M. Syndonia Bret-harte | University of Alaska Fairbanks (original) (raw)
Papers by M. Syndonia Bret-harte
AGU Fall Meeting Abstracts, Dec 1, 2013
AGU Fall Meeting Abstracts, Dec 18, 2014
<p>The terrestrial Arctic is subject to extreme climatic changes including increase... more <p>The terrestrial Arctic is subject to extreme climatic changes including increases in temperature and changes in precipitation patterns. At the heart of these developments lie changes in the land surface energy budget (SEB), which couples important earth system processes including the carbon and water cycles. However, despite the importance of the SEB, uncertainties in predictions of high-latitude SEBs persist, specifically for the SEB-components sensible and latent heat fluxes.</p><p>These uncertainties have in part been attributed to insufficient representation of Arctic vegetation in land surface components of Earth system models. However, to date, a quantitative understanding of the relative importance of Arctic vegetation for the SEB compared to other important SEB-drivers is missing.</p><p>Here we harmonize <em>in situ</em> observations from regional and global monitoring networks and provide a quantitative, circumpolar assessment of the magnitude and seasonality of observed SEB-components over treeless land >60&#176;N in the time period 1994-2021. Using a variance partitioning analysis, we identify vegetation type as an important predictor for SEB-components during Arctic summer, in comparison with other SEB-drivers including meteorological conditions, snow cover duration, topography, and permafrost extent. Differences among vegetation types are especially high for mean summer magnitudes of sensible and latent heat fluxes, where they reach up to 8% and 9% of the potential incoming shortwave radiation, respectively. Our comparison with SEB-observations across glacier sites show that importantly, these differences among vegetation types are of similar magnitude as differences between vegetation and glacier surfaces. In our seasonality synthesis we find that net radiation (Rnet), sensible (H) and ground (G) heat fluxes have an unexpected early start of summer-regime (when daily mean values > 0 Wm<sup>-2</sup>), preceding the end of snowmelt by 56, 33, and 39 days, respectively. An elevated variability among vegetation types in the estimated onset (and end) dates of net positive Rnet and H (and G) relative to snowmelt (and onset) date, suggests that vegetation types differentially affect the distribution, trapping and density of snow cover, with important consequences for the cumulative energy fluxes from and to the atmosphere. Finally, we find that long-term, year-round SEB data series of Arctic tundra are still very scarce, especially in the Arctic regions of Eastern Canada and Western Russia.</p><p>In conclusion, we provide quantitative evidence of the importance of vegetation types for predicting Arctic surface energy budgets at circumpolar scale. We highlight that substantial differences among vegetation types are not only found for mean magnitudes but also the seasonality of surface energy fluxes. We contend that the land surface components of Earth system models should account for Arctic vegetation types to improve climate projections in the rapidly changing terrestrial Arctic.</p>
Above ground plant and below ground stem biomass were measured in 2011 from three sites at and ar... more Above ground plant and below ground stem biomass were measured in 2011 from three sites at and around the Anaktuvuk River Burn: severely burned, moderately burned and unburned. These samples were analyzed for carbon and nitrogen concentrations.
In contribution to the Arctic Observing Network, the researchers have established two observatori... more In contribution to the Arctic Observing Network, the researchers have established two observatories of landscape-level carbon, water and energy balances at Imnaviat Creek, Alaska and at Pleistocene Park near Cherskii, Russia. These will form part of a network of observatories with Abisko (Sweden), Zackenburg (Greenland) and a location in the Canadian High Arctic which will provide further data points as part of the International Polar Year. This particular part of the project focuses on simultaneous measurements of carbon, water and energy fluxes of the terrestrial landscape at hourly, daily, seasonal and multi-year time scales. These are the major regulatory drivers of the Arctic climate system and form key linkages and feedbacks between the land surface, the atmosphere and the oceans. We will provide a comprehensive description of the state of the regional Arctic system with respect to these variables, its overall regulation and controlling features and its interaction with the gl...
