Holocene forest development and tree-limit changes in Ridalen, in the Røros mountains

Abstract

Holocene forest development, tree migration and tree-line fluctuations have been reconstructed from peat sequences, derived from two study localities situated along an altitudinal transect between the north boreal and low alpine zone in the mountain region of the Røros area. The local-scaled reconstructions are based on pollen analysis, stomata and megafossil findings. Additionally, an attempt to reconstruct the climate history has been made by the use of the indicator species approach. The reconstructions have further been compared to other palaeoecological studies in adjacent regions to Ridalen, in order to deduce regional patterns for the Holocene vegetation and climate. A pioneer flora dominated at the Lervahå mire at 770 m a.s.l. under relatively warm and dry conditions in early Holocene until ca. 10,500 cal. years BP. A rather coeval local presence of tree-birch (Betula pubescence) and pine (Pinus sylvestris) has been recorded at ca. 10,100 cal. years BP. Pine expanded rapidly thereafter, becoming the dominating tree in the Ridalen region until ca. 8,000-7,400 cal. years BP. Alnus grew locally in moister habitats at the Lervahå mire from ca. 9,200 cal. years BP and reached its maximum abundance around 7,400- 7,100 cal. years BP, after which a sub-alpine birch forest established at Reinskaret. In the cooler and moister period subsequent to ca. 5,000 cal. years BP the sub-alpine birch forest reached the Lervahå mire. Picea increased in abundance in regional forests after ca. 2,700 cal. years BP. In the period after ca. 3,700 cal. years BP, pine only grew as scattered individuals at 770 m a.sl. Tree-birch no longer grew at the Reinskaret locality after ca. 2,000 cal. years BP. The time of deglaciation could not be inferred for the area since the oldest layers could not be properly dated. Nevertheless, the stratigraphical sequence from the Lervahå mire is assumed to represent maximum the last ca. 11,500 years. The tree that first established in the study region is still uncertain.