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This study investigated the CO2 exchange over a 10-year period (2005–2014) inside and above a larch-dominant forest in the central Lena river basin, eastern Siberia. A wet-soil condition, such as that found in the active layer (seasonally thawed soil layer of upper permafrost), containing unusually high soil water close to saturation and partial surface waterlogging, was prolonged during the warm season of 2005–2009. In later years, the soil layer closer to the ground surface became dry (∼10% volumetric water content), although the deeper part remained relatively wet (∼30%). We quantitatively compared the whole forest and the understory CO2 exchanges to detect the separate effects of excessive soil waters on the overstory and understory vegetation. The conventional light and temperature response functions for half-hourly CO2 fluxes, that is, the net ecosystem exchange of daytime and night-time, respectively, were applicable to the understory observations. Comparison of the fitting parameters of the light response function at two levels revealed a smaller maximum net ecosystem exchange (NEE) under light saturation with a steep response under weak light conditions for the understory. The CO2 exchanges at the understory increased from the wet-soil period to the drying soil period by 46% (1.3 g C m−2 d−1) of gross primary production (GPP) and 29% (1.2 g C m−2 d−1) of ecosystem respiration (ER), while no trend was found in the ecosystem scale fluxes. These increases were due to an increasing understory biomass, changes in plentiful light and soil water in the inside-canopy environments, and enhanced turbulent mixing. The decline in the larch contribution could be compensated for by the understory growth and the remaining wetness of the active layer, which indicated that the interactions between the larch and the understory supported the stability of carbon cycles in this forest ecosystem.

Canopy-Scale methane (CH4) flux measurement over a larch forest in eastern Siberia was conducted by eddy covariance method using an open-path CH4 gas analyzer. Though the uncorrected flux showed strong CH4 uptake in the daytime, this changed to CH4 emission after density and spectroscopic effects were corrected. Random flux errors calculated from cross-covariance functions suggested that CH4 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 CH4 flux remained positive even after uncertainties due to random flux errors were taken into consideration. CH4 flux showed clear diurnal variation, representing emission in the daytime and near-zero in the nighttime, irrespective of wind direction. The daytime CH4 flux was dependent on both air temperature and volumetric soil water content. The CH4 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.

Water vapour and carbon dioxide fluxes, as well as environmental factors, were measured in an Eastern Siberian larch forest for 14 years from 1998 to 2011 to understand the water and carbon balances and the effects of environmental variables on these balances in this region. The overstory vegetation in this forest has been damaged by the unusual waterlogging that has occurred since 2005, and the number of living larch decreased by 15% from 1998 to 2011. During this period, the composition of floor vegetation changed from dense cowberry to grasses and shrubs with high water tolerance. This period was categorized into three phases according to annual precipitation: normal years from 1998 to 2000 and from 2009 to 2011, dry years from 2001 to 2004, and wet years from 2005 to 2008. Although the time series of atmospheric conditions were independent of that of annual precipitation, the behaviour of underground conditions such as soil temperature and soil water content followed the change in annual precipitation. The relationships of the evapotranspiration ratio and surface conductance to soil water content from 2007 were different from those prior to 2006, and the group from 2007 was smaller than that prior to 2006 for both relationships. The relationship between gross primary production and soil water content also changed, but this change occurred in 2008. Specifically, the change in characteristics of evapotranspiration preceded that in carbon dioxide by almost one year. The main driver of the interannual variation in water use efficiency was variation in evapotranspiration until 2007 but was variation in gross primary production since 2008. The time lag of the response of evapotranspiration to increased soil water content was nearly two years, whereas it was about three years in the case of gross primary production. Although drought stresses on forests are well known, this is the first report of water and carbon exchanges in Eastern Siberia indicating that the boreal forest has been damaged by wet stresses at the interannual scale.