Agricultural and Forest Meteorology, 2021
Abstract Vegetation indices derived from solar and photosynthetically active radiation (PAR) sens... more Abstract Vegetation indices derived from solar and photosynthetically active radiation (PAR) sensors (i.e. radiation derived) have been under-utilized in inferring ecosystem function, despite measurement capability at hundreds of sites. This under-utilization may be attributed to reported mismatches among the seasonality of radiation- and satellite-derived vegetation indices and canopy photosynthesis; herein referred to as measurement biases. Here biases in radiation derived reflectance and vegetation indices were assessed using a decadal record of satellite and ground based spectroradiometer data, ecosystem phenology and CO2 fluxes, and radiation derived vegetation indices (i.e. the Normalized Difference Vegetation Index [NDVI], the two band Enhanced Vegetation Index [EVI2]) from a high latitude tundra site (i.e. Imnaviat). At Imnaviat, we found poor correspondence between the three types of reflectance and vegetation indices, especially during the latter part of the growing season. Radiation derived vegetation indices resulted in incorrect estimates of phenological timing of up to a month and poor relationships with canopy photosynthesis (i.e. Gross Ecosystem Exchange (GEE)). These mismatches were attributed to solar position (i.e. solar zenith and azimuth angle) and a method, based on the diel visible and near-infrared albedo variation, was developed to improve the performance of the vegetation indices. The ability of radiation derived vegetation indices to infer GEE and phenological dates drastically improved once radiation derived vegetation indices were corrected for solar position associated biases at Imnaviat. Moreover, radiation derived vegetation indices became better aligned with MODerate resolution Imaging Spectroradiometer (MODIS) satellite estimates after solar position associated biases were corrected at Imnaviat and at 25 Fluxnet sites (~90 site years) across North America. Corrections developed here provide a way forward in understanding daily ecosystem function or filling large gaps in eddy covariance data at a significant number of Fluxnet sites.
Ecosphere, 2021
Future warming may alter plant stress at high‐elevation treelines and forests, thereby changing p... more Future warming may alter plant stress at high‐elevation treelines and forests, thereby changing plant–plant interactions. The relative importance of competition and facilitation may depend on the degree of resource or physical stress. According to the stress gradient hypothesis (SGH), physical stress on trees is more important at cold high elevations where facilitation predominates, and less important at low elevations where competition is the main interaction. Our goals were to investigate whether plant–plant interactions along elevational gradients corresponded to those predicted by the SGH, and to assess the effects of increasing temperatures on the growth of conifer seedlings in tundra–forest ecosystems in interior Alaska, USA. We established sites along two elevational gradients: one in tundra (four sites, 550–1170 m) and one in forest (three sites, 210–760 m). A field warming and neighbor removal experiment was conducted using transplanted seedlings of white spruce (Picea glau...
Frontiers in Earth Science, 2021
Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback im... more Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback implications for global climate. Stable isotopes (δ18O, δ2H, d-excess) are valuable tracers for constraining water cycle and climate processes through space and time. Yet, the paucity of well-resolved Arctic isotope data preclude an empirically derived understanding of the hydrologic changes occurring today, in the deep (geologic) past, and in the future. To address this knowledge gap, the Pan-Arctic Precipitation Isotope Network (PAPIN) was established in 2018 to coordinate precipitation sampling at 19 stations across key tundra, subarctic, maritime, and continental climate zones. Here, we present a first assessment of rainfall samples collected in summer 2018 (n = 281) and combine new isotope and meteorological data with sea ice observations, reanalysis data, and model simulations. Data collectively establish a summer Arctic Meteoric Water Line where δ2H = 7.6⋅δ18O–1.8 (r2 = 0.96, p <...
