Leif Kullman

20th Century Climate Warming and Tree-limit Rise in the Southern Scandes of Sweden

Abstract Climate warming by ca. 0.8°C between the late-19th and late-20th century, although with some fluctuations, has forced multispecies elevational tree-limit advance by >100 m for the principal tree species in the Swedish part of the Scandinavian mountain range. Predominantly, these processes imply growth in height of old established individuals and less frequently upslope migration of new individuals. After a slight retardation during some cooler decades after 1940, a new active phase of tree-limit advance has occurred with a series of exceptionally mild winters and some warm summers during the 1990s. The magnitude of total 20th century tree-limit rise varies with topoclimate and is mainly confined to wind-sheltered and snow-rich segments of the landscape. Thickening of birch tree stands in the “advance belt” has profoundly altered the general character of the subalpine/low alpine landscape and provides a positive feedback loop for further progressive change and resilience to short-term cooling episodes. All upslope tree-limit shifts and associated landscape transformations during the 20th century have occurred without appreciable time lags, which constitutes knowledge fundamental to the generation of realistic models concerning vegetation responses to potential future warming. The new and elevated pine tree-limit may be the highest during the past 4000 14C years. Thus, it is tentatively inferred that the 20th century climate is unusually warm in a late-Holocene perspective.

Holocene Tree‐Limit and Climate History from the Scandes Mountains, Sweden

Elevational tree-limit change of Pinus sylvestris, Betula pubescens ssp. tor- tuosa, and Alnus incana during the Holocene was studied from subfossil wood remains in the Scandes Mountains, Sweden. From 8250 yr BP to the present, the Pinus-limit descended at an average rate of 20 m per millennium (after adjustment for glacioisostatic land up- heaval). In contrast, Betula and Alnus ascended until -3000 yr BP, whereafter an altitudinal decline is apparent. The relative abundance of Pinus in the tree-limit ecotone has decreased, while that of Betula increased throughout much of the Holocene. The present-day subalpine belt of Betula developed after 7000 yr BP in response to long-term cooling interacting with short-term climatic disturbance of pine stands. The inferred tree-limit histories are consistent with climatic model simulations of solar radiation and the atmospheric circulation for the Holocene (the Milankovitch theory). Based on the current climate requirements of the tree- limit species, an early Holocene summer thermal optimum -1 .0C higher than present is inferred. Subsequently, tree-limit vegetation changed gradually, in dynamic equilibrium with progressive summer cooling and increasing humidity and snow cover, i.e., a decreas- ingly seasonal climate. Accelerated recession of Pinus, indicative of abrupt cooling, is recorded for a short period around 7200 yr BP A short-term advance of the Pinus tree- limit, presumably a consequence of climatic warming, occurred -4300 yr BP. Rapid and persistent reductions in the quantitative record of subfossils of all tree species, after 5500- 5000 yr BP, may result from fragmentation of the tree-limit ecotone. This response is inferred as the combined effect of long- and short-term cooling passing a threshold, which initiated periglaciation, podzolization, and paludification.

Dynamics of altitudinal tree-limits in Sweden: a review

The dynamic nature of Swedish (and Scandinavian) alpine tree-limits is reviewed in the perspective of recent research covering four timescales, viz. (1) the long-term Holocene perspective, (2) the Little Ice Age, (3) the post-Little Ice Age warming, and (4) the recent cooling. On all scales the tree-limits indicate a sensitive response in accordance with the tree-limit history and temperature trends (particularly seasonality) postulated for a wider geographical spectrum of the northern hemisphere. In detail, species have responded individualistically, particularly stressing the regenerative strategies. Vegetative regeneration and phenotypic plasticity provide relative stability to the altitudinal range limits of mountain birch, spruce and rowan, in contrast to the pine.

A coherent postglacial tree-limit chronology (Pinus sylvestris L.) for the Swedish Scandes: aspects of paleoclimate and "recent warming," based on megafossil evidence.

A coherent elevational tree-limit chronology, based on megafossil wood of Pinus sylvestris L. (Scots pine), is presented for the entire Holocene period in the Swedish Scandes. The chronology is arg...

Tree limit dynamics of Betula pubescens ssp. tortuosa in relation to climate variability: evidence from central Sweden

The tree limit of Betula pubescens ssp. tortuosa (mountain birch) in the southern Swedish Scandes was moni- tored during 1972-1992. The study included various aspects of growth, vigour and reproduction of Betula. The main focus was on the character and mechanics of the tree limit/climate equilibrium system. The tree limit, which changed in re- sponse to a temperature rise early this century, remained constant in position and tree physiognomy did not change, although the past 50 yr or so have been colder. Indeed, growth and reproductive effort and capacity decreased in this period. Obviously, most resources in Betula were used to resist stress in the mature phase, resulting in delayed recession of trees. Unless drastic warming occurs, stem dieback is predicted for the near future. Even a minorclimatic disturbance would have an effect, because resources are gradually being depleted. Radial growth correlated most closely with the mean tem- perature in July. Particularly at the present-day tree limit, the variance in annual growth could be largely explained by climatic factors. The hypothesis is that long-term tree limit dynamics during the late Holocene is merely a matter of fluctuations in vegetative vigour and stature of old individu- als. At the population level, response to climatic variability appears to be greatly delayed.

