增温与土地利用变化对青藏高原高寒草甸生态系统温室气体通量影响的研究

随着青藏高原气候变暖进程及放牧压力与开垦面积的逐年加大，非常有必要评价青藏高原高寒草甸生态系统温室气体通量对气候变暖、放牧（包括粪尿斑）和开垦的响应和反馈。与此同时，我们进行了粪尿斑及开垦对土壤理化性质影响研究。本研究对于青藏高原高寒草甸C、N循环关键过程对全球变化响应的认识具有重要意义。 本文通过2年（2006-2007）增温（白天+1.2℃，晚上+1.7℃）与放牧耦合试验，在生长季观测了增温与放牧对高寒草甸生态系统温室气体通量的影响。研究初步表明，增温试验显著增加10 cm土壤温度1℃左右；在放牧频度相对较高的2007年，放牧也显著增加了土壤温度1℃左右。总体上，增温对土壤湿度的影响不显著。 增温可以促进土壤对CH4的吸收，而放牧对CH4通量影响不大或起到一定程度抑制作用。在较低放牧频度的2006年，放牧显著降低了植物-土壤系统CO2总释放量，增温促进了CO2的释放。而在放牧频度相对较高的2007年生长季，没有发现增温及放牧对土壤-植被系统CO2释放显著的影响。2006年放牧后，增温促进了高寒草甸土壤N2O的释放；2007年，增温不放牧小区N2O总通量较对照增加了24.6%，同时放牧处理也促进了N2O释放。基于土壤温度和土壤湿度的线性回归模型可以解释55%-89%的CH4通量变异，而土壤湿度较土壤温度对CH4通量影响更大。土壤温度是影响CO2和N2O通量的主要因子，通过拟合的指数型曲线，土壤温度可以分别解释43%-63%CO2通量变异与65%-81%N2O通量变异。 在2005年与2006年夏季放牧期间，对牦牛粪尿斑处理对高寒草甸CH4、CO2和N2O通量进行了观测。牛粪小区2年观测期内CH4平均通量为687 μg m-2 h-1，而尿斑和对照土壤吸收CH4（平均通量分别为-34 μg m-2 h-1和-39 μg m-2 h-1）。牛粪小区CO2在2005年和2006年观测期内累积释放量较对照分别增加了35.8%和49.7%，而牛尿小区与对照累积释放量差异不显著。牛尿与牛粪小区N2O累积释放量显著高于对照，在2005年牛尿与牛粪小区N2O累积释放量较对照分别增加了3.7和3.5倍，而在2006年分别增加了2.1和1.8倍。因此，在估算放牧高寒草甸生态系统N2O释放时，来自牛粪斑释放量是不能被忽略的。但在中等放牧强度下（1.45头ha-1 y-1），有粪尿斑覆盖的高寒草甸在观测期内全球变暖潜势较相同面积没有粪尿斑覆盖的草甸仅增加了1%。土壤水分孔隙度（WFPS）可以解释牛尿小区35%和对照小区36%CH4通量变异。土壤温度是控制CO2释放的主要因子之一，它可以解释所有处理40-75% CO2的变异。牛尿处理（34%）、牛粪处理（48%）及对照（56%）N2O时间变异则同时受土壤温度和WFPS的驱动。在观测期内，牛尿可以显著提高土壤的pH值。粪尿斑对土壤微生物量碳氮没有产生显著性影响，但在一定时段内能显著增加土壤无机氮含量。 通过在青藏高原高寒草甸开展的人工草地试验，初步探讨了不同土地利用方式（种植燕麦、开垦后闲置及自然恢复）对CO2、CH4和N2O通量，以及土壤无机氮和微生物量氮的影响。燕麦地、自然恢复草地及开垦闲置地与天然草地相比，吸收CH4的能力均表现为增强（CH4的吸收总量分别增加了31.9%、57.2%和71.0%）。由于燕麦地生物量低于天然草地与恢复草地，造成了燕麦地土壤-植被CO2释放量低于天然草地和恢复草地。而闲置地几乎没有植被覆盖，其CO2释放量显著低于天然草地。草地恢复8年后，CO2释放基本恢复到天然草地的水平。闲置地N2O总通量显著高于天然草地，较天然草地增加了60.5%。观测期内燕麦地与天然草地相比，N2O总通量增加了24.3%，但没有达到显著性水平。开垦及种植燕麦，增加了土壤硝态氮的含量，而自然恢复地、燕麦地、天然草地和闲置地在观测期内土壤铵态氮平均含量没有观察到显著性差异。燕麦地土壤微生物量氮平均含量最低（119.7 mg N kg-1），而自然恢复草地、天然草地和闲置地土壤微生物量氮差异不大。