Past efforts to synthesize and quantify the magnitude and change in carbon dioxide (CO 2) fluxes ... more Past efforts to synthesize and quantify the magnitude and change in carbon dioxide (CO 2) fluxes in terrestrial ecosystems across the rapidly warming Arctic-boreal zone (ABZ) have provided valuable information but were limited in their geographical and temporal coverage. Furthermore, these efforts have been based on data aggregated over varying time periods, often with only minimal site ancillary data, thus limiting their potential to be used in large-scale carbon budget assessments. To bridge these gaps, we developed a standardized monthly database of Arctic-boreal CO 2 fluxes (ABCflux) that aggregates in situ measurements of terrestrial net ecosystem CO 2 exchange and its derived partitioned component fluxes: gross primary productivity and
Biogeosciences, 2020
Rapid Arctic warming, a lengthening growing season, and the increasing abundance of biogenic vola... more Rapid Arctic warming, a lengthening growing season, and the increasing abundance of biogenic volatileorganic-compound-emitting shrubs are all anticipated to increase atmospheric biogenic volatile organic compounds (BVOCs) in the Arctic atmosphere, with implications for atmospheric oxidation processes and climate feedbacks. Quantifying these changes requires an accurate understanding of the underlying processes driving BVOC emissions in the Arctic. While boreal ecosystems have been widely studied, little attention has been paid to Arctic tundra environments. Here, we report terpenoid (isoprene, monoterpenes, and sesquiterpenes) ambient mixing ratios and emission rates from key dominant vegetation species at Toolik Field Station (TFS; 68 • 38 N, 149 • 36 W) in northern Alaska during two back-to-back field campaigns (summers of 2018 and 2019) covering the entire growing season. Isoprene ambient mixing ratios observed at TFS fell within the range of values reported in the Eurasian taiga (0-500 parts per trillion by volume-pptv), while monoterpene and sesquiterpene ambient mixing ratios were respectively close to and below the instrumental quantification limit (∼ 2 pptv). Isoprene surface emission rates ranged from 0.2 to 2250 µgC m −2 h −1 (mean of 85 µgC m −2 h −1) and monoterpene emission rates remained, on average, below 1 µgC m −2 h −1 over the course of the study. We further quantified the temperature dependence of isoprene emissions from local vegetation, including Salix spp. (a known isoprene emitter), and compared the results to predictions from the Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1). Our observations suggest a 180 %-215 % emission increase in response to a 3-4 • C warming, and the MEGAN2.1 temperature algorithm exhibits a close fit with observations for enclosure temperatures in the 0-30 • C range. The data presented here provide a baseline for investigating future changes in the BVOC emission potential of the under-studied Arctic tundra environment.
Representing peatlands in global Earth System Models (ESMs) is a major challenge, but a crucial o... more Representing peatlands in global Earth System Models (ESMs) is a major challenge, but a crucial one since peatlands represent a significant component of the global carbon cycle.
Environmental Research Letters, 2020
Rising atmospheric CO2 concentration ([CO2]) enhances photosynthesis and reduces transpiration at... more Rising atmospheric CO2 concentration ([CO2]) enhances photosynthesis and reduces transpiration at the leaf, ecosystem, and global scale via the CO2 fertilization effect. The CO2 fertilization effect is among the most important processes for predicting the terrestrial carbon budget and future climate, yet it has been elusive to quantify. For evaluating the CO2 fertilization effect on land photosynthesis and transpiration, we developed a technique that isolated this effect from other confounding effects, such as changes in climate, using a noisy time series of observed land-atmosphere CO2 and water vapor exchange. Here, we evaluate the magnitude of this effect from 2000 to 2014 globally based on constraint optimization of gross primary productivity (GPP) and evapotranspiration in a canopy photosynthesis model over 104 global eddy-covariance stations. We found a consistent increase of GPP (0.138 ± 0.007% ppm−1; percentile per rising ppm of [CO2]) and a concomitant decrease in transpira...
