts; no non-native species were seen. However, bryophytes were mostly disturbance-adapted species, and non-vascular biomass had recovered less than vascular plant biomass. Soil nitrogen availability did not differ between burned and unburned sites. Graminoids showed allocation changes consistent with nitrogen stress. These patterns are similar to those seen following other, smaller tundra fires. Soil nitrogen limitation and the persistence of resprouters will likely lead to recovery of mixed shrub–sedge tussock tundra, unless permafrost thaws, as climate warms, more extensively than has yet occurred.
    Description: This work was supported by NSF (no. OPP-0632264) and NSF (no. OPP-1107892) to M. S. Bret-Harte, NSF (no. OPP-0856853) to G. R. Shaver and NSF (no. OPP-6737545) to M. C. Mack.
    Keywords: Alaskan tussock tundra ; Fire ; Vegetation recovery ; Permafrost ; Climate change ; Soil N availability
    Repository Name: Woods Hole Open Access Server
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  • 7
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    Nitrogen dynamics in a small arctic watershed: retention and downhill movement of 15N (2010)
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    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Monographs 80 (2010): 331-351, doi:10.1890/08-0773.1.
    Description: We examined short- and long-term nitrogen (N) dynamics and availability along an arctic hillslope in Alaska, USA, using stable isotope of nitrogen (15N), as a tracer. Tracer levels of 15NH4+ were sprayed once onto the tundra at six sites in four tundra types; heath (crest), tussock with high and low water flux (mid- and foot-slope), and wet sedge (riparian). 15N in vegetation and soil was monitored to estimate retention and loss over a 3-yr period. Nearly all 15NH4+ was immediately retained in the surface moss-detritus-plant layer and 〉 57 % of the 15N added remained in this layer at the end of the second year. Organic soil was the second largest 15N sink. By the end of the third growing season, the moss-detritus-plant layer and organic soil combined retained ≥ 87 % of the 15N added except at the mid-slope site with high water flux, where recovery declined to 68 %. At all sites, non-extractable and non-labile-N pools were the principal sinks for added 15N in the organic soil. Hydrology played an important role in downslope movement of dissolved 15N. Crest and mid-slope with high water flux sites were most susceptible to 15N losses via leaching perhaps because of deep permeable mineral soil (crest) and high water flow (mid-slope with high water flux). Late spring melt-season also resulted in downslope dissolved-15N losses, perhaps because of an asynchrony between N release into melt water and soil immobilization capacity. We conclude that separation of the rooting zone from the strong sink for incoming N in the moss detritus-plant layer, rapid incorporation of new N into relatively recalcitrant soil-N pools within the rooting zone, and leaching loss from the upper hillslope would all contribute to the strong N limitation of this ecosystem. An extended snow-free season and deeper depth of thaw under warmer climate may significantly alter current N dynamics in this arctic ecosystem.
    Description: Funding was provided by NSF grant #0444592. Additional support was provided by Toolik Field Station Long Term Ecological Research program, funded by National Science Foundation, Office of Polar Programs.
    Keywords: 15NH4 ; Arctic tundra watershed ; Total dissolved N ; Downhill transport of N ; Hydrolysable amino acids ; Hydrolysable amino sugars ; Mosses ; N dynamics ; N immobilization ; N leaching ; N limitation ; Snowmelt
    Repository Name: Woods Hole Open Access Server
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  • 8
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    Depleted 15N in hydrolysable-N of arctic soils and its implication for mycorrhizal fungi–plant interaction (2010)
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    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Biogeochemistry 97 (2009): 183-194, doi:10.1007/s10533-009-9365-1.
    Description: Uptake of nitrogen (N) via root-mycorrhizal associations accounts for a significant portion of total N supply to many vascular plants. Using stable isotope ratios (δ15N) and the mass balance among N pools of plants, fungal tissues, and soils, a number of efforts have been made in recent years to quantify the flux of N from mycorrhizal fungi to host plants. Current estimates of this flux for arctic tundra ecosystems rely on the untested assumption that the δ15N of labile organic N taken up by the fungi is approximately the same as the δ15N of bulk soil. We report here hydrolysable amino acids are more depleted in 15N relative to hydrolysable ammonium and amino sugars in arctic tundra soils near Toolik Lake, Alaska, USA. We demonstrate, using a case study, that recognizing the depletion in 15N for hydrolysable amino acids (δ15N = -5.6 ‰ on average) would alter recent estimates of N flux between mycorrhizal fungi and host plants in an arctic tundra ecosystem.
