A. S. Prokushkin

Root system development of Larix gmelinii trees affected by micro-scale conditions of permafrost soils in central Siberia

Spatial distributions of root systems of Larix gmelinii (Rupr.) Rupr. trees were examined in two stands in central Siberia: an even-aged stand (ca. 100 yrs-old) and a mature, uneven-aged (240–280 yrs-old) stand. Five larch trees of different sizes were sampled by excavating coarse roots (diameter > 5 mm) in each stand. Dimensions and ages of all first-order lateral roots were measured. Micro-scale conditions of soil temperature and soil water suction (each 10 cm deep) were also examined in relation to earth hummock topography (mound vs. trough) and/or ground floor vegetation types (moss vs. lichens). All larch trees developed superficial root systems, consisting of the aborted short tap root (10–40 cm in soil depth) and some well-spread lateral roots (n = 4 – 13). The root network of each tree was asymmetric, and its rooting area reached about four times the crown projection area. Lateral roots generally expanded into the upper soil layers of the mounds where summer soil temperature was 1–6 °C higher than inside nearby troughs. Chronological analysis indicated that lateral root expansion started successively from lower to upper parts of each aborted tap root, and some lateral roots occurred simultaneously at several decades after tree establishment. The process of root system development was likely to be primarily linked with post-fire dynamics of rhizosphere environment of the permafrost soils.

Labile pyrogenic dissolved organic carbon in major Siberian Arctic rivers: Implications for wildfire‐stream metabolic linkages

Biomass burning produces a spectrum of thermally altered materials that releases pyrogenic carbon (PyC) to terrestrial, atmospheric, and aquatic systems. Most studies focus on the refractory end of the PyC spectrum, derived from middle- to high-temperature combustion. Low-temperature PyC is produced during wildfires and has been found to be particularly labile and water soluble. Here we find that in each of the major Siberian watersheds, low-temperature fire-derived biomarkers are present in detectable concentrations during all flow regimes of the 2004–2006 sampling period, confirming that PyC is an intrinsic component of the dissolved organic carbon (DOC) pool mobilized by hydrologic events. Gymnosperm combustion, from the southern portions of these watersheds, is the primary source of this Py-DOC input. Using first-order degradation rates and transit times of water through these rivers, about half of the total estimated flux of this material may be remineralized during transport from fire source to river mouth (20–40 days), demonstrating the input of a labile source of PyC to these watersheds.

Seasonal and spatial variability of elemental concentrations in boreal forest larch foliage of Central Siberia on continuous permafrost

We measured the seasonal dynamics of major and trace elements concentrations in foliage of larch, main conifer species of Siberia, and we analyzed cryogenic soils collected in typical permafrost-dominated habitats in the Central Siberia. This region offers a unique opportunity to study element fractionation between the soil and the plant because of (i) the homogeneous geological substratum, (ii) the monospecific stands (Larix gmelinii) and (iii) the contrasted habitats (North-facing slope, South-facing slope, and Sphagnum peatbog) in terms of soil temperature, moisture, thickness of the active layer, tree biomass and rooting depth. The variation of these parameters from one habitat to the other allowed us to test the effects of these parameters on the element concentration in larch foliage considered with high seasonal resolution. Statistical treatment of data on larch needles collected 4 times in 3 locations during entire growing season (June–September) demonstrated that : (1) there is a high similarity of foliar chemical composition of larch trees in various habitats suggesting intrinsically similar requirements of larch tree growth for nutrients, (2) the variation of elemental concentrations in larch needles is controlled by the period (within the growing season) and not by the geographical location (South-facing slope, North-facing slope or bog zone) and (3) there are three groups of elements according to their patterns of elements concentration in needles over the growing season from June to September can be identified: (1): nutrient elements [P, Cu, Rb, K, B, Na, Zn, Ni and Cd] showing a decrease of concentration from June to September similar to the behaviour of major nutrients such as N, P and K; (2): accumulating elements [Ca, Mg, Mo, Co, Sr, Mn, Pb and Cr] showing an increase of concentration from June–July to September; (3): indifferent elements [Al, Zr, Fe, Ba, Ti, REEs (Pr, Nd, Ce, La, Gd, Er, Dy, Tb, Lu, Yb, Tm, Sm, Ho, Eu), Y, Th and U] showing a decrease of concentration from June to July and then an increase of concentration to September. A number of micronutrients (e.g., Cu, Zn) demonstrate significant resorption at the end of growing season suggesting possible limitation by these elements. Although the intrinsic requirement seems to be similar among habitats, the total amount of element stored within the different habitats is drastically different due to the differences in standing tree biomass. The partitioning coefficients between soil and larch appear to be among the lowest compared to other environments with variable plants, soils and climates. Applying the “space for time” substitution scenario, it follows that under ongoing climate warming there will be an increase of the element stock following enhanced above-ground biomass accumulation, even considering zero modification of element ratios and their relative mobility. In this sense, the habitats like south-facing slopes can serve as resultant of climate warming effect on element cycling in larch ecosystems for the larger territory of Central Siberia.