增温对川西北高寒草甸植物群落结构及几种主要植物生长和生理的影响

由于人类活动所引起的地球大气层中温室气体的富集已导致全球地表平均温度在20世纪升高了0.6 ℃，并预测在本世纪将上升1.4-5.8 ℃。气候变暖对陆地植物和生态系统影响深远，并已成为全球变化研究的重要议题。高海拔、高纬度地带的生态系统对气候变化最敏感。而在高原和高山极端环境影响下所形成的高寒草甸生态系统极其脆弱，对由于温室效应引起的全球气候变化极其敏感，对这些变化的响应更具有超前性。 本研究以川西北高寒草甸植物群落及几种主要物种为研究对象，采用国际山地综合研究中心（ITEX）普遍所采用的增温方法-----开顶式生长室（OTC）模拟气候变暖来研究增温对高寒草甸植物群落结构、物质分配及其主要物种生长和生理的影响，以探讨高寒草甸植物响应与适应气候变暖的生物学和生态学机制。主要研究结论如下： 1、OTC的增温效果 由于地温、地表温度和气温的平均值在OTC内分别高出对照样地0.28℃、0.46℃和1.4℃，这说明本研究所采用的开顶式生长室（OTC）起到了增温的作用；同时，由于温室内与温室外接受的降水量相同，温室内由于热量条件的改善，土壤蒸发和植被的蒸腾作用增强，直接导致了OTC内土壤表层相对湿度的减少。 2、群落结构对增温的响应 由于增温时间较短，增温内外样地的物种组成并未发生改变；但增温后一定程度上改变了植物群落的小气候环境，从而导致物种间的竞争关系被破坏，种间竞争关系的破坏引起群落优势种组成发生相应的改变，在对照样地，鹅绒委陵菜、甘青老鹳草、遏蓝菜和蚤缀是占绝对优势的物种，而在OTC内，小米草、尼泊尔酸模、垂穗披碱草、发草和羊茅的重要性显著增加。 禾草和杂草由于对增温的生物学特性及其资源利用响应的不同，加之增温造成土壤含水量下降等环境因子的改变。与对照样地相比较，OTC内禾草的盖度及生物量都显著增加，而杂草的盖度和生物量则显著下降。 3、植物生长期对增温的响应 OTC内立枯和调落物的生物量在生长季末（10月份）都要小于对照样地的立枯和调落物生物量，而OTC内的地上鲜体生物量在10月份却略高于对照样地。这说明OTC内植物的衰老或死亡得以延缓，而植物的生长期得以延长。 4、群落生物量及分配对增温的响应 OTC内的地上鲜体生物量(10月份除外)和地下0-30cm的根系生物量与对照样地相比较，都出现了不同程度的减少；土壤根系的分配格局也发生了明显的改变，其中，OTC内0-10cm土层的生物量分配比例增加，而20-30cm土层生物量分配比例的减少。 5、群落碳、氮对增温的响应 增温后，OTC内植物群落地上活体和地下活根的碳浓度不同程度的高于对照样地，植物群落的碳库在OTC内也略高于对照样地；而OTC内植物群落地上活体和地下活根的氮浓度不同程度的低于对照样地，其植物群落的氮库与对照样地相比也略有下降。 6、几种主要植物的生长及物质分配对增温的响应 垂穗披碱草在增温后株高、比叶面积和地上生物量均显著地增加；尼泊尔酸模在增温后比叶面积和单株平均生物量积累显著地增加，而各组分中，增温处理使叶的生物量显著增加，而根的生物量却显著下降；鹅绒委陵菜在增温后株高、比叶面积和单株平均生物量积累显著地减少，而各组分中，增温处理使叶和茎的生物量显著减少，根的生物量却显著地增加。 尼泊尔酸模的LMR、RMR、R/S、根部碳含量、碳和氮在叶片与根部的分配比例在增温后显著地增加，而SMR、根部氮含量、碳和氮在茎部的分配比例在增温后却显著地降低；鹅绒委陵菜的RMR、R/S、碳和氮在根部的分配比例在增温后显著地增加，而SMR、LMR、碳在叶片的分配比例在增温后却显著地降低 7、几种主要植物的光合生理过程对增温的响应 增温使垂穗披碱草和尼泊尔酸模叶片中的叶绿素a、叶绿素b、总叶绿素含量显著增加；而鹅绒委陵菜叶片的叶绿素a、叶绿素b、总叶绿素含量在增温后显著减少，类胡萝卜素含量在增温后却显著增加。 增温对3种植物的气体交换产生了显著影响。其中，垂穗披碱草和尼泊尔酸模叶片的光响应曲线在增温后明显高于对照处理，A、E、gs、Pmax、、Rday、AQY和LSP显著增加，而LCP则显著降低；鹅绒委陵菜的光响应曲线在增温后则明显的低于对照处理，A、E、gs、Pmax、、Rday、AQY和LSP显著减少，而LCP则显著增加。 增温后垂穗披碱草和尼泊尔酸模叶片的Fv/Fm、Yield和qP显著增加；而鹅绒委陵菜叶片的Fv/Fm、Yield和qP则显著减少，qN却显著地增加。 8、几种主要植物的抗氧化酶系统对增温的响应 增温使垂穗披碱草和尼泊尔酸模体内抗氧化酶活性和非酶促作用有所提高，植物膜脂过氧化作用降低；鹅绒委陵菜叶片中酶促反应和非酶促反应在增温后也显著提高，但可能由于增温后的土壤干旱超过了鹅绒委陵菜叶的抗氧化保护能力，抗氧化酶活性及非酶促反应（脯氨酸、类胡萝卜素）的提高不足以完全清除干旱诱导形成的过量活性氧，因此叶片的膜脂过氧化程度仍然显著提高。 Enrichment of atmospheric greenhouse gases resulted from human activities such as fossil fuel burning and deforestation has increased global mean temperature by 0.6 ℃ in the 20th century and is predicted to increase in this century by 1.4-5.8 ℃. The global warming will have profound, long-term impacts on terrestrial plants and ecosystems. The ecoologcial consequences arising from global warming have also become the very important issuses of global change research. The terrestrial habitats of high-elevation and high-latitude ecosystems are regarded as the most sensitive to changing climate. The alpine meadow ecosystme, which resulted from the composite effects of mountain extreme climatic factors in Tibetan Plateau, is thus thought to be especially vulnerable and sensitive to global warming. In this paper, the response of plant community and several main species in the alpine meadow of Northewst Sichuan to experimemtal warming was studied by using open-top chambers (OTC). The aim of the this study was to research the warming effects on plant community structure, substance allocation, growth and physiological processes of several mian species, and to explore the biological and ecological mechanism of how the alpine meadow plants acclimate and adapt to future global warming. The results were as follows: 1. Warming effects of OTC The mean soil temperature, soil surface temperature and air temperature in OTC manipulation increased by 0.28℃、0.46℃ and 1.4℃ compared to the control during the growing season. This suggested that the OTC used in our study had increased temperature there. Meanwhile, the OTC manipulation slightly altered thermal conditions, but the same amount of precipitation was supplied to both the OTC manipulation and the control, so higher soil evaporation and plant transpiration in OTC manipulation directly lead to the decrease of soil surface water content. 2. The reponse of community structure to experimental warming The species richness was not changed by the short-term effect of OTC manipulation. However, experimental warming changed the microenvironment of plant community, therefore competitive balances among species were shift, leading to changes in species dominance. In the present study, the dominant plant species in the control plots were some forbs including Potentilla anserine, Geranium pylzowianum, Thlaspi arvense and Arenaria serpyllifolia, however, the importance value of some gramineous grasses including Elymus nutans, Deschampsia caespitosa, Festuca ovina, and some forbs including Euphrasia tatarica and Rumex acetosa significantly increased in OTC. The different biology characteristics and resource utilizations between gramineous grasses and forbs, and enhanced temperature caused change in some environment factors such as soil water content. As a result, the coverage and biomass of gramineous grasses significantly increased in OTC compared to the control, however, the coverage and biomass of forbs singnifciantly decreased in OTC compared to the control. 3. The reponse of plant growing season to experimental warming Both the standing dead and fallen litter biomass in OTC were lower than those in the control in October, and the biomass of aboveground live-vegetation in OTC was higher than that of the control. The results indicated that the senescence of plants was postponed, and the growing season was prolonged in our research. 