摘要
长期以来,就营养物质和碳循环而言,北极生态系统降低了初级生产力;能量,水和温室气体交换的水平已引起了局部和区域性的小幅度降温。大气CO_2中的碳沉积在广袤而寒冷的有机土壤中,冰雪覆盖的低矮植被产生高的反射率,都影响了局部气候。然而,北极生态系统功能的许多方面都对气候变化及其产生的生物多样性影响敏感。当前的北极气候导致了低的有机物质分解速率,因此,尽管有机物和元素输入量较低,但北极生态系统还是趋向于积累有机物和元素,土壤中氮和磷等可利用元素结果成为促进碳固定以及生物量和有机物进一步积累的关键性限制因素。气候变暖可能增加特别是土壤中的碳和元素的周转,起初可能导致元素的丢失,但最后会慢慢的恢复。在北极生态系统中,单个物种和物种多样性已经明显地影响了元素的输入和滞留,另一方面,从长远来看,尽管CO_2和紫外线增加对植物组织化学、分解和氮固定的影响可能变得重要,但对整个生态系统来说, 影响可能很小。碳循环的示踪气体主要形式是CO_2和CH_4,大多数碳以CO_2的形式损失,这些CO_2是由植物和土壤生物产生。来自潮湿苔原生态系统以CH_4形式释放的碳大约是CO_2形式的5%,而且在没有任何其他变化的情况下,对变暖作出响应。冬天过程和植物类型也影响CH_4释放和能量在生物圈和大气之间的交换,因为反射率从冬末到夏天存在很大的变化,在冬末,雪反射了入射的大部分光线,在夏天,生态系统吸收了入射的大部分光线,所以在所有的陆地生态系统中,北极生态系统在能量交换方面表现出巨大的季节性变化。植被深刻地影响北极生态系统水和能量交换。在冰雪覆盖期间,反射率从苔原、森林苔原、落叶林、常绿林依次降低。灌木和树增加了雪的深度,反过来又使冬天的土壤温度增加,因此,由气候变化而引起的未来植被方面的变化很可能深远地改变区域的气候。
Species individualistic responses to warming and increased UV-B radiation are moderated by the responses of neighbors within communities,and trophic interactions within ecosystems.All of these responses lead to changes in ecosystem structure.Experimental manipulation of environmental factors expected to change at high latitudes showed that summer warming of tundra vegetation has generally led to smaller changes than fertilizer addition.Some of the factors manipulated have strong effects on the strueture of Arctic ecosystems but the effects vary regionally,with the greatest response of plant and invertebrate communities being observed at the coldest locations.Arctic invertebrate communities are very likely to respond rapidly to warming whereas microbial biomass and nutrient stocks are more stable.Experimentally enhanced UV-B radiation altered the community composition of gram-negative bacteria and fungi,but not that of plants.Increased plant productivity due to warmer summers may dominate foodweb dynamics.Trophic interactions of tundra and sub-Arctic forest plant-based food webs are centered on a few dominant animal species which often have cyclic population fluctuations that lead to extremely high peak abundances in some years.Population cycles of small rodents and insect defoliators such as the autumn moth affect the structure and diversity of tundra and forest-tundra vegetation and the viability of a number of specialist predators and parasites. Ice crusting in warmer winters is likely to reduce the accessibility of plant food to lemmings, while deep snow may protect them from snow-surface predators.In Fennoscandia,there is evidence already for a pronounced shift in small rodent community structure and dynamics that have resulted in a decline of predators that specialize in feeding on small rodents. Climate is also likely to alter the role of insect pests in the birch forest system:warmer winters may increase survival of eggs and expand the range of the insects.Insects that harass reindeer in the summer are also likely to become more widespread,abundant and active during wanner summers while refuges for reindeer/caribou on glaciers and late snow patcheswill probably disappear.