Journal of Geophysical Research: Biogeosciences, 2017
Eastern Siberia Russia is currently experiencing a distinct and unprecedented rate of warming. Th... more Eastern Siberia Russia is currently experiencing a distinct and unprecedented rate of warming. This change is particularly important given the large amounts of carbon stored in the yedoma permafrost soils that become vulnerable to thaw and release under warming. Data from this region pertaining to year-round carbon, water, and energy fluxes are scarce, particularly in sensitive ecotonal ecosystems near latitudinal treeline, as well as those already impacted by permafrost thaw. Here we investigated the interannual and seasonal carbon dioxide, water, and energy dynamics at an ecotonal forested site and a disturbed thermokarst-impacted site. The ecotonal site was approximately neutral in terms of CO 2 uptake/release, while the disturbed site was either a source or neutral. Our data suggest that high rates of plant productivity during the growing season at the disturbed site may, in part, counterbalance higher rates of respiration during the cold season compared to the ecotonal site. We also found that the ecotonal site was sensitive to the timing of the freezeup of the soil active layer in fall, releasing more CO 2 when freezeup occurred later. Both sites showed a negative water balance, although the ecotonal site appeared more sensitive to dry conditions. Water use efficiency at the ecotonal site was lower during warmer summers. Overall, these Siberian measurements indicate ecosystem sensitivity to warmer conditions during the fall and to drier conditions during the growing season and provide a better understanding of ecosystem response to climate in a part of the circumpolar Arctic where current knowledge is weakest. Plain Language Summary As Siberia warms, the frozen soils known as permafrost start to thaw, causing an irregular terrain of pits and mounds called thermokarst. Large amounts of carbon in Siberian soils have been locked away in permafrost for thousands of years, becoming vulnerable to release under thaw and thermokarst formation. This will potentially result in large amounts of additional greenhouse gases in the atmosphere, amplifying climate warming. We examined carbon dioxide (CO 2) fluxes over multiple years at two sites in northeastern Siberia, an ecotonal site that lies at the transition between the boreal forest and tundra biomes, and a site with thermokarst. We found that the ecotonal site is carbon neutral, consuming the same amount of CO 2 as it takes up from the atmosphere. However, this site releases greater amounts of CO 2 in years when soil freeze occurred later, which is expected to become common in the future. The thermokarst site released significantly more CO 2 , but it was also marked by greater plant growth, thereby offsetting some of the CO 2 lost. Due, in part, to a lack of data, models represent terrestrial ecosystem carbon dynamics in Siberia poorly and do not take into changes in carbon cycling that occur with thermokarst formation. warming scenario, with the winter and spring seasons contributing the most to this warming (Miao et al., 2014). Covering~3,125,000 km 2 , this region is underlain by extensive carbon-rich permafrost soils that are already exhibiting widespread thaw as temperatures warm (Romanovsky et al., 2010).
Functional Ecology, 2019
In arctic environments, drifting snow around large shrub patches during winter may enhance growth... more In arctic environments, drifting snow around large shrub patches during winter may enhance growth by insulating the soil and facilitating overwinter nutrient turnover, leading to increased summer plant growth and potentially widespread increases in shrub cover. To determine whether snow enhances growth of arctic plants, we examined the effect of 6 years of added snow on plant biomass allocation and growth of nine common vascular plant species collected in 2010 and 2011 on either side of snowfences established in 2005 across a gradient of shrub biomass and productivity near Toolik Lake, Alaska. The deciduous shrub Salix pulchra responded most positively to added snow showing an 88% increase in total ramet biomass, because increased secondary growth allowed plants to support more branches and leaves. Some graminoid species also showed growth increases, especially where they were more abundant and larger than nearby shrub species. Species sharing the same growth strategy (deciduous shr...
Ecology and Evolution, 2019
In Low Arctic tundra, thermal erosion of ice‐rich permafrost soils (thermokarst) has increased in... more In Low Arctic tundra, thermal erosion of ice‐rich permafrost soils (thermokarst) has increased in frequency since the 1980s. Retrogressive thaw slumps (RTS) are thermokarst disturbances forming large open depressions on hillslopes through soil wasting and vegetation displacement. Tall (>0.5 m) deciduous shrubs have been observed in RTS a decade after disturbance. RTS may provide conditions suitable for seedling recruitment, which may contribute to Arctic shrub expansion. We quantified in situ seedling abundance, and size and viability of soil seedbanks in greenhouse trials for two RTS chronosequences near lakes on Alaska's North Slope. We hypothesized recent RTS provide microsites for greater recruitment than mature RTS or undisturbed tundra. We also hypothesized soil seedbanks demonstrate quantity–quality trade‐offs; younger seedbanks contain smaller numbers of mostly viable seed that decrease in viability as seed accumulates over time. We found five times as many seedlings ...
