Skip to main content

Gaius Shaver

Followers

7

Following

21

Co-author

1

Public Views

Kansas State University

Christian-Albrechts-Universität zu Kiel

University of Rhode Island

University of North Carolina Wilmington

Wilkes University

Ramiro Riquelme-Bugueño

Universidad de Concepción

Adisel Montaña

University of East London

Gwen Robbins Schug

University of North Carolina at Greensboro

Gabriel Gutierrez-Alonso

University of Salamanca

Interests

Uploads

Papers by Gaius Shaver

Burn severity influences postfire CO<sub>2</sub>exchange in arctic tundra

Ecological Applications, Mar 1, 2011

Burned landscapes present several challenges to quantifying landscape carbon balance. Fire scars ... more 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 CO 2 (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.

Nutrient Addition Prompts Rapid Destabilization of Organic Matter in an Arctic Tundra Ecosystem

Ecosystems, Nov 17, 2007

Nutrient availability in the arctic is expected to increase in the next century due to accelerate... more Nutrient availability in the arctic is expected to increase in the next century due to accelerated decomposition associated with warming and, to a lesser extent, increased nitrogen deposition. To explore how changes in nutrient availability affect ecosystem carbon (C) cycling, we used radiocarbon to quantify changes in belowground C dynamics associated with long-term fertilization of graminoid-dominated tussock tundra at Toolik Lake, Alaska. Since 1981, yearly fertilization with nitrogen (N) and phosphorus (P) has resulted in a shift to shrub-dominated vegetation. These combined changes have altered the quantity and quality of litter inputs, the vertical distribution and dynamics of fine roots, and the decomposition rate of soil organic C. The loss of C from the deep organic and mineral soil has more than offset the C accumulation in the litter and upper organic soil horizons. In the litter and upper organic horizons, radiocarbon measurements show that increased inputs resulted in overall C accumulation, despite being offset by increased decomposition in some soil pools. To reconcile radiocarbon observations in the deeper organic and mineral soil layers, where most of the ecosystem C loss occurred, both a decrease in input of new root material and a dramatic increase of decomposition rates in centuries-old soil C pools were required. Therefore, with future increases in nutrient availability, we may expect substantial losses of C which took centuries to accumulate.

Burn severity influences post-fire CO2exchange in arctic tundra

Ecological Applications, Apr 28, 2010

Burned landscapes present several challenges to quantifying landscape carbon balance. Fire scars ... more 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 CO 2 (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.

Long-term nutrient addition alters arthropod community composition but does not increase total biomass or abundance

Oikos, Nov 13, 2017

A simple bottom-up hypothesis predicts that plant responses to nutrient addition should determine... more A simple bottom-up hypothesis predicts that plant responses to nutrient addition should determine the response of consumers: more productive and less diverse plant communities, the usual result of long-term nutrient addition, should support greater consumer abundances and biomass and less consumer diversity. We tested this hypothesis for the response of an aboveground arthropod community to an uncommonly long-term (24-year) nutrient addition experiment in moist acidic tundra in arctic Alaska. This experiment altered plant community composition, decreased plant diversity and increased plant production and biomass as a deciduous shrub (Betula nana) replaced graminoids, cryptogams and dwarf evergreen shrubs.became dominant Consistent with strong effects on the plant community, nutrient addition altered arthropod community composition, primarily through changes to herbivore taxa in the canopy-dwelling arthropod assemblage and detritivore taxa in the ground assemblage. Surprisingly, however, the loss of more than half of plant species was accompanied by negligible changes to diversity (rarefied richness) of arthropod taxa (which were primarily identified to family). Similarly, although long-term nutrient addition in this system roughly doubles plant production and biomass, arthropod abundance was either unchanged or decreased by nutrient addition, and total arthropod biomass was unaffected. Our findings differ markedly from the handful of terrestrial studies that have found bottom-up diversity cascades and productivity responses by consumers to nutrient addition. This is probably because unlike grasslands and salt marshes (where such studies have historically been conducted), this arctic tundra community becomes less palatable, rather than more so, after many years of nutrient addition due to increased dominance of B. nana. Additionally, by displacing insulating mosses and increasing the cover of shrubs that cool and shade the canopy microenvironment, fertilization may displace

