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 EU with evapotranspiration from the whole ecosystem above the overstory canopy EO. The EU/EO ratio had a seasonal trend with a flat-bottomed U-shape during the growing season (4 May–30 September). High-EU/EO ratios at the beginning and end of the growing season were observed because larch, one of the two sources of EO, was a deciduous tree, while the understory was the evergreen cowberry. The mean daily EU values during the foliated period of larch (1 June–31 August) were 0.8 and 0.9mmday−1, or 51.4 and 51.8% of EO 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 EU 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 EU was caused by the vapour pressure deficit above the forest floor.

The dependence of surface conductance Gs on various variables in a forest environment is important for characterising the spatiotemporal variations of water, energy and CO2 exchange between the forest and the atmosphere. Using a Jarvis-type model, we examined the variation of Gs in five different mature forests from three climate zones (boreal, cool- and warm-temperate) in the Far East. First, we applied the model using summertime Gs data from each site separately (within-site analysis). We evaluated the maximum surface conductance Gsmax and parameters related to the response of Gs to the environments. We found that these values differed among the locations. Second, we applied the model for pooled Gs data from all the sites and calculated a common parameter set (pooled analysis). In the pooled model, the difference in the observed Gsmax among the sites was expressed as a function of soil water content and the leaf area index. In addition, the responses of Gs to radiation, vapour pressure deficit and air temperature in different forests were also closely represented by common response functions. As a result, the pooled model could estimate the variation of Gs at each site using one parameter set with similar precision to the within-site models. These results suggest that the surface conductance of the various mature forests had the same maximum value and response properties, although we were not able to verify this. Our new parameterisation concept for pooled Gs data should be effective for simultaneous evaluations of the water, energy and CO2 exchanges of forests over wide regions.

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 2.9mmday−1) for temperate forests than for boreal forests (average 1.7mmday−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.

Introduction Canopy conductance (gc) is an important parameter that controls gas and heat exchanges between vegetation and the atmosphere. Two types of stomatal conductance (gs) models have been used to express gc: the Jarvis (1976) type model, which derives gs empirically as a function of several environmental variables, and the Ball et al. (1987) type model, which uses a correlation relationship between gs and the assimilation rate (A). A Jarvis-type model requires the maximum conductance value and many coefficients related to the stomatal response to environmental variables, while a Ball-type model requires only a dimensionless coefficient. Therefore, many recent land surface schemes have used Ball-type models (e.g., Baldocchi & Meyers, 1998). However, to improve the performance of Ball-type models, modified versions have imported empirical functions for the humidity deficit (Leuning, 1995) and soil moisture (e.g., Moriana et al., 2002). Consequently, the evaluation of the response of canopy conductance to environmental variables is very important for evaluating water, energy, and CO2 cycles between vegetation and the atmosphere. Studies have attempted to evaluate the spatial variation in canopy conductance, especially the maximum conductance (e.g., Schulze et al., 1995; Komatsu, 2003). As another parameter, Baldocchi & Meyers (1998) reported that the slope coefficient in the Ball-type model centres on 10 ± 20% in various vegetation types. These results are effective for modelling global gas exchange. However, the spatial distribution of the response characteristics of gc or gs to environmental variables has still not been clarified on a global scale. Although Jarvis-type models have been applied to various canopies and plants, there has been no clear comparison of the parameter values in the models because of the difference in the function form in the models used in different studies and in the investigation periods across studies. To allow a global evaluation of gc and the water and heat exchanges between vegetation and the atmosphere, the spatial distribution of the parameter values must be clarified. Here we investigated the variation in gc in five different forest types, which were distributed from middle to high northern latitudes in the Far East. In addition, we applied a Jarvis-type gc model to the data for each site for the 2003 or 2004 growing season, and then compared the response characteristics of gc to environmental factors.

Zero-plane displacement d and roughness length z0 were compared among boreal, cool-temperate, and warm-temperate forests. The normalized zero-plane displacement d/h (where h is the tree height) increased with stand density, whereas z0/h decreased. On the other hand, d/h showed a negative correlation with plant area index (PAI), which was inconsistent with parameterization of the models. The effect of the vertical structure of the forest might be included in PAI. Seasonal variations of d/h were observed in cool-temperate and warm-temperate forests. d/h decreased from autumn to winter in these sites which corresponded to the fall of leaves. However, d/h was underestimated by the models, and the decrease in d/h for cool-temperate forest appeared to be small compared with that predicted using the models with the large decrease in PAI.