Zn isotope fractionation in a pristine larch forest on permafrost-dominated soils in Central Siberia

Stable Zn isotopes fractionation was studied in main biogeochemical compartments of a pristine larch forest of Central Siberia developed over continuous permafrost basalt rocks. Two north- and south-oriented watershed slopes having distinctly different vegetation biomass and active layer depth were used as natural proxy for predicting possible future climate changes occurring in this region. In addition, peat bog zone exhibiting totally different vegetation, hydrology and soil temperature regime has been studied.The isotopic composition of soil profile from Central Siberia is rather constant with a δ66Zn value around 0.2‰ close to the value of various basalts. Zn isotopic composition in mosses (Sphagnum fuscum and Pleurozium schreberi) exhibits differences between surface layers presenting values from 0.14 to 0.2‰ and bottom layers presenting significantly higher values (0.5 – 0.7‰) than the underlain mineral surface. The humification of both dead moss and larch needles leads to retain the fraction where Zn bound most strongly thus releasing the lighter isotopes in solution and preserving the heavy isotopes in the humification products, in general accord with previous experimental and modeling works [GCA 75:7632–7643, 2011].The larch (Larix gmelinii) from North and South-facing slopes is enriched in heavy isotopes compared to soil reservoir while larch from Sphagnum peatbog is enriched in light isotopes. This difference may result from stronger complexation of Zn by organic ligands and humification products in the peat bog compared to mineral surfaces in North- and South-facing slope.During the course of the growing period, Zn followed the behavior of macronutrients with a decrease of concentration from June to September. During this period, an enrichment of larch needles by heavier Zn isotopes is observed in the various habitats. We suggest that the increase of the depth of rooting zone, and the decrease of DOC and Zn concentration in soil solution from the root uptake zone with progressively thawing soil could provoke heavy isotopes to become more available for the larch roots at the end of the vegetative season compared to the beginning of the season, because the decrease of DOC will facilitate the uptake of heavy isotope as it will be less retained in strong organic complexes.

The influence of heating on organic matter of forest litters and soils under experimental conditions

The specific features of changes in the content and mobility of organic matter in litters and cryogenic soils under heating were revealed. The thermal stability of the organic matter and litters is different. In the soils, the maximal loss of matter was recorded at a temperature of 300°C. In the litters, the maximal losses were found at 300, 400 and 550°C and depended inversely on the carbon content in them. The heating to 200°C caused insignificant changes in the mass of the litters and soils but increased the content of the water-soluble fraction of organic matter and the concentration of the water-soluble mineral nitrogen forms.

Productivity of Mosses and Organic Matter Accumulation in the Litter of Sphagnum Larch Forest in the Permafrost Zone

Productivity of the moss cover and necromass accumulation in the litter of a sphagnum larch forest have been estimated on the basis of tree age. It has been shown that the total carbon stock in the litter of a 100-year-old stand, including organic matter not destroyed by fire, exceeds the corresponding value for the tree stand itself by more than an order of magnitude. The accumulation of organic matter on the soil surface inhibits the growth of larch. In particular, this factor impairs hydrothermal conditions in the soil and causes a rise of the permafrost table; as a consequence, lower layers of the root system die off.

Dispersal limitation drives successional pathways in Central Siberian forests under current and intensified fire regimes.

Fire is a primary driver of boreal forest dynamics. Intensifying fire regimes due to climate change may cause a shift in boreal forest composition toward reduced dominance of conifers and greater abundance of deciduous hardwoods, with potential biogeochemical and biophysical feedbacks to regional and global climate. This shift has already been observed in some North American boreal forests and has been attributed to changes in site conditions. However, it is unknown if the mechanisms controlling fire-induced changes in deciduous hardwood cover are similar among different boreal forests, which differ in the ecological traits of the dominant tree species. To better understand the consequences of intensifying fire regimes in boreal forests, we studied postfire regeneration in five burns in the Central Siberian dark taiga, a vast but poorly studied boreal region. We combined field measurements, dendrochronological analysis, and seed-source maps derived from high-resolution satellite images to quantify the importance of site conditions (e.g., organic layer depth) vs. seed availability in shaping postfire regeneration. We show that dispersal limitation of evergreen conifers was the main factor determining postfire regeneration composition and density. Site conditions had significant but weaker effects. We used information on postfire regeneration to develop a classification scheme for successional pathways, representing the dominance of deciduous hardwoods vs. evergreen conifers at different successional stages. We estimated the spatial distribution of different successional pathways under alternative fire regime scenarios. Under intensified fire regimes, dispersal limitation of evergreen conifers is predicted to become more severe, primarily due to reduced abundance of surviving seed sources within burned areas. Increased dispersal limitation of evergreen conifers, in turn, is predicted to increase the prevalence of successional pathways dominated by deciduous hardwoods. The likely fire-induced shift toward greater deciduous hardwood cover may affect climate-vegetation feedbacks via surface albedo, Bowen ratio, and carbon cycling.