Contrasting effects of long term versus short-term nitrogen addition on photosynthesis and respiration in the Arctic

Plant Ecology, Aug 17, 2013

We examined the effects of short (<1 to 4 years) and long-term (22 years) nitrogen (N) and/or pho... more 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 CO 2 exchange parameters of the arctic species Betula nana and Eriophorum vaginatum in northern Alaska. Measured variables included: the carboxylation efficiency of Rubisco (V cmax ), electron transport capacity (J max ), dark respiration (R d ), 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 N increase was consistent throughout the whole canopy. However, despite this large increase in foliar N, no significant changes in V cmax and J max were observed. In contrast, R d 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.

Effects of long-term climate trends on the methane and CO2 exchange processes of Toolik Lake, Alaska

Frontiers in Environmental Science

Methane and carbon dioxide effluxes from aquatic systems in the Arctic will affect and likely amp... more Methane and carbon dioxide effluxes from aquatic systems in the Arctic will affect and likely amplify global change. As permafrost thaws in a warming world, more dissolved organic carbon (DOC) and greenhouse gases are produced and move from soils to surface waters where the DOC can be oxidized to CO2 and also released to the atmosphere. Our main study objective is to measure the release of carbon to the atmosphere via effluxes of methane (CH4) and carbon dioxide (CO2) from Toolik Lake, a deep, dimictic, low-arctic lake in northern Alaska. By combining direct eddy covariance flux measurements with continuous gas pressure measurements in the lake surface waters, we quantified the k600 piston velocity that controls gas flux across the air–water interface. Our measured k values for CH4 and CO2 were substantially above predictions from several models at low to moderate wind speeds, and only converged on model predictions at the highest wind speeds. We attribute this higher flux at low wi...

Forty Arctic Summers

Long-Term Ecological Research, 2016

I was committed to long-term, site-based, research long before the Arctic (ARC) Long-Term Ecologi... more I was committed to long-term, site-based, research long before the Arctic (ARC) Long-Term Ecological Research (LTER) site was established in 1987. Working with the LTER program since then has allowed me to reach my goals more easily than would have been possible otherwise. Because of my deep involvement in research in the LTER program, most of the examples I use in teaching now come from LTER sites. For the same reason, most of my communications with the public are about research in the LTER program. I learned the value of collaboration as a graduate student, from my earliest mentors and collaborators. Being a part of the LTER program has helped me to develop a wide array of enjoyable, comfortable, and productive collaborations. A message to students: be generous in all aspects of your research and professional life, because there is much more to be gained from generosity than there is to be lost. I helped set up the ARC site of the LTER program in 1987 and have made it the focus of...

Arctic Plant Physiological Ecology in an Ecosystem Context

Arctic Ecosystems in a Changing Climate, 1992

Leaf Angle and Light Absorptance of Arctostaphylos Species (Ericaceae) Along Environmental Gradients

Madroño; a West American journal of botany, 1978

AmeriFlux US-ICt Imnavait Creek Watershed Tussock Tundra

This is the AmeriFlux version of the carbon flux data for the site US-ICt Imnavait Creek Watershe... more This is the AmeriFlux version of the carbon flux data for the site US-ICt Imnavait Creek Watershed Tussock Tundra. Site Description - The Imnavait Creek Watershed Tussock Tundra (Biocomplexity Station) is located near Imnavait Creek in Alaska, north of the Brooks Range in the Kuparuk basin near Lake Toolik and the Toolik Field Station. The Kuparuk River has its headwaters in the Brooks Range and drains through northern Alaska into the Arctic Ocean. Within these headwaters lies the Imnavait basin at an average elevation of 930 m. Water tracks run down the hill in parallel zones with a spacing of approximately 10 m. The Biocomplexity Station was deployed in 2004, and it has been in operation during the melt seasons ever since.

