Leif Björkman

Late-Glacial and Holocene forest dynamics at Steregoiu in the Gutaiului Mountains, Northwest Romania

Pollen analyses and AMS C-14 measurements were performed on lacustrine sediments and peat deposits from the former crater lake Steregoiu in a mid-altitude area in the Gutaiului Mountains in NW Romania in order to provide a detailed reconstruction of the vegetational development of the area during the Late-Glacial and Holocene. The bottom sediments are siliciclastic and were deposited probably during the Full Glacial, which is characterised by open patchy vegetation consisting of a mosaic of shrubs, herbs, and ungrown grounds. Around 14 700 cal yr BP an open Pinus-Betula forest expanded, at the same time as organic-rich lake sediments started to be deposited. This change was likely a response to the first deglacial warming phase at the beginning of the Boiling period, or GI-1e, in the GRIP event stratigraphy. Between 13 750 and 12 950 cal yr BP an open forest dominated by Betula, Picea, Pinus, and Ulmus developed in the area. The forest composition implies rather warm climatic conditions, which may correspond to the Allerod, or GI-1c-GI-1a. Around 12950 cal yr BP the forest retracted, and at 12600 cal yr BP open patchy vegetation became re-established. These changes imply a return to significantly colder conditions, which may correspond to the Younger Dryas, or GS-1. At 11500 cal yr BP Betula, Pinus, and Alnus quickly responded to the temperature rise, characterising the beginning of the Holocene. Within a few hundred years, around 11300 cal yr BP, Ulmus and Picea became re-established and a mixed forest type expanded. Quercus, Tilia, and Fraxinus likely arrived at around 10 750 cal yr BP, and from about this time Betula, and Pinus started to diminish. Around 10 500 cal yr BP a dense mixed forest dominated by Ulmus, Picea, Quercus, Tilia, and Fraxinus had developed in the area. Corylus probably became locally established around 10 500 cal yr BP, but it did not expand until 10 200 cal yr BP. Tilia and Fraxinus were locally important until 8600 cal yr BP, when Picea gained renewed importance and the lake became completely overgrown. Picea was probably an important constituent in the carr forest at the site as well as in the upland forest. Around 4800 cal yr BP Fagus and Carpinus became important in the local stand, apparently without any major disturbance of the forest. Until around 3400 cal yr BP the forest was highly diverse, but when Fagus eventually expanded, Ulmus, Tilia, Picea, and Corylus diminished. From about 2200 cal yr BP onwards Fagus has been the local forest dominant, but some Quercus and Carpinus individuals have been present as well. Pollen evidence for human influence on the local vegetation is comparatively scant. The area may have been used for forest grazing from about 1050 cal yr BP, but the grazing pressure was probably low until ca. 300 cal yr BP

Pollen-based quantitative reconstructions of Holocene regional vegetation cover (plant-functional types and land-cover types) in Europe suitable for climate modelling

We present quantitative reconstructions of regional vegetation cover in north-western Europe, western Europe north of the Alps, and eastern Europe for five time windows in the Holocene [around 6k, 3k, 0.5k, 0.2k, and 0.05k calendar years before present (bp)] at a 1° × 1° spatial scale with the objective of producing vegetation descriptions suitable for climate modelling. The REVEALS model was applied on 636 pollen records from lakes and bogs to reconstruct the past cover of 25 plant taxa grouped into 10 plant-functional types and three land-cover types [evergreen trees, summer-green (deciduous) trees, and open land]. The model corrects for some of the biases in pollen percentages by using pollen productivity estimates and fall speeds of pollen, and by applying simple but robust models of pollen dispersal and deposition. The emerging patterns of tree migration and deforestation between 6k bp and modern time in the REVEALS estimates agree with our general understanding of the vegetation history of Europe based on pollen percentages. However, the degree of anthropogenic deforestation (i.e. cover of cultivated and grazing land) at 3k, 0.5k, and 0.2k bp is significantly higher than deduced from pollen percentages. This is also the case at 6k in some parts of Europe, in particular Britain and Ireland. Furthermore, the relationship between summer-green and evergreen trees, and between individual tree taxa, differs significantly when expressed as pollen percentages or as REVEALS estimates of tree cover. For instance, when Pinus is dominant over Picea as pollen percentages, Picea is dominant over Pinus as REVEALS estimates. These differences play a major role in the reconstruction of European landscapes and for the study of land cover-climate interactions, biodiversity and human resources.