4. The reponse of community biomass accumulation and its allocation to experimental warming Experimental warming caused the decrease of aboveground live biomass and belowground root biomass except for the aboveground live biomass in October. Experimental warming also had pronounced effects on the pattern of root biomass allocation. In the present study, the root biomass in 0-10cm soil layer increased in OTC manipulation compared to the control, however, the root biomass in the 20-30cm soil layer decreased in OTC manipulation compared to the control. 5. The reponse of community C and N content to experimental warming The C concentration and stock in aboveground live and belowground root both increased in OTC manipulation compared to the control. However, the N concentration and stock in aboveground live and belowground root both decreased in OTC manipulation compared to the control. 6. The reponse of gowth and biomass, C and N alloction of several species to experimental warming Experimental warming significantly increased the height, SLA (specific leaf area) and aboveground biomass of Elymus nutans in OTC manipulation compared to the control. The SLA and total biomass of Rumex acetosa also significantly increased in OTC manipulation compared to control, among the different components of Rumex acetosa, leaf biomass significantly increased, but root biomass significantly decreased in OTC manipulation compared to the control. However, the height, SLA and total biomass of Potentilla anserina significantly decreased in OTC manipulation compared to the control, among the different component of Potentilla anserina, leaf and stem biomass significantly decreased, but root biomass significantly increased in OTC manipulation compared to the control. The LMR (leaf mass ratio), RMR (root mass ratio), R/S (shoot/root biomass ration) and root C concentration of Rumex acetosa significantly increased in OTC manipulation compared to outside control, also, Rumex acetosa allocated relatively more C and N content to leaf and root in response to experimental warming, however, the SMR (stem mass ration) and root N concentration of Rumex acetosa significantly decreased in OTC manipulation compared to outside control, also, Rumex acetosa allocated relatively less C and N content to stem in response to experimental warming. The RMR and R/S of Potentilla anserina significantly increased in OTC manipulation compared to outside control, also, Potentilla anserina allocated relatively more C and N content to root in response to experimental warming, however, the SMR and LMR of Potentilla anserina significantly decreased in OTC manipulation compared to outside control, also, Potentilla anserina allocated relatively less C and N content to leaf in response to experimental warming. 7. The reponse of physiological processes of several species to experimental warming Experimental warming significantly increased chlorophyll a, chlorophyll b and total chlorophyll of Elymus nutans and Rumex acetosa in OTC manipulation compared to outside control. However, chlorophyll a, chlorophyll b, total chlorophyll and carotenoid of Potentilla anserina in OTC manipulation significantly decreased compared to outside control. Experimental warming had pronounced effects on gas exchange of Elymus nutans, Rumex acetosa and Potentilla anserine. In the present study, warming markedly increased the light response curves of Elymus nutans and Rumex acetosa in OTC manipulation compared to outside control, and also singnificantly increased A (net photosynthesis rate), E (transpiration rate), gs (stomatal conductance), Pmax (maximum net photosynthetic rate), Rday (dark respiration rate), AQY (apparent quantum yield) and LSP (light saturation point), but LCP (photosynthetic light compensation) of Elymus nutans and Rumex acetosa in OTC manipulation singnificantly decreased compared to outside control. However, warming markedly decreased the light response curves of Potentilla anserina in OTC manipulation compared to outside control, and also singnificantly decreased A, E, gs, Pmax, Rday, AQY and LSP, but LCP of Potentilla anserina in OTC manipulation singnificantly increased compared to outside control. Experimental warming singnificantly increased the chlorophyll fluorescence kinetics parameters such as Fv/Fm, Yield and qP of Elymus nutans and Rumex acetosa and qN of Potentilla anserina in OTC manipulation, but Fv/Fm, Yield and qP of Potentilla anserina in OTC manipulation singnificantly decreased. 8. The reponse of antioxidative systems of several species to experimental warming Experimental warming tended to increase the activities of antioxidative enzymes and stimulate the role of non-enzymes of Elymus nutans and Rumex acetosa. As a result, MDA content of Elymus nutans and Rumex acetosa decreased. The activities of antioxidative enzymes and non-enzymes of Potentilla anserina also significantly increased in OTC manipulation, but more O2- was produced because of lower soil water content, and the O2- accumulation exceeded the defense ability of antioxidative systems and non-enzymes fuctions. As a result, MDA content of Potentilla anserine still increased in OTC manipulation compared to outside control.