Synthesis of Winter In Situ Soil CO2 Flux in pan-Arctic and Boreal Regions, 1989-2017

This dataset provides a synthesis of winter non-growing season (NGS; September-April) in situ soi... more This dataset provides a synthesis of winter non-growing season (NGS; September-April) in situ soil CO2 flux measurement data from locations across pan-Arctic and Boreal permafrost regions. The in situ data were compiled from 66 published and 21 unpublished studies conducted from 1989-2017. The data sources (publication references) are provided. Sampling sites spanned pan-Arctic Boreal and tundra regions (>53 Deg N) in continuous, discontinuous, and isolated/sporadic permafrost zones. The CO2 flux measurements were aggregated at the monthly level, or seasonally when monthly data were not available, and are reported as the daily average (g C m-2 day-1) over the interval. Soil moisture and temperature data plus environmental and ecological model driver data (e.g., vegetation type and productivity, soil substrate availability) are also included based on gridded satellite remote sensing and reanalysis sources.

And Observed Changes in Climate

We manipulated light, temperature, and nutrients in moist tussock tundra near Toolik Lake, Alaska... more We manipulated light, temperature, and nutrients in moist tussock tundra near Toolik Lake, Alaska to determine how global changes in these parameters might affect community and ecosystem processes. Some of these manipulations altered nutrient avail- ability, growth-form composition, net primary production, and species richness in less than a decade, indicating that arctic vegetation at this site is sensitive to climatic change. In general, short-term (3-yr) responses were poor predictors of longer term (9-yr) changes in community composition. The longer term responses showed closer correspondence to pat- terns of vegetation distribution along environmental gradients. Nitrogen and phosphorus availability tended to increase in response to elevated temperature, reflecting increased mineralization, and in response to light attenuation, reflecting reduced nutrient uptake by vegetation. Nutrient addition increased biomass and production of deciduous shrubs but reduced growth of evergreen ...

Long-Term Release of Carbon Dioxide from Arctic Tundra Ecosystems in Alaska

Ecosystems, 2016

Releases of the greenhouse gases carbon dioxide (CO2) and methane (CH4) from thawing permafrost a... more Releases of the greenhouse gases carbon dioxide (CO2) and methane (CH4) from thawing permafrost are expected to be among the largest feedbacks to climate from arctic ecosystems. However, the current net carbon (C) balance of terrestrial arctic ecosystems is unknown. Recent studies suggest that these ecosystems are sources, sinks, or approximately in balance at present. This uncertainty arises because there are few long-term continuous measurements of arctic tundra CO2 fluxes over the full annual cycle. Here, we describe a pattern of CO2 loss based on the longest continuous record of direct measurements of CO2 fluxes in the Alaskan Arctic, from two representative tundra ecosystems, wet sedge and heath tundra. We also report on a shorter time series of continuous measurements from a third ecosystem, tussock tundra. The amount of CO2 loss from both heath and wet sedge ecosystems was related to the timing of freeze-up of the soil active layer in the fall. Wet sedge tundra lost the most CO2 during the anomalously warm autumn periods of September-December 2013-2015, with CH4 emissions contributing little to the overall C budget. Losses of C translated to approximately 4.1% and 1.4% of the total soil C stocks in active layer of the wet sedge and heath tundra, respectively, from 2008-2015. Increases in air temperature and soil temperatures at all depths may trigger a new trajectory of CO2 release, which will be a significant feedback to further warming if it is representative of larger areas of the Arctic.

Terrestrial Ecosystems at Toolik Lake, Alaska

Alaska's Changing Arctic, 2014

A Model of Carbohydrate, Nitrogen, Phosphorus Allocation and Growth in Tundra Production

Ecological Studies, 1978

Most models of plant production processes have been models of primary production and aboveground ... more Most models of plant production processes have been models of primary production and aboveground canopy processes and have included various environmental and physiological parameters in an attempt to predict photosynthesis and growth (Davidson and Philip, 1958; de Wit, 1965; Duncan et al., 1967; de Wit et al., 1970; Miller and Tieszen, 1972; Monsi and Saeki, 1953; Murphy and Knoerr, 1972; Paltridge, 1970; Lemon et al., 1971). Many have assumed that temperatures were optimal, most have assumed that water was not limiting growth, and all have assumed that minerals were not limiting growth. Horowitz (1958) developed mathematical formulations for alternative concepts of the mechanism of translocation. Thornley (1972) presented a primary production model which incorporated carbon and nitrogen to simulate the seasonal development of the root:shoot ratios. The model consisted of only two compartments, roots and shoots, composed of carbon and nitrogen. Although the model is fairly simple, it has the basic feedback mechanisms which have been associated with the carbon-nitrogen flow in plants. The lack of plant models which include minerals may reflect a current lack of a consistent set of hypotheses and data regarding the role of minerals in plant development and the mechanisms of their interaction.