AGU Fall Meeting Abstracts, Dec 1, 2013
AGU Fall Meeting Abstracts, Dec 18, 2014
<p>The terrestrial Arctic is subject to extreme climatic changes including increase... more <p>The terrestrial Arctic is subject to extreme climatic changes including increases in temperature and changes in precipitation patterns. At the heart of these developments lie changes in the land surface energy budget (SEB), which couples important earth system processes including the carbon and water cycles. However, despite the importance of the SEB, uncertainties in predictions of high-latitude SEBs persist, specifically for the SEB-components sensible and latent heat fluxes.</p><p>These uncertainties have in part been attributed to insufficient representation of Arctic vegetation in land surface components of Earth system models. However, to date, a quantitative understanding of the relative importance of Arctic vegetation for the SEB compared to other important SEB-drivers is missing.</p><p>Here we harmonize <em>in situ</em> observations from regional and global monitoring networks and provide a quantitative, circumpolar assessment of the magnitude and seasonality of observed SEB-components over treeless land >60&#176;N in the time period 1994-2021. Using a variance partitioning analysis, we identify vegetation type as an important predictor for SEB-components during Arctic summer, in comparison with other SEB-drivers including meteorological conditions, snow cover duration, topography, and permafrost extent. Differences among vegetation types are especially high for mean summer magnitudes of sensible and latent heat fluxes, where they reach up to 8% and 9% of the potential incoming shortwave radiation, respectively. Our comparison with SEB-observations across glacier sites show that importantly, these differences among vegetation types are of similar magnitude as differences between vegetation and glacier surfaces. In our seasonality synthesis we find that net radiation (Rnet), sensible (H) and ground (G) heat fluxes have an unexpected early start of summer-regime (when daily mean values > 0 Wm<sup>-2</sup>), preceding the end of snowmelt by 56, 33, and 39 days, respectively. An elevated variability among vegetation types in the estimated onset (and end) dates of net positive Rnet and H (and G) relative to snowmelt (and onset) date, suggests that vegetation types differentially affect the distribution, trapping and density of snow cover, with important consequences for the cumulative energy fluxes from and to the atmosphere. Finally, we find that long-term, year-round SEB data series of Arctic tundra are still very scarce, especially in the Arctic regions of Eastern Canada and Western Russia.</p><p>In conclusion, we provide quantitative evidence of the importance of vegetation types for predicting Arctic surface energy budgets at circumpolar scale. We highlight that substantial differences among vegetation types are not only found for mean magnitudes but also the seasonality of surface energy fluxes. We contend that the land surface components of Earth system models should account for Arctic vegetation types to improve climate projections in the rapidly changing terrestrial Arctic.</p>
Above ground plant and below ground stem biomass were measured in 2011 from three sites at and ar... more Above ground plant and below ground stem biomass were measured in 2011 from three sites at and around the Anaktuvuk River Burn: severely burned, moderately burned and unburned. These samples were analyzed for carbon and nitrogen concentrations.
In contribution to the Arctic Observing Network, the researchers have established two observatori... more In contribution to the Arctic Observing Network, the researchers have established two observatories of landscape-level carbon, water and energy balances at Imnaviat Creek, Alaska and at Pleistocene Park near Cherskii, Russia. These will form part of a network of observatories with Abisko (Sweden), Zackenburg (Greenland) and a location in the Canadian High Arctic which will provide further data points as part of the International Polar Year. This particular part of the project focuses on simultaneous measurements of carbon, water and energy fluxes of the terrestrial landscape at hourly, daily, seasonal and multi-year time scales. These are the major regulatory drivers of the Arctic climate system and form key linkages and feedbacks between the land surface, the atmosphere and the oceans. We will provide a comprehensive description of the state of the regional Arctic system with respect to these variables, its overall regulation and controlling features and its interaction with the gl...
