Assessing effects of permafrost thaw on C fluxes based on multiyear modeling across a permafrost thaw gradient at Stordalen, Sweden

Northern peatlands in permafrost regions contain a large amount of organic carbon (C) in the soil. Climate warming and associated permafrost degradation are expected to have significant impacts on the C balance of these ecosystems, but the magnitude is uncertain. We incorporated a permafrost model, Northern Ecosystem Soil Temperature (NEST), into a biogeochemical model, DeNitrificationDeComposition (DNDC), to model C dynamics in highlatitude peatland ecosystems. The enhanced model was applied to assess effects of permafrost thaw on C fluxes of a subarctic peatland at Stordalen, Sweden. DNDC simulated soil freeze–thaw dynamics, net ecosystem exchange of CO2 (NEE), and CH4 fluxes across three typical land cover types, which represent a gradient in the process of ongoing permafrost thaw at Stordalen. Model results were compared with multiyear field measurements, and the validation indicates that DNDC was able to simulate observed differences in seasonal soil thaw, NEE, and CH4 fluxes across the three land cover types. Consistent with the results from field studies, the modeled C fluxes across the permafrost thaw gradient demonstrate that permafrost thaw and the associated changes in soil hydrology and vegetation not only increase net uptake of C from the atmosphere but also increase the annual to decadal radiative forcing impacts on climate due to increased CH4 emissions. This study indicates the potential of utilizing biogeochemical models, such as DNDC, to predict the soil thermal regime in permafrost areas and to investigate impacts of permafrost thaw on ecosystem C fluxes after incorporating a permafrost component into the model framework

Issues Related to Incorporating Northern Peatlands into Global Climate Models

Northern peatlands cover ~3–4 million km2 (~10% of the land north of 45°N) and contain ~200–400 Pg carbon (~10–20% of total global soil carbon), almost entirely as peat (organic soil). Recent developments in global climate models have included incorporation of the terrestrial carbon cycle and representation of several terrestrial ecosystem types and processes in their land surface modules. Peatlands share many general properties with upland, mineral-soil ecosystems, and general ecosystem carbon, water, and energy cycle functions (productivity, decomposition, water infiltration, evapotranspiration, runoff, latent, sensible, and ground heat fluxes). However, northern peatlands also have several unique characteristics that will require some rethinking or revising of land surface algorithms in global climate models. Here we review some of these characteristics, deep organic soils, a significant fraction of bryophyte vegetation, shallow water tables, spatial heterogeneity, anaerobic biogeochemistry, and disturbance regimes, in the context of incorporating them into global climate models. With the incorporation of peatlands, global climate models will be able to simulate the fate of northern peatland carbon under climate change, and estimate the magnitude and strength of any climate system feedbacks associated with the dynamics of this large carbon pool

Longer growing seasons do not increase net carbon uptake in Northeastern Siberian tundra

With global warming, snowmelt is occurring earlier and growing seasons are becoming longer around the Arctic. It has been suggested that this would lead to more uptake of carbon due to a lengthening of the period in which plants photosynthesize. To investigate this suggestion, 8 consecutive years of eddy covariance measurements at a northeastern Siberian graminoid tundra site were investigated for patterns in net ecosystem exchange, gross primary production (GPP) and ecosystem respiration (Reco). While GPP showed no clear increase with longer growing seasons, it was significantly increased in warmer summers. Due to these warmer temperatures however, the increase in uptake was mostly offset by an increase in Reco. Therefore, overall variability in net carbon uptake was low, and no relationship with growing season length was found. Furthermore, the highest net uptake of carbon occurred with the shortest and the coldest growing season. Low uptake of carbon mostly occurred with longer or warmer growing seasons. We thus conclude that the net carbon uptake of this ecosystem is more likely to decrease rather than to increase under a warmer climate. These results contradict previous research that has showed more net carbon uptake with longer growing seasons. We hypothesize that this difference is due to site-specific differences, such as climate type and soil, and that changes in the carbon cycle with longer growing seasons will not be uniform around the Arcti

