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外文期刊>Nordic Journal of Botany
>Shrub encroachment interacts with environmental variation to reduce the albedo of alpine lichen heaths: an experimental study
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Shrub encroachment interacts with environmental variation to reduce the albedo of alpine lichen heaths: an experimental study
Global warming causes the replacement of lichens by shrubs in alpine and Arctic ecosystems. Since shrubs have lower albedo than lichens, this shrub encroachment can lead to a positive climatic feedback, resulting in higher temperatures in the surroundings. Therefore, gaining knowledge on the surface albedo of shrubs and lichens is important. Environmental factors also influence the surface albedo, but have often been neglected, potentially leading to biased results. In an experimental setup, we analyzed albedos of the lichen species Cladonia stellaris, Flavocetraria nivalis and Cetraria islandica, and how albedo changes with a stepwise replacement by the dwarf shrub Empetrum nigrum. Albedo was measured with radiometers in a paired set up. By setting certain environmental variables and species composition (monocultures) to be constant, we quantified the impact of environmental factors such as cloud cover, aspect and zenith angle on the surface albedo of two lichen species. Surface albedo (mean values +/- SD) differed between C. islandica (0.155 +/- 0.015), C. stellaris (0.364 +/- 0.019), F. nivalis (0.350 +/- 0.022) and E. nigrum (0.154 +/- 0.016), and an increase in shrub cover at the expense of lichen cover led to a corresponding decrease in albedo. A 0.6 reduction in clearness index (more clouds) produced a 0.054 albedo decline. On the north-facing aspect, albedo was 0.023 lower than on the south-facing aspect. Albedo increased by 0.032 with an increase in zenith angle of 15 degrees. Albedo variations caused by these studied environmental factors significantly affect the radiation budget of alpine and Arctic vegetation. We therefore stress the importance of considering environmental factors when surface albedos are estimated. Likewise, our species-specific measurements can be a basis for further studies of the impact of climate change on alpine and Arctic vegetation and species-related feedback mechanisms.
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