The soil emission is highly sensitive to changes in climatic variables and relatively small changes there may have a major influence on the magnitude of soil efflux. On a seasonal scale, soil CO2 efflux strongly depends on the changes in soil temperature when water is not a limiting factor. The scale of CO2 emission measurements includes a huge range from the permanent daily measurements to monthly data or even less often. The seasonal dependence of soil CO2 emissions on rainfall is still poorly understood since it area-specific process and sometimes observes the pulse-response – the fast and strong increase in emission rates. However, the accurate estimates of the emission rates and identification of the one-time emission releases require the daily scale which is more relevant. In this study, we observed the response of the soil CO2 emission to the rain events in the lichen pine forests of Central Siberia during the vegetation season. The results from this study suggest that big rain events (more than 10 mm per one time) enhanced the soil CO2 emission rates up to 12 times. The effect of such events on the emission rates was fixed also during three days after the big rain event. The pulse-response of soil emission was different during the season: in June and July – around 47% to the month emission, in August – 28%, and the highest impact in September – 65%.

Hydroclimatic shifts and anthropogenic-driven nitrogen deposition are major outcomes of global change that could compromise the functioning of many peatlands as a carbon sink. For Sphagnum-dominated peatlands, an emerging hypothesis is that these changes could trigger shifts in competitive dominance among plant functional groups, specifically from the currently predominant decay-resistant Sphagnum to the more decomposable vascular plants. However, these global change factors do not always occur concomitantly and the relationship between Sphagnum and vascular plant occurrence is notably complex, which includes facilitative interactions that are crucial to the productivity of Sphagnum and therefore carbon sequestration. We use a conceptual review to examine underlying mechanisms for the competitive exclusion hypothesis and the nature of facilitative interactions between Sphagnum and vascular plants under the potential global change conditions. We complement the review with an empirical study of peatlands with contrasting hydrology to provide some critical insights into the potential effects of change in plant communities on carbon sequestration. We then propose a conceptual model that presents probable combinations of global change factors and their implications for carbon sequestration. Vegetation structure in Sphagnum-dominated peatland appears to be driven largely by hydrology, rather than competition among plant functional groups. The peat deposit also exerts some controls (e.g., nutrient immobilization) on biotic structure, thereby acting as resistance against an abrupt shift in plant communities. However, peatland controls that constrain vegetation shifts have developed over a millennial timescale in many peatlands, and the pace of climate change may not allow enough time for the establishment of those mechanisms in younger peatlands. Thus, the persistence of a given peatland as a carbon sink also likely depends on the successional stage of the peatland.

Fire drives the distribution, accumulation and stability of globally-important pools of carbon within boreal landscapes across spatial and temporal scales. Repeat short-interval fires in Interior Alaska (occurring within 50 years or less) are a departure from historic norms of fire intervals and drive ecological transitions in forest communities from conifer to deciduous forests. However, uncertainty remains regarding how reburn-driven shifts in community composition may alter the distribution and abundance of carbon pools across the landscape. Specifically, understanding the fate of the Alaskan boreal as either a net sink or net source of carbon requires thorough investigation of post-fire and post-reburn carbon pool distribution at a stand scale. To examine the effect of short-interval reburning, we quantified above and belowground carbon in previously black spruce (Picea mariana) stands that have burned once, twice or three times within the last 70 years within short-intervals (50 years or less). These reburned stands are dominated by emerging deciduous populations of birch (Betula neoalaskana), aspen (Populus tremuloides) and willow (Salix). Here, we report initial results of aboveground biomass, soil carbon and soil pyrogenic carbon. We examine how differences in species-specific trends in community composition account for differences in aboveground biomass between reburns in an upland and lowland site and quantify the impact of reburning on the abundance of pyrogenic carbon within boreal soils. This work will help inform landscape-scale predictions of carbon dynamics within reburned Boreal Interior Alaska and expands on our understanding of the impact of short-interval fires in the boreal system.

Fire-frequency is increasing with climate warming in the Alaskan boreal forest with <50-year fire-return intervals becoming more common. Models predict a shift in vegetation to deciduous cover, reducing the accrual of an insulating moss-derived surface organic layer (SOL). This should warm the underlying soil, increasing decomposition rates and the potential for mineral soil C losses. In 2018/2019, we compared the effects of fire frequency on vegetation cover, root biomass, SOL depth, soil temperature, and CO2 efflux from heterotrophic soil respiration (RH). Treatments replicated across two study sites included areas burned 1x, 2x, and 3x over 60 years, with all burnt last in 2003-2005. Mature (100+ year old) black spruce (Picea mariana) forest served as a control.