    Description: This study was funded by NSF-DEB-0423385and NSF-DEB 0444592. Additional support was provided by Arctic Long Term Ecological Research program, funded by National Science Foundation, Division of Environmental Biology.
    Keywords: N-15 ; Arctic tundra ; Mycorrhizal fungi ; Decomposition ; Hydrolysable amino acids ; Plant-fungal interaction
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  • 9
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    Processing arctic eddy-flux data using a simple carbon-exchange model embedded in the ensemble Kalman filter (2011)
    Ecological Society of America
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    Publication Date: 2022-05-25
    Description: Author Posting. © Ecological Society of America, 2010. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Applications 20 (2010): 1285–1301, doi:10.1890/09-0876.1.
    Description: Continuous time-series estimates of net ecosystem carbon exchange (NEE) are routinely made using eddy covariance techniques. Identifying and compensating for errors in the NEE time series can be automated using a signal processing filter like the ensemble Kalman filter (EnKF). The EnKF compares each measurement in the time series to a model prediction and updates the NEE estimate by weighting the measurement and model prediction relative to a specified measurement error estimate and an estimate of the model-prediction error that is continuously updated based on model predictions of earlier measurements in the time series. Because of the covariance among model variables, the EnKF can also update estimates of variables for which there is no direct measurement. The resulting estimates evolve through time, enabling the EnKF to be used to estimate dynamic variables like changes in leaf phenology. The evolving estimates can also serve as a means to test the embedded model and reconcile persistent deviations between observations and model predictions. We embedded a simple arctic NEE model into the EnKF and filtered data from an eddy covariance tower located in tussock tundra on the northern foothills of the Brooks Range in northern Alaska, USA. The model predicts NEE based only on leaf area, irradiance, and temperature and has been well corroborated for all the major vegetation types in the Low Arctic using chamber-based data. This is the first application of the model to eddy covariance data. We modified the EnKF by adding an adaptive noise estimator that provides a feedback between persistent model data deviations and the noise added to the ensemble of Monte Carlo simulations in the EnKF. We also ran the EnKF with both a specified leaf-area trajectory and with the EnKF sequentially recalibrating leaf-area estimates to compensate for persistent model-data deviations. When used together, adaptive noise estimation and sequential recalibration substantially improved filter performance, but it did not improve performance when used individually. The EnKF estimates of leaf area followed the expected springtime canopy phenology. However, there were also diel fluctuations in the leaf-area estimates; these are a clear indication of a model deficiency possibly related to vapor pressure effects on canopy conductance.
    Description: This material is based upon work supported by the U.S. National Science Foundation under grants OPP-0352897, DEB-0423385, DEB-0439620, DEB-0444592, and OPP- 0632139.
    Keywords: Alaska, USA ; Data assimilation ; Ecosystem carbon balance ; Ecosystem models ; Eddy covariance ; Kalman filter ; Net ecosystem carbon exchange
    Repository Name: Woods Hole Open Access Server
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  • 10
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    Vegetation shifts observed in arctic tundra 1.5 decades after fire (2012)
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    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Taylor & Francis for personal use, not for redistribution. The definitive version was published in Remote Sensing Letters 3 (2012): 729-736, doi: 10.1080/2150704X.2012.676741.
    Description: With anticipated climate change, tundra fires are expected to occur more frequently in the future, but data on the longer term effects of fire on tundra vegetation composition are scarce. This study therefore addresses changes in vegetation structure that have persisted for 17 years after a tundra fire on the North Slope of Alaska. Fire-related shifts in vegetation composition were assessed from remote sensing imagery and ground observations of the burn scar and an adjacent control site. Early-season remotely sensed imagery from the burn scar exhibits a low vegetation index compared to the control site, while the late-season signal is slightly higher. The range and maximum vegetation index is greater in the burn scar, although the mean annual values do not differ among the sites. Ground observations revealed a greater abundance of graminoid species and an absence of Betula nana in the post-fire tundra sites, which is a likely explanation for the spectral differences observed in the remotely sensed imagery. Additional differences in vegetation composition in the burn scar include less moss cover and a greater cover of herbaceous species. The partial replacement of tundra by graminoid-dominated ecosystems has been predicted by the ALFRESCO model of disturbance, climate, and vegetation succession.
    Description: 2013-04-19
    Keywords: Fire ; Tundra ; North Slope ; Grass ; NDVI ; GIMMS ; Vegetation shift ; Succession
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