A consistent measure of forest canopy height is required for among-site comparisons of meteorological properties and for comparison of model results such as distributions of wind and other scalars calculated by large eddy simulations with observations. The aerodynamic canopy height has been proposed as a useful measure of canopy height, which is determined by micrometeorological methods without depending on stand inventory data. However, the aerodynamic canopy height is obtainable only if the necessary meteorological instruments are installed. In this study, we explored an alternative measure of forest canopy height based on tree inventory data. When cumulative trunk basal area from the shortest tree is plotted against corresponding individual tree height, it shows a sigmoidal curve with its inflection point occurring at a height class. The above-ground height of this inflection point (hereafter cumulative basal area inflection (CuBI) height) is defined as forest canopy height, as many tall trees with relatively large basal area belong to this height class. The difference between the aerodynamic canopy height and the CuBI height was always smaller than 12%, whereas the differences exceeded 20% for at least one of the forests when canopy height was determined using alternate methods. This suggests that, unlike other canopy height definitions, CuBI height is a consistent index of the aerodynamic canopy height, little affected by criterion of the trees composing the upper canopy or by the presence/absence of many shorter understory trees. Because chronological changes in size-structure of tree height and trunk diameter have long been studied for many forests, CuBI height may be used as an index of chronological change in the aerodynamic canopy height.

We measured evapotranspiration in an eastern Siberian boreal forest, in which the understory was cowberry and the overstory was larch, during the entire growing seasons of 2005 and 2006. We compared evapotranspiration from the understory vegetation above the forest floor E U with evapotranspiration from the whole ecosystem above the overstory canopy E O . The E U / E O ratio had a seasonal trend with a flat-bottomed U-shape during the growing season (4 May–30 September). High- E U / E O ratios at the beginning and end of the growing season were observed because larch, one of the two sources of E O , was a deciduous tree, while the understory was the evergreen cowberry. The mean daily E U values during the foliated period of larch (1 June–31 August) were 0.8 and 0.9 mm day −1 , or 51.4 and 51.8% of E O in 2005 and 2006, respectively. The understory vegetation was one of the most important components of the hydrologic cycle in this forest. A significant amount of E U was caused by plant physiological control, due to the aerodynamic conductance, which was much larger than the surface conductance, leading to a smaller decoupling coefficient. We found that 71% of E U was caused by the vapour pressure deficit above the forest floor.

This paper summarizes and synthesises the results obtained in several recent studies on water and energy exchange of East Siberian forests located on permafrost. It is found that annual evaporation of these forests shows relatively small inter-annual variation (147–196 mm). The availability of stored water in the permafrost probably functions as a buffering resource that is only used occasionally in very dry years. Surface soil water content shows a 1-year memory to precipitation in Eastern Siberia because the precipitation conditions in previous summers carry over in a frozen condition to the next spring. Modelling shows that evaporation was larger for southern forests than for northern forests. Compared to tundra, taiga forest shows a relatively small Bowen ratio. In an alas system, observations show considerable impact of the distribution of lakes in the vicinity on the observed evaporation fluxes. There appeared to be little difference in the observed Bowen ratio between a young and old larch forest. Surface conductance modelling and upscaling studies note that leaf area index (LAI) was a controlling factor in evaporation. With the permafrost area under strong pressure from climate warming, the need to understand the ecophysiological controls of these forests in East Siberia is paramount. With the data and analysis presented in this special issue becoming available we are now in a better position to model the effects of permafrost degradation on water and energy balances.

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 169–220 mm compared with the wide range in annual precipitation (111–347 mm year −1 ). The evapotranspiration rate was 1.49–2.30 mm day −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 127 cm before 2003 to over 200 cm 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.

To elucidate the energy balance over a forest, seasonal changes in the water and energy exchanges over a young larch forest (E-YL) in eastern Siberia were investigated and compared with the reported characteristics of a mature larch forest (W-ML) in the same region. Observations of the energy fluxes were conducted during the growing season from 15 April through 14 September 2000. The sum of the turbulent heat fluxes H and λE, ground heat flux G and heat storage change rate ΔS was generally in good agreement with the net radiation flux RNET, except when the friction velocity u* was 0.3 m s−1 by the CF. The averaged features of the CF relationships with u* and atmospheric stability ζ were similar to those reported for Canadian boreal forests. In contrast, hourly CF and hourly EBR were widely scattered. The probability functions of EBR for different wind velocities were consistent with previous results from large eddy simulation. To determine the seasonal course of daily flux values, the Bowen ratio energy budget method was used to estimate the turbulent heat fluxes. The daily latent heat flux maintained a value of approximately 50 W m−2 during the summer, whereas the daily sensible heat flux varied with changes in the available energy. The Bowen ratio had a clear “U” shape over the course of the growing season. The minimum values of approximately 1.0 occurred in the summer from June to August. The general features of seasonal changes at E-YL in 2000 were similar to those at W-ML in 1998, except for the lower Bowen ratio in August at E-YL.