Depth of the SOL was highest in controls (16-19 cm) and declined with fire frequency: 1x = 9 cm, 2x = 4-7 cm, and 3x = <1 cm. Vegetation basal area and root biomass in mineral soil was lowest and spruce-dominant in 1x (0-2 m2 ha-1, 1-2 Mg ha-1), higher and mixed spruce/deciduous in 2x (0-3 m2 ha-1, 2-4 Mg ha-1), and highest and predominantly deciduous in 3x (5-9 m2 ha-1, 7-8 Mg ha-1). Mean soil temperatures at 10 cm for DOY 140-260 (TS10) were lowest for 1x/controls (7-8°C) and increased in 2x (9-10°C) and 3x (10-11°C). Annual RH efflux was highest in controls (5 Mg C ha-1), similar but variable in 3x (4-6 Mg C ha -1 yr-1), and lower amongst 1x/2x burns (2-4 Mg C ha-1 yr-1). TS10 effectively predicted RH but differences accounted for <10% of elevated RH in 2x and 3x burns, suggesting that decomposition rates increased with burn frequency but depended on multiple factors. Varying results were correlated with vegetation cover/root biomass and SOL depth, suggesting that active-layer depth, deciduous leaf-litter, and root turnover may have also stimulated decomposition.

Climate change is altering vegetation and disturbance dynamics in boreal ecosystems. However, the aggregate impact of these changes on boreal carbon budgets is not well understood. Here we combined multiple satellite datasets to estimate annual stocks and changes in aboveground biomass (AGB) across boreal northwestern North America. From 1984 to 2014, the 2.82 x 106 km2 study region gained +434 ± 176 Tg AGB. Fires resulted in losses of -789 ± 48 Tg, which were mostly compensated by post-fire recovery of +642 ± 86 Tg. Timber harvests contributed to losses of -74 ± 5 Tg, which were partly offset by post-harvest recovery of +32 ± 9 Tg. Earth system models overestimated AGB accumulation by a factor of 3 (+1,519 ± 171 Tg), which suggests these models overestimate the vegetation carbon sink in boreal ecosystems and highlights the need to improve representation of fire and other disturbance processes in these models.

How the current net CO2 sink strength of boreal ecosystems will respond to climate warming still remains unclear. Warmer air temperature can enhance both photosynthesis and respiration with changes in net ecosystem CO2 exchange (NEE) resulting from the combined changes in both processes. Recently, warming has been found to cause net CO2 loss using a whole-ecosystem warming experiment (“Spruce and Peatland Responses Under Changing Environments” [SPRUCE]). However, field observations of warming responses of NEE are still rare. Here, we use a pan-boreal multi-year eddy covariance NEE synthesis dataset from boreal forest and peatland ecosystems to quantify the effect of air temperature on interannual variability in monthly NEE. We find that temperature sensitivity varies seasonally with increasing net CO2 uptake with warming in the spring and decreasing net CO2 uptake with warming in late summer and fall. Then, we test the seasonal relationship between temperature sensitivity of NEE and vegetation greening (EVI/NDVI) and dryness index. Furthermore, we compare the estimated NEE responses to current and projected warming rates from the CRU TS climate records and from CMIP6 model simulations, respectively, using (a) seasonally resolved warming trends and (b) uniform annual warming rates, as simulated in the SPRUCE project. The results highlight the importance of seasonally varying warming trends on future C cycle responses in the boreal biome.

Boreal forests provide an important global carbon sink. Since their productivity is commonly constrained by nitrogen availability, the current view is that increased nitrogen supply enhances their ecosystem carbon sink-strength. While this understanding relies primarily on observed responses from aboveground tree biomass and soil carbon stocks, empirical data evaluating effects on the ecosystem-scale carbon balance are lacking. Here we use a unique long-term experiment including paired stands with eddy covariance measurements to explore ecosystem-scale nitrogen addition effects on the ecosystem carbon balance of a boreal pine stand. We find that one decade of nitrogen addition altered the sensitivity of carbon fluxes to environmental conditions. Furthermore, while the net C uptake initially increased, it stabilized after one decade indicating a transient impact of N addition. Our results reveal that the limited long-term effect results from a tight seasonal coupling due to which enhanced growing season carbon uptake are compensated by increased emission during the subsequent winter period. Thus, our study highlights the need to account for whole ecosystem responses to perturbations to improve our understanding on global change-carbon cycle feedbacks in boreal forests.

