Water level drawdown makes boreal peatland vegetation more responsive to weather conditions.

Climate warming and projected increase in summer droughts puts northern peatlands under pressure by subjecting them to a combination of gradual drying and extreme weather events. The combined effect of those on peatland functions is poorly known. Here, we studied the impact of long-term water level drawdown (WLD) and contrasting weather conditions on leaf phenology and biomass production of ground level vegetation in boreal peatlands. Data were collected during two contrasting growing seasons from a WLD experiment including a rich and a poor fen and an ombrotrophic bog. Results showed that WLD had a strong effect on both leaf area development and biomass production, and these responses differed between peatland types. In the poor fen and the bog, WLD increased plant growth, while in the rich fen, WLD reduced the growth of ground level vegetation. Plant groups differed in their response, as WLD reduced the growth of graminoids, while shrubs and tree seedlings benefited from it. In addition, the vegetation adjusted to the lower WTs, was more responsive to short-term climatic variations. The warmer summer resulted in a greater maximum and earlier peaking of leaf area index, and greater biomass production by vascular plants and Sphagnum mosses at WLD sites. In particular, graminoids benefitted from the warmer conditions. The change towards greater production in the WLD sites in general and during the warmer weather in particular, was related to the observed transition in plant functional type composition towards arboreal vegetation.

How do forest fires affect soil greenhouse gas emissions in upland boreal forests? A review.

Wildfires strongly regulate carbon (C) cycling and storage in boreal forests and account for almost 10% of global fire C emissions. However, the anticipated effects of climate change on fire regimes may destabilize current C-climate feedbacks and switch the systems to new stability domains. Since most of these forests are located in upland soils where permafrost is widespread, the expected climate warming and drying combined with more active fires may alter the greenhouse gas (GHG) budgets of boreal forests and trigger unprecedented changes in the global C balance. Therefore, a better understanding of the effects of fires on the various spatial and temporal patterns of GHG fluxes of different physical environments (permafrost and nonpermafrost soils) is fundamental to an understanding of the role played by fire in future climate feedbacks. While large amounts of C are released during fires, postfire GHG fluxes play an important role in boreal C budgets over the short and long term. The timescale over which the vegetation cover regenerates seems to drive the recovery of C emissions after both low- and high-severity fires, regardless of fire-induced changes in soil decomposition. In soils underlain by permafrost, fires increase the active layer depth for several years, which may alter the soil dynamics regulating soil GHG exchange. In a scenario of global warming, prolonged exposition of previously immobilized C could result in higher carbon dioxide emission during the early fire succession. However, without knowledge of the contribution of each respiration component combined with assessment of the warming and drying effects on both labile and recalcitrant soil organic matter throughout the soil profile, we cannot advance on the most relevant feedbacks involving fire and permafrost. Fires seem to have either negligible effects on methane (CH4) fluxes or a slight increase in CH4 uptake. However, permafrost thawing driven by climate or fire could turn upland boreal soils into temporary CH4 sources, depending on how fast the transition from moist to drier soils occurs. Most studies indicate a slight decrease or no significant change in postfire nitrous oxide (N2O) fluxes. However, simulations have shown that the temperature sensitivity of denitrification exceeds that of soil respiration; thus, the effects of warming on soil N2O emissions may be greater than on C emissions.

Wildfire effects on BVOC emissions from boreal forest floor on permafrost soil in Siberia.

One of the effects of climate change on boreal forest will be more frequent forest wildfires and permafrost thawing. These will increase the availability of soil organic matter (SOM) for microorganisms, change the ground vegetation composition and ultimately affect the emissions of biogenic volatile organic compounds (BVOCs), which impact atmospheric chemistry and climate. BVOC emissions from boreal forest floor have been little characterized in southern boreal region, and even less so in permafrost soil, which underlies most of the northern boreal region. Here, we report the long-term effects of wildfire on forest floor BVOC emission rates along a wildfire chronosequence in a Larix gmelinii forest in central Siberia. We determined forest floor BVOC emissions from forests exposed to wildfire 1, 23 and > 100 years ago. We studied how forest wildfires and the subsequent succession of ground vegetation, as well as changes in the availability of SOM along with the deepened and recovered active layer, influence BVOC emission rates. The forest floor acted as source of a large number of BVOCs in all forest age classes. Monoterpenes were the most abundant BVOC group in all age classes. The total BVOC emission rates measured from the 23- and >100-year-old areas were ca. 2.6 times higher than the emissions from the 1-year-old area. Lower emissions were related to a decrease in plant coverage and microbial decomposition of SOM after wildfire. Our results showed that forest wildfires play an important indirect role in regulating the amount and composition of BVOC emissions from post-fire originated boreal forest floor. This could have a substantial effect on BVOC emissions if the frequency of forest wildfires increases in the future as a result of climate warming.

Temperature sensitivity of soil organic matter decomposition after forest fire in Canadian permafrost region.