Agricultural and Forest Meteorology, 2021
Abstract Vegetation indices derived from solar and photosynthetically active radiation (PAR) sens... more Abstract Vegetation indices derived from solar and photosynthetically active radiation (PAR) sensors (i.e. radiation derived) have been under-utilized in inferring ecosystem function, despite measurement capability at hundreds of sites. This under-utilization may be attributed to reported mismatches among the seasonality of radiation- and satellite-derived vegetation indices and canopy photosynthesis; herein referred to as measurement biases. Here biases in radiation derived reflectance and vegetation indices were assessed using a decadal record of satellite and ground based spectroradiometer data, ecosystem phenology and CO2 fluxes, and radiation derived vegetation indices (i.e. the Normalized Difference Vegetation Index [NDVI], the two band Enhanced Vegetation Index [EVI2]) from a high latitude tundra site (i.e. Imnaviat). At Imnaviat, we found poor correspondence between the three types of reflectance and vegetation indices, especially during the latter part of the growing season. Radiation derived vegetation indices resulted in incorrect estimates of phenological timing of up to a month and poor relationships with canopy photosynthesis (i.e. Gross Ecosystem Exchange (GEE)). These mismatches were attributed to solar position (i.e. solar zenith and azimuth angle) and a method, based on the diel visible and near-infrared albedo variation, was developed to improve the performance of the vegetation indices. The ability of radiation derived vegetation indices to infer GEE and phenological dates drastically improved once radiation derived vegetation indices were corrected for solar position associated biases at Imnaviat. Moreover, radiation derived vegetation indices became better aligned with MODerate resolution Imaging Spectroradiometer (MODIS) satellite estimates after solar position associated biases were corrected at Imnaviat and at 25 Fluxnet sites (~90 site years) across North America. Corrections developed here provide a way forward in understanding daily ecosystem function or filling large gaps in eddy covariance data at a significant number of Fluxnet sites.
Ecosphere, 2021
Future warming may alter plant stress at high‐elevation treelines and forests, thereby changing p... more Future warming may alter plant stress at high‐elevation treelines and forests, thereby changing plant–plant interactions. The relative importance of competition and facilitation may depend on the degree of resource or physical stress. According to the stress gradient hypothesis (SGH), physical stress on trees is more important at cold high elevations where facilitation predominates, and less important at low elevations where competition is the main interaction. Our goals were to investigate whether plant–plant interactions along elevational gradients corresponded to those predicted by the SGH, and to assess the effects of increasing temperatures on the growth of conifer seedlings in tundra–forest ecosystems in interior Alaska, USA. We established sites along two elevational gradients: one in tundra (four sites, 550–1170 m) and one in forest (three sites, 210–760 m). A field warming and neighbor removal experiment was conducted using transplanted seedlings of white spruce (Picea glau...
Frontiers in Earth Science, 2021
Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback im... more Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback implications for global climate. Stable isotopes (δ18O, δ2H, d-excess) are valuable tracers for constraining water cycle and climate processes through space and time. Yet, the paucity of well-resolved Arctic isotope data preclude an empirically derived understanding of the hydrologic changes occurring today, in the deep (geologic) past, and in the future. To address this knowledge gap, the Pan-Arctic Precipitation Isotope Network (PAPIN) was established in 2018 to coordinate precipitation sampling at 19 stations across key tundra, subarctic, maritime, and continental climate zones. Here, we present a first assessment of rainfall samples collected in summer 2018 (n = 281) and combine new isotope and meteorological data with sea ice observations, reanalysis data, and model simulations. Data collectively establish a summer Arctic Meteoric Water Line where δ2H = 7.6⋅δ18O–1.8 (r2 = 0.96, p <...
