Shaver Gaius R.

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Shaver

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Gaius R.

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Contrasting soil thermal responses to fire in Alaskan tundra and boreal forest

(John Wiley & Sons, 2015-02-24) Jiang, Yueyang ; Rocha, Adrian V. ; O’Donnell, Jonathan A. ; Drysdale, Jessica A. ; Rastetter, Edward B. ; Shaver, Gaius R. ; Zhuang, Qianlai

Recent fire activity throughout Alaska has increased the need to understand postfire impacts on soils and permafrost vulnerability. Our study utilized data and modeling from a permafrost and ecosystem gradient to develop a mechanistic understanding of the short- and long-term impacts of tundra and boreal forest fires on soil thermal dynamics. Fires influenced a variety of factors that altered the surface energy budget, soil moisture, and the organic-layer thickness with the overall effect of increasing soil temperatures and thaw depth. The postfire thickness of the soil organic layer and its impact on soil thermal conductivity was the most important factor determining postfire soil temperatures and thaw depth. Boreal and tundra ecosystems underlain by permafrost experienced smaller postfire soil temperature increases than the nonpermafrost boreal forest from the direct and indirect effects of permafrost on drainage, soil moisture, and vegetation flammability. Permafrost decreased the loss of the insulating soil organic layer, decreased soil drying, increased surface water pooling, and created a significant heat sink to buffer postfire soil temperature and thaw depth changes. Ecosystem factors also played a role in determining postfire thaw depth with boreal forests taking several decades longer to recover their soil thermal properties than tundra. These factors resulted in tundra being less sensitive to postfire soil thermal changes than the nonpermafrost boreal forest. These results suggest that permafrost and soil organic carbon will be more vulnerable to fire as climate warms.
Nitrate is an important nitrogen source for Arctic tundra plants

(National Academy of Sciences, 2018-03-27) Liu, Xue-Yan ; Koba, Keisuke ; Koyama, Lina A. ; Hobbie, Sarah E. ; Weiss, Marissa S. ; Inagaki, Yoshiyuki ; Shaver, Gaius R. ; Giblin, Anne E. ; Hobara, Satoru ; Nadelhoffer, Knute J. ; Sommerkorn, Martin ; Rastetter, Edward B. ; Kling, George W. ; Laundre, James A. ; Yano, Yuriko ; Makabe, Akiko ; Yano, Midori ; Liu, Cong-Qiang

Plant nitrogen (N) use is a key component of the N cycle in terrestrial ecosystems. The supply of N to plants affects community species composition and ecosystem processes such as photosynthesis and carbon (C) accumulation. However, the availabilities and relative importance of different N forms to plants are not well understood. While nitrate (NO3−) is a major N form used by plants worldwide, it is discounted as a N source for Arctic tundra plants because of extremely low NO3− concentrations in Arctic tundra soils, undetectable soil nitrification, and plant-tissue NO3− that is typically below detection limits. Here we reexamine NO3− use by tundra plants using a sensitive denitrifier method to analyze plant-tissue NO3−. Soil-derived NO3− was detected in tundra plant tissues, and tundra plants took up soil NO3− at comparable rates to plants from relatively NO3−-rich ecosystems in other biomes. Nitrate assimilation determined by 15N enrichments of leaf NO3− relative to soil NO3− accounted for 4 to 52% (as estimated by a Bayesian isotope-mixing model) of species-specific total leaf N of Alaskan tundra plants. Our finding that in situ soil NO3− availability for tundra plants is high has important implications for Arctic ecosystems, not only in determining species compositions, but also in determining the loss of N from soils via leaching and denitrification. Plant N uptake and soil N losses can strongly influence C uptake and accumulation in tundra soils. Accordingly, this evidence of NO3− availability in tundra soils is crucial for predicting C storage in tundra.
Nitrogen dynamics in arctic tundra soils of varying age : differential responses to fertilization and warming

( 2013-03) Yano, Yuriko ; Shaver, Gaius R. ; Rastetter, Edward B. ; Giblin, Anne E. ; Laundre, James A.

