Search

A simple stochastic representation of the spatial variability in thaw depth is proposed. Thaw depth distribution measured in the two larch‐type forests in eastern Siberia, Spasskaya Pad and Elgeeii, showed different spatial, seasonal, and interannual variability, respectively. Year‐to‐year variation in active‐layer thickness was minor in Spasskaya Pad compared to Elgeeii. A gamma distribution adequately represented both sites' thaw depth spatial variability as the cumulative probability. Thus, we developed a simple model using the gamma distribution that illustrates the spatial variability in thaw depth at any thawing stage as a function of a given mean thaw depth. A hierarchy of models was introduced that sequentially considered the constant state, linearity, and nonlinearity in the dependence of the rate parameter of the gamma distribution on the mean thaw depth. Although the requirements of the model levels differed between Spasskaya Pad and Elgeeii, the proposed model successfully represented the spatial variability in thaw depth at both sites during different thaw seasons. [ABSTRACT FROM AUTHOR]

The tree-ring width index (RWI) and satellite-derived vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), are used as long-term indicators of the past forest carbon uptake. However, fundamental questions remain with respect to what is represented by the RWI and NDVI at the ecosystem level. To address this question, we compared tree-ring parameters (RWI and the carbon isotope ratio: δ13C) and NDVI products with forest ecosystem CO 2 fluxes estimated using the eddy covariance method, at a larch forest in eastern Siberia. The RWI and tree-ring δ13C correlated well with the ecosystem gross primary production (GPP), and their temporal stabilities were high during 2004–2014. However, the NDVI products did not show any temporally stable relationship with the GPP. This could be ascribed to significant changes in the understory vegetation in this forest, i.e., from dense cowberry to shrubs and moisture-tolerant grasses, because of an excessively moist environment during 2007–2008. Changes in the understory vegetation could be reflected by the NDVI products but not by the GPP. Our results indicate that it is more feasible to study forest carbon uptake using tree-ring parameters than using satellite-derived vegetation indices in such larch-dominated forest ecosystems in eastern Siberia. [ABSTRACT FROM AUTHOR]

Aims Boreal forests play an important role in the global carbon cycle. Compared with the boreal forests in North America and Europe, relatively few research studies have been conducted in Siberian boreal forests. Knowledge related to the role of Siberian forests in the global carbon balance is thus essential for a full understanding of global carbon cycle. Methods This study investigated the net ecosystem exchange (NEE) during growing season (May-September) in an eastern Siberian boreal larch forest for a 3-year period in 2004-2006 with contrasting meteorological conditions. Important Findings The study found that the forest served as a carbon sink during all of the 3 studied years; in addition, the meteorological conditions essentially influenced the specific annual value of the strength of the carbon sinks in each year. Although 2005 was the warmest year and much wetter than 2004, 2005 also featured the greatest amount of ecosystem respiration, which resulted in a minimum value of NEE. The study also found that the phenological changes observed during the three study years had a relatively small effect on annual NEE. Leaf expansion was 26 days earlier in 2005 than in the other 2 years, which resulted in a longer growing season in 2005. However, the NEE in 2005 was counterbalanced by the large rate of ecosystem respiration that was caused by the higher temperatures in the year. This study showed that meteorological variables had larger influences on the interannual variations in NEE for a Siberian boreal larch forest, as compared with phenological changes. The overall results of this study will improve our understanding of the carbon balance of Siberian boreal larch forests and thus can help to forecast the response of these forests to future climate change. [ABSTRACT FROM AUTHOR]