岷江上游高山林线附近优势树种土壤种子库和幼苗更新特征的研究

岷江上游地区高山/亚高山植被分布的坡向性分异显著，阴阳坡高山林线不仅物种组成差异明显，并且分布海拔呈现出阴坡高阳坡低的格局．阳坡林线树种主要是圆柏属乔木，林线类型多为渐变型，海拔高度大约在3 400m~3 800m；阴坡林线树种主要是冷杉，林线类型多为骤变型，海拔高度约在3 800m~4 400m．本研究采用土壤种子库物理筛选、室内萌发实验及野外群落调查等方法，对岷江上游地区阴坡岷江冷杉和阳坡祁连圆柏两类林线树种不同海拔梯度上土壤种子库以及幼苗库特征进行了调查，从土壤种子库和幼苗更新特征的角度对林线乔木树种种群更新特征进行了分析，进而对该地区高山林线在阴阳坡分布差异的原因进行了探讨，结果显示： 1．土壤种子库 阴坡：阴坡高山林线附近岷江冷杉土壤种子的平均密度大约为50.96粒/m2，其中树线以上10m处土壤种子密度为1.00粒/m2，树线处大约19.33粒/m2，林线交错带内土壤种子密度最高为136.83粒/m2，郁闭林内种子密度小于林线交错带，只有30.50粒/m2，种子平均空壳率为52％，霉变率达34%，完好种子只有6%．土壤种子库垂直分布特征为地被物层含种子比重最大，大约在67.50%左右；其次为0~2cm层，约18.84%左右；2~5cm层所占种子比例最小，约13.66%左右．霉变种子数量与土壤深度呈负相关． 阳坡：阳坡祁连圆柏土壤种子的平均密度为60.16粒/m2．树线以上10m处密度为1.92粒/m2，树线位置大约108.16粒/m2，林线交错带内平均为75.80粒/m2，郁闭林内种子密度小于林线交错带，只有20.00粒/m2．种子平均空壳率为36％，完好种子占49%，霉变率较低，大约为10%．阴阳坡林线树种土壤种子库垂直分布特征为：地被物层含种子最多，其次为0~4cm层，4~10cm层所占种子比例最小，霉变种子数量与土壤深度也呈负相关． 2． 幼苗库调查 阳坡：在树线以上区域没有发现幼苗，林线交错带内幼苗密度平均达3 250株/hm2，郁闭林内仅2 750株/ hm2．整个样地内1～2a幼苗很少甚至没有出现，3～10a的幼苗相对较多．空间分布上，祁连圆柏幼苗在林线交错带内接近随机分布，郁闭林内则介于随机分布和均匀分布之间． 阴坡：在树线以上幼苗密度为1 250株/ hm2，全部为1~2a幼苗，林线交错带内幼苗密度平均达7 000株/ hm2，郁闭林内达6 250株/ hm2．林线附近岷江冷杉幼苗丰富度以及幼苗的出现频率明显高于祁连圆柏，年龄结构也较祁连圆柏完整．岷江冷杉幼苗空间分布除了树线处幼苗的分布为随机分布，其他海拔则为集群分布． 3．从不同土壤深度的种子总量和幼苗数量的相关性检验发现，当年生幼苗数量跟表层种子总量相关性极显著, 但是两年生幼苗的数量与底层种子数量相关性显著．土壤种子在土壤中的垂直分布格局从一定程度上可以反映种子库的年际特征．岷江冷杉土壤种子库较丰富，种子散布后的存活力随着时间的变化逐渐下降，属于季节性瞬时种子库；祁连圆柏土壤种子散布格局为集群型分布，成熟种子大部分散布在母株冠幅内，属于永久性土壤种子库． 4．在阴坡林线交错带及以上区域还存在较为丰富的乔木土壤种子，并且在树线以上区域还发现了少量的岷江冷杉幼苗．从样地乔木的年龄结构发现，在林线交错带内上部到树线位置主要以幼龄林为主，且年龄结构完整，基本符合入侵性林线特征；阳坡林线交错带内幼苗出现频率很低，树线以上区域虽然存在种子库，但是没有幼苗出现，在林线交错带内乔木径级差距很大，年龄结构异常不完整，这种特征的林线将会面临两个可能结果：一种是维持现有状态，保持平衡；另外一种就是退化，但阳坡林线的实际动态趋势还有待长期定点研究． Treelines on the upper region of Minjiang River differ between the north aspect and the south aspect in their appearances, altitudes and tree species. On the north aspect, trees of Abies form a sharp and abrupt treeline ranging from 3800m to 4400m, while on the south the treeline is generally lower(3 400~3 800m), more open and gradual and mostly composed of Sabina. In this study, we examined the altitudinal gradients of soil seed banks and seedling recruitments at the treeline ecotones of a N-aspect and a S-aspect by using soil sieving, germination experiment and field investigations, analyzed the characteristics of population regeneration of tree species at the transitional zone and presented a analysis of the causes to the aspect-related difference in treeline patterns in the study area. Major results of our study include: 1． Soil seed bank N-aspect: Of the 50 plots investigated, the average density of soil seeds is 50.96/m2, in which well-formed seeds account for 6%, empty seeds 52%, parasitized seeds34%, and seeds damaged by animals 8%. The size of soil seed bank varies along altitude, being 1.00 seeds /m2 at the 10m above the treeline and ca.19.33 seeds/m2 at the upper limit of treeline. The highest density (136.83 seeds/m2) occurs at the treeline ecotone. By contrast, the density of soil seed for the closed forest is only 30.50 seeds/m2. In terms of vertical strata, 67.50% of the total seeds are at the surface layer, 18.84% at the middle layer (0~2cm) and 13.66% at deeper layer (2~5cm). The number of parasitized seeds is negatively correlated to soil depth. S-aspect: Of the 50 plots investigated, the average density of soil seeds is 60.16 seeds/m2, and the well-formed seeds account for 49%, empty seeds 36%, parasitized seeds10%, and seeds damaged by animals 1%. The size of soil seed bank varies along altitude, with 1.92 seeds/m2 recorded at the10m above the treeline，108.16 seeds/m2 at the upper limit of treeline, and 75.80 seeds/m2 at the treeline ecotone, while that for the closed forest is 20.00 seeds/m2. The number of seeds decreases with the depth of soil. As is on the N-aspect, the size of soil bank, from large to small, follows the order of the surface layer, the middle layer (0~4cm) and the bottom layer (4~10cm). The number of parasitized seeds is also negatively correlated to the depth of the soil. 2． Seedling bank N-aspect: A mean maximum seedling abundance of 31 000 seedlings/hm2 was recorded near alpine treeline at growing season. The density of seedlings is 1 250 seedlings/ha (all being 1 or 2 years old) at the alpine meadow 10m away above treeline, 7 000 seedlings/ha at treeline ecotone and 6 250 seedlings/ha for closed forest．The spatial distribution of Abies faxoniana seedlings is random at the upper limit of the treeline but clumped at other altitudes． S-aspect: No seedlings were found at the alpine meadow 10m away from the treeline. The density of seedlings was 3 250 seedlings/ha at treeline ecotone and 2 750 seedlings/ha for the closed forest．Hardly any 1 year current and 2 year-old seedlings appeared at the plots. The spatial distribution of Sabina przewalskii seedlings is random at treeline ecotone and between “random” and “even” forest closed forest． 3．Correlation tests of seedling population and seed bank at different soil layers indicated that the emergents were strongly correlated to seed bank at surface layer while the number of two-year seedlings was significantly correlated to the seed bank at the bottom of soil layer, indicating that germination mainly occurs at the soil surface while the middle or bottom layer was the reserve for non-germination or dead seeds. It can thus be postulated that Abies faxoniana soil seed bank is of seasonal transient type. By contrast, the soil seed bank of Sabina przewalskii is of persistent type and the soil seeds and seedlings of this species occurred more frequently near the islands of adult trees. 4．A good many soil seeds of both tree species were found near the treeline ecotone and above at N- and S-aspects. A few young seedlings were found above the Abies treeline. Investigation of five altitudinal transects respectively on N- and S-aspects indicated that Abies faxoniana has a more complete age structure than the stands of Sabina przewalskii. The age of firs decreased from closed forest to the upper limit of treeline, which suggests that the Abies treeline is advancing to higher altitude. While on the south aspect, only few Sabina przewalskii soil seeds and nearly no seedlings were found above the treeline ecotone. The stands exhibit extremely great difference in diameter classes with significantly incomplete age structure. This would lead to two possible results for the treelines: maintaining an equilibrium state at the current position or degenerating. But more studies should be carried out at longer time scales or larger spatial scales to understand whether the Sabina treeline is degenerating.

