Tight coupling between leaf area index and foliage N content in arctic plant communities

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Oecologia 142 (2005): 421-427, doi:10.1007/s00442-004-1733-x.The large spatial heterogeneity of arctic landscapes complicates efforts to quantify key processes of these ecosystems, for example productivity, at the landscape level. Robust relationships that help to simplify and explain observed patterns, are thus powerful tools for understanding and predicting vegetation distribution and dynamics. Here we present the same linear relationship between leaf area index and total foliar nitrogen, the two factors determining the photosynthetic capacity of vegetation, across a wide range of tundra vegetation types in both Northern-Sweden and Alaska between leaf area indices of 0 and 1 m2 m-2, which is essentially the entire range of leaf area index values for the Arctic as a whole. Surprisingly, this simple relationship arises as an emergent property at the plant community level, whereas at the species level a large variability in leaf traits exists. As the relationship between LAI and foliar N exists among such varied ecosystems, the arctic environment must impose tight constraints on vegetation canopy development. This relationship simplifies the quantification of vegetation productivity of arctic vegetation types as the two most important drivers of productivity can now be estimated reliably from remotely sensed NDVI images.This work was funded by the US National Science Foundation

Interannual, summer, and diel variability of CH4 and CO2 effluxes from Toolik Lake, Alaska, during the ice-free periods 2010-2015

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Eugster, W., DelSontro, T., Shaver, G. R., & Kling, G. W. Interannual, summer, and diel variability of CH4 and CO2 effluxes from Toolik Lake, Alaska, during the ice-free periods 2010-2015. Environmental Science: Processes & Impacts, 22(11), (2020): 2181-2198, doi: 10.1039/D0EM00125B.Accelerated warming in the Arctic has led to concern regarding the amount of carbon emission potential from Arctic water bodies. Yet, aquatic carbon dioxide (CO2) and methane (CH4) flux measurements remain scarce, particularly at high resolution and over long periods of time. Effluxes of methane (CH4) and carbon dioxide (CO2) from Toolik Lake, a deep glacial lake in northern Alaska, were measured for the first time with the direct eddy covariance (EC) flux technique during six ice-free lake periods (2010–2015). CO2 flux estimates from the lake (daily average efflux of 16.7 ± 5.3 mmol m−2 d−1) were in good agreement with earlier estimates from 1975–1989 using different methods. CH4 effluxes in 2010–2015 (averaging 0.13 ± 0.06 mmol m−2 d−1) showed an interannual variation that was 4.1 times greater than median diel variations, but mean fluxes were almost one order of magnitude lower than earlier estimates obtained from single water samples in 1990 and 2011–2012. The overall global warming potential (GWP) of Toolik Lake is thus governed mostly by CO2 effluxes, contributing 86–93% of the ice-free period GWP of 26–90 g CO2,eq m−2. Diel variation in fluxes was also important, with up to a 2-fold (CH4) to 4-fold (CO2) difference between the highest nighttime and lowest daytime effluxes. Within the summer ice-free period, on average, CH4 fluxes increased 2-fold during the first half of the summer, then remained almost constant, whereas CO2 effluxes remained almost constant over the entire summer, ending with a linear increase during the last 1–2 weeks of measurements. Due to the cold bottom temperatures of this 26 m deep lake, and the absence of ebullition and episodic flux events, Toolik Lake and other deep glacial lakes are likely not hot spots for greenhouse gas emissions, but they still contribute to the overall GWP of the Arctic.We acknowledge support received from the Arctic LTER grants NSF-DEB-1637459, 1026843, 1754835, NSF-PLR 1504006, and supplemental funding from the NSF-NEON and OPP-AON programs. W. E. acknowledges additional funding received from ETH Zurich scientific equipment grants 0-43350-07 and 0-43683-11. James Laundre is thanked for technical support, Jason Dobkowski for supervising deployment and removal of the float to and from the lake, and Randy Fulweber for his GIS support. Many thanks also go to Toolik Field Station staff members for support

Nitrogen dynamics in arctic tundra soils of varying age : differential responses to fertilization and warming

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Oecologia 173 (2013): 1575-1586, doi:10.1007/s00442-013-2733-5.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.Primary financial support came from NSF grant #DEB-0444592 to the MBL, and additional logistical support from NSF-OPP