Eastern Siberia is characterized by continuous permafrost, and has recently been exposed to the effects of climate change. Larch, which is the dominant tree species, has been subject to major environmental changes including fluctuations in soil water content. The purpose of this study was to clarify the responses of mature larch tree roots to changes in soil water conditions. We established a treatment plot in a larch forest, and artificially changed the soil water conditions by covering the ground surface with a vinyl sheet, and from 2004 to 2006 monitored root development through root windows. The vinyl sheet maintained high levels of soil water content, even though the ambient conditions varied from dry in 2004 to wet in 2005 and dry in 2006. In the treatment plot the plants adapted to the wet conditions by decreasing vertical root development. In contrast, roots of plants in the control plot developed to the subsurface layer, even in 2005, and did not develop vertically in 2006 despite the drought. We conclude that larch adapted to the annual changes in soil water content by changing the vertical distribution of roots, and that this reflected a memory effect. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study evaluated the effect of recent eastern Siberian land surface changes, such as water surface expansion, on water-energy fluxes and precipitation and focused on land surface parameters using a three-dimensional atmospheric model [the Japan Meteorological Agency Nonhydrostatic model (JMA-NHM)]. Five parameters were set (viz., surface albedo, evaporative efficiency, roughness length, heat capacity, and thermal conductivity), and a response of evaporation and precipitation was evaluated. Increased precipitation corresponded to 75% of the increased evaporation on interparameter average, indicating strong land-atmosphere coupling. Water-energy flux and precipitation responses to water surface expansion were evaluated by two methods: JMA-NHM and the parameter sensitivity method. The latter method used a linear combination of parameter sensitivity on the fluxes and precipitation and parameter changes with land surface change. JMA-NHM demonstrated an increase in evaporation and precipitation and a decrease in downward shortwave radiation with low-level cloud increases. The parameter sensitivity method gave the same order as JMA-NHM in the estimation. This method has minimal calculation cost; thus, water-energy flux and precipitation response with further water surface expansion and decreases in forest area were simulated, producing various land surface data. The enhancement of the precipitation response to evaporation was weak for further water surface expansion in the largely expanded water surface area; however, the ratio increased dramatically for the small water surface expanding area, indicating intense water cycle enhancement at the beginning of water surface expansion. Although grassland formation from forest has minimal impact, if incoming downward shortwave radiation were to increase because of the disappearance of the forest shading effect and the water surface formed by permafrost melting, the water cycle would be enhanced intensely. [ABSTRACT FROM AUTHOR]

In a deciduous larch forest in eastern Siberia, the mean and standard deviation of the total evapotranspiration ( E) during May to September (day of year (DOY) = 121-274) for 2003-2006 were 181.5 and 26.4 mm, respectively. The interannual variation (IAV) in the total E was caused by the IAV in E for the canopy-foliated period (DOY = 164-253), not by the IAV in the dates of leaf expansion and leaf fall. For the years with higher total E, E in the canopy-foliated period was consistently higher, which corresponded to the higher soil water content in these years. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Water vapor, energy fluxes, and environmental conditions were measured in an eastern Siberian larch forest for 7 water years, from 1998 to 2006, to understand the water-balance characteristics and interannual variation (IAV). The latent heat flux accounted for 38–67% of the sum of turbulent heat fluxes in June, July, and August, a relatively moderate fraction was compared to values measured at mid- and low latitudes. More than 70% of the annual precipitation evaporated during May to September. Annual evapotranspiration, including interception loss, was relatively steady at 169–220mm compared with the wide range in annual precipitation (111–347mmyear−1). The evapotranspiration rate was 1.49–2.30mmday−1 on a daily basis from May to September above a dry canopy. This feature is one of the remarkable characteristics of the water balance in eastern Siberian forests. The thawing depth of the permafrost has been rapidly deepening since 2004, such that the maximal thawing depth varied from 127cm before 2003 to over 200cm after 2004. At the same time, there was a very large increase in the moisture content of the surface soil. This increase could not be explained by the amount of annual precipitation alone and may have been due to inflow from the deeper thawing layer. The IAV of evapotranspiration was small, but the yearly evapotranspiration coefficient (the ratio of evapotranspiration to potential evaporation) ranged from 0.30 to 0.45. These results indicate that the IAV of evapotranspiration is controlled by regulation of the land surface rather than by atmospheric demand. Soil-moisture content was the most important variable among the factors determining the evapotranspiration coefficient at an interannual temporal scale. This result differs somewhat from previous satellite-based findings that air temperature was a major variable for plant activity. This difference might result from the fact that the IAV of soil water content did not correspond to that of the precipitation amount because of the presence of the permafrost. By contrast, the soil water content was strongly affected by precipitation in the previous summer. [Copyright &y& Elsevier]