The flux of carbon from terrestrial ecosystems to the atmosphere in 1980 due to changes in land use: geographic distribution of the global flux

Tellus B, 1987

Nitrate is an important nitrogen source for Arctic tundra plants

Proceedings of the National Academy of Sciences, 2018

Significance How terrestrial plants use N and respond to soil N loading is central to evaluating ... more Significance How terrestrial plants use N and respond to soil N loading is central to evaluating and predicting changing ecosystem structure and function with climate warming and N pollution. Here, evidence from NO 3 − in plant tissues has uncovered the uptake and assimilation of soil NO 3 − by Arctic tundra plants, which has long been assumed negligible. Soil NO 3 − contributed about one-third of the bulk N used by tundra plants of northern Alaska. Accordingly, the importance of soil NO 3 − for tundra plants should be considered in future studies on N and C cycling in Arctic ecosystems where C sequestration is strongly determined by N availability.

Arctic Ecosystems in a Changing Climate: An Ecophysiological Perspective

Arctic and alpine research, May 1, 1993

ABSTRACT This book is an international synthesis of studies on arctic ecosystems, a region where ... more ABSTRACT This book is an international synthesis of studies on arctic ecosystems, a region where climatic change is greatest, presenting the interrelationship between climate change and ecosystems. In addition to chapters dealing specifically with climatic change issues, important background information on arctic ecosystems and vegetation is given. Individual contributions are arranged into four parts: The Arctic System; Carbon Balance; Water and Nutrient Balance; and Interactions. An brief introduction, summary, and a useful index are also included.

Deforestation measured by landsat: steps toward a method

Biomass and production of tundra vegetation under three eddy covariance towers at Imnavait Creek, Alaska

Burn severity influences postfire CO<sub>2</sub>exchange in arctic tundra

Ecological Applications, Mar 1, 2011

Burned landscapes present several challenges to quantifying landscape carbon balance. Fire scars ... more 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 CO 2 (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.

Nutrient Addition Prompts Rapid Destabilization of Organic Matter in an Arctic Tundra Ecosystem

Ecosystems, Nov 17, 2007

Nutrient availability in the arctic is expected to increase in the next century due to accelerate... more Nutrient availability in the arctic is expected to increase in the next century due to accelerated decomposition associated with warming and, to a lesser extent, increased nitrogen deposition. To explore how changes in nutrient availability affect ecosystem carbon (C) cycling, we used radiocarbon to quantify changes in belowground C dynamics associated with long-term fertilization of graminoid-dominated tussock tundra at Toolik Lake, Alaska. Since 1981, yearly fertilization with nitrogen (N) and phosphorus (P) has resulted in a shift to shrub-dominated vegetation. These combined changes have altered the quantity and quality of litter inputs, the vertical distribution and dynamics of fine roots, and the decomposition rate of soil organic C. The loss of C from the deep organic and mineral soil has more than offset the C accumulation in the litter and upper organic soil horizons. In the litter and upper organic horizons, radiocarbon measurements show that increased inputs resulted in overall C accumulation, despite being offset by increased decomposition in some soil pools. To reconcile radiocarbon observations in the deeper organic and mineral soil layers, where most of the ecosystem C loss occurred, both a decrease in input of new root material and a dramatic increase of decomposition rates in centuries-old soil C pools were required. Therefore, with future increases in nutrient availability, we may expect substantial losses of C which took centuries to accumulate.