Burn severity influences postfire CO2 exchange in Arctic tundra

Author Posting. © Ecological Society of America, 2011. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Applications 21 (2011): 477–489, doi:10.1890/10-0255.1.Burned landscapes present several challenges to quantifying landscape carbon balance. Fire scars are composed of a mosaic of patches that differ in burn severity, which may influence postfire carbon budgets through damage to vegetation and carbon stocks. We deployed three eddy covariance towers along a burn severity gradient (i.e., severely burned, moderately burned, and unburned tundra) to monitor postfire net ecosystem exchange of CO2 (NEE) within the large 2007 Anaktuvuk River fire scar in Alaska, USA, during the summer of 2008. Remote sensing data from the MODerate resolution Imaging Spectroradiometer (MODIS) was used to assess the spatial representativeness of the tower sites and parameterize a NEE model that was used to scale tower measurements to the landscape. The tower sites had similar vegetation and reflectance properties prior to the Anaktuvuk River fire and represented the range of surface conditions observed within the fire scar during the 2008 summer. Burn severity influenced a variety of surface properties, including residual organic matter, plant mortality, and vegetation recovery, which in turn determined postfire NEE. Carbon sequestration decreased with increased burn severity and was largely controlled by decreases in canopy photosynthesis. The MODIS two-band enhanced vegetation index (EVI2) monitored the seasonal course of surface greenness and explained 86% of the variability in NEE across the burn severity gradient. We demonstrate that understanding the relationship between burn severity, surface reflectance, and NEE is critical for estimating the overall postfire carbon balance of the Anaktuvuk River fire scar.This work was supported by NSF grants #0632139 (OPP-AON), #0808789 (OPP-ARCSS SGER), #0829285 (DEB-NEON SGER), and #0423385 (DEBLTER) to the Marine Biological Laboratory

Seasonal patterns of carbon dioxide and water fluxes in three representative tundra ecosystems in northern Alaska

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 3, no 1 (2012): art4, doi:10.1890/ES11-00202.1.Understanding the carbon dioxide and water fluxes in the Arctic is essential for accurate assessment and prediction of the responses of these ecosystems to climate change. In the Arctic, there have been relatively few studies of net CO2, water, and energy exchange using micrometeorological methods due to the difficulty of performing these measurements in cold, remote regions. When these measurements are performed, they are usually collected only during the short summer growing season. We established eddy covariance flux towers in three representative Alaska tundra ecosystems (heath tundra, tussock tundra, and wet sedge tundra), and have collected CO2, water, and energy flux data continuously for over three years (September 2007–May 2011). In all ecosystems, peak CO2 uptake occurred during July, with accumulations of 51–95 g C/m2 during June–August. The timing of the switch from CO2 source to sink in the spring appears to be regulated by the number of growing degree days early in the season, indicating that warmer springs may promote increased net CO2 uptake. However, this increased uptake in the spring may be lost through warmer temperatures in the late growing season that promote respiration, if this respiration is not impeded by large amounts of precipitation or cooler temperatures. Net CO2 accumulation during the growing season was generally lost through respiration during the snow covered months of September–May, turning the ecosystems into net sources of CO2 over measurement period. The water balance from June to August at the three ecosystems was variable, with the most variability observed in the heath tundra, and the least in the tussock tundra. These findings underline the importance of collecting data over the full annual cycle and across multiple types of tundra ecosystems in order to come to a more complete understanding of CO2 and water fluxes in the Arctic.This research was funded by the National Science Foundation Office of Polar Programs (OPP 0632264), with a grant during the International Polar Year, ‘Collaborative Research on Carbon, Water, and Energy Balance of the Arctic Landscape at Flagship Observatories and in a PanArctic Network’. Trac

Long-term experimental warming and nutrient additions increase productivity in tall deciduous shrub tundra

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 5 (2014): art72, doi:10.1890/ES13-00281.1.Warming Arctic temperatures can drive changes in vegetation structure and function directly by stimulating plant growth or indirectly by stimulating microbial decomposition of organic matter and releasing more nutrients for plant uptake and growth. The arctic biome is currently increasing in deciduous shrub cover and this increase is expected to continue with climate warming. However, little is known how current deciduous shrub communities will respond to future climate induced warming and nutrient increase. We examined the plant and ecosystem response to a long-term (18 years) nutrient addition and warming experiment in an Alaskan arctic tall deciduous shrub tundra ecosystem to understand controls over plant productivity and carbon (C) and nitrogen (N) storage in shrub tundra ecosystems. In addition, we used a meta-analysis approach to compare the treatment effect size for aboveground biomass among seven long-term studies conducted across multiple plant community types within the Arctic. We found that biomass, productivity, and aboveground N pools increased with nutrient additions and warming, while species diversity decreased. Both nutrient additions and warming caused the dominant functional group, deciduous shrubs, to increase biomass and proportional C and N allocation to aboveground stems but decreased allocation to belowground stems. For all response variables except soil C and N pools, effects of nutrients plus warming were largest. Soil C and N pools were highly variable and we could not detect any response to the treatments. The biomass response to warming and fertilization in tall deciduous shrub tundra was greater than moist acidic and moist non-acidic tundra and more similar to the biomass response of wet sedge tundra. Our data suggest that in a warmer and more nutrient-rich Arctic, tall deciduous shrub tundra will have greater total deciduous shrub biomass and a higher proportion of woody tissue that has a longer residence time, with a lower proportion of C and N allocated to belowground stems.This research was supported by NSF grants DEB-0516041, DEB-0516509 and the Arctic LTER (DEB-0423385)

