【目的】研究祁连山区重要的水源涵养林———青海云杉林的生物量随海拔(2700～3300 m)的变化规律，为预测祁连山区的产水能力和实施黑河流域植被－水的综合管理提供科学依据。【方法】在祁连山中段北坡的排露沟小流域，沿海拔梯度，选取生长状况良好、具有代表性的林分布设样地，进行植被调查。在2700～2800，2800～2900，2900～3100和3100～3300 m 4个海拔区段共设置15块20 m ×20 m 样地。利用已有的青海云杉生物量估算方程，计算研究区森林生物量的平均值、各海拔区段内的森林生物量以及不同径级树木对总生物量的贡献量，据此分析森林生物量的大小及其随海拔变化规律的主要影响因素。【结果】研究区青海云杉林的生物量为(128.61±50.25) t·hm －2，中树(12.5 cm ＜DBH≤22.5 cm )和大树( DBH＞22.5 cm)是森林生物量的主要来源，二者所占比例分别为44%和35%，小树(5 cm ＜DBH≤12.5 cm)的生物量较少，仅占21%。随海拔升高，青海云杉林生物量呈“单峰”变化，在海拔2700～2800 m为(135.43±21.25)t·hm －2，在海拔2800～2900 m达到最大为(152.33±73.01)t·hm －2，在海拔2900～3100 m为(124.28±39.54) t·hm －2，在海拔3100～3300 m最小为(100.48±65.69) t·hm －2。不同海拔区段内，森林生物量的主要来源不同，在海拔2700～2800 m，以中树生物量最多，为63.54 t·hm －2(占该海拔区段内森林生物量的48%)；在海拔2800～2900 m，中树生物量继续增加，达到90.53 t·hm －2(占该海拔区段内森林生物量的59%)，在海拔2900～3100 m，大树成为森林生物量的主要来源，其生物量为54.46 t·hm －2(占该海拔区段内森林生物量的45%)；在海拔3100～3300 m，大树生物量继续增加，达到71.46 t·hm －2(占该海拔区段内森林生物量的71%)。与附近地区相比，研究区青海云杉林的生物量(128.61 t·hm －2)与祁连山哈溪林场(132.90 t·hm －2)、祁连山平均值(169.80 t·hm －2)和全国平均值(156.66 t·hm －2)相近，但比甘肃省肃南县(282.54 t·hm －2)、祁连山寺大隆林区(217.41 t·hm －2)和祁连山北坡山区的平均值(209.24 t·hm －2)小。【结论】林分密度、温度和降水共同决定了森林生物量的大小及其海拔变化规律，海拔2800～2900 m是本研究区青海云杉林生长的最适宜场所。%Objective]In this study, biomass changes in Qinghai spruce ( Picea crassifolia ) forests along an altitudinal gradient ( from 2 700 -3 300 m) were studied. The results will be helpful to the prediction of water yield in Qilian Mountains and the integrated management of vegetation,soil and water in Heihe Basin. [Method]In Pailugou small watershed,located in the middle section of the northern slope of Qilian Mountains,vegetation was investigated by setting up sample plots in selected representative stands with good growth along the altitude gradients. A total of 15 plots in a size of 20 m × 20 m were set up at the altitudes of 2 700 -2 800 ,2 800 -2 900 ,2 900 -3 100 ,and 3 100 -3 300 m. The forest biomass and its distribution among different size classes were calculated using the existing equations on the relationship among biomass,tree height and tree diameter on breast height ( DBH) ,which were built up for the study area. Key factors attributing to the forest biomass and its changes along the altitudinal increase were analyzed.[Result]The total biomass of Qinghai spruce forest was ( 128 . 61 ± 50 . 25 ) t·hm -2 ,which came from the middle-sized trees (12. 5 cm < DBH≤22. 5 cm) and the big trees (DBH > 22. 5 cm). The biomass of the middle-sized trees (12. 5 cm < DBH≤22. 5 cm) and the big trees (DBH >22. 5 cm) evenly accounted for 44% and 35% of total forest biomass,respectively. The contribution of saplings (5 cm < DBH < 12. 5 cm) to forest biomass was limited with the value of 21% of total forest biomass. Along the altitudinal increase,the total biomass of spruce forest showed an“unimodal”change,and peaked at the altitudes of 2 800 -2 900 m. The total biomass of spruce forest at the altitudes of 2 700 -2 800,2 800 -2 900,2 900 -3 100,and 3 100 -3 300 m were( 135. 43 ± 21. 25),(152. 33 ± 73 . 01 ) ,( 124 . 28 ± 39 . 54 ) ,and ( 100 . 48 ± 65 . 69 ) t·hm -2 ,respectively. Along the altitudinal increase,key contributors to forest biomass shifted among tree size classes. At the altitudes of 2 700 -2 800 m,middle tree size class was key contributor to the total biomass of spruce forest with the biomass of 63 . 54 t·hm -2 ,which accounted for 48% of the total forest biomass. At the altitudes of 2 800 -2 900 m,the biomass of middle-sized trees increased and reached 90. 53 t·hm -2 ( accounting for 59% of the total). Nevertheless,at the altitudes of 2 900 - 3 100 m,the key contributor to forest biomass shifted to big tree size class with a biomass of 54. 46 t·hm -2 ( 45% of the total) . At the altitudes of 3 100 -3 300 m,the biomass of big trees reached 71. 46 t·hm -2(71% of the total). The forest biomass (128. 61 t·hm -2) in the study area was close to that in Haxi Forest Farm of Qilian Mountains (132. 90 t·hm -2),the average biomass in Qilian Mountains (169. 80 t·hm -2) and the national average of forest biomass (156. 66 t·hm -2). However,it was less than the biomass of forests at Sunan County,Gansu Province (282. 54 t·hm -2),Sidalong Forest Farm of Qilian Mountains (217. 41 t·hm -2),and the average forest biomass of 209. 24 t·hm -2 at the northern slope of Qilian Mountains. [Conclusion]The change of forest biomass along altitudinal gradients strongly relies on stand density, temperature and precipitation. The altitudes of 2 800 -2 900 mis the most suitable area for Qinghai spruce forests in the study area.
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