Past efforts to synthesize and quantify the magnitude and change in carbon dioxide (CO 2) fluxes ... more Past efforts to synthesize and quantify the magnitude and change in carbon dioxide (CO 2) fluxes in terrestrial ecosystems across the rapidly warming Arctic-boreal zone (ABZ) have provided valuable information but were limited in their geographical and temporal coverage. Furthermore, these efforts have been based on data aggregated over varying time periods, often with only minimal site ancillary data, thus limiting their potential to be used in large-scale carbon budget assessments. To bridge these gaps, we developed a standardized monthly database of Arctic-boreal CO 2 fluxes (ABCflux) that aggregates in situ measurements of terrestrial net ecosystem CO 2 exchange and its derived partitioned component fluxes: gross primary productivity and
Biogeosciences, 2020
Rapid Arctic warming, a lengthening growing season, and the increasing abundance of biogenic vola... more Rapid Arctic warming, a lengthening growing season, and the increasing abundance of biogenic volatileorganic-compound-emitting shrubs are all anticipated to increase atmospheric biogenic volatile organic compounds (BVOCs) in the Arctic atmosphere, with implications for atmospheric oxidation processes and climate feedbacks. Quantifying these changes requires an accurate understanding of the underlying processes driving BVOC emissions in the Arctic. While boreal ecosystems have been widely studied, little attention has been paid to Arctic tundra environments. Here, we report terpenoid (isoprene, monoterpenes, and sesquiterpenes) ambient mixing ratios and emission rates from key dominant vegetation species at Toolik Field Station (TFS; 68 • 38 N, 149 • 36 W) in northern Alaska during two back-to-back field campaigns (summers of 2018 and 2019) covering the entire growing season. Isoprene ambient mixing ratios observed at TFS fell within the range of values reported in the Eurasian taiga (0-500 parts per trillion by volume-pptv), while monoterpene and sesquiterpene ambient mixing ratios were respectively close to and below the instrumental quantification limit (∼ 2 pptv). Isoprene surface emission rates ranged from 0.2 to 2250 µgC m −2 h −1 (mean of 85 µgC m −2 h −1) and monoterpene emission rates remained, on average, below 1 µgC m −2 h −1 over the course of the study. We further quantified the temperature dependence of isoprene emissions from local vegetation, including Salix spp. (a known isoprene emitter), and compared the results to predictions from the Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1). Our observations suggest a 180 %-215 % emission increase in response to a 3-4 • C warming, and the MEGAN2.1 temperature algorithm exhibits a close fit with observations for enclosure temperatures in the 0-30 • C range. The data presented here provide a baseline for investigating future changes in the BVOC emission potential of the under-studied Arctic tundra environment.
Representing peatlands in global Earth System Models (ESMs) is a major challenge, but a crucial o... more Representing peatlands in global Earth System Models (ESMs) is a major challenge, but a crucial one since peatlands represent a significant component of the global carbon cycle.
Environmental Research Letters, 2020
Rising atmospheric CO2 concentration ([CO2]) enhances photosynthesis and reduces transpiration at... more Rising atmospheric CO2 concentration ([CO2]) enhances photosynthesis and reduces transpiration at the leaf, ecosystem, and global scale via the CO2 fertilization effect. The CO2 fertilization effect is among the most important processes for predicting the terrestrial carbon budget and future climate, yet it has been elusive to quantify. For evaluating the CO2 fertilization effect on land photosynthesis and transpiration, we developed a technique that isolated this effect from other confounding effects, such as changes in climate, using a noisy time series of observed land-atmosphere CO2 and water vapor exchange. Here, we evaluate the magnitude of this effect from 2000 to 2014 globally based on constraint optimization of gross primary productivity (GPP) and evapotranspiration in a canopy photosynthesis model over 104 global eddy-covariance stations. We found a consistent increase of GPP (0.138 ± 0.007% ppm−1; percentile per rising ppm of [CO2]) and a concomitant decrease in transpira...
