北极陆地生态系统、气候及紫外线辐射的历史演变

在末次盛冰期,地球上很多大陆都被大量的冰层所覆盖,一些浅海域的海床露出水面将先前分离的大陆连接起来.尽管存在一些适宜于动植物生长而未被冰层覆盖的地区,但其年平均气温仍比更新世时期低10～13℃.在盏冰期的几千年时间内冰川开始消融,其显著特征是气候在大约18000～11400年以前出现了一系列的波动.气候在更新世度过一个温暖期后,开始了一个缓慢的全面变冷的过程,这导致了一系列为期几百年至几千年的气候波动,例如发生在大约13世纪晚期至19世纪早期的"小冰期".在最近150000年的气候变化过程中,北极的各种生态系统和生物组成在近10000年接近其最低分布范围.大冰期结束时的全球大范围急剧升温导致了许多物种的消失,这使北极地区的生物多样性大大降低.因此,北极生态系统以及大型脊椎动物等北极生物的生存正在受到威胁,尤其是目前以及将来的全球变暖都会进一步给它们带来重大灾难.已有的证据表明,就像更新世早期的情形那样,北极地区的树线很有可能会进一步向北发展,并迅速进入到苔原地区,从而减小苔原带,这就会进一步增加北极地区物种灭绝的可能性.一些物种将很有可能向北扩大它们的领地,并取代该地区原有的物种.在更新世早期,由于北极地区的海平面相对较低,当树线入侵到现在的海岸地区时,苔原带至少能够在北极圈的一部分低地区域生存,而从目前来看,未来的海平面极有可能上升,这将会对北极苔原带和其它无树生态系统的分布施加进一步的限制.很显然,全球现在的气候状况对北极生态系统带来的负面影响超出更新世的任何时期,很有可能是巨大的,尤其是当各种环境变化(例如紫外线B的增加,大气中氮化合物的沉积,重金属和酸污染,放射性污染物,生物栖息地破碎化)共同作用于北极生态系统时的影响也是前所未有之际.     

环境变化背景下北极生物的多样性、分布及其适应性

生物个体是对气候变化和紫外线B(UV－B)辐射变化产生反应的基础,而且这种反应会在各种时间尺度上发生.北极地区的动物、植物以及微生物种类的多样性从表面上看是低的,而且从北方针叶林到极地荒漠逐渐减少,但其原始物种却很丰富.与这种物种多样性随纬向梯度减少的趋势相反,一些空间分布范围很广的单一优势物种的优势度则呈增长趋势.全球气候变暖可能会减少该地区的物种多样性,并限制到这些物种的分布范围,尤其是在该地区生物分布的北部边缘,一些极地特有的动物和植物种类会面临着灭绝的危险.最有可能侵入苔原地带的物种是那些目前生存在极地外缘的北方地区生物.许多植物都具有自身的特征使它们能够在以下环境中生存短暂的无冰雪覆盖的生长季节,低的太阳高度角,永久冻结地带及低的土壤温度,贫乏的养分获取条件以及极少的物理扰动.以上这些特征有些可能会限制当地物种对气候变暖的反应,但其最主要的因素是这些物种与那些潜在的入侵物种相比缺乏竞争能力.北极地区陆生动物拥有许多适应特性,这使它们能够适应北极地区剧烈的温度变化.许多动物通过冬眠或迁移来逃避极地地区的恶劣天气和资源短缺.北极地区动物生存的生物环境则相对简单几乎没有天敌、竞争者、疾病、寄生生物,但同时食物资源也很短缺.极地陆生动物可能对由气候变化带来的温暖而干旱的夏季非常不适应,这种变化将会影响到动物的迁移路线、途中栖息地,并会改变冬季积雪的状况和冻融的循环过程.气候变化还会改变动物繁殖和发育的季节,并会引来新的竞争者、捕食者、寄生生物以及疾病等.极地微生物也能很好地适应该地区的气候一些微生物甚至在-39℃的低温下还能进行代谢活动.蓝藻细菌和藻类生物有着很广泛的适应策略,这能够使它们避免(至少可以减少)紫外线的伤害.微生物能够忍受许多环境条件,而且其生长周期很短,这些特点将使它们能很快适应新的生存环境.与此形成对比的是,极地植物和动物很可能通过改变其分布范围而不是积极的生物进化来适应环境的变暖.     

