Arctic warming on two continents has consistent negative effects on lichen diversity and mixed effects on bryophyte diversity

Abstract

Little is known about the impact of changing temperature regimes on composition and diversity of cryptogam communities in the A rctic and S ubarctic, 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 S wedish subarctic birch forest and subarctic/subalpine tundra to A laskan arctic tussock tundra. In both S weden and A laska, 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, S phagnum 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 S wedish subarctic birch forest where amount of litter constituted the best explanatory variable. In a warming experiment in moist acidic tussock tundra in A laska, 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 S phagnum girgensohnii , H ylocomium splendens and P leurozium 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, N 2 ‐fixation, trophic interactions), these changes will have important feedback consequences for ecosystem functions and climate.

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