青藏高原东部植物生态特征在季节性雪厚度梯度上的变化

本文以青藏高原东部的高山草甸为研究对象，设置早融、中间及晚融三个融雪部位，采用实验室测量、野外测量、野外样方调查相结合的 方法，从个体、种群和群落的水平上比较研究了高山雪场植物在同一雪场样地中不同融雪梯度上的特征变异及适应，结果表明： 从早融到晚融的梯度上，随着融雪时间的逐渐推迟，表土日温差降低，冻融交替的强度减弱，土壤水份逐渐增加，总N、总P、总K 以及 可溶性的N、P 和pH 变化不明显，土壤有机质及可溶性的K 和Ca 逐渐降低。冻融交替强度上的差异以及土壤水分差异被认为是融雪梯度上 影响植物生长的主要原因。 从早融到晚融的梯度上，伴随着生态因子的改变，几种常见植物的个体特征也发生相应的变化。首先，物候期推迟。植物开始生长的时间 一般要推迟将近二十天，但同一种植物在不同的融雪部位上的衰老期趋于一致，这预示着在晚融部位同一植物的生长期要缩短。其次，个体生 长特性发生改变。黑褐穗苔草（Carex atrofusca subsp. minor (Boott) T.Koyama）和西北黄芪（Astragalus fenzelianus Pet.-Stib.）的个体生长（株高、单株叶数、单叶面积和地上生物量）表现为逐渐增加的趋势；斑唇马先蒿（Pedicularis longiflora Rudolph var. tubiformis (Klotz.) Tsoong）和川西小黄菊（Pyrethrum tatsienense (Bur. et Franch.) Ling ex Shih.）则表现为逐渐降低的趋势；长叶火绒草（Leontopodium longifolium Ling）在融雪梯度上的变化趋势不明显。再次，从繁殖特性来看，大卫马先蒿（Pedicularis davidii var. pentodon Tsoong）的单株花数、单花种子数、种子千粒重及种子萌发率随融雪的推迟呈现为逐渐增加的趋势；圆穗蓼（Polygonum macrophyllum D.Don）的种子（小坚果）千粒重和萌发率也表现为逐渐增加，其余繁殖特征变化不明显。 在种群层次上，几个常见物种的分布格局随着融雪的推迟都发生一定的变化，基本上表现为从早融的集群分布到中间或晚融部位的随机分布。物种间的联结性也发生较大的变化，由早融部位的总体上的正关联逐步过度到晚融部位上的总体上的负关联。特定种对间的联结性也发生较大的变化。恶劣环境条件（如剧烈的冻融交替）的影响以及对恶劣条件适应被认为是分布格局及种间联结性发生变化的主要原因。 在群落层次上，物种多样性的变化表现为单峰曲线的格局，即在中间部位多样性最高。早融部位强烈的冻融交替和晚融部位缩短的生长季是早融及晚融部位物种多样性不高的重要原因。几乎所有的只出现在一个融雪部位（雪深级别）上的物种都发生在中间融雪部位。这说明，中等的雪深更有利于许多高山植物的存活，而过浅过深的积雪都不利于植物的生存。另外，相距较近的融雪梯度之间的物种相似性较大，而相距较远的梯度之间物种的替代率较高，物种的相似性较小。在群落的生物量方面，地上生物量随融雪的推迟而升高，地下生物量随融雪的推迟而下降，地上与地下生物量之总和随着融雪的推迟而下降，地下生物量与地上生物量之比随着融雪的推迟而下降。早融部位的地上生物量主要集中于地上0-10cm 的范围内，表明在早融部位植物地上部分有变矮的趋势；早融部位的地下生物量在土壤各深度分布相对较均一，而晚融部位地下生物量则主要集中于地下0-10cm 的范围内。生物量的变化趋势主要与雪场中各部位的土壤水分含量及地表日温度差异有关，是植物适应特定环境的结果。 To detect the plants’ responses to snow-cover gradients in an alpine meadow of eastern Tibetan plateau, laboratory method and field sample plot method were employed, and three gradeients （early-, medium and late-melting）were established in a natural snowbed. The measurements were carried out for two years and was done on three levels——individual, population and community. The results are shown as follows : From early- to late-melting gradients, daily ground temperature difference between day and night decreased, amplitude of freeze-thaw alternation weakened, soil organic matter contents and soluble K and Ca decreased, while soil water content increased. Total N, total P, total K，pH soluble N and soluble P kept constant from early- to late-melting portions. Among these factors, the changes of intense freeze-thaw alternation and soil water contents were considered as main factors affecting plants’ growth. From early- to late-melting portions, all phenological phases postponed, e.g. phase of plant emergence postponed almost twenty days. However, the same species’ individuals at different portions withered in step, which implied that the individuals at late-melting portion possessed shorter growing season length. Along the same gradient, both Carex atrofusca subsp. minor (Boott) T. Koyama and Astragalus fenzelianus Pet.-Stib. increased their individual growth, whereas Pedicularis longiflora Rudolph var. tubiformis (Klotz.) Tsoong and Pyrethrum tatsienense (Bur. et Franch.) Ling ex Shih. decreased their individual growth. Unlike the four plants mentioned above, Leontopodium longifolium L. did not show any evident change. As to reproductive charateristics, the flowers per individual, the number of seeds per flower, the thousand seed weight and the seed germination rate of Pedicularis davidii var. pentodon showed an increasing trend; and Polygonum macrophyllum D.Don also increased its thousand seed weight and seed germination rate along the same gradient. However, the other reproductive charateristics of Polygonum macrophyllum D.Don did not change significantly. At population level, the distribution pattern of several selected species changed from cluster pattern to random pattern as the snowmelt postponed. Overall association among the species changed from positive to negative along the same gradient. Further, interspecific association also changed evidently. Adverse circumstances such as intense freeze-thaw alternation were considered as primary factors resulting in changes of population distribution pattern and interspecific association. At the level of community, species diversity showed a pattern of a unimodal trend, i.e. the highest diversity occurred at medium snow depth，perhaps because of intense freeze-thaw alternation at early-melting portions and the shortest growing season at late-melting portions. Almost all species that only appeared at one snowmelt portion occurred at medium portion, indicating that medium snow depth was more suitable for many species’ survival. Species replacement from one snowmelt portion to its neighboring portion seldom took place. However, while distance between two portions became farther, species replacement between the two portions occurred more frequently. As for biomass, aboveground biomass increased from early- to late-melting portions, whereas belowground biomass, total biomass and the ratio of belowground to aboveground all decreased along the same snow gradient. A majority of aboveground biomass distributed in a height range of 0-10 cm, suggesting that height of plants inhabiting early-melting portion be shorter compared with other portions. In addition, belowground biomass at early-melting portion was evenly distributed at different soil depth in comparison with aboveground biomass, whereas belowground biomass at late-melting portion concentrated 0-10cm soil layer below ground. The changing trend of biomass was also related to two factors. One was soil water content, and the other topsoil temperature difference between day and night.