The research was conducted in the Republic of Karelia, Russia (N61.8 E33.8). The objects of the study were plant communities on former agricultural lands on Retisols and Podzols: hay meadow, deciduous forests of 15–20 years, pine and spruce forests of 65 years, pine and spruce forests of 110 years. For each sample plots, data on the absolute dry weight of the aboveground part of vascular plants were obtained, divided into functional groups (graminoides, herbs, shrubs, ferns, legumes).

The regularities of the successional development of plant communities are manifested in changes in the absolute and relative values of the aboveground biomass of certain functional groups of plants in the total biomass. In meadows, the greatest contribution to the composition of the ground cover is made by plants of the group of graminoides (from 52%) and various grasses (from 13%). In young forests, the main contribution to the total biomass is made by graminoides (from 21%) and various grasses (from 28%), as well as shrubs. The middle-aged communities are characterized by a large participation of ferns (Pteridium aquilinum) and various grasses, which is an indicator of anthropogenic disturbance. In the old-growth forest community, forest shrubs predominate (more than 60%). Based on the data on the biomass of different plant groups in each sample plot, a cluster analysis was carried out, which showed the degree of their similarity. Plant communities that are at the same stage of development, regardless of the type of soil, have the greatest similarity. That is, two meadows are combined in one cluster, young trees in the other, and middle-aged forests in the third. Thus, it can be concluded that the stage of development of the plant community has a greater influence on the structure of the ground cover than the habitat conditions.

Boreal forests contain ~30% of the global soil organic carbon (SOC) stock within a region expecting large increases in temperature and precipitation over the next century. Yet, the controls on SOC reservoirs remain poorly understood. In the mineral soil, weathering products and other soil characteristics may offer a mechanism for stabilizing organic material within moist forests, but detailed studies in boreal forests are lacking. The Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect is a climate transect that exhibits an increase in temperature and precipitation with decreasing latitude similar to that expected in the next century in this region. Average surface mineral SOC stocks are similar among regions despite a 3-fold increase in dissolved organic carbon inputs and >50% increase in litterfall with decreasing latitude. To explain this, here, the relationships between mineral soil characteristics and ecosystem parameters with SOC were assessed using an information theoretic approach to rank models. Organically complexed metals, a weathering product, were a component of all plausible models. Models including mineral surface area were also plausible, however, strong correlations with organically complexed metals suggest these complexes make up a significant portion of this surface area. The organically complexed Al ranked higher than Fe, potentially explained by the greater solubility of reduced Fe. We suggest that pockets of reducing conditions may occur even in these upland soils, particularly beneath the winter snowpack, or within the saturated soils of the spring melt. Thus, Fe-organic complexes may be removed while Al complexes remain largely unaffected. These results suggest that mineralogically-controlled mechanisms explain the observed constancy of SOC stocks despite the increased inputs across this transect, but also that such a mechanism and its influence on SOC may be impacted by climate effects (e.g. shifts in snowpack dynamics) that control forest soil redox conditions.

We studied the long-term effects of moth-damage on the ecosystem and soil carbon stocks in the Pulmankijärvi area in Finnish Lapland. This area is characterized by high year-round grazing pressure of reindeer, which hinders the recovery of mountain birch (B. pubescens Ehrh. ssp. czerepanovii) trees after damage caused by moth (Epirrita autumnata and Operophtera brumata) outbreaks. Severe moth outbreaks in the 1960’s and between 2006-2008 have resulted in death of trees in the area. The moths can affect large areas, and these periodic outbreaks are predicted to occur more frequently in the future with climate change. Thus, it would be important to understand their effects on ecosystem and soil C stocks at the mountain birch treeline. We compared soil C stocks under living trees, and trees that died in the 1960’s outbreak, and in the 2006-2008 outbreak to a treeless tundra control using a small-scale space-for-time substitution approach on three sites (1 ha each), where all these tree statuses were present in the same area in the patchy landscape. Soil C stocks under living trees were significantly higher (4.1 ± 2.1 kg m2) than in the treeless tundra (2.4 ± 0.6 kg m2), and still remained higher even decades after the trees died (after 12 years: 3.7 ± 1.7 kg m2 or after 55 years: 4.9 ± 3.0 kg m2). Our results indicate that live mountain birch trees accumulate soil C stocks, even though they have been suggested to cause losses of C if treelines advance on the treeless tundra, as their litter is easily decomposable and they may prime organic matter decomposition to access enough N for their growth in these N-limited ecosystems. Our results indicate that such priming-induced soil C losses could not exceed the overall positive effect of living trees on soil C stocks in this area.