Spatio-temporal variations in the energy budget and evapotranspiration (ET) in eastern Siberia were examined for a period of 19 years, from 1986 to 2004, for the Lena and Kolyma watersheds, which are underlain by permafrost. The energy budget and ET were calculated with a land surface model [Yamazaki, T., Yabuki, H., Ishii, Y., Ohta, T., Ohata, T., 2004. Water and energy exchanges at forests and a grassland in eastern Siberia evaluated using a one-dimensional land surface model. J. Hydrometeol. 5, 504–515] that considered vegetation, snow, and soil processes. Station data were used to compile gridded time series of forcing data. The spatial distribution of the energy budget showed that the latent heat flux (QE) was a significant term in the surface energy budget in forested regions, while the sensible heat (QH) was significant in the tundra region. A similar result was also obtained for the seasonal variation; in the Kolyma watershed, for instance, QH dominated the seasonal energy budget, even during the growing season. In the Lena watershed, 53% of precipitation (PG) was lost through ET, which occurred during a very sharply pronounced growing season. Transpiration (ET) was a major component of ET in the Lena watershed, while in the Kolyma watershed, evaporation from the forest floor (ES) was dominant. This suggested clear relations between the energy budget and the difference in land surface and climate. In the Lena and Kolyma watersheds, a time series of PG exhibited strong annual variations with an increasing trend during 1986–2004. ET also exhibited an increasing trend in the two watersheds. However, the interannual variation was weaker than that of PG, reflecting the influence of extensive permafrost in the area. The ET components also showed an increasing trend. In contrast, ES decreased during 1986–2004, the trend of ES thus being inversely correlated to that of PG. Interestingly, annual ET also exhibited an inverse correlation with PG in the Lena and Kolyma watersheds, although this relationship was weak. In contrast, ET displayed a strong correlation with air temperature (Ta) and the timing of the disappearance of snow.

Roughness length and zero-plane displacement over boreal, cool- and warm-temperate forests were observed and parameterised using forest structure data. Previous models for roughness length and zero-plane displacement using leaf area index and frontal area index did not describe intersite differences, and the model for zero-plane displacement did not express seasonal variations with the change of leaf area that was smaller in dense forest than in sparse forest. The observed results show that intersite differences of normalised zero-plane displacement were related to stand density, and seasonal variations were related to leaf area index at each forest, with the degree depending on stand density. From these observations, a new concept is proposed for normalised zero-plane displacement: the basal part is primarily determined by the density of stems and branches (stand density), while the seasonal variation depends on the density of leaves (leaf area index), which is limited to the residual of the basal part. Based on this concept, a new model was developed and verified to express both intersite differences and seasonal variations in observed roughness length and zero-plane displacement.

We measured sensible and latent heat fluxes (H and λE) in five forests located in boreal, cool-temperate, and warm-temperate zones of the Far East concurrently over several years to clarify their energy-consumption characteristics and the variation in and factors controlling evapotranspiration. The consumption of energy for evapotranspiration was larger at the southern sites than at the northern sites, and evapotranspiration in summer (July–August) was larger (average 2.9 mm day−1) for temperate forests than for boreal forests (average 1.7 mm day−1). Differences in energy-consumption characteristics between the forest types (e.g., deciduous vs. coniferous) were not as distinct as those by location. This inter-locational difference resulted from differing evapotranspiration restrictions caused by land-surface characteristics, rather than differing atmospheric evaporation demand.

We examined the dependence of stomatal conductance on physiological properties and meteorological variables using a Jarvis-type stomatal conductance model that included a function representing leaf chlorophyll concentration as a physiological property. We sampled the leaves of 5- and 10-year-old Quercus serrata trees. A low leaf chlorophyll concentration imposed a restriction on the opening capacity of the stomata. The stomatal conductance variability depended markedly on chlorophyll function; the degree of dependence was almost equal to that on solar radiation or vapour pressure deficit. The characteristics of stomatal conductance response to meteorological or physiological changes exhibited little seasonal variation (except relative to temperature). Stomatal conductance variability depended on the following meteorological parameters in decreasing order: radiation, vapour pressure deficit, leaf temperature, and soil moisture; this order did not vary seasonally. The dependence of stomatal conductance variability on the seasonal change in chlorophyll concentration was larger in spring and autumn than in summer. These results indicate that the consideration of seasonal changes in plant physiological properties is important in evaluating the water, energy, and CO2 cycles between plants and the atmosphere.