The boreal forest (called taiga in Eurasia), the largest terrestrial biome in the world, comprises 11% of the Earth's vegetation (about 16 million km2) and 26% of the total global vegetation carbon stocks [Larsen, 1980; Bonan and Shugart, 1989; Dixon et al., 1994; Prentice et al., 2001]. It is located in the circumpolar region spanning most of interior Alaska, Canada, Scandinavia and Finland, Scotland, Russia, Kasachstan, northern Mongolia, and northeastern China [Bonan and Shugart, 1989]. Owing to its large distribution area, it plays a critical role in balancing the global carbon budget [Goulden et al., 1997; Myneni et al., 1997; Field et al., 1998; Houghton et al., 1998; Malhi et al., 1999; Schulze et al., 1999; Ciais et al., 2000; Geider et al., 2001] and mitigating global climate [Foley et al., 1994; Bonan et al., 1995; Dixon et al., 1996; Beerling, 1999; Kasischke and Stocks, 2000; Foley et al., 2003]. The boreal forest is also one of the most intensively studied ecosystem in the flux measurement community, for instance, in the Boreal Ecosystem-Atmosphere Study (BOREAS) [Sellers et al., 1997; Hall, 1999, 2001], the Northern Hemisphere Climate-Processes Land-Surface Experiment (NOPEX) [Halldin et al., 1999], and the EuroSiberian Carbonflux project [Schulze et al., 2002]. The boreal forests can act as either sources or sinks for atmospheric carbon dioxide [Malhi et al., 1999; Chapin et al., 2000; Valentini et al., 2000]. The source or sink strength of boreal forests shows large interannual variability with respect to climate [Goulden et al., 1998; Lindroth et al., 1998; Schulze et al., 1999; Baldocchi et al., 2001; Law et al., 2002], and large differences among different forest types [Apps et al., 1993; Bonan, 1993; Gower et al., 2001]. Summer drought or water stress may switch a boreal forest from a sink to a source of CO2 [Price and Black, 1990; Price and Black, 1991; Hollinger et al., 1998]. Increasing temperatures may reduce the sink strength of the boreal forests [Plochl and Cramer, 1995; Lindroth et al., 1998]. Source strength (soil respiration) of some boreal forests was shown to be a function of photosynthetic assimilation [Högberg et al., 2001]. Disturbances, for example, fire and logging, may have a considerable influence on the CO2 uptake ability of some boreal forests [Kurz and Apps, 1999; Schulze et al., 1999; Amiro, 2001; Law et al., 2002; Thornton et al., 2002; Hicke et al., 2003]. There are also many other long-term factors that must be kept in mind in quantifying the source/sink strength of the boreal forests, for example, the increase in atmospheric CO2 concentration and air temperature [Melillo et al., 1993; Dixon et al., 1994; Gifford, 1994; Amthor, 1995; Goulden et al., 1998; Chen et al., 2000; White et al., 2000; Hicke et al., 2002], nitrogen deposition [Cao and Woodward, 1998; Schulze et al., 1999; Chen et al., 2000], and the length of the growing season [Myneni et al., 1997; Goulden et al., 1998; Randerson et al., 1999; Schulze et al., 1999; Black et al., 2000; Baldocchi et al., 2001]. Clearly, until now we have accumulated much knowledge on the CO2 exchange between the boreal forests and the atmosphere in both the western and eastern hemispheres, but there is a paucity of information on NEE of the southern limit of the Siberian boreal forest, which is adjacent to the vast area of Asian steppe biome. Our study on the NEE of a marginal area of the boreal forest should have implications for quantifying the role of the entire boreal ecosystem in the global carbon cycling and its sensitivity to climate.
 The boreal forest in Mongolia covers about 5% of the Mongolian land area, is distributed primarily in the mountain areas such as the Khentii Mountains and the Khangai Mountains of northern Mongolia, and is dominated by Siberian larch (Larix sibirica Ledeb.) which covers more than 70% of the forested areas [Hilbig, 1995; Gunin et al., 1999]. The cold continental climate in these areas is characterized by long, cold and dry winters versus short, cool and moist summers. Low temperatures and a short growing season are the major limiting factors for plant growth [Hilbig, 1995; Gunin et al., 1999]. Although most of the Mongolian boreal forests remain pristine, the area that is adjacent to the steppe (the forest-steppe transition zone) has become seriously threatened over the past 2 decades owing to growing commercial exploitation of the forest resources, mainly by logging and mining [Erdenesaikhan and Erdenetuya, 1999; Goldammer, 2002]. Fire hazard, pest outbreak, and unsustainable excessive logging are major disturbances impacting the dynamics of Mongolian boreal forests in general [Intergovernmental Panel on Climate Change (IPCC), 1997; Goldammer, 2002]. The forest-steppe transition zone is ecologically fragile and vulnerable to environmental changes and human activities. A raising concern is thus the possible influences from the projected global warming, namely the shrinking extent of the boreal forest and the recession of the treeline to higher altitudes in Mongolia [Goldammer, 2002]. An analysis of instrumental meteorological data by Dagvadorj and Mijiddorj  shows that since the 1940s, annual temperatures have increased by 1.8°C in western, 1.0°C in central, and 0.3°C in eastern Mongolia. In addition, their results also indicate that concomitantly the growing season length has increased by about 10 to 20 days, and summer precipitation, especially in August, has increased [Dagvadorj and Mijiddorj, 1996]. Recent dendrochronological research provides strong supporting evidence for these trends in climate change [Jacoby et al., 1996, 1999; D'Arrigo et al., 2000; Pederson et al., 2001]. Rising temperatures may favor carbon uptake by the Mongolian boreal forests by increasing the growing season length, but at the same time this may also lower soil moisture via increased evapotranspiration, and increase additional release of conserved plant and soil carbon via respiration. Sustained drying induced by such a temperature increase may also increase the risk of forest fires and pest outbreaks and thereby further accelerate forest area losses [IPCC, 1997]. Furthermore, the Mongolian boreal forest plays an important role in biodiversity conservation, carbon and water cycling, regional climate regulation, and sustainable social-economic development in Mongolia [Batjargal and Enkhbat, 1998; Bastian, 2000]. Therefore it is necessary to obtain a better scientific understanding of the exchange mechanisms of water, energy and CO2 over the Mongolian boreal forest and their responses to climate change and anthropogenic disturbances. The present study's goal is to close this gap in current knowledge. It was part of a 5-year international project (2002–2006), the Rangelands Atmosphere-hydrosphere-biosphere Interaction Study Experiment in Northeastern Asia (RAISE), which is currently in progress in the region along the Kherlen River, the second longest river in Mongolia. This paper reports one-full-year-long NEE measurements (from 25 March 2003 to 24 March 2004) over a montane larch forest, located in the upper reach of the Kherlen River, by means of the eddy covariance (EC) technique. The major issues covered here include (1) the dynamics of NEE across daily and seasonal scales and (2) the environmental controls on NEE.