Spatial and temporal relationships between precipitation and ANPP of four types of grasslands in northern China

Precipitation is considered to be the primary resource limiting terrestrial biological activity in water-limited regions. Its overriding effect on the production of grassland is complex. In this paper, field data of 48 sites (including temperate meadow steppe,temperate steppe, temperate desert steppe and alpine meadow) were gathered from 31 published papers and monographs to analyze the relationship between above-ground net primary productivity (ANPP) and precipitation by the method of regression analysis. The results indicated that there was a great difference between spatial pattern and temporal pattern by which precipitation influenced grassland ANPP. Mean annual precipitation (MAP) was the main factor determining spatial distribution of grassland ANPP (r~2 = 0.61,P ＜ 0.01); while temporally, no significant relationship was found between the variance of AN PP and inter-annual precipitation for the four types of grassland. However, after dividing annual precipitation into monthly value and taking time lag effect into account, the study found significant relationships between ANPP and precipitation. For the temperate meadow steppe, the key variable determining inter-annual change of ANPP was last August-May precipitation (r~2= 0.47, P = 0.01); for the temperate steppe, the key variable was July precipitation (r~2 = 0.36, P = 0.02); for the temperate desert steppe, the key variable was April-June precipitation (r~2 = 0.51, P ＜0.01); for the alpine meadow, the key variable was last September-May precipitation (r~2 = 0.29, P ＜ 0.05). In comparison with analogous research, the study demonstrated that the key factor determining inter-annual changes of grassland ANPP was the cumulative precipitation in certain periods of that year or the previous year.

高寒草甸矮嵩草种群繁殖对策的研究

This paper studied the reproductive strategies of Kobresia humilis in alpine meadow about its sexual reproduction ,vegetative reproduction and reproductive efforts respectively. The results show that the seed output of Kobresia humilis is 715. 5per unit (m2) . The germination is above 60 % in fit condition ,while only about 3 % in field. There is only 16. 13 % seed which get into seed bank and remain vitality till grass greening. So the seedling is only 3. 46 per unit (m2) in field.But the new ramets of Kobresia humilis are 711. 34 unit (m2 ) from vegetative reproduction. In addition ,vegetative reproductive effort is more than sexual reproductive effort . It constitutes 93. 5 % of all reproductive efforts. Therefore ,the main reproductive strategy of Kobresia humilis is vegetative reproduction ,and sexual reproduction is secondary in alpine condition.

Diurnal and seasonal variations of UV radiation on the northern edge of the Qinghai-Tibetan Plateau

We monitored UVA, UVB, and solar radiation from August 2001 to 2003 on the northern Qinghai-Tibetan Plateau to characterize the diurnal and seasonal variations of UV radiation on the world's highest plateau. Daily UVB radiation and the ratio of UVB to total solar radiation increased significantly when the atmospheric ozone concentration decreased as estimated by the total ozone mapping spectrometer (TOMS), as well as when cloud coverage decreased. The UVB/UVA ratio also showed a significant increase when the TOMS ozone concentration decreased in the morning. The seasonal variation pattern of UVB, however, was closely correlated with solar elevation but was little affected by the seasonal pattern of the atmospheric ozone amount. Compared to observations from the central plateau, the magnitude of the UVB increase attributed to ozone depletion was smaller at the northern edge. The study suggests that the temporal variation of ground UV radiation is determined by both solar elevation and the ozone amount, but the spatial difference on the plateau is likely to be ascribed mainly to the spatial variation of the ozone amount. (c) 2007 Published by Elsevier B.V.

