Tree-limit landscape evolution at the southern fringe of the Swedish Scandes (Dalarna province) – Holocene and 20th century perspectives

Abstract

Tree-limit and climate evolution at the southern extremity of the Swedish Scandes have been reconstructed for the entire Holocene and for the past century. The main objective was to test the reproducibility of a similar study further north in the Swedish Scandes. The long-term history relies mainly on radiocarbondated megafossil tree remains preserved in peat and mineral soil. The more recent perspective was analysed from age distributions in the most marginal, extant tree populations. Pinus sylvestris (pine) immigrated to the study region already during the Late-Glacial, 10,500 14C yr BP, when it grew in the summit areas of fi rst deglaciated mountains. The pine tree-limit peaked ca. 9200 14C yr BP, followed by a 345 m gradual descent (summer cooling + land uplift) until about a century ago. Presence of tree species assemblages without modern analogs at high elevations during the early-Holocene, Pinus sylvestris (dominant), Betula pubescens ssp. tortuosa, Picea abies, Larix sibirica, Quercus robur and Corylus avellana, strengthens the view of a warm and dry summer climate, although with a different seasonal distribution of incoming radiant energy than today. The longterm natural trend of tree-limit retreat and summer cooling was anomalously broken about a century ago. During the 20th century, the pine tree-limit has shifted 140 m uphill, in congruence with instrumentally recorded summer warming by ca. 1 ºC. In a perspective of the entire Holocene tree-limit chronology, the modern tree-limit rise (and climate warming) is anomalous for the past 4000–7000 years. Except for tree-limit rise, recent warming has contributed to changes in the alpine plant cover, e.g. vanishing snow-bed communities, expansion of deciduous dwarf-shrubs, graminoids, and invasion of exotic and thermophilic plant species. Overall, this study has reproduced processes and patterns previously reported further north in the Scandes.