In the northern foothills of the Brooks Range, Alaska, a series of glacial retreats has created a landscape that varies widely in time since deglaciation (= soil age), from ~10k years to more than 2M years. Productivity of the moist tundra that covers most of this landscape is generally N-limited, but varies widely, as do plant-species composition and key soil properties such as pH. These differences might be altered in the future because of the projected increase in N availability under a warmer climate. We hypothesized that future changes in productivity and vegetation composition across soil ages might be mediated through changes in N availability. To test this hypothesis, we compared readily available-N (water-soluble ammonium, nitrate, and amino acids), moderately-available N (soluble proteins), hydrolysable-N, and total-N pools across three tussock-tundra landscapes with soil ages ranging from 11.5k to 300k years. We also compared the effects of long-term fertilization and warming on these N pools for the two younger sites, in order to assess whether the impacts of warming and increased N availability differ by soil age. Readily available N was largest at the oldest site, and amino acids (AA) accounted for 80-89 % of this N. At the youngest site, however, inorganic N constituted the majority (80-97%) of total readily-available N. This variation reflected the large differences in plant functional-group composition and soil chemical properties. Long-term (8-16 years) fertilization increased soluble inorganic N by 20-100 fold at the intermediate-age site, but only by 2-3 fold at the youngest-soil site. Warming caused small and inconsistent changes in the soil C:N ratio and soluble AA, but only in soils beneath Eriophorum vaginatum, the dominant tussock-forming sedge. These differential responses suggest that the impacts of warmer climates on these tundra ecosystems are more complex than simply elevated N mineralization, and that the response of the N cycling might differ strongly depending on the ecosystem’s soil age, initial soil properties, and plant-community composition.
Terrestrial C sequestration at elevated CO2 and temperature : the role of dissolved organic N loss

( 2004-06-07) Rastetter, Edward B. ; Perakis, Steven S. ; Shaver, Gaius R. ; Agren, Goran I.

We used a simple model of carbon–nitrogen (C–N) interactions in terrestrial ecosystems to examine the responses to elevated CO2 and to elevated CO2 plus warming in ecosystems that had the same total nitrogen loss but that differed in the ratio of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) loss. We postulate that DIN losses can be curtailed by higher N demand in response to elevated CO2, but that DON losses cannot. We also examined simulations in which DON losses were held constant, were proportional to the amount of soil organic matter, were proportional to the soil C:N ratio, or were proportional to the rate of decomposition. We found that the mode of N loss made little difference to the short-term (<60 years) rate of carbon sequestration by the ecosystem, but high DON losses resulted in much lower carbon sequestration in the long term than did low DON losses. In the short term, C sequestration was fueled by an internal redistribution of N from soils to vegetation and by increases in the C:N ratio of soils and vegetation. This sequestration was about three times larger with elevated CO2 and warming than with elevated CO2 alone. After year 60, C sequestration was fueled by a net accumulation of N in the ecosystem, and the rate of sequestration was about the same with elevated CO2 and warming as with elevated CO2 alone. With high DON losses, the ecosystem either sequestered C slowly after year 60 (when DON losses were constant or proportional to soil organic matter) or lost C (when DON losses were proportional to the soil C:N ratio or to decomposition). We conclude that changes in long-term C sequestration depend not only on the magnitude of N losses, but also on the form of those losses.
Depleted 15N in hydrolysable-N of arctic soils and its implication for mycorrhizal fungi–plant interaction

( 2009-08) Yano, Yuriko ; Shaver, Gaius R. ; Giblin, Anne E. ; Rastetter, Edward B.

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.
Processing arctic eddy-flux data using a simple carbon-exchange model embedded in the ensemble Kalman filter

(Ecological Society of America, 2010-07) Rastetter, Edward B. ; Williams, Mathew ; Griffin, Kevin L. ; Kwiatkowski, Bonnie L. ; Tomasky, Gabrielle ; Potosnak, Mark J. ; Stoy, Paul C. ; Shaver, Gaius R. ; Stieglitz, Marc ; Hobbie, John E. ; Kling, George W.