Lateglacial and early Holocene vegetation development in the Gutaiului Mountains, northwestern Romania

Pollen analyses and AMS 14 C measurements were performed on lacustrine sediments and peat sequences from two former crater lakes (Preluca Tiganului, Steregoiu) situated in the Gutaiului Mountains in northwestern Romania, in order to reconstruct the vegetation development during the Lateglacial and Early Holocene. Before c. 14,700 cal. years BP (GS-2) open vegetation with shrubs (Salix, Juniperus), grasses and herbs (e.g. Artemisia and Chenopodiaceae) prevailed in the area. Around c. 14,700 cal. years BP and coinciding with the beginning of GI-1e, Pinus expanded, and patchy vegetation with scattered Pinus stands developed. The last 100 years of GI-1e (14,150–14,050 cal. years BP) were characterised by an increase in Pinus and Betula and an expansion of open forest communities. This development was shortly interrupted between 14,050 and 13,800 cal. years BP (likely corresponding to the cold phase GI-1d), when the tree cover became reduced and open vegetation with scattered Pinus individuals became frequent. The period with a significant expansion of Betula and Picea and the formation of an open forest (including Pinus and Ulmus), which took place between 13,800 and c. 12,950 cal. years BP, is tentatively correlated with GI-1c-a. A renewed reduction in tree cover (decrease of Picea and Betula, disappearance of Ulmus) started at B12,950 cal. years BP and at 12,600 cal. years BP forest stands were rapidly replaced by open vegetation communities with low shrubs (Salix, Juniperus), grasses and herbs (e.g. Artemisia and Chenopodiaceae). The period between 12,900 and 11,500 is correlated with the cold phase GS-1. At 11,500 cal. years BP, most likely as a response to the warmer climatic conditions at the beginning of the Holocene, an expansion of Betula and Alnus and, slightly later, also of Ulmus can be observed. Between 11,500 and 11,250 cal. years BP, open forests with Betula, Pinus and Ulmus were widespread in the area. At 11,250 cal. years BP dense forests dominated by Ulmus replaced the open forest type. Around 10,700 cal. years BP Quercus, Tilia and Fraxinus expanded strongly, and Acer and Corylus became established. Mixed deciduous forest with Picea dominated the upland vegetation between 10,700 and 10,150 cal. years BP. At 10,150 cal. years BP Corylus increased significantly and between 10,150 and 8500 cal. years BP, dense mixed deciduous forests with Picea and Corylus were abundant in the area. Climatic fluctuations seem to have been the driving force behind vegetation changes during the Lateglacial period, while the forest development during the Early Holocene was mainly driven by migrational and successional processes. r 2002 Elsevier Science Ltd. All rights reserved.

A long-term record of Quercus decline, logging and fires in a southern Swedish Fagus-Picea forest

