Ulf Segerström

Holocene climatic change reconstructed from diatoms, chironomids, pollen and near-infrared spectroscopy at an alpine lake (Sjuodjijaure) in northern Sweden

The results of a multiproxy study reconstructing the climate history of the last 9300 years in northern Sweden are presented. It is based on diatom, chironomid and pollen analyses, as well as near-infrared spec troscopy (NIRS), of a radiocarbon dated sediment core from Sjuodjijaure (67°22N, 18°04E), situated 100 m above tree-line in the Scandes mountains. Mean July air temperature was reconstructed using transfer functions established for the region. The biological proxies show significant changes in composition during the Holocene and the inferred temperatures all follow the same general trend. For the period between about 9300 to 7300 cal. BP the reconstructions should be interpreted with caution due to the lack of convincing modern analogues in the training set. However the reconstruction suggest that July temperature was on average about the same as today, with several rapid short-term cold and warm periods. Cold periods were dated to about 8500, 8200 and 7600 cal. years BP and a warm period to about 7700 cal. BP. About 7300 cal. BP, a major shift to a warmer climate occurred. Pine migrated into the area, which was previously covered with birch forest. From the mid-Holocene until today the sediment record suggests a descending tree-limit and a gradual lowering of July temperature.

BOREAL SWAMP FORESTS : BIODIVERSITY HOTSPOTS IN AN IMPOVERISHED FOREST LANDSCAPE

wamp forests in the boreal part of Sweden are dominated by Norway spruce (Picea abies). These forests typically grow on peat deposits with a water table that is often situated well below the ground surface (Figure 1). At present, only a few small stands of old-growth swamp forests are still found in remote areas because of extensive logging over the past two centuries. The primeval appearance of these forests, the wet conditions, the lack of charred stumps, and the absence of fire scars on trees have led to the

Disturbance history of a swamp forest refuge in Northern Sweden

Abstract Swedish swamp forests have a considerable conservation value because of their rich biodiversity which includes many threatened species. They have been interpreted as refugia from fire with long histories of forest continuity. Palaeoecological investigation of a small swamp forest in northern Sweden shows that the forested area was under cultivation 500 years ago. The present forest has only developed during the last 300 years as a consequence of changes in land-use. Palaeoecology can be used to test assumptions about stability, disturbance and ‘natural’ vegetation. Results from such studies suggest that communities are highly dynamic, and conservation policy should create a framework within which dynamic processes can operate, in addition to species preservation. Our method is a potentially valuable tool in the formulation of future nature conservation policy.

Do Diatom, Chironomid, and Pollen Records Consistently Infer Holocene July Air Temperature? A Comparison Using Sediment Cores from Four Alpine Lakes in Northern Sweden

Abstract The aim of this study is to assess the performance of diatom, chironomid, and pollen transfer functions for inferences of July air temperature during the Holocene using sediments from four alpine lakes in an area with low human impact in northern Sweden. The study demonstrates that diatom, chironomid, and pollen assemblages in the sediment cores contain climate information so that present-day temperature at each lake can be inferred with reasonable confidence for most proxies. Most proxy records from the sites consistently infer a long-term decreasing trend in July air temperature from ca. 6000 cal yr BP until the present. However, there are also large variations in the temporal patterns of the inferred temperatures during some periods, especially before 7000 cal yr BP, when there are also nonsynchronous changes in loss-on-ignition in the four lakes. This variability indicates that local conditions in the catchments (influence of snowfields, soil-forming processes) had a large impact on the organism assemblages in the early Holocene. Long-distance transport of pollen into high alpine lakes makes temperature inferences from pollen transfer functions unreliable. Due to the uncertainties of the methods, predictive errors of the transfer functions, and variability caused by local catchment/lake characteristics, only long-term trends in climate can be inferred. High-resolution studies using diatoms, chironomids, and pollen for climate reconstruction are probably not meaningful during periods with small changes in climate (<1°C). Future research should concentrate on low-resolution, multiproxy, and multilake studies to further understand the relationship between the proxies and climate.

Long-term dynamics of vegetation and disturbance of a southern boreal spruce swamp forest

Abstract. Analysis of pollen, charcoal and loss-on-ignition in peat cores from a Picea aeies-dominated swamp forest in central Sweden show the vegetation changes and disturbance patterns over 9500 yr. Six major sequences of local vegetation development are identified: (A) Pinus period, ca. 9500–7000 cal. BP; (B) Open mire period (ca. 7000–4500 cal. BP; (C) Betula period, ca. 4500–2300 cal. BP; (D) Picea period (ca. 2300–1000 cal. BP; (E) Human impact period (ca. 1000–100 cal. BP); and (F) Period of human abandonment during the last ca. 100 yr. The swamp forest has been highly dynamic in response to various natural and anthropogenic disturbance agencies. Several fires have heavily influenced the vegetation development. During the last ca. 900 yr human influence has been important, initially from grazing and trampling by domesticated animals (ca. 1000–500 cal. BP), and subsequently small-scale cereal growing (ca. 400–100 BP). Cutting, burning and animal browsing influenced the structure and dynamics of the swamp forest by creating a more open stand and suppressing tree regeneration. Recent cessation of human impact has led to increased tree regeneration and a denser swamp forest stand. The present high biodiversity, and subsequent conservation interest does not result from long-term stability or absence of fire and human impact. However, in spite of repeated disturbances, a continuity of old and senescent trees produced a forest type with abundant dead wood. With the relatively minor importance of fire over long periods of time, the swamp forest developed a structure maintaining a high biological diversity. An important issue for maintaining long-term biodiversity in the boreal landscape must be to create a mosaic where different forest types are present, with a variety of structures, substrates and processes, to provide a certain degree of freedom for species to move around in the landscape.