在物种水平上生物对气候和紫外线B辐射预期变化的响应

控制实验表明,不同物种对每个环境因子变化变量产生的响应也存在着差异.植物往往对营养元素的变化反应最为强烈,尤其是氮素的增加.夏季增温实验表明,木本植物对温度的升高表现出了积极的响应,而地衣、苔藓类植物的丰富度却因增温而降低.物种对增温的响应主要受水分有效性和雪覆盖程度控制.在气候保持湿润的情况下,伴随着夏季温度的升高,许多无脊椎动物种群的数量都有所增加.实验表明,CO2浓度和紫外线B(UV-B)辐射的增加对植物和动物影响较小,但是,一些微生物和真菌却对紫外线B辐射的增加非常敏感,甚至可能会因此产生一些诱导突变而引起流行传染病的爆发.苔原土壤的加温、CO2浓度的升高以及矿物质营养的改善一般都会增加微生物的活动.在温带气候中,藻类往往比蓝藻细菌更占优势.冬季结冰-解冻过程的增加会导致冻壳的形成,从而会大大降低许多陆生动物的冬季存活率,改变这些动物群体的动态过程.厚的积雪会使驯鹿等植食性动物很难采食到雪下的草类植物,同时也不利于其逃避食肉动物的追捕.而无雪期的提前到来则可能会加速植物的生长.物种对气候变化的响应最初可能出现在亚种这一水平上一个具有很高遗传/群系多样性的北极植物或动物物种,演化历史已经使其具有一种适应不同环境条件的能力,这将使它们能够很快适应未来的环境变化.本土知识(IK)、航空照片和卫星图像表明一些物种的分布已经发生了变化北极植被更加趋向灌木化,而且生长也更加旺盛；北极驯鹿的分布范围最近也发生了变化；一些原来在树线以南区域活动的害虫和鸟类也在北极被发现.与此相对应,大多数在北极地区进行繁殖鸟类的数量却都在下降.根据一些模型的预测,随着气候的变暖,苔原带鸟类的数量将会大幅度地下降.据物种-气候响应模型预测,由于受到气候变暖的影响,北极地区现有物种在未来的潜在分布范围都将大大缩小和向北退缩,而一些无脊椎动物和微生物则很可能会迅速向北扩展到北极地区.     

Environmental Monitoring and Research in the Abisko Area—An Overview

This article gives an overview of the studies on the environment surrounding the Abisko Scientific Research Station in Swedish Lapland. The long-term monitoring of the Station on processes related to the climate, and to the physical, biotic, and chemical environmental conditions is particularly addressed. Some variables are recorded since more than 100 years. The obtained data in combination with results from short-term studies and manipulation experiments are important to understand past and future conditions of the ecosystems. This has practical applications for the planning of tourism, transports, reindeer herding, and for societal purposes.     

Watershed land use effects on lake water quality in Denmark

Mitigating nutrient losses from anthropogenic nonpoint sources is today of particular importance for improving the water quality of numerous freshwater lakes worldwide. Several empirical relationships between land use and in-lake water quality variables have been developed, but they are often weak, which can in part be attributed to lack of detailed information about land use activities or point sources. We examined a comprehensive data set comprising land use data, point-source information, and in-lake water quality for 414 Danish lakes. By excluding point-source-influenced lakes (n = 210), the strength in relationship (R2) between in-lake total nitrogen (TN) and total phosphorus (TP) concentrations and the proportion of agricultural land use in the watershed increased markedly, from 10-12% to 39-42% for deep lakes and from 10-12% to 21-23% for shallow lakes, with the highest increase for TN. Relationships between TP and agricultural land use were even stronger for lakes with rivers in their watershed (55%) compared to lakes without (28%), indicating that rivers mediate a stronger linkage between landscape activity and lake water quality by providing a "delivery" mechanism for excess nutrients in the watershed. When examining the effect of different near-freshwater land zones in contrast to the entire watershed, relationships generally improved with size of zone (25, 50, 100, 200, and 400 m from the edge of lake and streams) but were by far strongest using the entire watershed. The proportion of agricultural land use in the entire watershed was best in explaining lake water quality, both relative to estimated nutrient surplus at agricultural field level and near-lake land use, which somewhat contrasts typical strategies of management policies that mainly target agricultural nutrient applications and implementation of near-water buffer zones. This study suggests that transport mechanisms within the whole catchment are important for the nutrient export to lakes. Hence, the whole watershed should be considered when managing nutrient loadings to lakes, and future policies should ideally target measures that reduce the proportion of cultivated land in the watershed to successfully improve lake water quality.     