Climate warming in arctic/subarctic ecosystems will result in increased frequency of forest fires, elevated soil temperatures and thawing of permafrost, which have implications for soil organic matter (SOM) decomposition rates, the CO2 emissions and globally significant soil C stocks in this region. It is still unclear how decomposability and temperature sensitivity of SOM varies in different depths and different stages of succession following forest fire in permafrost regions and studies on long term effects of forest fires in these areas are lacking. To study this question, we took soil samples from 5, 10 and 30 cm depths from forest stands in Northwest Canada, underlain by permafrost, that were burnt by wildfire 3, 25 and over 100 years ago. We measured heterotrophic soil respiration at 1, 7, 13 and 19 °C. Fire had a significant effect on the active layer depth, and it increased the temperature sensitivity (Q10) of respiration in the surface (5 cm) and in the deepest soil layer (30 cm) in the 3-year-old area compared to the 25- and more than 100-year-old areas. Also the metabolic quotient (qCO2) of soil microbes was increased after fire. Though fires may facilitate the SOM decomposition by increasing active layer depth, they also decreased SOM quality, which may limit the rate of decomposition. After fire all of these changes reverted back to original levels with forest succession.

Changes in fluxes of carbon dioxide and methane caused by fire in Siberian boreal forest with continuous permafrost.

Rising air temperatures and changes in precipitation patterns in boreal ecosystems are changing the fire occurrence regimes (intervals, severity, intensity, etc.). The main impacts of fires are reported to be changes in soil physical and chemical characteristics, vegetation stress, degradation of permafrost, and increased depth of the active layer. Changes in these characteristics influence the dynamics of carbon dioxide (CO2) and methane (CH4) fluxes. We have studied the changes in CO2 and CH4 fluxes from the soil in boreal forest areas in central Siberia underlain by continuous permafrost and the possible impacts of the aforementioned environmental factors on the emissions of these greenhouse gases. We have used a fire chronosequence of areas, with the last fire occurring 1, 23, 56, and more than 100 years ago. The soils in our study acted as a source of CO2. Emissions of CO2 were lowest at the most recently burned area and increased with forest age throughout the fire chronosequence. The CO2 flux was influenced by the pH of the top 5 cm of the soil, the biomass of the birch (Betula) and alder (Duschekia) trees, and by the biomass of vascular plants in the ground vegetation. Soils were found to be a CH4 sink in all our study areas. The uptake of CH4 was highest in the most recently burned area (forest fire one year ago) and the lowest in the area burned 56 years ago, but the difference between fire chronosequence areas was not significant. According to the linear mixed effect model, none of the tested factors explained the CH4 flux. The results confirm that the impact of a forest fire on CO2 flux is long-lasting in Siberian boreal forests, continuing for more than 50 years, but the impact of forest fire on CH4 flux is minimal.

Carbon dioxide, methane and nitrous oxide fluxes from a fire chronosequence in subarctic boreal forests of Canada.

Forest fires are one of the most important natural disturbances in boreal forests, and their occurrence and severity are expected to increase as a result of climate warming. A combination of factors induced by fire leads to a thawing of the near-surface permafrost layer in subarctic boreal forest. Earlier studies reported that an increase in the active layer thickness results in higher carbon dioxide (CO2) and methane (CH4) emissions. We studied changes in CO2, CH4 and nitrous oxide (N2O) fluxes in this study, and the significance of several environmental factors that influence the greenhouse gas (GHG) fluxes at three forest sites that last had fires in 2012, 1990 and 1969, and we compared these to a control area that had no fire for at least 100years. The soils in our study acted as sources of CO2 and N2O and sinks for CH4. The elapsed time since the last forest fire was the only factor that significantly influenced all studied GHG fluxes. Soil temperature affected the uptake of CH4, and the N2O fluxes were significantly influenced by nitrogen and carbon content of the soil, and by the active layer depth. Results of our study confirm that the impacts of a forest fire on GHGs last for a rather long period of time in boreal forests, and are influenced by the fire induced changes in the ecosystem.

Nitrogen balance along a northern boreal forest fire chronosequence.

Fire is a major natural disturbance factor in boreal forests, and the frequency of forest fires is predicted to increase due to climate change. Nitrogen (N) is a key determinant of carbon sequestration in boreal forests because the shortage of N limits tree growth. We studied changes in N pools and fluxes, and the overall N balance across a 155-year non stand-replacing fire chronosequence in sub-arctic Pinus sylvestris forests in Finland. Two years after the fire, total ecosystem N pool was 622 kg ha-1 of which 16% was in the vegetation, 8% in the dead biomass and 76% in the soil. 155 years after the fire, total N pool was 960 kg ha-1, with 27% in the vegetation, 3% in the dead biomass and 69% in the soil. This implies an annual accumulation rate of 2.28 kg ha-1 which was distributed equally between soil and biomass. The observed changes in N pools were consistent with the computed N balance +2.11 kg ha-1 yr-1 over the 155-year post-fire period. Nitrogen deposition was an important component of the N balance. The biological N fixation increased with succession and constituted 9% of the total N input during the 155 post-fire years. N2O fluxes were negligible (≤ 0.01 kg ha-1 yr-1) and did not differ among post-fire age classes. The number and intensity of microbial genes involved in N cycling were lower at the site 60 years after fire compared to the youngest and the oldest sites indicating potential differences in soil N cycling processes. The results suggest that in sub-arctic pine forests, the non-stand-replacing, intermediate-severity fires decrease considerably N pools in biomass but changes in soil and total ecosystem N pools are slight. Current fire-return interval does not seem to pose a great threat to ecosystem productivity and N status in these sub-arctic forests.