高寒灌丛草甸和草甸均是青藏高原广泛分布的植被类型,在生态系统碳通量和区域碳循环中具有极其重要的作用。然而迄今为止,对其碳通量动态的时空变异还缺乏比较分析,对碳通量的季节和年际变异的主导影响因子认识还不够清晰,不利于深入理解生态系统碳通量格局及其形成机制。该研究选取位于青藏高原东部海北站高寒灌丛草甸和高原腹地当雄站高寒草原化草甸年降水量相近的5年(2004–2008年)的涡度相关CO_2通量连续观测数据,对生态系统净初级生产力(NEP)及其组分,包括总初级生产力(GPP)和生态系统呼吸的季节、年际动态及其影响因子进行了对比分析。结果表明:灌丛草甸的CO_2通量无论是季节还是年际累积量均高于草原化草甸,并且连续5年表现为"碳汇",平均每年NEP为70 g C·m^(–2)·a^(–1),高寒草原化草甸平均每年NEP为–5 g C·m^(–2)·a^(–1),几乎处于碳平衡状态,但其源/汇动态极不稳定,在2006年–88 g C·m^(–2)·a^(–1)的"碳源"至2008年54 g C·m^(–2)·a^(–1)的"碳汇"之间转换,具有较大的变异性。这两种高寒生态系统源/汇动态的差异主要源于归一化植被指数(NDVI)的差异,因为NDVI无论在年际水平还是季节水平都是NEP最直接的影响因子;其次,灌丛草甸还具有较高的碳利用效率(CUE,CUE=NEP/GPP),而年降水量和NDVI是决定两生态系统CUE大小的关键因子。两地区除了CO_2通量大小的差异外,其环境影响因子也有所不同。采用结构方程模型进行的通径分析表明,灌丛草甸生长季节CO_2通量的主要限制因子是温度,NEP和GPP主要受气温控制,随着气温升高而增加;而草原化草甸的CO_2通量多以季节性干旱导致的水分限制为主,其次才是气温的影响,受二者的共同限制。此外,两生态系统生长季节生态系统呼吸主要受GPP和5 cm土壤温度的直接影响,其中GPP起主导作用,非生长季节生态系统
Fine root biomass was measured by soil core sampler in four community type on Tibetan Plateau,of which include two coppice of Betula platyphylla and Quercus aquifolioides,one shrub of Salix oritrepha,and one coniferous woodland of Pinus densata.The fine root density were Betula platyphylla of(785.9±290.4)g·m-2,Quercus aquifolioides of(801.0±279.5)g·m-2,Salix oritrepha of(376.0±146.0)g·m-2 and Pinus densata of(431.2±171.1)g·m-2.Result showed that the fine root density of two coppices was significantly higher than that of shrub and coniferous woodland.Dead fine root was 16.2% of the total fine root in Betula platyphylla,and the percentage were about 25% in other three type of forest.Distribution of fine root had the same pattern that high density of fine root appeared at 0～10 cm soil depth.
