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The study focuses on structural and compositional dynamics of high-elevation (boreal) mixed Picea-Betula forest in the Swedish Scandes. Data from seven years' monitoring of crown condition (defoliation), seed viability, soil and air temperatures, a tree-ring chronology and the age structure of Picea are presented. Real long-term dynamism of the tree cover was inferred from radiocarbon dates of buried subfossil wood. At all time-scales, the forest has responded sensitively to thermal variability. Picea seed viability, demography and radial growth correlated positively with summer temperature. The current Picea population largely results from warming during the first half of the 20th century. However, cooler climate in the past c. five decades has led to reduced population and radial growth as well as accelerating crown defoliation. This decline is related primarily to winter soil temperatures. The mechanism of needle loss is deduced to be winter desiccation and resulting cavitation of the xylem. It is hypothesized that previous similar episodes have been balanced by periods of spruce expansion. The successional history leading up to the present stage includes fire disturbance and subsequent cooling during the Little Ice Age, which prolonged and consolidated the early post-fire Betula phase. Pinus was incapable of penetrating these unusually dense birch stands, whereas the shade-intolerant spruce could establish progressively as climate warmed. This course of structural and compositional dynamism relates to complex interactions of disturbance, climate change and reproductive responses. [ABSTRACT FROM AUTHOR]

Elevational distribution, site characteristics and regeneration patterns of grey alder [Alnus incana (L.) Moench] have been studied in subalpine forest of the southern Swedish Scandes, mainly as a basis for paleoecological inferences. Alder occurs very sparsely in the mountain birch forest and is strictly confined to sites with fresh fine-grained soils with an insignificant mor humus layer. It frequently grows in close association with moving water (inundation in the spring). The potential climatic tree-limit is inferred to appear about 30 m below that of mountain birch. Within a more than 200 m broad vertical zone below the tree-limit, sexual reproduction does not take place at the present. Conceivably, the alders here are old relicts from a warmer period in the past. In contrast to other tree species, the range-limit did not advance altitudinally in response to the 20th century warming (up to the 1930s/1940s) and a non-equilibrium relationship with climate is hypothesized to have developed after the maximum range extension in the early- or mid-Holocene. The virtual extinction of alder in the upper subalpine forest during the late Holocene is understandable in terms of pronounced thermophily of generative life-cycle stages and a narrow range of soil preference. Both these properties are acting regressively on reproductive success as a consequence of climatic cooling and long-term soil deterioration. [ABSTRACT FROM AUTHOR]

In 1987 particularly, but also in 1988, primaeval montane coniferous forests of northern Scandinavia experienced needle loss on an unprecedented scale, which affected the character of the entire landscape in certain regions. This study elucidates the dieback event for Norway spruce [Picea abies (L.) Karst.] in an area of the Swedish Scandes. The study includes visual assessment of percentage needle loss, age structure, radial growth and various environmental parameters, along altitudinal transects and within a larger sample plot. Frequency and magnitude of foliage damage were found to be related to elevation. The dieback process of closed forest stands was interpreted as a response to short-term climatic variability. Damage mainly developed during one of the coldest early winters (December-January 1986/7) of this century. In combination with sub-normal depth of the snow cover, the severe cold promoted exceptionally deep and persistent soil frost in spruce forest stands. It is hypothesized that damage was mainly due to winter/spring drought (frost-drought). Phenotypically closed forest stands may change in just a few years in response to cooling, a feature which has not been realized previously. The possibility of major retrogressive responses to short-term climatic events should therefore he considered in studies of historical biogeography and when modelling vegetation succession. The forest studied was probably established after a severe wind storm in 1837. The restocking process was facilitated by the post-Little Ice Age warming, but the stands are at present suffering severe dieback due to the recent cooling of the climate. The results stress the potential importance of short-term climatic disturbance in boreal forest succession. Reasonably, rapid cold-induced dieback of forest stands may have been a recurrent feature of Holocene forest history in the Boreal zone. [ABSTRACT FROM AUTHOR]

The Holocene history of the forest-alpine tundra ecotone in Central Sweden (Scandes Mountains) is inferred from radiocarbon analyses of subfossil wood remains. Pinus sylvestris was the dominant subalpine tree species during the early Holocene, when it ascended almost 200 m higher than currently. A short climatic episode (less than 100 years) is postulated to have triggered erosional processes around 6300 B.P., and extinguished the upper part of the subalpine pine woodland. Subsequently, a subalpine belt of Betula pubescens ssp. tortuosa and Alnus incana developed. A Holocene thermal optimum occurred around 6100 B.p., when the birch/alder belt flourished and the tree-limits probably reached their highest levels during the Holocene. Shortly after 6000 B.P., a long-term pine forest retrogression started and the birch/alder belt was disrupted by expanding snow-beds. Pine receded slightly at its tree-limit, but the uppermost belt of closed pine forest (presently dominated by birch) remained intact until c. 3300 B.p., when a severe climatic deterioration occurred. The present-day subalpine belt of pure birch forest developed successively and increased in vertical extent after c. 5300 B.P., when summer temperature declined. The evolution of the birch belt is postulated to have been ultimately a response to decreased seasonality, which favoured birch at the expense of pine. Because of the 'inertia' characterizing the highest pine forest, the birch belt was relatively narrow until a major thermal decline c. 3300 B.P., when it made a massive downslope expansion. The latest phase of pine recession was during the Little Ice Age, 800-300B.P. [ABSTRACT FROM AUTHOR]

The natural decline of a virgin high-altitude Pinus sylvestris forest during the Little Ice Age (approximately AD 1300-1850) was studied in the Swedish Scandes. Methods included 14C and cross-dating of wood remnants, soil analyses, year-ring chronologies and age structures. A closed pine forest started a gradual decline in the 11th century or somewhat later, which proceeded until the mid-19th century. Long-term failure of regeneration and premature death of trees are postulated to have caused the forest dieback, mainly owing to a negative shift of the carbon budget, including weather-induced loss of photosynthetic tissue and root disfunction. Temperature measurements on the study site stressed its current marginality. Especially thermal conditions in and close to the ground were deduced to be decisive. The decline of the forest is considered to indicate a long-term lowering during the Little Ice Age of the summer (June-September) mean temperature by 1 °C or more, compared with the 1931-1960 mean. [ABSTRACT FROM AUTHOR]