Journal of Geophysical Research: Biogeosciences, 2017
Eastern Siberia Russia is currently experiencing a distinct and unprecedented rate of warming. Th... more Eastern Siberia Russia is currently experiencing a distinct and unprecedented rate of warming. This change is particularly important given the large amounts of carbon stored in the yedoma permafrost soils that become vulnerable to thaw and release under warming. Data from this region pertaining to year-round carbon, water, and energy fluxes are scarce, particularly in sensitive ecotonal ecosystems near latitudinal treeline, as well as those already impacted by permafrost thaw. Here we investigated the interannual and seasonal carbon dioxide, water, and energy dynamics at an ecotonal forested site and a disturbed thermokarst-impacted site. The ecotonal site was approximately neutral in terms of CO 2 uptake/release, while the disturbed site was either a source or neutral. Our data suggest that high rates of plant productivity during the growing season at the disturbed site may, in part, counterbalance higher rates of respiration during the cold season compared to the ecotonal site. We also found that the ecotonal site was sensitive to the timing of the freezeup of the soil active layer in fall, releasing more CO 2 when freezeup occurred later. Both sites showed a negative water balance, although the ecotonal site appeared more sensitive to dry conditions. Water use efficiency at the ecotonal site was lower during warmer summers. Overall, these Siberian measurements indicate ecosystem sensitivity to warmer conditions during the fall and to drier conditions during the growing season and provide a better understanding of ecosystem response to climate in a part of the circumpolar Arctic where current knowledge is weakest. Plain Language Summary As Siberia warms, the frozen soils known as permafrost start to thaw, causing an irregular terrain of pits and mounds called thermokarst. Large amounts of carbon in Siberian soils have been locked away in permafrost for thousands of years, becoming vulnerable to release under thaw and thermokarst formation. This will potentially result in large amounts of additional greenhouse gases in the atmosphere, amplifying climate warming. We examined carbon dioxide (CO 2) fluxes over multiple years at two sites in northeastern Siberia, an ecotonal site that lies at the transition between the boreal forest and tundra biomes, and a site with thermokarst. We found that the ecotonal site is carbon neutral, consuming the same amount of CO 2 as it takes up from the atmosphere. However, this site releases greater amounts of CO 2 in years when soil freeze occurred later, which is expected to become common in the future. The thermokarst site released significantly more CO 2 , but it was also marked by greater plant growth, thereby offsetting some of the CO 2 lost. Due, in part, to a lack of data, models represent terrestrial ecosystem carbon dynamics in Siberia poorly and do not take into changes in carbon cycling that occur with thermokarst formation. warming scenario, with the winter and spring seasons contributing the most to this warming (Miao et al., 2014). Covering~3,125,000 km 2 , this region is underlain by extensive carbon-rich permafrost soils that are already exhibiting widespread thaw as temperatures warm (Romanovsky et al., 2010).
Functional Ecology, 2019
In arctic environments, drifting snow around large shrub patches during winter may enhance growth... more In arctic environments, drifting snow around large shrub patches during winter may enhance growth by insulating the soil and facilitating overwinter nutrient turnover, leading to increased summer plant growth and potentially widespread increases in shrub cover. To determine whether snow enhances growth of arctic plants, we examined the effect of 6 years of added snow on plant biomass allocation and growth of nine common vascular plant species collected in 2010 and 2011 on either side of snowfences established in 2005 across a gradient of shrub biomass and productivity near Toolik Lake, Alaska. The deciduous shrub Salix pulchra responded most positively to added snow showing an 88% increase in total ramet biomass, because increased secondary growth allowed plants to support more branches and leaves. Some graminoid species also showed growth increases, especially where they were more abundant and larger than nearby shrub species. Species sharing the same growth strategy (deciduous shr...
Ecology and Evolution, 2019
In Low Arctic tundra, thermal erosion of ice‐rich permafrost soils (thermokarst) has increased in... more In Low Arctic tundra, thermal erosion of ice‐rich permafrost soils (thermokarst) has increased in frequency since the 1980s. Retrogressive thaw slumps (RTS) are thermokarst disturbances forming large open depressions on hillslopes through soil wasting and vegetation displacement. Tall (>0.5 m) deciduous shrubs have been observed in RTS a decade after disturbance. RTS may provide conditions suitable for seedling recruitment, which may contribute to Arctic shrub expansion. We quantified in situ seedling abundance, and size and viability of soil seedbanks in greenhouse trials for two RTS chronosequences near lakes on Alaska's North Slope. We hypothesized recent RTS provide microsites for greater recruitment than mature RTS or undisturbed tundra. We also hypothesized soil seedbanks demonstrate quantity–quality trade‐offs; younger seedbanks contain smaller numbers of mostly viable seed that decrease in viability as seed accumulates over time. We found five times as many seedlings ...