Holocene peat and carbon accumulation rates in the southern taiga of western Siberia

Although recent studies have recognized peatlands as a sink for atmospheric CO₂, little is known about the role of Siberian peatlands in the global carbon cycle. We have estimated the Holocene peat and carbon accumulation rate in the peatlands of the southern taiga and subtaiga zones of western Siberia. We explain the accumulation rates by calculating the average peat accumulation rate and the long-term apparent rate of carbon accumulation (LORCA) and by using the model of Clymo (1984, Philosophical Transactions of the Royal Society of London Series B 303, 605-654). At three key areas in the southern taiga and subtaiga zones we studied eight sites, at which the dry bulk density, ash content, and carbon content were measured every 10 cm. Age was established by radiocarbon dating. The average peat accumulation rate at the eight sites varied from 0.35 ± 0.03 to 1.13 ± 0.02 mm yr⁻¹ and the LORCA values of bogs and fens varied from 19.0 ± 1.1 to 69.0 ± 4.4 g C m⁻² yr⁻¹. The accumulation rates had different trends especially during the early Holocene, caused by variations in vegetation succession resulting in differences in peat and carbon accumulation rates. The indirect effects of climate change through local hydrology appeared to be more important than direct influences of changes in precipitation and temperature. River valley fens were more drained during wetter periods as a result of deeper river incision, while bogs became wetter. From our dry bulk density results and our age-depth profiles we conclude that compaction is negligible and decay was not a relevant factor for undrained peatlands. These results contribute to our understanding of the influence of peatlands on the global carbon cycle and their potential impact on global change.     

Growth and physiological responses of larch trees to climate changes deduced from tree-ring widths and δ13C at two forest sites in eastern Siberia

Tree-ring chronologies of ring width and stable carbon isotope ratios (δ¹³C) over the past 160 years were developed using living larch trees at two forest sites, each with different annual precipitation, in eastern Siberia: Spasskaya Pad (SP) (62°14′N, 129°37′E); and Elgeeii (EG) (60°0′N, 133°49′E). Intrinsic water-use efficiency (iWUE) was derived from tree-ring δ¹³C. The physiological responses of the larch trees to climate varied between these sites and over time. Ring widths correlated negatively with summer temperatures at SP, where summer precipitation is lower than at EG, probably due to temperature-induced water stress. Since the 1990s, however, the negative effect of warming has been more severe at EG, where the productivity of larch trees is higher than at SP. A greater reduction of larch tree growth and higher increase rate of iWUE at EG reflects greater temperature-induced water stress, which is incident to the larger forest biomass. Our results suggest that effect of increase in atmospheric CO₂ on larch tree growth is not sufficient to compensate for temperature-induced water stress on larch growth in eastern Siberia and differences in precipitation and forest productivity largely affect the larch tree response to changing climate in eastern Siberia.     

Global greenhouse to icehouse and back again: The origin and future of the Boreal Forest biome

The Boreal Forest biome (Taiga), dominated by evergreen and deciduous coniferous trees (Pinaceae), is circumpolar in its present distribution, covering a significant part of the total land area of the Northern Hemisphere and representing perhaps a third of the total forest area of the planet. Nothing comparable to this extant biome existed during the global “greenhouse” interval of the Late Mesozoic and Paleogene. Latitudinal temperature gradients should have confined boreal taxa to extremely high latitudes, but evergreen taxa do not appear to have been competitive in the lowlands of the high arctic, where the vegetation consisted of a unique circumpolar forest dominated by deciduous conifers and broad-leaved taxa.Probable sources for the pinaceous taxa that now characterize boreal latitudes were the Paleogene evergreen montane coniferous forests of the western North American Cordillera. Taphonomic factors limit the fossil record for such forests, but assemblages such as the Eocene Thunder Mountain (Idaho) and Bull Run (Nevada) floras were dominated by evergreen and deciduous Pinaceae that dominate extant montane, subalpine, and Boreal Forest associations. In response to post-Eocene global cooling, such forests presumably would have migrated to lower elevations, eventually spreading across high-latitude North America, subsequently reaching Eurasia via the Beringian corridor. This high-diversity coniferous forest was differentially winnowed and modified during subsequent migration southward in both the New and Old World. Despite its extensive geographic distribution, the Boreal Forest may be the youngest of the major forest biomes.If global warming ultimately results in a significant redistribution of terrestrial vegetation, the history of the Boreal Forest may well be reversed. Northward migration of the Boreal Forest may be characterized by loss of taxa and extensive community reorganization as individual taxa are pushed to their limits with respect to rates of migration and biotic stress takes its toll in the form of insect predation and disease. If evergreen taxa are unable to survive at low elevations at high polar latitudes, such conifers might once again become restricted to montane refugia and the lowlands of the high arctic would be populated by a larch-dominated deciduous conifer forest of low diversity and limited geographic extent. Given the biogeographic significance of the Boreal Forest biome, such a consequence would represent a profound ecological transformation.