Horizontal wind speed and vertical wind speed fluctuation data obtained from aircraft during the GEWEX Asian Monsoon Experiment-Siberia (GAME-Siberia) project revealed the development of a local circulation in the atmospheric boundary layer (ABL) near the Lena River, Siberia. Horizontal wind speed distributions showed regions where wind direction was opposite that of the synoptically dominant wind flow. Horizontal wind speed distributions and horizontally averaged vertical wind speed fluctuations with a 4-km cut-off distance were used to derive air stream vectors. The vectors showed the development of local circulations only over the upwind side of the Lena River on some days. Criteria for local circulation detection are suggested. Tower-based energy imbalances were observed at two flux-tower sites. Turbulent heat fluxes were underestimated relative to the available energy when mean downward flow dominated at the flux tower. This tower-observed energy imbalance might be related to the development of local circulations near the Lena River. [ABSTRACT FROM AUTHOR]

For a larch forest site near Yakutsk, energy and water fluxes are estimated and their variations are investigated using a one-dimensional land surface model with daily routine data. The estimation period is 1986-2000, including the period before the start of tower observation (1997) on the left bank of the Lena River. The land surface model includes three submodels: vegetation, snow cover, and soil. The model can calculate water and energy fluxes above and within the forest if meteorological data over the forest are given as input. Data used in this study are Baseline Meteorological Data in Siberia (BMDS) Version 3. This set comprises daily data of main meteorological elements. The procedure is as follows: (1) preparation of equations between routine Yakutsk data and larch tower data; (2) estimation of past meteorological data over the taiga using those equations; and (3) estimation of fluxes using the one-dimensional model. The date of leaf-out start is parameterized with soil temperature and accumulated air temperature. It corresponds to the green-up time obtained from satellite normalized difference vegetation index (NDVI) data. Estimated monthly deviation of net radiation, Rn is 10 Wm-2, its maximum is 20 Wm-2. The monthly deviation of latent heat flux, LE, is less than 12 Wm-2. Its warm season average is less than 7 Wm -2. Although the magnitude of LE is almost zero in December and January, and several Wm-2 aside from these 2 months, the sensible heat flux, H, sometimes becomes negative during the cold season. The variation of evapotranspiration is considerably smaller than that of precipitation. The evapotranspiration (E) normalized by potential evaporation (Ep), E/Ep is 0.37 0-44 in the warm season; it tends to be large when leaf-out starts early. The amount of evapotranspiration in the warm season can be estimated from Ep within an error of 5 mm using this relationship. [ABSTRACT FROM AUTHOR]

Water and energy exchanges are evaluated for two larch forests, one pine forest, and one grassland area in eastern Siberia near Yakutsk using a one-dimensional land surface model. Diurnal and seasonal variations of fluxes are simulated reasonably with general stomatal parameters at all sites. In the grassland site, the Bowen ratio is 0.2 in midsummer; it is smaller than that in forest sites (about 1). Sensitivity tests indicate that leaf area should be given accurately along with total plant area index including stem and branch areas. If both plant and leaf areas are given, the outline of seasonal heat balance can be simulated using the same stomatal parameters for forests and grassland sites with the model. In the larch site on the left bank of the Lena River, although input precipitation varies widely from 82 to 236 mm year to year from 1998 through 2000, calculated total evapotranspiration varies only within a range of 50 mm around 238 mm in the larch site. Understory evapotranspiration contributes 37%–44% to total evapotranspiration; interception is 15%–21% of precipitation. Evapotranspiration normalized by potential evaporation is 0.37 for larch sites almost independent of year; for grassland it is 0.52. At some sites, evapotranspiration in the warm season exceeds precipitation, thereby implying either a warm-season depletion of water storage in the soil column (most likely melted water from the thawing of the soil) or a horizontal transport of subsurface melt water from neighboring areas, or both. [ABSTRACT FROM AUTHOR]