北极4个亚区的影响分析

在评价北极陆地生态系统影响时,人们常常强调物种和生态系统对环境变化响应的地理变化,这种变化往往与气候、生物多样性、植被带、生态系统结构和功能的南-北梯度相关联,可是,环境、生态系统的功能和结构上,以及环境史和当前气候变化的明显东-西变化显然也很重要.尽管一些地方变得温暖,但另一些地方却降温了,海洋、群岛和山脉等地理屏障的东西差异过去也对物种和植被带响应气候变化而改变分布区的能力产生了很大影响,同时,这些地理屏障为种群遗传分化和生物多样性热点区的形成提供了必要的隔离条件,这些屏障在未来气候变暖时,也将影响物种重新分布的能力.为了说明这种东西向的变化,同时也避免过分笼统或过于专业化,基于大尺度的天气和气候形成因素,北极气候影响评价项目确定了4个主要亚区.通过模拟与4个北极气候影响评价亚区有关的主要信息,导致物种分布区发生改变的地理屏障,特别是大陆的分布和海洋产生的隔离,明显会影响植被带的向北移动.对植被区向北移动的地理限制或者促进将影响将来碳的贮存和释放,以及生物圈与大气之间水和能量的交换.此外,气候变化使受威胁物种数量在各个亚区之间差别很大(白令海地区别尤其是热点),各个植被亚区重新分布的能力差异将影响每个区的生物多样性.总而言之,亚区分析表明,在整个北极地区水平上概括生态系统结构和功能的反应、物种的丧失,以及生物圈对气候系统的反馈的趋势是困难的,说明需要对北极陆地生态系统对于气候变化响应的空间变化性有深刻的认识.     

Responses to projected changes in climate and UV-B at the species level

Environmental manipulation experiments showed that species respond individualistically to each environmental-change variable. The greatest responses of plants were generally to nutrient, particularly nitrogen, addition. Summer warming experiments showed that woody plant responses were dominant and that mosses and lichens became less abundant. Responses to warming were controlled by moisture availability and snow cover. Many invertebrates increased population growth in response to summer warming, as long as desiccation was not induced. CO2 and UV-B enrichment experiments showed that plant and animal responses were small. However, some microorganisms and species of fungi were sensitive to increased UV-B and some intensive mutagenic actions could, perhaps, lead to unexpected epidemic outbreaks. Tundra soil heating, CO2 enrichment and amendment with mineral nutrients generally accelerated microbial activity. Algae are likely to dominate cyanobacteria in milder climates. Expected increases in winter freeze-thaw cycles leading to ice-crust formation are likely to severely reduce winter survival rate and disrupt the population dynamics of many terrestrial animals. A deeper snow cover is likely to restrict access to winter pastures by reindeer/caribou and their ability to flee from predators while any earlier onset of the snow-free period is likely to stimulate increased plant growth. Initial species responses to climate change might occur at the sub-species level: an Arctic plant or animal species with high genetic/racial diversity has proved an ability to adapt to different environmental conditions in the past and is likely to do so also in the future. Indigenous knowledge, air photographs, satellite images and monitoring show that changes in the distributions of some species are already occurring: Arctic vegetation is becoming more shrubby and more productive, there have been recent changes in the ranges of caribou, and "new" species of insects and birds previously associated with areas south of the treeline have been recorded. In contrast, almost all Arctic breeding bird species are declining and models predict further quite dramatic reductions of the populations of tundra birds due to warming. Species-climate response surface models predict potential future ranges of current Arctic species that are often markedly reduced and displaced northwards in response to warming. In contrast, invertebrates and microorganisms are very likely to quickly expand their ranges northwards into the Arctic.