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.
Ecosystem responses to climate change at a Low Arctic and a High Arctic long-term research site

(Springer, 2017-01-23) Hobbie, John E. ; Shaver, Gaius R. ; Rastetter, Edward B. ; Cherry, Jessica E. ; Goetz, Scott J. ; Guay, Kevin C. ; Gould, William A. ; Kling, George W.

Long-term measurements of ecological effects of warming are often not statistically significant because of annual variability or signal noise. These are reduced in indicators that filter or reduce the noise around the signal and allow effects of climate warming to emerge. In this way, certain indicators act as medium pass filters integrating the signal over years-to-decades. In the Alaskan Arctic, the 25-year record of warming of air temperature revealed no significant trend, yet environmental and ecological changes prove that warming is affecting the ecosystem. The useful indicators are deep permafrost temperatures, vegetation and shrub biomass, satellite measures of canopy reflectance (NDVI), and chemical measures of soil weathering. In contrast, the 18-year record in the Greenland Arctic revealed an extremely high summer air-warming of 1.3°C/decade; the cover of some plant species increased while the cover of others decreased. Useful indicators of change are NDVI and the active layer thickness.
Incident radiation and the allocation of nitrogen within Arctic plant canopies : implications for predicting gross primary productivity

( 2012-01) Street, Lorna E. ; Shaver, Gaius R. ; Rastetter, Edward B. ; van Wijk, Mark T. ; Kaye, Brooke A. ; Williams, Mathew

Arctic vegetation is characterized by high spatial variability in plant functional type (PFT) composition and gross primary productivity (P). Despite this variability, the two main drivers of P in sub-Arctic tundra are leaf area index (LT) and total foliar nitrogen (NT). LT and NT have been shown to be tightly coupled across PFTs in sub-Arctic tundra vegetation, which simplifies up-scaling by allowing quantification of the main drivers of P from remotely sensed LT. Our objective was to test the LT–NT relationship across multiple Arctic latitudes and to assess LT as a predictor of P for the pan-Arctic. Including PFT-specific parameters in models of LT–NT coupling provided only incremental improvements in model fit, but significant improvements were gained from including site-specific parameters. The degree of curvature in the LT–NT relationship, controlled by a fitted canopy nitrogen extinction co-efficient, was negatively related to average levels of diffuse radiation at a site. This is consistent with theoretical predictions of more uniform vertical canopy N distributions under diffuse light conditions. Higher latitude sites had higher average leaf N content by mass (NM), and we show for the first time that LT–NT coupling is achieved across latitudes via canopy-scale trade-offs between NM and leaf mass per unit leaf area (LM). Site-specific parameters provided small but significant improvements in models of P based on LT and moss cover. Our results suggest that differences in LT–NT coupling between sites could be used to improve pan-Arctic models of P and we provide unique evidence that prevailing radiation conditions can significantly affect N allocation over regional scales.
The footprint of Alaskan tundra fires during the past half-century : implications for surface properties and radiative forcing

(IOP Publishing, 2012-12-19) Rocha, Adrian V. ; Loranty, Michael M. ; Higuera, Philip E. ; Mack, Michelle C. ; Hu, Feng Sheng ; Jones, Benjamin M. ; Breen, Amy L. ; Rastetter, Edward B. ; Goetz, Scott J. ; Shaver, Gaius R.