Seasonal changes in the water and energy exchanges over a pine forest in eastern Siberia were investigated and compared with published data from a nearby larch forest. Continuous observations (April to August 2000) were made of the eddy‐correlation sensible heat flux and latent heat flux above the canopy. The energy balance was almost closed, although the sum of the turbulent fluxes sometimes exceeded the available energy flux (Rn − G) when the latent heat flux was large; this was related to the wind direction. We examined the seasonal variation in energy balance components at this site. The seasonal variation and magnitude of the sensible heat flux (H) was similar to that of the latent heat flux (λE), with maximum values occurring in mid‐June. Consequently, the Bowen ratio was around 1·0 on many days during the study period. On some clear days just after rainfall, λE was very large and the sum of H and λE exceeded Rn − G. The evapotranspiration rate above the dry canopy from May to August was 2·2 mm day−1. The contributions of understory evapotranspiration (Eu) and overstory transpiration (Eo) to the evapotranspiration of the entire ecosystem (Et) were both from 25 to 50% throughout the period analysed. These results suggest that Eu plays a very important role in the water cycle at this site. From snowmelt through the tree growth season (23 April to 19 August 2000), the total incoming water, comprised of the sum of precipitation and the water equivalent of the snow at the beginning of the melt season, was 228 mm. Total evapotranspiration from the forest, including interception loss and evaporation from the soil when the canopy was wet, was 208–254 mm. The difference between the incoming and outgoing amounts in the water balance was from +20 to −26 mm. The water and energy exchanges of the pine and larch forest differed in that λE and H increased slowly in the pine forest, whereas λE increased rapidly in the larch forest and H decreased sharply after the melting season. Consequently, the shape of the Bowen ratio curves at the two sites differed over the period analysed, as a result of the differences in the species in each forest and in soil thawing. Copyright © 2003 John Wiley & Sons, Ltd.

The water and energy exchanges in forests form one of the most important hydro-meteorological systems. There have been far fewer investigations of the water and heat exchange in high latitude forests than of those in warm. humid regions. There have been few observations of this system in Siberia for an entire growing season, including the snowmelt and leaf-fall seasons. In this study, the characteristics of the energy and water budgets in an eastern Siberian larch forest were investigated from the snowmelt season to the leaf-fall season. The latent heat flux was strongly affected by the transpiration activity of the larch trees and increased quickly as the larch stand began to foliate. The sensible heat dropped at that time, although the net all wave radiation increased. Consequently, the seasonal variation in the Bowen ratio was clearly 'U'-shaped. and the minimum value (1.0) occurred in June and July. The Bowen ratio was very high (10-25) in early spring, just before leaf opening. The canopy resistance for a big leaf model far exceeded the aerodynamic resistance and fluctuated over a much wider range. The canopy resistance was strongly restricted by the saturation deficit, and its minimum value was 100 s m(-1) (10 mm s(-1) in conductance). This minimum canopy resistance is higher than values obtained for forests in warm, humid regions, but is similar to those measured in other boreal conifer forests. It has been suggested that the senescence of Leaves also affects the canopy resistance, which was higher in the leaf-fall season than in the foliated season. The mean evapotranspiration rate from 21 April 1998 to 7 September 1998 was 1.16 mm day(-1). and the maximum rate, 2.9 mm day(-1), occurred at the beginning of July. For the growing season from 1 June to 31 August, this rate was 1.5 mm day(-1). The total evapotranspiration from the forest (151 mm) exceeded the amount of precipitation (106 mm) and was equal to 73% of the total water input (211 mm), including the snow water equivalent. The understory evapotranspiration reached 35% of the total evapotranspiration, and the interception evaporation was 15% of the gross precipitation. The understory evapotranspiration was high and the interception evaporation was low because the canopy was sparse and the leaf area index was low. Copyright (C) 2001 John Wiley & Sons, Ltd.

東シベリアヤクーツク周辺の北方林において, 林分構造に関する調査を行った。 Larix gmelinii, Pinus sylvestrisが優占する林分の成長はきわめて緩慢であったが, 両種の最大樹高はほぼ上限値に達していた。伐採や森林火災後に成林したと考えられるBetula platyphyllaが優占する林分および, L.gmeliniiとB. platvphyllaが混交する林分は過密な状態にあり, 樹高成長が旺盛であった。Betula platyphylla優占林分および, L.gmeliniiとB. platyphyllaが混交する林分の樹高階分布と直径階分布から, 撹乱後に成立したBetula platyphylla優占林から, L.gmelinii林へと移行していく過程が検出できた。

蒸散活動における樹体内水分の生理的・生態的意義を明らかにするため, 地際で切断された樹体地上部の水分量を無給水状態の広葉樹と針葉樹樹体について調べた。樹体重量と葉の水ポテンシャル, 気孔コンダクタンスを計測した結果, 広葉樹 (アキニレ, クスノキ) では針葉樹 (スギ) に比べて吸水や蒸散によって置き換えられる水分の割合が大きかった。また気孔閉鎖を起こすときの葉の水ポテンシャルはアキニレよりスギで高かった。また, 立木時の葉面積あたりの通水抵抗はアキニレよりスギで著しく大きかった。スギは日中の蒸散によって樹体内に生じた水欠差の影響を受けやすいため, 葉内水分の減少過程の早い段階から気孔を閉じることが示唆された。

Introduction Boreal forest, which widely extends over northern Eurasia from 45° to 70°, occupy one third of the total forest over the world in area. Although we often think that boreal forest is uniform, the climate condition (e.g. temperature, precipitation) is greatly different in the area. Thus, probably also vegetation response, and the characteristics of water and energy cycle widely vary. Here, the question is whether the physiological characteristics themselves change or just superficial response change. This study will discuss water and energy cycle using a land surface model over/in forests in Yakutsk, Moshiri and Seto, we will base on in situ physiological and hydrometeorological observation. This study is a part of the JST/CREST project “Parameterization of the relationship between the water cycle system and plant eco-physiological properties in boreal forest areas”. The outline of the project and site description can be found in Ohta (in this issue). Spatio-temporal variations of energy and water fluxes in Eastern Siberia is reported by Park et al. (in this issue).