Burn severity influences post-fire CO2exchange in arctic tundra

Ecological Applications, Apr 28, 2010

Burned landscapes present several challenges to quantifying landscape carbon balance. Fire scars ... more 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 CO 2 (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.

Long-term nutrient addition alters arthropod community composition but does not increase total biomass or abundance

Oikos, Nov 13, 2017

A simple bottom-up hypothesis predicts that plant responses to nutrient addition should determine... more A simple bottom-up hypothesis predicts that plant responses to nutrient addition should determine the response of consumers: more productive and less diverse plant communities, the usual result of long-term nutrient addition, should support greater consumer abundances and biomass and less consumer diversity. We tested this hypothesis for the response of an aboveground arthropod community to an uncommonly long-term (24-year) nutrient addition experiment in moist acidic tundra in arctic Alaska. This experiment altered plant community composition, decreased plant diversity and increased plant production and biomass as a deciduous shrub (Betula nana) replaced graminoids, cryptogams and dwarf evergreen shrubs.became dominant Consistent with strong effects on the plant community, nutrient addition altered arthropod community composition, primarily through changes to herbivore taxa in the canopy-dwelling arthropod assemblage and detritivore taxa in the ground assemblage. Surprisingly, however, the loss of more than half of plant species was accompanied by negligible changes to diversity (rarefied richness) of arthropod taxa (which were primarily identified to family). Similarly, although long-term nutrient addition in this system roughly doubles plant production and biomass, arthropod abundance was either unchanged or decreased by nutrient addition, and total arthropod biomass was unaffected. Our findings differ markedly from the handful of terrestrial studies that have found bottom-up diversity cascades and productivity responses by consumers to nutrient addition. This is probably because unlike grasslands and salt marshes (where such studies have historically been conducted), this arctic tundra community becomes less palatable, rather than more so, after many years of nutrient addition due to increased dominance of B. nana. Additionally, by displacing insulating mosses and increasing the cover of shrubs that cool and shade the canopy microenvironment, fertilization may displace

Contrasting effects of long term versus short-term nitrogen addition on photosynthesis and respiration in the Arctic

Plant Ecology, Aug 17, 2013

We examined the effects of short (<1 to 4 years) and long-term (22 years) nitrogen (N) and/or pho... more 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 CO 2 exchange parameters of the arctic species Betula nana and Eriophorum vaginatum in northern Alaska. Measured variables included: the carboxylation efficiency of Rubisco (V cmax ), electron transport capacity (J max ), dark respiration (R d ), 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 N increase was consistent throughout the whole canopy. However, despite this large increase in foliar N, no significant changes in V cmax and J max were observed. In contrast, R d 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.

Effects of long-term climate trends on the methane and CO2 exchange processes of Toolik Lake, Alaska

Frontiers in Environmental Science

Methane and carbon dioxide effluxes from aquatic systems in the Arctic will affect and likely amp... more Methane and carbon dioxide effluxes from aquatic systems in the Arctic will affect and likely amplify global change. As permafrost thaws in a warming world, more dissolved organic carbon (DOC) and greenhouse gases are produced and move from soils to surface waters where the DOC can be oxidized to CO2 and also released to the atmosphere. Our main study objective is to measure the release of carbon to the atmosphere via effluxes of methane (CH4) and carbon dioxide (CO2) from Toolik Lake, a deep, dimictic, low-arctic lake in northern Alaska. By combining direct eddy covariance flux measurements with continuous gas pressure measurements in the lake surface waters, we quantified the k600 piston velocity that controls gas flux across the air–water interface. Our measured k values for CH4 and CO2 were substantially above predictions from several models at low to moderate wind speeds, and only converged on model predictions at the highest wind speeds. We attribute this higher flux at low wi...

Forty Arctic Summers

Long-Term Ecological Research, 2016

I was committed to long-term, site-based, research long before the Arctic (ARC) Long-Term Ecologi... more I was committed to long-term, site-based, research long before the Arctic (ARC) Long-Term Ecological Research (LTER) site was established in 1987. Working with the LTER program since then has allowed me to reach my goals more easily than would have been possible otherwise. Because of my deep involvement in research in the LTER program, most of the examples I use in teaching now come from LTER sites. For the same reason, most of my communications with the public are about research in the LTER program. I learned the value of collaboration as a graduate student, from my earliest mentors and collaborators. Being a part of the LTER program has helped me to develop a wide array of enjoyable, comfortable, and productive collaborations. A message to students: be generous in all aspects of your research and professional life, because there is much more to be gained from generosity than there is to be lost. I helped set up the ARC site of the LTER program in 1987 and have made it the focus of...