We summarized our recently-published papers on greenhouse gas exchanges at two important boreal regions underlain by permafrost: eastern Siberia and interior Alaska. Relevant literatures were also referred to, and future research directions on the high-latitude terrestrial processes were suggested. Long-term monitoring of CO 2 fluxes at the boreal forests revealed that anomalous weather and disturbances changed the CO 2 balance. More than a decade is required to return to a CO 2 sink at burned forests in interior Alaska. Anomalous high precipitation altered the forest structure in eastern Siberia, shifting the overstory/understory contributions to the CO 2 balance. The CH 4 emissions were higher in the two boreal forests than in the other boreal forests. Upscaling the in-situ observations and comparisons with top-down approaches revealed considerable inconsistencies exist among the approaches. We recommend the following directions in future research. First, long-term monitoring is indispensable to detect the effect of climate change on ecosystems. Second, disturbance impacts, including fire, thermokarst, and wet spells, need to be quantified. Third, further observations are necessary for constraining CH 4 exchange models. Finally, reconciling top-down and bottom-up approaches is required to reduce uncertainty. Prompt sharing of observed data and model products is crucial to improve our understanding of high-latitude processes. [ABSTRACT FROM AUTHOR]

Detecting year-to-year variability of the timing of the start of the growing season (SGS) and the end of the growing season (EGS) is an important task in accurately evaluating ecosystem functions and services under climate change in vulnerable ecosystems in Siberia. We constructed a degree-day model for estimating the SGS and EGS dates at a deciduous coniferous forest site in Siberia, based on the relationship between daily phenology images and daily mean air temperature between 2013 and 2017. We tested the model by applying it to another similar site in Siberia. The model successfully estimated the SGS and EGS dates from the cumulative effective temperature, derived from daily mean air temperatures exceeding a best-fit threshold value of 2 °C (root mean square error: RMSE = 1.00) and falling below a best-fit threshold value of 1 °C, 0 °C or −1 °C (RMSE = 2.29), respectively. The modelled SGS and EGS dates closely matched the observed dates of leaf flush and leaf fall, respectively, in the larch overstory. [ABSTRACT FROM AUTHOR]

This article explains the Global Energy and Water Cycle Experiment (GEWEX)-Asian Monsoon Experiment (GAME). GEWEX was conducted under the umbrella of the World Climate Research Program and was led by Professor Yasunari from Japan. GAME research focused on water and energy cycles of Eurasia. Intensive field work carried out in eastern Siberia, the Tibetan Plateau, Central China, and Thailand has resulted in new insights into hydrological and meteorological processes. The papers in this issue focus on the hydrological characteristics of the GAME region.

• A canopy-scale daily-mean CH 4 flux over a Cajander larch forest was net emission. • CH 4 flux was nearly the same as the upper boundary of the limit of flux detection. • Daytime CH 4 flux showed dependencies on both temperature and soil moisture. Canopy-Scale methane (CH 4) flux measurement over a larch forest in eastern Siberia was conducted by eddy covariance method using an open-path CH 4 gas analyzer. Though the uncorrected flux showed strong CH 4 uptake in the daytime, this changed to CH 4 emission after density and spectroscopic effects were corrected. Random flux errors calculated from cross-covariance functions suggested that CH 4 flux was nearly the same as the upper boundary of the limit of flux detection at the 95th percentile, being barely resolved by the measurement system; and that most of the daytime CH 4 flux remained positive even after uncertainties due to random flux errors were taken into consideration. CH 4 flux showed clear diurnal variation, representing emission in the daytime and near-zero in the nighttime, irrespective of wind direction. The daytime CH 4 flux was dependent on both air temperature and volumetric soil water content. The CH 4 flux from May 29, to June 12, was calculated as net emissions of 4.9–13.8 nmol m − 2 s − 1 in daily average, ranging between the forest floor and a mesotrophic fen near this site measured by static chambers in a previous study. [ABSTRACT FROM AUTHOR]