The Torneträsk System - A basis for predicting future subarctic ecosystems

Arctic and subarctic areas have experienced a rapid warming and substantial increases in precipitation in recent decades. The frequency and intensity of some extreme events, such as fires, winter warming events, extreme rainfall, and droughts, has also increased. These climatic changes and other anthropogenic factors have caused profound changes in arctic and subarctic ecosystems with important implications for the local residents and for the global population, which are likely to exacerbate under the predicted climate change scenarios. Thus, a better understanding of potential future ecosystem changes is paramount for defining climate change mitigation goals and adaptation strategies.Dynamic ecosystem models are powerful tools to study the influences of climatic and other drivers on ecosystem processes. Nevertheless, predictions of ecosystem changes still hold large uncertainties, arising mostly from insufficient observational data, lack of process understanding, difficulties in quantifying the effects of different ecosystem processes and their interactions, and/or model limitations in representing these interacting processes. The Torneträsk area, in the Swedish subarctic, has an unrivalled history of environmental observations spanning over 100 years, and is one of the most studied sites in the Arctic. The area has undergone substantial climatic and ecosystem changes. By studying its rapidly-transforming ecosystems we can obtain critically important information needed to improve our understanding and predictions of future ecosystem changes at a larger scale. This thesis summarized and ranked the direct and indirect drivers of ecosystem change in the Torneträsk area, and proposed research priorities to improve predictions of ecosystem change. Winter warming events (WWEs) were the top-ranked research priority. Hence, this thesis further examined the impacts of WWEs on subarctic ecosystems using monitoring data, manipulation experiments and modelling. The monitoring and manipulation data suggest an increasingly strong warming effect of WWEs on permafrost, especially rain on snow events occurrying in the presence of thick snowpacks. The modeling experiments in LPJ-GUESS indicated a strong cooling effect of WWEs on ground temperature, driven mostly by changes in snow insulation, which resulted in profound changes in the biogeochemical fluxes of magnitudes comparable to long-term climatic changes. We identified several modeling gaps that may explain the mismatch between the model- and the observational-based impacts of WWEs on ground temperatures, including 1) the lack of surface energy balance in LPJ-GUESS, the model’s daily timestep that neglects sub-daily freeze-thaw cycles within the snowpack, and 3) the model’s simplistic water retention scheme that minimizes the amount of water retained in the snowpack and hence the amount of latent heat release upon freezing. Addressing these issues is paramount for accurately estimating future ecosystem changes and their inplications on the arctic’s carbon balance. Climate change, including long term changes and short-lasting events such as WWEs, affected lowland permafrost sites in the Torneträsk area differently, depending on the site-specific climatíc and environmental conditions. This resulted in permafrost thaw rates decreasing Eastwards. This thesis revealed, through metagenomic sequencing and greenhouse gas measurements in three peatlands across this thaw gradient, that different rates and stages of permafrost degradation influence greenhouse gas exchange through an altered taxonomic structure and function of the microbial communities. This highlights the need for expanding the monitoring of peatland fluxes and microbial dynamics that is currently based on very few sites

Tundra shrubification and tree-line advance amplify arctic climate warming:results from an individual-based dynamic vegetation model

One major challenge to the improvement of regional climate scenarios for the northern high latitudes is to understand land surface feedbacks associated with vegetation shifts and ecosystem biogeochemical cycling. We employed a customized, Arctic version of the individual-based dynamic vegetation model LPJ-GUESS to simulate the dynamics of upland and wetland ecosystems under a regional climate model-downscaled future climate projection for the Arctic and Subarctic. The simulated vegetation distribution (1961-1990) agreed well with a composite map of actual arctic vegetation. In the future (2051-2080), a poleward advance of the forest-tundra boundary, an expansion of tall shrub tundra, and a dominance shift from deciduous to evergreen boreal conifer forest over northern Eurasia were simulated. Ecosystems continued to sink carbon for the next few decades, although the size of these sinks diminished by the late 21st century. Hot spots of increased CH4 emission were identified in the peatlands near Hudson Bay and western Siberia. In terms of their net impact on regional climate forcing, positive feedbacks associated with the negative effects of tree-line, shrub cover and forest phenology changes on snow-season albedo, as well as the larger sources of CH4, may potentially dominate over negative feedbacks due to increased carbon sequestration and increased latent heat flux

Twenty-Two Years of Warming, Fertilisation and Shading of Subarctic Heath Shrubs Promote Secondary Growth and Plasticity but Not Primary Growth

