Species occurrence and richness of lichen, bryophytes and vascular plants near Abisko, Sweden and Toolik lake, Alaska

Little is known about the impact of changing temperature regimes on composition and diversity of cryptogam communities in the Arctic and Subarctic, despite the well-known importance of lichens and bryophytes to the functioning and climate feedbacks of northern ecosystems. We investigated changes in diversity and abundance of lichens and bryophytes within long-term (9-16 years) warming experiments and along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra. In both Sweden and Alaska, lichen diversity responded negatively to experimental warming (with the exception of a birch forest) and to higher temperatures along climatic gradients. Bryophytes were less sensitive to experimental warming than lichens, but depending on the length of the gradient, bryophyte diversity decreased both with increasing temperatures and at extremely low temperatures. Among bryophytes, Sphagnum mosses were particularly resistant to experimental warming in terms of both abundance and diversity. Temperature, on both continents, was the main driver of species composition within experiments and along gradients, with the exception of the Swedish subarctic birch forest where amount of litter constituted the best explanatory variable. In a warming experiment in moist acidic tussock tundra in Alaska, temperature together with soil ammonium availability were the most important factors influencing species composition. Overall, dwarf shrub abundance (deciduous and evergreen) was positively related to warming but so were the bryophytes Sphagnum girgensohnii, Hylocomium splendens and Pleurozium schreberi; the majority of other cryptogams showed a negative relationship to warming. This unique combination of intercontinental comparison, natural gradient studies and experimental studies shows that cryptogam diversity and abundance, especially within lichens, is likely to decrease under arctic climate warming. Given the many ecosystem processes affected by cryptogams in high latitudes (e.g. carbon sequestration, N2-fixation, trophic interactions), these changes will have important feedback consequences for ecosystem functions and climate.

尽管地衣（lichens）与苔藓植物（bryophytes）对北方生态系统的功能及气候反馈具有公认的重要性，但目前学界对北极与亚北极地区温度格局变化对隐花植物（cryptogam）群落组成与多样性的影响仍知之甚少。本研究依托长期（9-16年）增温实验，并沿自然气候梯度开展调查，探究了地衣与苔藓植物的多样性及丰度变化，调查梯度涵盖瑞典亚北极桦木林、亚北极/亚高山苔原以及阿拉斯加北极薹草丘苔原。在瑞典与阿拉斯加两地，地衣多样性对实验增温（桦木林站点除外）以及气候梯度上的更高温度均呈负响应。苔藓植物对实验增温的敏感性低于地衣，但根据气候梯度的跨度不同，苔藓植物多样性随温度升高以及在极低温度条件下均出现下降。在苔藓植物中，泥炭藓属（Sphagnum）植物的丰度与多样性对实验增温均表现出极强的抗性。在两大洲的实验站点与气候梯度中，温度均是驱动物种组成的核心因素，但瑞典亚北极桦木林站点除外，该站点枯落物量为最佳解释变量。在阿拉斯加湿润酸性薹草丘苔原的增温实验中，温度与土壤铵态氮有效性是影响物种组成的最关键因素。总体而言，矮灌木丰度（落叶与常绿类群）与增温呈正相关，苔藓植物中的吉氏泥炭藓（Sphagnum girgensohnii）、塔藓（Hylocomium splendens）以及偏蒴藓（Pleurozium schreberi）亦是如此；而绝大多数其他隐花植物则与增温呈负相关关系。本研究通过跨大洲对比、自然梯度调查与实验研究的独特结合，表明在北极气候变暖背景下，隐花植物的多样性与丰度（尤其是地衣类群）大概率会出现下降。鉴于高纬度地区诸多生态过程（如碳固存、生物固氮、营养级相互作用）均受隐花植物影响，这些变化将对生态系统功能与气候产生重要的反馈效应。