Arctic Plant Physiological Ecology in an Ecosystem Context

Arctic Ecosystems in a Changing Climate, 1992

Leaf Angle and Light Absorptance of Arctostaphylos Species (Ericaceae) Along Environmental Gradients

Madroño; a West American journal of botany, 1978

AmeriFlux US-ICt Imnavait Creek Watershed Tussock Tundra

This is the AmeriFlux version of the carbon flux data for the site US-ICt Imnavait Creek Watershe... more This is the AmeriFlux version of the carbon flux data for the site US-ICt Imnavait Creek Watershed Tussock Tundra. Site Description - The Imnavait Creek Watershed Tussock Tundra (Biocomplexity Station) is located near Imnavait Creek in Alaska, north of the Brooks Range in the Kuparuk basin near Lake Toolik and the Toolik Field Station. The Kuparuk River has its headwaters in the Brooks Range and drains through northern Alaska into the Arctic Ocean. Within these headwaters lies the Imnavait basin at an average elevation of 930 m. Water tracks run down the hill in parallel zones with a spacing of approximately 10 m. The Biocomplexity Station was deployed in 2004, and it has been in operation during the melt seasons ever since.

Synthesis of Winter In Situ Soil CO2 Flux in pan-Arctic and Boreal Regions, 1989-2017

This dataset provides a synthesis of winter non-growing season (NGS; September-April) in situ soi... more This dataset provides a synthesis of winter non-growing season (NGS; September-April) in situ soil CO2 flux measurement data from locations across pan-Arctic and Boreal permafrost regions. The in situ data were compiled from 66 published and 21 unpublished studies conducted from 1989-2017. The data sources (publication references) are provided. Sampling sites spanned pan-Arctic Boreal and tundra regions (>53 Deg N) in continuous, discontinuous, and isolated/sporadic permafrost zones. The CO2 flux measurements were aggregated at the monthly level, or seasonally when monthly data were not available, and are reported as the daily average (g C m-2 day-1) over the interval. Soil moisture and temperature data plus environmental and ecological model driver data (e.g., vegetation type and productivity, soil substrate availability) are also included based on gridded satellite remote sensing and reanalysis sources.

And Observed Changes in Climate

We manipulated light, temperature, and nutrients in moist tussock tundra near Toolik Lake, Alaska... more We manipulated light, temperature, and nutrients in moist tussock tundra near Toolik Lake, Alaska to determine how global changes in these parameters might affect community and ecosystem processes. Some of these manipulations altered nutrient avail- ability, growth-form composition, net primary production, and species richness in less than a decade, indicating that arctic vegetation at this site is sensitive to climatic change. In general, short-term (3-yr) responses were poor predictors of longer term (9-yr) changes in community composition. The longer term responses showed closer correspondence to pat- terns of vegetation distribution along environmental gradients. Nitrogen and phosphorus availability tended to increase in response to elevated temperature, reflecting increased mineralization, and in response to light attenuation, reflecting reduced nutrient uptake by vegetation. Nutrient addition increased biomass and production of deciduous shrubs but reduced growth of evergreen ...