Population dynamics and responses to management of plateau pikas Ochotona curzoniae

1. Plateau pikas Ochotona curzoniae are considered a pest species on the Tibetan Plateau because they compete with livestock for forage and their burrowing could contribute to soil erosion. The effectiveness of pest control programmes in Tibet has not been measured, and it is not known whether changes in livestock management have exacerbated problems with plateau pikas or compromised their control. This study measured the impact of control programmes and livestock management for forage conservation on populations of plateau pikas in alpine meadow in Naqu District, central Tibet, during 2004 and 2005.2. Current techniques for controlling plateau pikas in spring cause large reductions in abundance, but high density-dependent rates of increase result in no differences between treated and untreated populations by the following autumn. Rates of increase from spring to autumn are not influenced by standing plant biomass or concurrent grazing by yaks Bos grunniens and Tibetan sheep Ovis aries.3. In autumn there was significantly lower biomass outside fenced areas with year-round livestock grazing compared with inside fenced areas with equivalent or higher numbers of plateau pikas but predominantly winter grazing by livestock. Inside fenced areas, control of plateau pikas in spring produced no detectable effect on standing plant biomass at the end of the following summer compared with uncontrolled populations of plateau pikas.4. Regardless of their initial density, populations of plateau pikas declined rapidly over winter outside fenced areas where there was very low standing plant biomass in autumn. However, inside fenced areas with higher plant biomass in autumn, low-density populations of plateau pikas declined more slowly than high-density populations.5. Synthesis and applications. Current control programmes have limited effect because populations of plateau pikas can recover in one breeding season. There was no apparent increase in forage production in areas where plateau pikas were controlled. However, plateau pikas appear to benefit from changes in grazing management, with low-density populations declining less over winter inside fenced areas than elsewhere. It was not evident that control programmes are warranted or that they will improve the livelihoods of Tibetan herders.

Enrichment of stable carbon and nitrogen isotopes of plant populations and plateau pikas along altitudes

Stable carbon and nitrogen isotope ratios were measured in plant populations and plateau pikas to determine enrichment in stable isotopes of three alpine meadow ecosystems at different elevations in the Qinghai-Tibet Plateau. The results indicated that stable carbon isotope signatures of plant populations at the three locations showed significant variations, delta C-13 of plant populations showed an enrichment of 0.86 parts per thousand per 1000 in over the linear proportion of the altitudinal response, while stable nitrogen isotopes showed no apparent difference. The stable nitrogen isotopes of plateau pikas became significantly isotopically heavier along altitudinal gradients and showed an enrichment of 3.17 parts per thousand/km. Stable carbon isotopes showed no significance, however, and the enrichment of 0.448 parts per thousand/km. delta C-13 and delta N-15 in plateau pikas were not significantly correlated. There appeared to be segregation between the metabolism of stable carbon and nitrogen isotopes of plateau pikas. Variances in metabolic rate at lower water availability and temperatures are presumed to be the main cause of enrichment of stable nitrogen isotopes along altitudinal gradients. Attention should be paid to construct trophic positions and to trace food chain information based oil an isotopic enrichment model.

Characterization of photosynthetic pathway of plant species growing in the eastern Tibetan plateau using stable carbon isotope composition

The photosynthetic pathway of plant species collected at Menyuan, Henan, and Maduo sites, east of Tibetan Plateau, China, during the growing season were studied using stable carbon isotopes in leaves. The 232 samples leaves analyzed belonged to 161 species, 30 families, and 94 genera. The delta(13)C values (from -24.6 to -29.2 %o) indicated that all the considered species had a photosynthetic C-3 pathway. The absence of plant species with C-4 photosynthetic pathway might be due to the extremely low air temperature characterizing the Tibetan Plateau. The average delta(13)C value was significantly (p < 0.05) different between annuals and perennials at the three considered study sites. Hence the longer-lived species had greater water-use efficiency (WUE) than shorter-lived species, that is, longer-lived species are better adapted to the extreme environmental conditions of the Tibetan Plateau.

Phylogeography of the Qinghai-Tibetan Plateau endemic Juniperus przewalskii (Cupressaceae) inferred from chloroplast DNA sequence variation

The vegetation of the northeast Qinghai-Tibetan Plateau is dominated by alpine meadow and desert-steppe with sparse forests scattered within it. To obtain a better understanding of the phylogeography of one constituent species of the forests in this region, we examined chloroplast trnT-trnF and trnS-trnG sequence variation within Juniperus przewalskii, a key endemic tree species. Sequence data were obtained from 392 trees in 20 populations covering the entire distribution range of the species. Six cpDNA haplotypes were identified. Significant population subdivision was detected (G(ST) = 0.772, N-ST = 0.834), suggesting low levels of recurrent gene flow among populations and significant phylogeographic structure (N-ST > G(ST), P < 0.05). Eight of the nine disjunct populations surveyed on the high-elevation northeast plateau were fixed for a single haplotype (A), while the remaining, more westerly population, contained the same haplotype at high frequency together with two low frequency haplotypes (C and F). In contrast, most populations that occurred at lower altitudes at the plateau edge were fixed or nearly fixed for one of two haplotypes, A or E. However, two plateau edge populations had haplotype compositions different from the rest. In one, four haplotypes (A, B, D and E) were present at approximately equivalent frequencies, which might reflect a larger refugium in the area of this population during the last glacial period. Phylogenetic analysis indicated that the most widely distributed haplotype A is not ancestral to other haplotypes. The contrasting phylogeographic structures of the haplotype-rich plateau edge area and the almost haplotype-uniform plateau platform region indicate that the plateau platform was recolonized by J. przewalskii during the most recent postglacial period. This is supported by the findings of a nested clade analysis, which inferred that postglacial range expansion from the plateau edge followed by recent fragmentation is largely responsible for the present-day spatial distribution of cpDNA haplotypes within the species.