Environmental history: a piece in the puzzle for establishing plans for environmental management.

Establishment of plans for environmental planning and management requires that a number of natural and societal factors must be taken into consideration. Insights into the inherent dynamics of nature as well as the role that past human activities have played for establishing the current condition of the landscape and the natural environment in general are essential. Many natural and man-made changes occur over time scales of decades or centuries, and these are difficult to comprehend without a historical perspective. Such a perspective can be obtained using palaeoecological studies, i.e. by geochemical and biological analyses of lake sediment and peat deposits. To illustrate the long-term dynamics of nature and particularly the role of man, we present here five case studies from Sweden concerning pollution, lake acidification, lake eutrophication, biodiversity, and landscape dynamics and conservation--topics of broad interests--and discuss benefits of including a longer time perspective in environmental management.

Climatic Reflection in Varved Lake Sediments

The individual thicknesses of about 1300 varves (230 A.D. — 1552 A.D.) of the varved lake sediment from Lake Judesjon, N. Sweden, were measured to study short-term climatic changes. The results indicate a cyclic variation in thickness, with a period length of 30–40 years, dependent on variation in the amount of organic material deposited annually. This variation is most likely to be due to changes in the primary production of the lake caused by variation in the amount of insolation during the summer of different years.

Calculating net annual accumulation rates of sediment components exemplified by pollen

The net annual accumulation rate of a sediment component can easily be estimated by determining the total content of the specific component (x) in a sediment sample, which represents a known number of years (yr) and certain area of the lake bottom (a), using the formula: annual accumulation = x/(yr·a). To illustrate and test this procedure we have calculated mean net annual accumulation rates for sediment dry matter, mineral matter, organic matter and pollen grains of Scots Pine (Pinus sylvestris L.) for one short sequence of a varved lake sediment core. This paper includes descriptions of a simple method for sub-sampling fresh cores of consolidated sediments and an alternative method for making absolute pollen counts. The test results clearly show that in varved sediments quantitative sediment samples can be cut out with very high precision, i.e. the variable a and in this case also yr can be very accurately determined. They also show that dry sediment, mineral and organic matter and pine pollen grain contents (i.e. x) and thus, mean net annual accumulation values could be estimated with a good precision (C < 3%). This procedure for estimating net annual accumulation rates is, of course, applicable in non-varved sediments and for many other sediment compounds provided that a reliable chronology can be determined.

Tracing modern environmental conditions to their roots in early mining, metallurgy, and settlement in Gladhammar, southeast Sweden: Vegetation and pollution history outside the traditional Bergslagen mining region

We present results from a multidisciplinary project using lake sediment as a natural archive in combination with archaeology to investigate the earliest history of the Gladhammar mining area, southeastern Sweden. The aim was to identify and trace human impacts on the landscape, specifically in connection with settlement and metal production. Sediment records from two lakes linked to different processes in metal production were analyzed; Tjursbosjön down-slope of the mining area and Hyttegöl situated downstream of an excavated blast furnace, 1.8 km from the mines. The sediment analyses included multi-element geochemistry (WD-XRF), stable lead isotopes, pollen, and charcoal. Although historical documents record activities beginning in AD 1526, the archaeological study found indications that mining and metal production likely predated this period. The known historical period is well reflected in the sediment records, such as a 500-fold increase in copper, stream erosion, loss of forest cover and an expansion in agriculture. More importantly, already in the 12th–13th centuries, there was a 2- to 10-fold increase in lead, copper, and charcoal particles and evidence of erosion linked to the establishment of a blast furnace. Lead isotopes reveal a change from natural conditions to an input of lead from regional ores as early as the 9th–10th centuries. Settlement in the form of agriculture can be seen from 2000 BP. This sediment evidence of early mining or metallurgy during the 9th–15th centuries is supported by a few radiocarbon dates from the excavated mining fields, which on their own were considered as vague or improbable outliers by archaeologists.

Fire as an important factor for the genesis of boreal Picea abies swamp forests in Fennoscandia

The initial establishment of Picea abies in Sweden and Norway on a landscape level, between 3000 and 1000 years ago, was often preceded by recurrent fire and thereafter the influence of fire decreased. However, in some swamp forests, the absence of fire over the last 3500 years has promoted the continuous presence of deciduous trees, i.e. Picea has not established although it has been present regionally for over 3000 years. Our objective was to study long-term vegetation development and fire history in a Picea swamp forest located close (c. 600 m) to a deciduous swamp forest with a documented fire-free history in northernmost Sweden. The study included analyses of charred particles, pollen and ignition residues. Principal component analysis was applied to identify major changes in the pollen spectra. Our results showed that the current Picea swamp forest has developed from a deciduous fen and that fires affected the fen between 6700 and 2300 cal. yr BP. Picea abies established on the fen around 2200 cal. yr BP, following the last local on-site fire. The main factors responsible for the local vegetation development have been: fire (6700 to 2300 cal. yr BP); autogenous processes and climate (2300 to 1000 cal. yr BP); autogenous processes or anthropogenic impact (1000 to 300 cal. yr BP); anthropogenic impact through selective cutting and grazing (300 to 100 cal. yr BP); and autogenous processes and grazing (100 cal. yr BP to present). We conclude that fire facilitated the initial Picea abies establishment. Once established, Picea abies created local conditions that in combination with a colder and wetter climate prevented fire and the establishment of other tree species.