One of the most common descriptors of the relationship between root and shoot biomass is the root : shoot ratio, which has become a core method for estimating root biomass from the more easily measured shoot biomass. Previous reviews have examined root : shoot ratio data, but have only considered particular vegetation types and have not always critically reviewed the data used. Reliable root : shoot ratios are needed for a wide range of vegetation types in order to improve the accuracy of root biomass estimates, including those required for estimating the effects of land management and land use change in National Greenhouse Gas Inventories.This study reviewed root : shoot ratios in terrestrial biomes. A key facet of our analysis was a critical methodological review, through which unreliable data were identified and omitted on the basis of specific criteria. Of the 786 root : shoot ratio observations collated, 62% were omitted because of inadequate or unverifiable root sampling methods. When only the reliable data were examined, root : shoot ratios were found to be negatively related to shoot biomass, mean annual precipitation, mean annual temperature, forest stand age, and forest stand height.Although a single allometric equation derived in this study reliably predicted root biomass from shoot biomass for forests and woodlands, in general, the use of vegetationspecific root : shoot ratios were found to be a more accurate method for predicting root biomass. When the root : shoot ratio data collated here were applied to an analysis of the global carbon budget, there was a 50% increase in estimated global root carbon stock, and a 12% increase in estimated total carbon stock of terrestrial vegetation. The use of the vegetation-specific root : shoot ratios presented in this study is likely to substantially improve the accuracy of root biomass estimates for purposes such as carbon accounting and for studies of ecosystem dynamics.

forests. The three large Siberian rivers, Lena, Yenisei, and Ob, which also have the highest 58 proportion of forests within their watersheds, contribute about 90% of the total lignin discharge 59 to the Arctic Ocean. The composition of river DOC is also characterized by elevated levels of p-60 hydroxybenzenes, particularly during the low flow season, which indicates a larger contribution 61 from mosses and peat bogs. The lignin composition was strongly related to the average 14 C-age 62 of DOC supporting the abundance of young, boreal-vegetation-derived leachates during spring 63 flood, and older, soil-, peat-, and wetland-derived DOC during groundwater dominated low flow 64 conditions, particularly in the Ob and Yukon Rivers. We observed significant differences in 65 DOC concentration and composition between the rivers over the seasonal cycles with the 66 Mackenzie River being the most unique, the Lena River being similar to the Yenisei, and the 67 Yukon being most similar to the Ob. The observed relationship between the lignin phenol 68 composition and watershed characteristics suggests that DOC discharge from these rivers could 69 increase in a warmer climate under otherwise undisturbed conditions. 70 71 4

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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.

Little is known about the dynamics of concentrations and carbon isotope ratios of individual carbohydrates in leaves in response to climatic and physiological factors. Improved knowledge of the isotopic ratio in sugars will enhance our understanding of the tree ring isotope ratio and will help to decipher environmental conditions in retrospect more reliably. Carbohydrate samples from larch (Larix gmelinii) needles of two sites in the continuous permafrost zone of Siberia with differing growth conditions were analysed with the Compound-Specific Isotope Analysis (CSIA). We compared concentrations and carbon isotope values (δ(13) C) of sucrose, fructose, glucose and pinitol combined with phenological data. The results for the variability of the needle carbohydrates show high dynamics with distinct seasonal characteristics between and within the studied years with a clear link to the climatic conditions, particularly vapour pressure deficit. Compound-specific differences in δ(13) C values as a response to climate were detected. The δ(13) C of pinitol, which contributes up to 50% of total soluble carbohydrates, was almost invariant during the whole growing season. Our study provides the first in-depth characterization of compound-specific needle carbohydrate isotope variability, identifies involved mechanisms and shows the potential of such results for linking tree physiological responses to different climatic conditions.

Tree-ring width of Larix gmelinii (Rupr.) Rupr., ratios of stable isotopes of C (delta(13)C) and O (delta(18)O) of whole wood and cellulose chronologies were obtained for the northern part of central Siberia (Tura, Russia) for the period 1864-2006. A strong decrease in the isotope ratios of O and C (after atmospheric delta(13)C corrections) and tree-ring width was observed for the period 1967-2005, while weather station data show a decrease in July precipitation, along with increasing July air temperature and vapor pressure deficit (VPD). Temperature at the end of May and the whole month of June mainly determines tree radial growth and marks the beginning of the vegetation period in this region. A positive correlation between tree-ring width and July precipitation was found for the calibration period 1929-2005. Positive significant correlations between C isotope chronologies and temperatures of June and July were found for whole wood and cellulose and negative relationships with July precipitation. These relationships are strengthened when the likely physiological response of trees to increased CO(2) is taken into account (by applying a recently developed delta(13)C correction). For the O isotope ratios, positive relationships with annual temperature, VPD of July and a negative correlation with annual precipitation were observed. The delta(18)O in tree rings may reflect annual rather than summer temperatures, due to the late melting of the winter snow and its contribution to the tree water supply in summer. We observed a clear change in the isotope and climate trends after the 1960s, resulting in a drastic change in the relationship between C and O isotope ratios from a negative to a positive correlation. According to isotope fractionation models, this indicates reduced stomatal conductance at a relatively constant photosynthetic rate, as a response of trees to water deficit for the last half century in this permafrost region.

Abstract. The relative role of fire and of climate in determining canopy species composition and aboveground carbon stocks were investigated. Measurements were made along a transect extending from the dark taiga zone of central Siberia, where Picea and Abies dominate the canopy, into the Larix zone of eastern Siberia. We test the hypotheses that the change in canopy species composition is based (1) on climate-driven performance only, (2) on fire only, or (3) on fire-performance interactions. We show that the evergreen conifers Picea obovata and Abies sibirica are the natural late-successional species both in central and eastern Siberia, provided there has been no fire for an extended period of time. There are no changes in performance of the observed species along the transect. Fire appears to be the main factor explaining the dominance of Larix and of soil carbon. Of lesser influence were longitude as a proxy for climate, local hydrology and active-layer thickness. We can only partially explain fire return frequency, which is not only related to climate and land cover, but also to human behavior.Stand-replacing fires decreased from 300 to 50 yrs between the Yenisei Ridge and the upper Tunguska. Repeated non-stand-replacing surface fires eliminated the regeneration of Abies and Picea. With every 100 yrs since the last fire, the percentage of Larix decreased by 20 %.Biomass of stems of single trees did not show signs of age-related decline. Relative diameter increment was 0.41 ± 0.20 % at breast height and stem volume increased linearly over time with a rate of about 0.36 t C ha −1 yr −1 independent of age class and species. Stand biomass reached about 130 t C ha −1 (equivalent to about 520 m 3 ha −1 ). Individual trees of Larix were older than 600 yrs. The maximum age and biomass seemed to be limited by fungal rot of heart wood. 60 % of old Larix and Picea and 30 % of Pinus sibirica trees were affected by stem rot. Implications for the future role of fire and of plant diseases are discussed.