Tree-limits and montane forests in the Swedish Scandes: sensitive biomonitors of climate change and variability

• Published in: Ambio 1998-06, Vol.27 (4), p.312-321
• Main Author: Kullman, L. (Umeaa Univ. (Sweden). Naturgeografiska Inst.)
• Format: Article
• Language: English
• Subjects: ARBOLES FORESTALES; ARBRE FORESTIER; BOREAL FORESTS; BOSQUE BOREAL; CAMBIO CLIMATICO; CHANGEMENT CLIMATIQUE; Climate change; Climate models; CLIMATIC CHANGE; Coniferous forests; EFECTO INVERNADERO; EFFET DE SERRE; ENVIRONMENTAL IMPACT ASSESSMENT; EVALUACION DEL IMPACTO AMBIENTAL; EVALUATION IMPACT SUR ENVIRONNEMENT; Forest regeneration; Forest stands; FOREST TREES; FORET BOREALE; GREENHOUSE EFFECT; Hardwood trees; HIGHLANDS; Holocene climatic optimum; INDICATOR PLANTS; Montane forests; Paleoclimatology; PLANT RESPONSE; PLANTAS INDICADORAS; PLANTE INDICATRICE; REGION D'ALTITUDE; REPONSE DE LA PLANTE; RESPUESTA DE LA PLANTA; SUECIA; SUEDE; SWEDEN; ZONA DE MONTANA
• ISSN: 0044-7447; 1654-7209

The elevational tree-limit constitutes an ideal and sensitive proxy indicator of climate change and variability, i.e. an essential part of monitoring systems focusing on global climate change. That contention is purported by multi-scale records and reconstructions of changes in altitudinal tree-limits and northern boreal forests. Climatically forced trends in their position, structure and composition have occurred at all temporal scales throughout the Holocene. A progressive elevational descent of Pinus sylvestris tree-limit since the earliest Holocene, concurs with the deterministic theory of millennial climate forcing by changes in the Earth's orbital parameters. The successively less seasonal climate with cooler, more humid summers and winters with increasing snow cover has preconditioned the emergence of a subalpine birch forest belt during the past ca. 7000 yrs BP as well as the growing geo-ecological prominence of Picea abies. Superimposed on this long-term trend, climatic anomalies of shorter duration have been inferred from the tree-limit chronology. Some exceptionally warm and stable centuries, with high tree-limits and dense montane forests occurred during the Medieval period. Thereafter, the Little Ice Age prevailed until the late 19th century. Northern and high-elevation ecosystems were profoundly stressed, disturbed and destabilized by cold, windy and highly variable climate conditions. An episode of warmer climate during the the first half of the present century imposed some recovery of structures decayed by the Little Ice Age. However, tree-limits and high-elevation forests were far from restored to their medieval levels. During the past 4-5 decades, a more martime and slightly cooler climate has been instrumentally recorded. High-elevation arboreal vegetation has responded retrogressively by defoliation, retarded growth, ceasing regeneration and locally some tree-limit retraction. Neoglacial processes have been resumed, e.g. dieback of subalpine/alpine dwarf shrub heaths, followed by deflation of humus and surface mineral soils. These processes are readily monitored in a unique regional network, with baseline data since the early 20th century.