Long-term release of carbon dioxide from Arctic tundra ecosystems in Alaska

Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Ecosystems 20 (2017): 960–974, doi:10.1007/s10021-016-0085-9.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.This work was funded by the National Science Foundation Division of Polar Programs Arctic Observatory Network grant numbers 856864, 1304271, 0632264, and 1107892. This study was also partially funded by the NSF Alaska Experimental Program to Stimulate Competitive Research award number OIA-1208927.2017-11-2

Shrub encroachment in Arctic tundra : Betula nana effects on above- and belowground litter decomposition

Author Posting. © Ecological Society of America, 2017. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecology 98 (2017): 1361–1376, doi:10.1002/ecy.1790.Rapid arctic vegetation change as a result of global warming includes an increase in the cover and biomass of deciduous shrubs. Increases in shrub abundance will result in a proportional increase of shrub litter in the litter community, potentially affecting carbon turnover rates in arctic ecosystems. We investigated the effects of leaf and root litter of a deciduous shrub, Betula nana, on decomposition, by examining species-specific decomposition patterns, as well as effects of Betula litter on the decomposition of other species. We conducted a 2-yr decomposition experiment in moist acidic tundra in northern Alaska, where we decomposed three tundra species (Vaccinium vitis-idaea, Rhododendron palustre, and Eriophorum vaginatum) alone and in combination with Betula litter. Decomposition patterns for leaf and root litter were determined using three different measures of decomposition (mass loss, respiration, extracellular enzyme activity). We report faster decomposition of Betula leaf litter compared to other species, with support for species differences coming from all three measures of decomposition. Mixing effects were less consistent among the measures, with negative mixing effects shown only for mass loss. In contrast, there were few species differences or mixing effects for root decomposition. Overall, we attribute longer-term litter mass loss patterns to patterns created by early decomposition processes in the first winter. We note numerous differences for species patterns between leaf and root decomposition, indicating that conclusions from leaf litter experiments should not be extrapolated to below-ground decomposition. The high decomposition rates of Betula leaf litter aboveground, and relatively similar decomposition rates of multiple species below, suggest a potential for increases in turnover in the fast-decomposing carbon pool of leaves and fine roots as the dominance of deciduous shrubs in the Arctic increases, but this outcome may be tempered by negative litter mixing effects during the early stages of encroachment.National Science Foundation Grant Numbers: OPP-0909507, OPP-0807639, ARC-0806451; Arctic LTER Project. Grant Number: DEB-102684

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

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Earth Surface 120 (2015): 363–378, doi:10.1002/2014JF003180.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.We are pleased to acknowledge funding from the US National Science Foundation, grants DEB-1026843 and EF-1065587, to the Marine Biological Laboratory. Additional logistical support was provided by Toolik Field Station and CH2MHill, funded by NSF's Office of Polar Programs.2015-08-2

Differential physiological responses to environmental change promote woody shrub expansion

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecology and Evolution 3 (2013): 1149–1162, doi:10.1002/ece3.525.Direct and indirect effects of warming are increasingly modifying the carbon-rich vegetation and soils of the Arctic tundra, with important implications for the terrestrial carbon cycle. Understanding the biological and environmental influences on the processes that regulate foliar carbon cycling in tundra species is essential for predicting the future terrestrial carbon balance in this region. To determine the effect of climate change impacts on gas exchange in tundra, we quantified foliar photosynthesis (Anet), respiration in the dark and light (RD and RL, determined using the Kok method), photorespiration (PR), carbon gain efficiency (CGE, the ratio of photosynthetic CO2 uptake to total CO2 exchange of photosynthesis, PR, and respiration), and leaf traits of three dominant species – Betula nana, a woody shrub; Eriophorum vaginatum, a graminoid; and Rubus chamaemorus, a forb – grown under long-term warming and fertilization treatments since 1989 at Toolik Lake, Alaska. Under warming, B. nana exhibited the highest rates of Anet and strongest light inhibition of respiration, increasing CGE nearly 50% compared with leaves grown in ambient conditions, which corresponded to a 52% increase in relative abundance. Gas exchange did not shift under fertilization in B. nana despite increases in leaf N and P and near-complete dominance at the community scale, suggesting a morphological rather than physiological response. Rubus chamaemorus, exhibited minimal shifts in foliar gas exchange, and responded similarly to B. nana under treatment conditions. By contrast, E. vaginatum, did not significantly alter its gas exchange physiology under treatments and exhibited dramatic decreases in relative cover (warming: −19.7%; fertilization: −79.7%; warming with fertilization: −91.1%). Our findings suggest a foliar physiological advantage in the woody shrub B. nana that is further mediated by warming and increased soil nutrient availability, which may facilitate shrub expansion and in turn alter the terrestrial carbon cycle in future tundra environments.This study was supported by the National Science Foundation #0732664; Australian Research Council DP0986823; and Marsden Fund of the Royal Society of New Zealand