Winter soil frost conditions in boreal forests control growing season soil CO2 concentration and its atmospheric exchange

The impact of changes in winter soil frost regime on soil CO₂ concentration and its atmospheric exchange in a boreal Norway spruce forest was investigated using a field‐scale soil frost manipulation experiment. The experiment comprised three treatments: deep soil frost, shallow soil frost and control plots (n= 3). Winter soil temperatures and soil frost distribution were significantly altered by the different treatments. The average soil CO₂ concentrations during the growing season were significantly lower in plots with deep soil frost than in plots with shallow soil frost. The average CO₂ soil–atmosphere exchange rate exhibited the same pattern, and differences in soil respiration rates among the treatments were statistically significant. Both the variation in soil CO₂ concentration and the CO₂ soil–atmosphere exchange rate could statistically be explained by the differences in the maximum soil frost depth during the previous winter. A response model for growing season soil respiration rates suggests that every 1 cm change in winter soil frost depth will change the emission rates by ca. 0.01 g CO₂ m^?2 day^?1, corresponding to 0.2–0.5% of the estimated net ecosystem productivity (NEP). This suggests that the soil frost regime has a significant influence on the C balance of the system, because interannual variations in soil frost up to 60 cm have been recorded at the site. We conclude that winter climate conditions can be important in controlling C balances in northern terrestrial ecosystems, and also that indirect effects of the winter season must be taken into account, because these can affect the prevailing conditions during the growing season.     

Peat carbon stocks in the southern Mackenzie River Basin: uncertainties revealed in a high-resolution case study

The organic carbon (C) stocks contained in peat were estimated for a wetland‐rich boreal region of the Mackenzie River Basin, Canada, using high‐resolution wetland map data, available peat C characteristic and peat depth datasets, and geostatistics. Peatlands cover 32% of the 25 119 km² study area, and consist mainly of surface‐ and/or groundwater‐fed treed peatlands. The thickness of peat deposits measured at 203 sites was 2.5 m on average but as deep as 6 m, and highly variable between sites. Peat depths showed little relationship with terrain data within 1 and 5 km, but were spatially autocorrelated, and were generalized using ordinary kriging. Polygon‐scale calculations and Monte Carlo simulations yielded a total peat C stock of 982–1025 × 10¹² g C that varied in C mass per unit area between 53 and 165 kg m^?2. This geostatistical approach showed as much as 10% more peat C than calculations using mean depths. We compared this estimate with an overlapping 7868 km² portion of an independent peat C stock estimate for western Canada, which revealed similar values for total peatland area, total C stock, and total peat C mass per unit area. However, agreement was poor within ～875 km² grids owing to inconsistencies in peatland cover and little relationship in peat depth between estimates. The greatest disagreement in mean peat C mass per unit area occurred in grids with the largest peatland cover, owing to the spatial coincidence of large cover and deep peat in our high‐resolution assessment. We conclude that total peat C stock estimates in the southern Mackenzie Basin and perhaps in boreal western Canada are likely of reasonable accuracy. However, owing to uncertainties particularly in peat depth, the quality of information regarding the location of these large stocks at scales as wide as several hundreds of square kilometers is presently much more limited.     

Sedimentation in Boreal Lakes—The Role of Flocculation of Allochthonous Dissolved Organic Matter in the Water Column

We quantified sedimentation of organic carbon in 12 Swedish small boreal lakes (<0.48 km²), which ranged in dissolved organic carbon (DOC) from 4.4 to 21.4 mg C l⁻¹. Stable isotope analysis suggests that most of the settling organic matter is of allochthonous origin. Annual sedimentation of allochthonous matter per m² lake area was correlated to DOC concentration in the water (R ² = 0.41), and the relationship was improved when sedimentation data were normalized to water depth (R ² = 0.58). The net efflux of C as CO₂ from the water to the atmosphere was likewise correlated to DOC concentration (R ² = 0.52). The losses of organic carbon from the water column via mineralization to CO₂ and via sedimentation were approximately of equal importance throughout the year. Our results imply that DOC is a precursor of the settling matter, resulting in an important pathway in the carbon cycle of boreal lakes. Thus, flocculation of DOC of terrestrial origin and subsequent sedimentation could lead to carbon sequestration by burial in lake sediments.     