Recent large and frequent fires above the Alaskan arctic circle have forced a reassessment of the ecological and climatological importance of fire in arctic tundra ecosystems. Here we provide a general overview of the occurrence, distribution, and ecological and climate implications of Alaskan tundra fires over the past half-century using spatially explicit climate, fire, vegetation and remote sensing datasets for Alaska. Our analyses highlight the importance of vegetation biomass and environmental conditions in regulating tundra burning, and demonstrate that most tundra ecosystems are susceptible to burn, providing the environmental conditions are right. Over the past two decades, fire perimeters above the arctic circle have increased in size and importance, especially on the North Slope, indicating that future wildfire projections should account for fire regime changes in these regions. Remote sensing data and a literature review of thaw depths indicate that tundra fires have both positive and negative implications for climatic feedbacks including a decadal increase in albedo radiative forcing immediately after a fire, a stimulation of surface greenness and a persistent long-term (>10 year) increase in thaw depth. In order to address the future impact of tundra fires on climate, a better understanding of the control of tundra fire occurrence as well as the long-term impacts on ecosystem carbon cycling will be required.
Carbon turnover in Alaskan tundra soils : effects of organic matter quality, temperature, moisture and fertilizer

( 2006-02-15) Shaver, Gaius R. ; Giblin, Anne E. ; Nadelhoffer, Knute J. ; Thieler, K. K. ; Downs, M. R. ; Laundre, James A. ; Rastetter, Edward B.

Soils of tundra and boreal ecosystems contain large organic matter stocks, typically as a layer of peat that blankets the underlying mineral soil. Despite the low productivity of northern vegetation, organic matter accumulates as peat because decomposition of plant litter is limited by low soil temperatures and often wet, anaerobic conditions (Heal et al. 1981, Jonasson et al. 2001). The total C storage in this northern peat is globally significant, accounting for about one third of the global soil C stock if one includes both tundras and boreal forests (Oechel and Billings 1992, Callaghan et al. 2004a). Soils of northern ecosystems also contain large amounts of organic N that is currently unavailable to plants, but is potentially available and could support higher productivity if mineralized (Shaver et al. 1991, Nadelhoffer et al. 1992, Weintraub and Schimel 2005 a). Controls on soil C stocks and turnover, therefore, are key issues for understanding C exchanges between northern ecosystems and the atmosphere. In this paper, we determine how C losses from peaty soil organic matter are related to its chemical composition, and how that composition changes as the organic matter decomposes. To address these issues we compared four soil organic matter types from three tundra ecosystems near Toolik Lake, Alaska. The comparison included both unfertilized soils and soils that were fertilized annually for eight years before sampling. Under laboratory conditions, we determined how temperature and moisture conditions affect C losses from these organic matter types. The experiment also allowed us to determine how the chemical composition of different types of organic matter changed over four simulated “seasons” of decomposition. The chemical composition or “quality” of soil organic matter is a useful predictor of C turnover (Ågren and Bosatta 1996) although a wide range of definitions and fractionation schemes have been used (Sollins et al. 1999, Harmon and Lajtha 1999). In general, high-quality organic matter is defined as that which is more readily processed by microbes and has a higher rate of decomposition. Fresh plant litter and newly-formed organic matter are expected to be of higher quality than older, more fully decomposed organic matter in which the more labile components have been metabolized (Aerts 1997, Berg 2000). Species composition of the vegetation may also have a strong influence on litter and organic matter “quality” (Berendse 1994, Cornelissen 1996, Hobbie 1996, Hobbie and Gough 2004). In this research we characterized organic matter quality with a widely used sequential extraction procedure (Ryan et al. 1990, Harmon and Lajtha 1999) that breaks soil organic matter into 4 fractions: (1) a “non-polar extractable” (NPE) fraction extracted in methylene chloride, (2) a “water-soluble” (WS) fraction extracted in boiling water, (3) an “acid-soluble’ (AS) fraction extracted in H2SO4, and (4) an “acid-insoluble” (AIS) residue.
Nitrogen dynamics in a small arctic watershed: retention and downhill movement of 15N

( 2009-06-12) Yano, Yuriko ; Shaver, Gaius R. ; Giblin, Anne E. ; Rastetter, Edward B. ; Nadelhoffer, Knute J.

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.

Shaver Gaius R.

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