Most manipulation experiments simulating global change in tundra were short-term or did not measure plant growth directly. Here, we assessed the growth of three shrubs (Cassiope tetragona, Empetrum hermaphroditum and Betula nana) at a subarctic heath in Abisko (Northern Sweden) after 22 years of warming (passive greenhouses), fertilisation (nutrients addition) and shading (hessian fabric), and compare this to observations from the first decade of treatment. We assessed the growth rate of current-year leaves and apical stem (primary growth) and cambial growth (secondary growth), and integrated growth rates with morphological measurements and species coverage. Primary- and total growth of Cassiope and Empetrum were unaffected by manipulations, whereas growth was substantially reduced under fertilisation and shading (but not warming) for Betula. Overall, shrub height and length tended to increase under fertilisation and warming, whereas branching increased mostly in shaded Cassiope. Morphological changes were coupled to increased secondary growth under fertilisation. The species coverage showed a remarkable increase in graminoids in fertilised plots. Shrub response to fertilisation was positive in the short-term but changed over time, likely because of an increased competition with graminoids. More erected postures and large, canopies (requiring enhanced secondary growth for stem reinforcement) likely compensated for the increased light competition in Empetrum and Cassiope but did not avoid growth reduction in the shade intolerant Betula. The impact of warming and shading on shrub growth was more conservative. The lack of growth enhancement under warming suggests the absence of long-term acclimation for processes limiting biomass production. The lack of negative effects of shading on Cassiope was linked to morphological changes increasing the photosynthetic surface. Overall, tundra shrubs showed developmental plasticity over the longer term. However, such plasticity was associated clearly with growth rate trends only in fertilised plots

Impacts of extreme winter warming events on litter decomposition in a sub-Arctic heathland

Arctic climate change is expected to lead to a greater frequency of extreme winter warming events. During these events, temperatures rapidly increase to well above 0 degrees C for a number of days, which can lead to snow melt at the landscape scale, loss of insulating snow cover and warming of soils. However, upon return of cold ambient temperatures, soils can freeze deeper and may experience more freeze-thaw cycles due to the absence of a buffering snow layer. Such loss of snow cover and changes in soil temperatures may be critical for litter decomposition since a stable soil microclimate during winter (facilitated by snow cover) allows activity of soil organisms. Indeed, a substantial part of fresh litter decomposition may occur in winter. However, the impacts of extreme winter warming events on soil processes such as decomposition have never before been investigated. With this study we quantify the impacts of winter warming events on fresh litter decomposition using field simulations and lab studies. Winter warming events were simulated in sub-Arctic heathland using infrared heating lamps and soil warming cables during March (typically the period of maximum snow depth) in three consecutive years of 2007, 2008, and 2009. During the winters of 2008 and 2009, simulations were also run in January (typically a period of shallow snow cover) on separate plots. The lab study included soil cores with and without fresh litter subjected to winter-warming simulations in climate chambers. Litter decomposition of common plant species was unaffected by winter warming events simulated either in the lab (litter of Betula pubescens ssp. czerepanovii), or field (litter of Vaccinium vitis-idaea, and B. pubescens ssp. czerepanovii) with the exception of Vaccinium myrtillus (a common deciduous dwarf shrub) that showed less mass loss in response to winter warming events. Soil CO2 efflux measured in the lab study was (as expected) highly responsive to winter warming events but surprisingly fresh litter decomposition was not. Most fresh litter mass loss in the lab occurred during the first 3-4 weeks (simulating the period after litter fall). In contrast to past understanding, this suggests that winter decomposition of fresh litter is almost nonexistent and observations of substantial mass loss across the cold season seen here and in other studies may result from leaching in autumn, prior to the onset of "true" winter. Further, our findings surprisingly suggest that extreme winter warming events do not affect fresh litter decomposition. Crown Copyright (c) 2009 Published by Elsevier Ltd. All rights reserved

Potential for Abrupt Changes in Atmospheric Methane

Methane (CH4) is the second most important greenhouse gas that humans directly influence, carbon dioxide (CO2) being first. Concerns about methane’s role in abrupt climate change stem primarily from (1) the large quantities of methane stored as solid methane hydrate on the sea floor and to a lesser degree in terrestrial sediments, and the possibility that these reservoirs could become unstable in the face of future global warming, and (2) the possibility of large-scale conversion of frozen soil in the high- latitude Northern Hemisphere to methane producing wetland, due to accelerated warming at high latitudes. This chapter summarizes the current state of knowledge about these reservoirs and their potential for forcing abrupt climate change

Peatland dynamics in response to past and potential future climate change : A regional modelling approach