Carbon Quality and Stocks in Organic Horizons in Boreal Forest Soils

We investigated the mechanisms that determine the quality and quantity of organic carbon (C) stocks in boreal forest soils by analyzing both qualitative and quantitative changes in the organic fractions in the soil organic matter (OM) in a vertical gradient in the decomposition continuum of the organic horizon [litter layer (L), fermentation layer (F), and humus layer (H)] in forest soils using a sequential fractionation method at two forest types along a climatic gradient in Finland. We predicted that the concentrations of water-soluble (WSE) and non-polar (NPE) extractives should decrease and those of the acid-soluble (AS) fraction and acid-insoluble residue (AIR) should increase from the L to the F, and from the F to the H layers, but the C/N ratio of soil OM should stay constant after reaching the critical quotient. We also predicted that the AIR concentrations should be higher in the south than north boreal, and in sub-xeric than mesic forests. Consistent with our hypothesis, the concentrations of WSE and NPE fractions decreased and concentrations of AIR increased in the vertical soil gradient. The highest concentrations of the AS fraction were found in the F layer. The C/N ratio was lowest in the F layer, and the highest in the H layer, indicating that soil OM is depleted in N in relation to C along the vertical soil gradient. Concentrations of WSE and NPE were lower, and concentrations of AIR were higher in the south than in north boreal forests, which is in agreement with our hypothesis that higher soil temperatures may enhance accumulation of slowly decomposable OM in the soil. The concentrations of AIR were higher in the sub-xeric than mesic forests. Contrary to our expectations, however, the differences in the chemical quality in soil OM between the site types were amplified from the L to the H layer. The size of the C storage was significantly larger in south than north boreal sites, and larger in the mesic than in the sub-xeric sites.     

Plant Community Composition as a Predictor of Regional Soil Carbon Storage in Alaskan Boreal Black Spruce Ecosystems

The boreal forest is the largest terrestrial biome in North America and holds a large portion of the world’s reactive soil carbon. Therefore, understanding soil carbon accumulation on a landscape or regional scale across the boreal forest is useful for predicting future soil carbon storage. Here, we examined the relationship between floristic composition and ecosystem parameters, such as soil carbon pools, the carbon-to-nitrogen (C/N) ratio of live black spruce needles, and normalized basal area increment (NBAI) of trees in black spruce communities, the most widespread forest type in the boreal forest of Alaska. Variability in ecosystem properties among black spruce stands was as large as that which had previously been documented among all forest types in the central interior of Alaska; we found an eightfold range in NBAI and fivefold range in mineral soil carbon and nitrogen pools. Acidic black spruce communities had significantly more carbon in the organic soil horizon than did nonacidic black spruce communities, but did not differ in any other measured ecosystem parameter. We explained 48% of the variation in total soil carbon with a combination of plant community indices and abiotic and biotic factors. Plant community composition was at least as effective as any single environmental factor or stand characteristic in predicting soil C pools in Alaskan black spruce ecosystems. We conclude that among the community properties analyzed, the presence of key groups of species, overall species composition, and diversity of certain functional types, especially Sphagnum moss species, are important predictors of soil carbon sequestration in the black spruce forest type.     

The greening and browning of Alaska based on 1982–2003 satellite data

Aim To examine the trends of 1982–2003 satellite‐derived normalized difference vegetation index (NDVI) values at several spatial scales within tundra and boreal forest areas of Alaska. Location Arctic and subarctic Alaska. Methods Annual maximum NDVI data from the twice monthly Global Inventory Modelling and Mapping Studies (GIMMS) NDVI 1982–2003 data set with 64‐km² pixels were extracted from a spatial hierarchy including three large regions: ecoregion polygons within regions, ecozone polygons within boreal ecoregions and 100‐km climate station buffers. The 1982–2003 trends of mean annual maximum NDVI values within each area, and within individual pixels, were computed using simple linear regression. The relationship between NDVI and temperature and precipitation was investigated within climate station buffers. Results At the largest spatial scale of polar, boreal and maritime regions, the strongest trend was a negative trend in NDVI within the boreal region. At a finer scale of ecoregion polygons, there was a strong positive NDVI trend in cold arctic tundra areas, and a strong negative trend in interior boreal forest areas. Within boreal ecozone polygons, the weakest negative trends were from areas with a maritime climate or colder mountainous ecozones, while the strongest negative trends were from warmer basin ecozones. The trends from climate station buffers were similar to ecoregion trends, with no significant trends from Bering tundra buffers, significant increasing trends among arctic tundra buffers and significant decreasing trends among interior boreal forest buffers. The interannual variability of NDVI among the arctic tundra buffers was related to the previous summer warmth index. The spatial pattern of increasing tundra NDVI at the pixel level was related to the west‐to‐east spatial pattern in changing climate across arctic Alaska. There was no significant relationship between interannual NDVI and precipitation or temperature among the boreal forest buffers. The decreasing NDVI trend in interior boreal forests may be due to several factors including increased insect/disease infestations, reduced photosynthesis and a change in root/leaf carbon allocation in response to warmer and drier growing season climate. Main conclusions There was a contrast in trends of 1982–2003 annual maximum NDVI, with cold arctic tundra significantly increasing in NDVI and relatively warm and dry interior boreal forest areas consistently decreasing in NDVI. The annual maximum NDVI from arctic tundra areas was strongly related to a summer warmth index, while there were no significant relationships in boreal areas between annual maximum NDVI and precipitation or temperature. Annual maximum NDVI was not related to spring NDVI in either arctic tundra or boreal buffers.     

