| Larch forest was got more attention by the international flux researchers because of its fast growth and possible carbon sink capacity, particularly the larch plantations in the middle and high latitudes. Based on the long-term data (2004～2006) measured by the eddy covariance method at Laoshan flux observation tower in Experimental Forests of Northeast Forestry University (45°20' N, 127°34' E), tried to find the soil heat flux characteristics, the influenes of wind direction on the estimation of CO2 flux, possible influences of air humidity (vapor pressure deficit,VPD) on the stand photosynthesis and respiration, and finally annual carbon sink capacity of this cold temperate larch plantation was estimated. Following conclusions were got:1) From April to August, soil heat flux(SHF) of the larch plantation was positive, with the process of absorbing heat, whereas most of the SHF was negative from September to the following March with the process of giving off heat. Fluctuations of annual cumulative SHF from 2003～2005 was in the range of-190.44 to 30 MJ·m-2·year-1, indicating a nearly balance between absorbed and given off heat for the soil. Net radiation had significant influence on SHF and soil heat conductivity(K), and the impact would be greater onκ. In winter, SHF withκand net radiation at 4～5 hours before showed a significant correlation, however, SHF withκand net radiation at 2～3 hours before showed significant correlation in summer. This finding suggests that 4～5 hours in winter but 2～3 hours time-lag effect in summer is observed in the influences of radiation on SHF.κwas very stable in summer and winter, butκabruptly fluctuated during the period soil freezing and soil-thaw in the spring and fall. Multivariate statistical analysis onκin different months in 3 years showed that there was no notable difference between most months (p＞0.05), butκmeasured between every March and April often existed significant difference. Thus, when SHF was estimated with data of soil temperature, it is supposed to consider the largeκfluctuation from March to April. Furthermore, significant linear correlation between soil water content and SHF were also observed in spring, autumn and winter, but quite weak relationship in summer, indicating that soil water may also affect soil heat flux.2) The net ecosystem exchange (NEE) calculated by the different wind direction data was quite different. From the view of megarelief, NEE calculated utilized the data from westerly where was human gathered district was much lower than that in opposite direction from east wind (large area of forests). NEE in westerly was positive but negative in easterly in 2004 and 2005. NEE in westerly was negative in 2006, but it was only 80% of that in easterly. From the view of microtopography, NEE of the southerly was (southerly and southeasterly wind) bigger than NEE of northerly (northerly and northwest wind). NEE calculated by the north wind in 2004 and NEE calculated by the northwest wind in 2004 and 2005 were all positive. Despite NEE calculated by the northerly was negative in 2005 and 2006, but was only 60% and 3% of the southerly. In 2006, although NEE calculated from data of northwest wind was negative, but was only 33%.of southeast wind. Thus topography had important effect on the results of NEE calculation.3) Air humidity had great impact on the ecosystem photosynthetic results, but less affected on ecosystem respiration results. From 2004 to 2006, when vapor pressure deficit (VPD)＜10 hPa, the maximum photosynthetic rate ranged from -25 to -30μmol·m-2·s-1, and when VPD＞15 hPa, maximum photosynthetic rate was about -15μmol.m-2·s-1. VPD had little impact on the results of respiration, which may ultimately lead to VPD had a significant response to the forest carbon sink.4) Seasonal variation of larch forest CO2 flux of was obvious, which the maximum respiration usually appeared in the non-growing season of March to April, and maximum photosynthesis generally occurred in the vegetative seasona of June to July. From 2004 to 2006, carbon sequestration capacity of larch plantation was 182.47, 176.44 and 309.46 gC·m-2·year-1, respectively.
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