New and Firm Evidence for Mid-Holocene Appearance of Picea Abies in the Scandes Mountains, Sweden

1 A subalpine clonal population of Picea abies in the Scandes Mountains (Sweden) was analysed with respect to age structure, height and radial growth, foliage vigour and radiocarbon dates of subfossil wood fragments. 2 A tree-ring chronology yielded consistently low annual increments from AD 1600 to the mid-1800s. Subsequently, radial growth, initiation of new stems and height growth increased in accord with climate warming and possibly deeper snow cover. The growth form changed from krummholz to erect tree-size. During the past c. 50 years growth has declined and supra-nival stems have eroded. The reason is climate cooling mediated by the complex temperature/snow cover/ground frost, which appears as a critical determinant of the Holocene Picea spread. 3 Subfossil Picea wood was dated to c. 4800-4700 radiocarbon years BP (two dates), i.e. Picea grew here more than 2000 years prior to inferences from pollen data. Obviously, stray finds of pollen may represent local presence and it is hypothesized that Picea immigrated to specific microhabitats even earlier than the date established here, possibly soon after the regional deglaciation. Much later it could spread regionally as climate gradually became less seasonal, damper and more snow-rich, in response to orbital forcing of insolation. 4 Early immigration, well before local or regional dominance, precludes migrational lag and rather suggests that in a landscape perspective a dynamic equilibrium between Picea abundance and climate has existed for most of the Holocene. The elevational range-limit, however, may have been out-of-phase with climate for centuries or millennia, tuned to climate mainly by phenotypic responses.

Recent reversal of Neoglacial climate cooling trend in the Swedish Scandes as evidenced by mountain birch tree-limit rise

Abstract An intensive study reconstructs the local late-Holocene tree-limit history ( Betula pubescens ssp. tortuosa ) in the southern Swedish Scandes, as a proxy palaeoclimatic indicator. Elevational tree-limit rise by 75 m, in accordance with instrumentally recorded summer warming, took place over the past century as evidenced by a combination of historical records and modern age–structure analyses. This regionally representative development was put into long-term perspective by radiocarbon-dating of a large sample of megafossil wood remains (trunks and roots), preserved and recovered at the same location and elevation as the new and higher tree-limit. The magnitude and rate of 20th century tree-limit advance (implicitly climate warming) were without precedent for the past ca. 3500 calendar years. The course of development conforms to an increasingly emergent worldwide pattern, suggesting that warming over the past century is a fundamental reversal of a long-term (Neoglacial) cooling trend. The results are of relevance for current efforts to attribute causes of recent warming in terms of natural versus anthropogenic forcing. The highly anomalous nature of the past century, with respect to climate and ecological change, could argue that at least in part, the warming may be human-induced. However, definite conclusions have to await more global-scale data.

A Richer, Greener and Smaller Alpine World: Review and Projection of Warming-Induced Plant Cover Change in the Swedish Scandes

Alpine plant life is proliferating, biodiversity is on the rise and the mountain world appears more productive and inviting than ever. Upper range margin rise of trees and low-altitude (boreal) plant species, expansion of alpine grasslands and dwarf-shrub heaths are the modal biotic adjustments during the past few decades, after a century of substantial climate warming in the Swedish Scandes. This course of biotic landscape evolution has reached historical dimensions and broken a multi-millennial trend of plant cover retrogression, alpine tundra expansion, floristic and faunal impoverishment, all imposed by progressive and deterministic neoglacial climate cooling. Continued modest warming over the present century will likely be beneficial to alpine biodiversity, geoecological stability, resilience, sustainable reindeer husbandry and aesthetic landscape qualities. These aspects are highlighted by an integrative review of results from long-term monitoring of subalpine/alpine vegetation in the Swedish Scandes. This forms the basis for some tentative projections of landscape transformations in a potentially warmer future. Notably, these results and projections are not necessarily valid in other regions and differ in some respects from model predictions. Continued monitoring is mandatory as a basis for generation of more realistic vegetation and ecosystem models.

Long-Term Dynamics of High-Altitude Populations of Pinus sylvestris in the Swedish Scandes

The age structure of Scots pine (Pinus sylvestris L.) was studied, on a regional basis, in a narrow altitudinal zone well below the altitudinal tree-limit in the Swedish Scandes. The study period covered the last c. 400 years. Regeneration success was related to changes in summer mean temperature and thermal conditions during the late winter and spring. A positive impact of the twentieth-century climatic improvement was found. A marked regeneration gap occurred during the climax (AD 1680-1720) of the Little Ice Age. Possibly the coldness of this period increased mortality rates even of adult pines. The age structure suggests that within the altitudinal and temporal interval studied pine populations are in a dynamic steady-state condition. Regeneration takes place even during climatically unfavourable periods. This differs from both more and slightly less marginal sites where populations are more vulnerable due to climate and climate-fire interactions, respectively. The existence of this zone of relative population stability is an important feature when discussing Holocene biogeographical changes.

Structural change in a subalpine birch woodland in North Sweden during the past century

An isolated subalpine woodland with predominant mountain birch (Betula pubescens Ehrh. subsp. tortuosa (Ledeb.) Nyman) was studied with respect to structural change during the past century. Present-day age and size data of tree populations and the field-layer flora were analysed against the perspective of an early twentieth century account of the forest structure and flora. Air temperature recordings during some periods of the 1980s provided information on the local thermal characteristics in relation to the physiological tolerances of the tree species. Extensive invasion of spruce (Picea abies (L.) Karst.), and to a lesser extent pine (Pinus sylvestris L.), peaked in the 1930s, coinciding with the general summer warming (about 1?C) of the first half of the twentieth century. The subalpine birch forest changed mainly by increased height and proliferation of basal shoots. The spacing structure of birch individuals, however, was largely unaffected by climatic change. Presumably, increased vegetative growth related to breakage of main stems from rime ice, snow and wind in combination (loss of apical dominance), and increased nutrient mineralization. Conceivably, mechanical snow impact has selected for mountain birch predominance and restricted the abundance of coniferous tree species. Tree invasion into an open forest occurred despite the increased vigour of the pre-existing tree dominant, although it was possibly preceded and facilitated by disturbance in the form of transient defoliation from insect attack (Epirrita autumnata Bkh.) or some other agent.