Forested wetlands play a vital role in carbon (C) sequestration. In this study, we compared the short-term effects (5 years) of two wildfire intensities, three selective harvest intensities and the mid-term effects (25 years) of four groundwater table depth (WTD) drainage for forestry on ecosystem C storage in Daxing’anling, northeast of China. We found that: Low intensity harvest led to a mild increase in ecosystem C storage while moderate- and high-intensity harvest resulted in significant reductions (33.2-41.6%) compared with the control natural forested wetlands stand (274.54 t C·ha-1), and light- and heavy-intensity burn caused the ecosystem C storage decreases by 27. 8% and 45. 2%. As for the drainage for forestry, the ecosystem C storage was higher at the low WTD (316.78 t C·ha-1), and decreased significantly by 24.1-28.1% with the increasing WTD on the forested wetland plantation transect. Significantly greater NPP (69.1-83.2%) and annual C sequestration (52.0-78.7%) were in the low- and moderate-intensity harvest stands compared to the high intensity harvest and control (8.28 t ha-1 yr-1, 5.08 t C·ha-1 yr-1 and 6.48 t ha-1 yr-1, 3.52 t C·ha-1 yr-1). Increases by 48.6% and 35.8% and decreases by 22.1% and 38.8% in NPP and annual C sequestration were in light- and heavy-intensity burn stands, respectively. The ecosystem C storage under the drainage for forestry treatment was higher at low WTD (316.78 t C·ha-1) and decreased significantly by 24.1-28.1% with increasing WTD, while the NPP and annual C sequestration (3. 67-10. 34 t ha-1 yr-1 and 1. 59-4. 87 t C·ha-1 yr-1) showed a significant increasing trend with increasing WTD, respectively. Therefore, it appears that low intensity wildfire and harvest may be capable of sustaining the ecosystem C sink and drainage for forestry is an effect way to restore C sequestration for this forested wetland type.

The aim of the study was to estimate with modelling approach the effect of continuous-cover forestry on the emissions of greenhouse gases (GHG) from nutrient-rich drained peatland sites. The simulated management scenarios of Norway spruce dominated stands in southern Finland were constructed by varying harvesting interval and the post-harvest basal area. A process-based model of soil organic matter dynamics used for estimation of mineralization of fresh litter was coupled with empirical routines for peat CO2 and CH4 emissions. Simulations showed that the forest ecosystems on peatland sites represent carbon sinks with low and middle harvesting intensity, while they turned into carbon sources with high harvesting intensities (with post-harvest basal area lower than 8 m2 ha‚àí1). This was mostly because intensive harvestings raised the water levels, thus decreasing tree growth and increasing soil methane emissions. Carbon dioxide emissions from peat and litter, in turn, correlated negatively with the intensity of harvestings. Correlation of site carbon balance with harvested roundwood indicated that there is a significant trade-off between maintaining carbon in drained peatland forests and providing harvest revenues. We also concluded that the ditch network maintenance may be needed to sustain the stand growth especially when the harvest intensity is high. The management solutions assume trade-off between profitability, including income from timber and harvesting costs, and the necessity of maintaining other monetary and non-monetary ecosystem services. The simulations provide novel results and fill a gap of knowledge in ecosystem responses to alternative management regimes in continuous-cover forestry on drained peatlands. The study was supported by the Academy of Finland, projects #310203, #322972 and #312912 (collecting and processing of the experimental data and reporting results), and by the Russian Science Foundation, project #18-14-00362 (performing the simulation experiments and reporting results).