Long-Term Release of Carbon Dioxide from Arctic Tundra Ecosystems in Alaska

Ecosystems, 2016

Releases of the greenhouse gases carbon dioxide (CO2) and methane (CH4) from thawing permafrost a... more Releases of the greenhouse gases carbon dioxide (CO2) and methane (CH4) from thawing permafrost are expected to be among the largest feedbacks to climate from arctic ecosystems. However, the current net carbon (C) balance of terrestrial arctic ecosystems is unknown. Recent studies suggest that these ecosystems are sources, sinks, or approximately in balance at present. This uncertainty arises because there are few long-term continuous measurements of arctic tundra CO2 fluxes over the full annual cycle. Here, we describe a pattern of CO2 loss based on the longest continuous record of direct measurements of CO2 fluxes in the Alaskan Arctic, from two representative tundra ecosystems, wet sedge and heath tundra. We also report on a shorter time series of continuous measurements from a third ecosystem, tussock tundra. The amount of CO2 loss from both heath and wet sedge ecosystems was related to the timing of freeze-up of the soil active layer in the fall. Wet sedge tundra lost the most CO2 during the anomalously warm autumn periods of September-December 2013-2015, with CH4 emissions contributing little to the overall C budget. Losses of C translated to approximately 4.1% and 1.4% of the total soil C stocks in active layer of the wet sedge and heath tundra, respectively, from 2008-2015. Increases in air temperature and soil temperatures at all depths may trigger a new trajectory of CO2 release, which will be a significant feedback to further warming if it is representative of larger areas of the Arctic.

Terrestrial Ecosystems at Toolik Lake, Alaska

Alaska's Changing Arctic, 2014

A Model of Carbohydrate, Nitrogen, Phosphorus Allocation and Growth in Tundra Production

Ecological Studies, 1978

Most models of plant production processes have been models of primary production and aboveground ... more Most models of plant production processes have been models of primary production and aboveground canopy processes and have included various environmental and physiological parameters in an attempt to predict photosynthesis and growth (Davidson and Philip, 1958; de Wit, 1965; Duncan et al., 1967; de Wit et al., 1970; Miller and Tieszen, 1972; Monsi and Saeki, 1953; Murphy and Knoerr, 1972; Paltridge, 1970; Lemon et al., 1971). Many have assumed that temperatures were optimal, most have assumed that water was not limiting growth, and all have assumed that minerals were not limiting growth. Horowitz (1958) developed mathematical formulations for alternative concepts of the mechanism of translocation. Thornley (1972) presented a primary production model which incorporated carbon and nitrogen to simulate the seasonal development of the root:shoot ratios. The model consisted of only two compartments, roots and shoots, composed of carbon and nitrogen. Although the model is fairly simple, it has the basic feedback mechanisms which have been associated with the carbon-nitrogen flow in plants. The lack of plant models which include minerals may reflect a current lack of a consistent set of hypotheses and data regarding the role of minerals in plant development and the mechanisms of their interaction.

The flux of carbon from terrestrial ecosystems to the atmosphere in 1980 due to changes in land use: geographic distribution of the global flux

Tellus B, 1987

Nitrate is an important nitrogen source for Arctic tundra plants

Proceedings of the National Academy of Sciences, 2018

Significance How terrestrial plants use N and respond to soil N loading is central to evaluating ... more Significance How terrestrial plants use N and respond to soil N loading is central to evaluating and predicting changing ecosystem structure and function with climate warming and N pollution. Here, evidence from NO 3 − in plant tissues has uncovered the uptake and assimilation of soil NO 3 − by Arctic tundra plants, which has long been assumed negligible. Soil NO 3 − contributed about one-third of the bulk N used by tundra plants of northern Alaska. Accordingly, the importance of soil NO 3 − for tundra plants should be considered in future studies on N and C cycling in Arctic ecosystems where C sequestration is strongly determined by N availability.

Arctic Ecosystems in a Changing Climate: An Ecophysiological Perspective

Arctic and alpine research, May 1, 1993

ABSTRACT This book is an international synthesis of studies on arctic ecosystems, a region where ... more ABSTRACT This book is an international synthesis of studies on arctic ecosystems, a region where climatic change is greatest, presenting the interrelationship between climate change and ecosystems. In addition to chapters dealing specifically with climatic change issues, important background information on arctic ecosystems and vegetation is given. Individual contributions are arranged into four parts: The Arctic System; Carbon Balance; Water and Nutrient Balance; and Interactions. An brief introduction, summary, and a useful index are also included.

Deforestation measured by landsat: steps toward a method

Biomass and production of tundra vegetation under three eddy covariance towers at Imnavait Creek, Alaska