立地因子对青海祁连圆柏林生态系统碳密度的影响

Effects of site factors on organic carbon density in Juniperus przewalskii forest ecosystem in Qinghai

【目的】估算青海祁连圆柏(Juniperus przewalskii)林生态系统碳密度,分析其空间分异特征及随不同立地因子的变化规律,为青海祁连圆柏林的科学经营管理提供依据。【方法】在青海省祁连圆柏天然分布较为广泛的泽库、兴海、都兰、乌兰、祁连和德令哈6个市(县),按照不同立地条件(海拔、坡向、坡度、坡位)设置20 m×20 m的样地96个,依据每木检尺数据和祁连圆柏数量,在每个样地内选择标准木3~5株,采集其树枝、树叶、树干和树根,称量其鲜质量,并收集以上各器官样品200~500 g;同时在各样地按“S”形取样法设置土壤样点5个,每10 cm为一层,分层采集0~60 cm土层土样。分别测定祁连圆柏各器官含碳率和各层土壤有机碳含量,再根据乔木层生物量和土壤体积质量,估算祁连圆柏林生态系统碳密度,分析其空间分异特征及随立地因子的变化规律,采用逐步回归分析来筛选影响祁连圆柏林生态系统碳密度的主导地形因子。【结果】①青海祁连圆柏林生态系统碳密度均值为291.28 t/hm2,变异系数为0.38,表明祁连圆柏林生态系统碳密度空间分异较大,与我国其他区域松柏科森林生态系统碳密度相比,青海祁连圆柏林生态系统碳密度处于较高水平。②在青海祁连圆柏林生态系统中,土壤层碳密度占比达71.95%,约为乔木层的2.5倍,表明土壤有机碳密度是构成该生态系统碳密度的主体。③不同地域之间祁连圆柏林生态系统碳密度存在一定差异,以兴海县(382.25 t/hm^(2))最大,泽库县(213.20 t/hm^(2))最小。④青海祁连圆柏林生态系统碳密度随着海拔的增大呈先升高后降低的趋势,其中海拔>3500~≤3700 m的地区最大,为365.69 t/hm^(2),海拔>2900~≤3100 m的地区最小,为196.40 t/hm^(2);随坡度的增大而减小,坡度>5°~≤15°缓坡碳密度最大,为386.72 t/hm2,坡度>35°~≤45°的急坡最小,为212.52 t/hm^(2);下坡位(350.56 t/hm^(2))高于中坡位(288.28 t/hm^(2))和上坡位(208.16 t/hm^(2)),阳坡(293.27 t/hm^(2))略高于阴坡(284.29 t/hm^(2))。逐步回归分析结果显示,坡位和海拔的确定系数增量(ΔR2)均明显高于坡度、坡向。【结论】海拔、坡位、坡度、坡向对青海祁连圆柏林生态系统碳密度有一定影响,其中坡位和海拔是影响青海祁连圆柏林生态系统碳密度的主导因子。

【Objective】This study explored organic carbon density(OCD)of Juniperus przewalskii forest ecosystem and its relationship with site conditions to provide basis for better management of these ecosystems.【Method】The research was carried out in Zeku,Xinghai,Dulan,Wulan,Qilian and Delingha counties in Qinghai.The six counties had large distribution of natural J.przewalskii forests.Ninety-six sampling plots of 20 m×20 m were set up in different site conditions(elevation,slope aspect,slope position and slope gradient).At each sampling plot,three to five standard trees were selected according to the survey results.Fresh weights of tree organs including trunk,branch,leaves and roots were obtained,and 200-500 g sample was collected for each organ.Five soil profiles were also selected at each sampling plot and 6 samples were collected for depths of 0-60 cm at interval of 10 cm.The carbon contents of soil samples and tree organ samples were analyzed.Arbor layer biomass and soil bulk density of each sampling plot were also monitored to obtain OCD of the J.przewalskii forest ecosystem.The spatial variation of the forest ecosystem OCD and its relationship with different site conditions were then analyzed.Stepwise regression analysis was used to quantify the dominant factors of OCD.【Result】(1)The average OCD of J.przewalskii forest ecosystem was 291.28 t/hm^(2) with coefficient of variations 0.38,indicating large spatial variations.Compared with Pinaceae and Cupressaceae forest ecosystems in other regions in China,the carbon density of J.przewalskii forest ecosystem in Qinhai was at a higher level.(2)The soil pool OCD accounted for 71.95% of the total ecosystem OCD as the main contributor,which was about 2.5 times as that of the arbor layer.(3)The OCD of J.przewalskii forest ecosystem varied among different regions with the highest value of 382.25 t/hm2 in Xinghai and the lowest of 213.20 t/hm^(2) in Zeku.(4)The forest ecosystem OCD firstly increased and then decreased with the increase of elevation.The highest OCD of 365.69 t/hm^(2) was at>3500-≤3700 m above sea level,while the lowest of 196.40 t/hm^(2) was at>2900-≤3100 m above sea level.The forest ecosystem OCD decreased from 386.72 t/hm^(2) to 212.52 t/hm^(2) with the increase of slope gradient.The down slopes had higher OCD(350.56 t/hm2)than middle(288.28 t/hm^(2))and up slopes(208.16 t/hm^(2)),and the sunny slopes had higher value(293.27 t/hm^(2))than shady slopes(284.29 t/hm^(2)).Stepwise regression analysis showed that the increases in coefficient of determination(ΔR2)of slope position and elevation were higher than those of slope gradient and slope aspect.【Conclusion】Elevation,slope position,slope gradient and slope aspect had impacts on the spatial variation of forest ecosystem OCD.Slope position and elevation were the dominant topographic factors affecting OCD of J.przewalskii forest ecosystem in Qinghai.