Enhanced forest growth can contribute to the climate change mitigation. Forest soil treatment with fertilizer, likewise drainage and thinning can result in enhanced forest growth. This study summarizes the results of forest soil fertilization with wood ash and ammonium nitrate, in Latvia. The experimental objects are established in 61 stands on dry mineral soil, drained mineral soil and drained organic soil, where the dominant tree species are Norway spruce Picea abies (L.) H.Karst., Scots pine Pinus sylvestris L. and Birch Betula ssp.. We analyzed the organic carbon content in soil samples collected from different layers. The results were evaluated within each experimental group determined by the fertilizer applied (wood ash, ammonium nitrate, combined fertilizer). The mean detected organic carbon stock at 0 – 40 cm depth in objects treated with wood ash is 120 t ha-1 (control) and 119 t ha-1 (fertilized) in stands on mineral soil and 497 t ha-1 (control) and 648 t ha-1 (fertilized) in stands on organic soil. The mean organic carbon stock at 0 – 40 cm depth in objects treated with ammonium nitrate is 86 t ha-1 (control) and 88 t ha-1 (fertilized) in stands on mineral soil and 152 t ha-1 (control) and 125 t ha-1 (fertilized) in stands on organic soil. The mean organic carbon stock at 0 – 40 cm depth in objects treated with combined fertilizer is 182 t ha-1 (control) and 164 t ha-1 (fertilized) in stands on drained mineral soil and 333 t ha-1 (control) and 265 t ha-1 (fertilized) in stands on organic soil.

One of the most rapid pathways through which climate warming could alter the structure and function of high northern latitude ecosystems is through intensification of wildfire disturbance. Here, we plan to investigate the impacts of intensifying wildfire regimes on the long-term carbon (C) storage of boreal and tundra ecosystems. More severe and frequent wildfires can shift northern ecosystems across a C cycle threshold: from a net accumulation of C from the atmosphere over centuries and fire cycles to a net loss. This shift can occur when organic soil C that escaped burning in previous fires or that has accumulated over centuries, termed ‘legacy carbon’, combusts. We know that more frequent wildfires can lead to legacy C loss and expect that this will vary with fire severity and soil drainage, primary factors shaping C storage in high latitude ecosystems. Here, we propose to determine the frequency of legacy C loss across a diverse array of ecosystems throughout the circumpolar region. We have developed an organic soil collection protocol and are currently collaborating with 12 distinct research groups with sites across North America, Europe, and Russia. Each research group has agreed to collect and send soil samples from 10 recently burned forest or tundra sites within their respective study regions. At Northern Arizona University, we will use radiocarbon dating to assess legacy C presence and combustion and hold a data synthesis workshop for all collaborators. We invite anyone interested in participating in the Legacy Carbon Collaborative to contact Xanthe Walker at Northern Arizona University (email: xanthe.walker@gmail.com).

The attributes of dead wood (DW) are an important determinant of numerous forest ecosystem functions, ranging from wildlife habitat to carbon cycling. Despite this, information regarding the DW in the boreal forests of Alaska has been limited until a systematic forest inventory of the Tanana region of interior Alaska was completed by the USDA Forest Service and partners from 2014-2018. In order to more fully evaluate the results of this DW inventory, numerous DW attributes (DW biomass by species, decay, and size class) were investigated for the boreal forests of the Tanana region in comparison to the DW attributes derived from the same inventory program across the coterminous US. Overall, initial results suggest DW characteristics unique to boreal forests when compared to temperate forests with important implications against a backdrop of global change.

Boreal forest soils contain a large pool of soil organic carbon (C) that is sensitive to changes in climate and disturbance. Destabilization of boreal forest soil C would provide a positive feedback to climate in the form of rising atmospheric CO2 concentration. Current estimates of soil C in Interior Alaska vary and are often limited to study sites near the road system, leading to uncertainty in current and future soil C stocks. In this study, we leveraged an extensive collection of soil cores from 520 Forest Inventory and Analysis (FIA) plots across ~13.5 million hectares in the Tanana Valley to gain insight into the factors governing soil C pools in four dominant boreal forest types. Using complementary random forest and linear mixed-effects analyses, we examined the relative influence of ecological, topographic and climatic factors contributing to soil C pools across the region. Total soil C was greatest in black spruce, least in aspen and intermediate in Alaska paper birch and white spruce forests. Soil C in the mineral layer accounted for a substantial fraction (42-51%) of total soil C but varied strongly among forest types. In black and white spruce stands, we detected smaller soil C pools on south-facing aspects, while slope angle had a minor effect on soil C pools on north-facing sites. Our analysis provides important insights to the vertical and spatial distribution of soil C from under-sampled regions and forest communities in Interior Alaska. The FIA soil C data offer a valuable opportunity to improve understanding of soil C stocks in the Tanana watershed and throughout boreal Alaska.