| Summary: | How fast can vegetation respond to rapid climate change? To answer this question, we require long-term vegetational data and an independent climate record. Both can be obtained from multi-proxy palaeoecological studies involving pollen analysis and plant macrofossil analysis (vegetational data) and chironomid analysis (climate record). Late-glacial climate changed rapidly and passed critical vegetation thresholds in western Norway. The interstadial (Allerod) vegetation at Krakenes on the west coast was analogous to low- or mid-alpine vegetation in the west Norwegian mountains today. There was a marked vegetational response over ~10 years to the Younger Dryas cooling, even though mean July air temperature, as inferred from the independent fossil chironomid record, only decreased by about 2 °C. Together with the prevailing precipitation, this was sufficient to allow a cirque glacier to develop above Krakenes Lake during the Younger Dryas. As summer temperatures increased rapidly at the opening of the Holocene, plants responded immediately. Warmth-intolerant arctic-alpines rapidly succumbed. Warmth-tolerant arctic-alpine species expanded until declining as a result of increasing competition. Successional processes proceeded through to damp and dry grassland, the development of tall-fern vegetation, and the expansion of Empetrum-dominated dwarf-shrub heath. Tree-birch Betula pubescens showed a migrational lag of ~500 years before forming birch woodland. This study illustrates how floristic and vegetational patterns recorded in sedimentary sequences can be interpreted in terms of ecological processes if an independent palaeoclimate record is available; here, from fossil chironomid assemblages from the same sediment core.
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