The majority of the northern peatlands developed during the Holocene as a result of a positive mass balance between net primary productivity (NPP) and heterotrophic decomposition rates. Over that time they have sequestered a huge amount of carbon in terrestrial ecosystems. A significant proportion of these areas also coincides with areas underlain with permafrost and shows a diverse range of peat accumulation patterns. Thus, for predicting and understanding the long-term evolution of peatland carbon stocks across the pan-Arctic, mechanistic representations of both peatland and permafrost dynamics are needed in the modelling framework. In this thesis, a novel implementation of dynamic multi-layer peatland and permafrost dynamics in the individual- and patch- based dynamic vegetation and ecosystem model (LPJ-GUESS) is described. The major emphasis of this work goes into enhancing the current understanding of the processes involved in the long-term peat accumulation and its internal dynamics, including how these systems are influenced by small-scale heterogeneity, vegetation dynamics and interactions with underlying permafrost. A simple two-dimensional microtopographical (2-DMT) model was also developed to address the established hypotheses concerning stability, behaviour and transformation of these microstructures and the effects of this small-scale heterogeneity on the coupled dynamics of vegetation, hydrology and peat accumulation. LPJ-GUESS was calibrated and validated using data from a mire in Stordalen, northern Sweden, and evaluated using data from multiple sites in Scandinavia and from Mer Bleue, Canada. It was subsequently applied across the pan-Arctic to advance the existing knowledge on carbon accumulation rates at different spatial and temporal scales, and also to demonstrate the potential implications of current warming on these climate sensitive ecosystems. Both of the models developed in this thesis performed satisfactorily when confronted with experimental data.LPJ-GUESS is quite robust in capturing peat accumulation and permafrost dynamics including reasonable vegetation and hydrological conditions at temporal and spatial scales across various climate gradients. The simulations improved our knowledge of peatland functioning in the past, present and future. It was found that Stordalen mire will continue to accumulate carbon in the coming decades but later will turn into a carbon source. It was also found that permafrost-free regions that are predicted to experience reduced rates of precipitation may lose significant amount of carbon in the future due to reductions in soil moisture. Conversely, peatlands currently underlain with permafrost could gain carbon due to an initial increase in soil moisture as a result of permafrost thawing. My modelling results also suggest that peatlands can show diverse range of behaviour with alternative compositional and structural dynamics depending on the initial topographical, climatic conditions, and plant characteristics, therefore, it will be challenging to represent such dynamics in current Earth System Models (ESMs). With the inclusion of aforementioned processes, LPJ-GUESS has now become quite robust. The resultant model can now be coupled with ESM where it can address issues related to peatland-mediated biogeochemical and biophysical feedbacks to climate change in the Arctic and globally

Responses to projected changes in climate and UV-B at the species level

Environmental manipulation experiments showed that species respond individualistically to each environmental-change variable. The greatest responses of plants were generally to nutrient, particularly nitrogen, addition. Summer warming experiments showed that woody plant responses were dominant and that mosses and lichens became less abundant. Responses to warming were controlled by moisture availability and snow cover. Many invertebrates increased population growth in response to summer warming, as long as desiccation was not induced. CO2 and UV-B enrichment experiments showed that plant and animal responses were small. However, some microorganisms and species of fungi were sensitive to increased UV-B and some intensive mutagenic actions could, perhaps, lead to unexpected epidemic outbreaks. Tundra soil heating, CO 2 enrichment and amendment with mineral nutrients generally accelerated microbial activity. Algae are likely to dominate cyanobacteria in milder climates. Expected increases in winter freeze-thaw cycles leading to ice-crust formation are likely to severely reduce winter survival rate and disrupt the population dynamics of many terrestrial animals. A deeper snow cover is likely to restrict access to winter pastures by reindeer/caribou and their ability to flee from predators while any earlier onset of the snow-free period is likely to stimulate increased plant growth. Initial species responses to climate change might occur at the sub-species level: an Arctic plant or animal species with high genetic/racial diversity has proved an ability to adapt to different environmental conditions in the past and is likely to do so also in the future. Indigenous knowledge, air photographs, satellite images and monitoring show that changes in the distributions of some species are already occurring: Arctic vegetation is becoming more shrubby and more productive, there have been recent changes in the ranges of caribou, and "new" species of insects and birds previously associated with areas south of the treeline have been recorded. In contrast, almost all Arctic breeding bird species are declining and models predict further quite dramatic reductions of the populations of tundra birds due to warming. Species-climate response surface models predict potential future ranges of current Arctic species that are often markedly reduced and displaced northwards in response to warming. In contrast, invertebrates and microorganisms are very likely to quickly expand their ranges northwards into the Arctic