摘要
落叶松林由于其速生的特性越来越多的受到国际通量研究领域的关注，特别是处于中高纬度地区的落叶松人工林更是国际通量界关注的焦点。本文以东北林业大学老山实验站通量观测塔(45°20′N，127°34′E)2004～2006年期间涡度相关法观测到的通量数据为基础，对寒温带落叶松人工林生态系统土壤热通量变化特征、风向对林分碳汇估计的影响、空气饱和蒸汽压亏缺对林分和呼吸的影响以及林分CO_2通量的年季变化规律进行了研究，并得出如下结论：
1)落叶松人工林土壤热通量，4月至8月为正值，是吸收热量过程，而9月至次年3月的绝大部分时间为负值，波动平缓，是放出热量的过程；年累积值波动范围在-190.44至30MJ·m~(-2)·year~(-1)之间，说明土壤吸收热量和放出热量趋于平衡；土壤热通量和土壤热导率(κ)受净辐射影响显著，且净辐射对κ的影响要大于热通量。在冬季土壤热通量和K与4～5小时前的净辐射呈现显著相关关系，而在夏季与2～3小时前的净辐射呈现显著相关关系，说明冬季存在4～5小时左右的延迟效应而在夏季这一延迟时间为2～3小时。κ在夏季与冬季变化十分平稳，但在春季和秋季土壤冻融交替期间存在很大的波动，对不同月份κ值进行多元统计分析表明，κ在所测定的3年间多数月份间无明显差异(p＞0.05)，而每年3月和4月间所测定的κ值往往与其它月份存在显著差异，当使用土壤温度数据进行土壤热通量计算时，应该充分考虑3月和4月间κ值波动对通量计算的影响。此外，土壤含水率也对土壤热通量有影响，在春、秋、冬季与土壤含水率呈显著的线性相关关系，而在夏季相关性较弱。
2)使用不同风向数据计算的净生态系统交换量(NEE)的结果差异较大。从大地形角度来看，来自人类聚集区风-西风向计算值(NEE)要远远低于非碳源风向-东风向数据的计算值(NEE)，2004和2005年西风向甚至为正值，2006年西风向虽然为负值，但仅为东风向是的80％。从小地形来看，沟谷上行风(南风和东南风)NEE的结果远大于下行风(北风和西北风)NEE的结果，2004年的北风向及2004和2005年的西北风向数据拟合计算出的NEE甚至为正值。2005和2006年北风向的NEE虽然为负值，但仅为南风的60％和3％。2006年西北风向的NEE虽然为负值，但仅为东南风向NEE的33％。由此可见，地形地势及地理位置对落叶松林NEE的观测结果的影响很大。
3)空气湿度对生态系统光合测定结果存在很大的影响，而对呼吸测定结果影响较小。从2004到2006年的结果来看，VPD＜10h Pa时的最大光合碳固定速率在-25～-30μmol·m~(-2)·s~(-1)之间，而VPD＞15hPa时，最大光合碳固定速率在-15μmol·m~(-2)·s~(-1)左右。VPD对呼吸测定结果影响不大，这可能最终导致VPD显著的响应林分的碳汇大小。
4)落叶松林CO_2通量的年季变化规律十分明显，其中NEE日平均值的正向最大值一般出现在非生长季的3～4月份左右，负向最大值一般出现在6～7月份，此时森林生态系统的光合作用能力最强。2004～2006年落叶松人工林年固碳能力分别为182.47、176.44和309.46 gC·m~(-2)·year~(-1)。
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 CO_2 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 CO_2 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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