Climate change and the northern Russian treeline zone

The Russian treeline is a dynamic ecotone typified by steep gradients in summer temperature and regionally variable gradients in albedo and heat flux. The location of the treeline is largely controlled by summer temperatures and growing season length. Temperatures have responded strongly to twentieth-century global warming and will display a magnified response to future warming. Dendroecological studies indicate enhanced conifer recruitment during the twentieth century. However, conifers have not yet recolonized many areas where trees were present during the Medieval Warm period (ca AD 800-1,300) or the Holocene Thermal Maximum (HTM; ca 10,000-3,000 years ago). Reconstruction of tree distributions during the HTM suggests that the future position of the treeline due to global warming may approximate its former Holocene maximum position. An increased dominance of evergreen tree species in the northern Siberian forests may be an important difference between past and future conditions. Based on the slow rates of treeline expansion observed during the twentieth century, the presence of steep climatic gradients associated with the current Arctic coastline and the prevalence of organic soils, it is possible that rates of treeline expansion will be regionally variable and transient forest communities with species abundances different from today's may develop.     

Non-equilibrium succession dynamics indicate continued northern migration of lodgepole pine

Because species affect ecosystem functioning, understanding migration processes is a key component of predicting future ecosystem responses to climate change. This study provides evidence of range expansion under current climatic conditions of an indigenous species with strong ecosystem effects. Surveys of stands along the northern distribution limit of lodgepole pine (Pinus contorta var. latifolia) in central Yukon Territory, Canada showed consistent increases in pine dominance following fire. These patterns differed strongly from those observed at sites where pine has been present for several thousand years. Differences in species thinning rates are unlikely to account for the observed increases in pine dominance. Rates of pine regeneration at its range limits were equivalent to those of spruce, indicating a capacity for rapid local population expansion. The study also found no evidence of strong climatic limitation of pine population growth at the northern distribution limit. We interpret these data as evidence of current pine expansion at its range limits and conclude that the northern distribution of lodgepole pine is not in equilibrium with current climate. This study has implications for our ability to predict vegetation response to climate change when populations may lag in their response to climate.     

Minor Changes in Vegetation and Carbon Gas Balance in a Boreal Mire under a Raised CO<Subscript>2</Subscript> or NH<Subscript>4</Subscript>NO<Subscript>3</Subscript> Supply

Increasing concentrations of carbon dioxide (CO₂) in the atmosphere or continuous nitrogen (N) deposition might alter the carbon (C) cycle in boreal mires and thus have significant impacts on the development of climate change. The atmospheric impact of the C cycle in mires is twofold: C accumulation attenuates and CH₄ release strengthens the natural greenhouse effect. We studied the effects of an increased supply of CO₂ or NH₄NO₃ on the vegetation and annual CO₂ exchange in lawns of a boreal oligotrophic mire in eastern Finland over a 2-year period. Ten study plots were enclosed with mini-FACE (Free Air Carbon Dioxide Enrichment) rings. Five plots were vented with CO₂-enriched air (target 560 ppmv), while their controls were vented with ambient air; five plots were sprayed with NH₄NO₃, corresponding to a cumulative addition of 3 g N m⁻² a⁻¹, while their controls were sprayed with distilled water only. A raised NH₄NO₃ supply seemed to affect the composition of the moss layer. Raised CO₂ did not affect the vegetation, but gross photosynthesis increased significantly. The change in net CO₂ exchange depended on the annual weather conditions. Our results suggest that C accumulation may increase in wet years and compensate for the warming effect caused by the increase in CH₄ release from this mire. In contrast, a relatively dry and warm growing period favors decomposition and can even make the CO₂ balance negative. Along with the increased CH₄ release under raised CO₂, the decreased C accumulation then increases the radiative forcing of boreal mires. Received 22 October 2001; accepted 13 May 2002.