Nutrient uptake along a fire gradient in boreal streams of Central Siberia

Fire can transform the boreal forest landscape, thereby leading to potential changes in the loading of organic matter and nutrients to receiving streams and in the retention or transformation of these inputs within the drainage network. We used the Tracer Additions for Spiraling Curve Characterization (TASCC) method to conduct 17 nutrient-addition experiments (9 single additions of NO3– and 8 combined additions of NH4+ and PO43–) in 5 boreal headwater streams underlain by continuous permafrost and draining watersheds with a range of burn histories (4–>100 y since last burn) in the Nizhnyaya Tunguska River watershed in Central Siberia. Hydrology, ambient nutrient concentration, and the ratio of dissolved organic C (DOC) to nutrients drove rates of nutrient uptake in the streams. Nutrients were taken up with greater efficiency and magnitude under conditions with high flow and reduced diffusive boundary layer (DBL), regardless of watershed burn history. Ambient molar ratio of DOC∶PO43– explained some variation in ambient uptake velocity (υf) for NH4+ and PO43–. We also observed tight coupling between ambient rates of NH4+ and PO43– uptake across the watershed burn-history gradient. These data suggest that fire-driven changes in stream chemistry may alter N and P retention and subsequent export of materials to downstream receiving waters. Climate change is likely to enhance the frequency and intensity of boreal forest fires and alter the extent of permafrost. Therefore, understanding the interactions among C, N, and P in these Arctic systems has important implications for global biogeochemical cycling.

Global Warming and Dissolved Organic Carbon Release from Permafrost Soils

Global riverine transport of organic carbon (OC) is estimated to be 0.4–0.9 Pg annually (Meybeck 1982; Hope et al. 1994; Aitkenhead-Peterson et al. 2005). Therefore, the riverine export of OC from drainage basins to the ocean represents a major component of the global carbon cycle (Spitzy and Leenheer 1991; Hedges et al. 1997). Recent evidence from Northern Europe about increased dissolved organic carbon (DOC) concentrations in surface waters draining upland areas and wetlands (Freeman et al. 2001; Frey and Smith 2005), highlights the importance of understanding the transfer of C between soil and freshwater systems. Although the magnitude of the fluxes involved in land–atmosphere C exchange is significantly larger than that associated with surface waters, rates of DOC transport in streams draining subarctic catchments rich in organic soils are comparable to rates of C sequestration in the soil–plant system of high latitudes (Hope et al. 1994; Billet et al. 2006). The Arctic drainage basin (∼24 × 10 km) processes about 11% of both global runoff and DOC (Lobbes 2000; Lammers et al. 2001). Heavily influenced by permafrost, arctic river basins demonstrate the highest susceptibility to climate change. With 23–48% of the world’s soil organic carbon (SOC) stored in the high-latitude region, the arctic/subarctic river basins have an enormous potential to mobilize and transport terrestrial OC to the Arctic Ocean (Guo and Macdonald 2006). The response of permafrost soils to warming is crucial for understanding potential change in terrestrial C export to rivers. High hydraulic conductivity, low mineral content, and low DOC sorption capacity of the shallow soil active layer overlying impermeable permafrost together lead to quick DOC transport to streams and rivers, with limited microbial transformation, especially during snowmelt. As the depth, temperature and seasonal duration of the active layer increase with climate warming, new inputs of DOC may derive from thawed permafrost and/or vegetation changes (Sturm et al. 2001; Neff et al. 2006). However, significant differences in geomorphology, hydrology, permafrost distribution, soil types and

Specific features of xylogenesis in Dahurian larch, Larix gmelinii (Rupr.) Rupr., growing on permafrost soils in Middle Siberia

Processes of xylem formation in Dahurian larch have been studied at three sites differing in the hydrothermal regime of soils in the permafrost zone of Middle Siberia. It has been found that the start and end dates of different phases of tree ring formation may differ between the sites by up to 14 days, depending on site conditions. The data obtained contribute to knowledge of possible changes in larch forest phytomass production and provide the possibility of predicting its dynamics under conditions of climate change.