Analysis of diets of upland buzzards using stable carbon and nitrogen isotopes

We measured the stable carbon and nitrogen isotope ratios for muscles of the upland buzzards (Buteo hemilasius) and their potential food sources, plateau pikas (Ochotona curzoniae), Qinghai voles (Lasiopodomys fuscus), plateau zokors (Myospalax fontanierii), and several passerine bird species at the alpine meadow in Maduo county, Guoluo prefecture of Qinghai province, People's Republic of China, to provide diet information of upland buzzards, highlighting different diet composition of upland buzzards exposed to different locations. The results demonstrated that stable carbon isotope ratios of upland buzzards, passerine birds, plateau pikas, plateau zokors, and Qinghai voles were -24.42 +/- 0.25parts per thousand, -22.89 +/- 1.48parts per thousand, -25.30 +/- 1.47parts per thousand, -25.78 +/- 0.22parts per thousand, and -25.41 +/- 0.01parts per thousand, respectively, and stable nitrogen isotope ratios were 7.89 +/- 0.38parts per thousand, 8.37 +/- 2.05parts per thousand, 5.83 +/- 1.10parts per thousand, 5.23 +/- 0.34parts per thousand, and 8.86 +/- 0.06parts per thousand, respectively. Fractionation of stable carbon and nitrogen isotope ratios between upland buzzards and their food were 1.03parts per thousand and 2.11parts per thousand, respectively. Based on mass balance principle of stable isotopes and the Euclidean distance mixing model, upland buzzards depended mainly on plateau pikas as food (74.56%). Plateau zokors, Qinghai voles, and passerine birds only contributed a small proportion (25.44%) to diets of upland buzzards. The results were closely accordant with analyses of stomach contents and food pellets, which firmly supported the feasibility of using stable carbon and nitrogen isotope ratios to investigate diet information of upland buzzards. Another study based on stable carbon isotopes showed that upland buzzards living in the Haibei prefecture (another prefecture located in the southeast Qinghai province) mainly preyed on passerine birds (64.96% or more) as food supply. We were alarmed by the preliminary results that widespread poisoning activities of small mammals could reshape the food composition of upland buzzards, influencing the stability and sustainability of the alpine meadow. Bio-control on rodent pests should be carried out rather than the chemical measures.

Palynology on sediment profile Tso Kar in Ladakh

Palynological investigation of a 410 cm long core section from Tso Kar (33°10'N, 78°E, 4527 m a.s.l.), an alpine lake situated in the arid Ladakh area of NW India at the limit of the present-day Indian summer monsoon, was performed in order to reconstruct post-glacial regional vegetation and climate dynamics. The area was covered with alpine desert vegetation from ca. 15.2 to 14 kyr BP (1 kyr=1000 cal. years), reflecting dry and cold conditions. High influx values of long-distance transported Pinus sylvestris type pollen suggest prevailing air flow from the west and northwest. The spread of alpine meadow communities and local aquatic vegetation is a weak sign of climate amelioration after ca. 14 kyr BP. Pollen data (e.g. influx values of Pinus roxburghii type and Quercus) suggest that this was due to a strengthening of the summer monsoon and the reduced activity of westerly winds. The further spread of Artemisia and species-rich meadows occurred in response to improved moisture conditions between ca. 12.9 and 12.5 kyr BP. The subsequent change towards drier desert-steppe vegetation likely indicates more frequent westerly disturbances and associated snowfalls, which favoured the persistence of alpine meadows on edaphically moist sites. The spread of Chenopodiaceae-dominated vegetation associated with an extremely weak monsoon occurred at ca. 12.2-11.8 kyr BP during the Younger Dryas interstadial. A major increase in humidity is inferred from the development of Artemisia-dominated steppe and wet alpine meadows with Gentianaceae after the late glacial/early Holocene transition in response to the strengthening of the summer monsoon. Monsoonal influence reached maximum activity in the Tso Kar region between ca. 10.9 and 9.2 kyr BP. The subsequent development of the alpine meadow, steppe and desert-steppe vegetation points to a moderate reduction in the moisture supply, which can be linked to the weaker summer monsoon and the accompanying enhancement of the winter westerly flow from ca. 9.2 to 4.8 kyr BP. The highest water levels of Tso Kar around 8 kyr BP probably reflect combined effect of both monsoonal and westerly influence in the region. An abrupt shift towards aridity in the Tso Kar region occurred after ca. 4.8 kyr BP, as evidenced by an expansion of Chenopodiaceae-dominated desert-steppe. Low pollen influx values registered ca. 2.8-1.3 kyr BP suggest scarce vegetation cover and unfavourable growing conditions likely associated with a further weakening of the Indian Monsoon.

The effect of land management on plant community composition, species diversity, and productivity of alpine Kobersia steppe meadow

Grassland degradation is widespread and severe on the Tibet Plateau. To explore management approaches for sustainable development of degraded and restored ecosystems, we studied the effect of land degradation on species composition, species diversity, and vegetation productivity, and examined the relative influence of various rehabilitation practices (two seeding treatments and a non-seeded natural recovery treatment) on community structure and vegetation productivity in early secondary succession. The results showed: (1) All sedge and grass species of the natural steppe meadow had disappeared from the severely degraded land. The above-ground and root biomass of severely degraded land were only 38 and 14.7%, respectively, of those of the control. So, the original ecosystem has been dramatically altered by land degradation on alpine steppe meadow. (2) Seeding measures may promote above-ground biomass, particularly grass biomass, and ground cover. Except for the grasses seeded, however, other grass and sedge species did not occur after seeding treatments in the sixth year of seeding. Establishment of grasses during natural recovery treatment progressed slowly compared with during seeding treatments. Many annual forbs invaded and established during the 6 years of natural recovery. In addition, there was greater diversity after natural recovery treatment than after seeding treatments. (3) The above-ground biomass after seeding treatment and natural recovery treatment were 114 and 55%, respectively, of that of the control. No significant differences in root biomass occurred among the natural recovery and seeded treatments. Root biomass after rehabilitation treatment was 23-31% that of the control.

Surface energy partitioning in alpine swamp meadow in the Qinghai Tibetan Plateau

Based on surface energy flux data measured by eddy covariance methods from China Flux in alpine swamp meadow of the Qinghai Tibetan Plateau in 2005, the daily and seasonal dynamic of surface energy fluxes and their partitioning, as well as abiotic factors effects were analyzed. The results suggested that LE （Latent heat flux） was the largest consumer of the incoming energy. Rn （Net radiation flux） and LE showed clear seasonal variations in sharp hump and up to their maximums in August and July, respectively. H （Sensible heat flux） increased to its peak in August whereafter declined slowly. Precipitation could reduce the components of surface energy. As to Rn and LE, their correlations with abiotic factors were evident while it was not significant in H. Average EBR （Energy balance ratio） was 50.7 %, which was much larger in growing season than non-growing season.