Abstract We reconstructed forest development and disturbance events (fire and logging) during the last 1000 yr with tree-ring data, pollen and charcoal analysis from a semi-natural Fagus sylvatica-Picea abies forest (ca. 1 km2) in the hemiboreal zone. According to pollen analysis, Quercus robur together with Pinus sylvestris was abundant in the forest until the turn of the 18th/19th centuries when these species disappeared completely (Quercus) or nearly completely (Pinus) and were replaced by Fagus and Picea. The disappearance of Quercus was corroborated by the remarkable discovery of a single Quercus stump that had been cut in the 18th century and had become overgrown and preserved by a very old Picea. In total 11 fires were dated from 1555 to 1748 from fire scars in several Pinus stumps cut 100 - 200 yr ago. Since the last fire in 1748, no Quercus or Pinus have regenerated in the core of the reserve apart from single pines in neighbouring managed forest (80 yr ago). During the period of documented fires Fagus was protected from fires in a refuge made up of large boulders. Picea colonized the region at the time when the fires ceased 250 yr ago. We hypothesize that most of the fires were probably of human origin because of their patchiness and high frequency compared to the natural background levels of lightning ignitions in the region. On a 300-yr time scale, logging and fire suppression seem to strongly overshadow the effect of climate change on forest composition and dynamics. Nomenclature: Tutin et al. (1964–1976). Abbreviation: LID = Lightning Ignition Density.

The establishment of Fagus sylvatica at the stand-scale in southern Sweden

Pollen analysis was carried out on peat profiles from small forest hollows at four sites in southern Sweden in order to investigate the establishment of Fagus sylvatica at the stand-scale. Viewed on a continental scale the migration pattern of Fagus can be correlated with climatic change, but at finer scales such a correlation is weaker. At the stand-scale there are factors other than climate that are crucial for the establishment of Fagus(e.g., disturbance, seed dispersal, human activities). The establishment of Fagus does not show any regional coherence in southern Sweden, and this may imply that climate was not the limiting factor for its establishment. Fagus seeds are highly dependent on ground disturbance for successful establishment. A semi-open cultural landscape may be optimal for Fagus establishment, as cultural activities may create conditions particularly suitable for its regeneration. At two of the studied sites cultural activities probably created conditions that favoured the establishment of Fagus at 400 and 900 BP respectively. At the other sites the local forest stands were relatively unaffected by cultural activities prior to the establishment of Fagus at 950 BP and 1450 BP respectively, where, fires (natural?) or slight human influence were effective. The present-day distribution of Fagus in southern Sweden suggests migration with a discontinuous front and outlying populations, and this model probably applies to its past distribution. This type of migration means that the landscape becomes filled in by dispersal from outlying stands. The timing of stand-scale establishment is then largely influenced by site-specific factors and chance. Fagus may still be migrating northwards in Sweden. It grows well in its outlying area, and it seems that present-day land use, not climate, is the limiting factor for the local expansion of Fagus. The northern distribution limits of Fagus probably still represent an active front, and outlying stands act as‘infection centres’.

Long-term population dynamics of Fagus sylvatica at the northern limits of its distribution in southern Sweden: a palaeoecological study

Regional pollen diagrams indicate that Fagus may have been present close to its northern distri butional limits in southern Sweden for as long as 1000 years. Fagus has probably always been rare in this region, except for some small and scattered stands. Many of these outlying Fagus stands have been regarded as planted but no previous studies have investigated this in detail. At Mattarp, a small outpost Fagus stand in northern Småland, the pollen record of the deposit of a small hollow indicates that Fagus became established c. 400 BP. The local establishment was associated with an earlier phase of woodland clearance and the former system of land-use. Local factors seem to have controlled the immigration and establishment at the stand-scale at this site. Regional climate had probably been favourable for Fagus for almost 2000 years and cannot be regarded as the direct cause for establishment at this site. The complex south Swedish cultural landscape may have favoured Fagus establishment in many areas, particularly when there has been a temporary reduction in human influence. Picea became regionally established around 800 BP, but a local expansion did not occur until c. 400 BP, coinciding with the establishment of Fagus. Picea expansion also seems to have been favoured by a moderate human influence at this site. Fagus pollen influx values have steadily increased since the time of establishment. It seems likely that the Fagus stand has continually increased in size, and probably would continue to do so if this were not prevented by human activities.

The history of Fagus forest in southwestern Sweden during the last 1500 years

In the recent past, Fagus forests in southwestern Sweden were more common and widespread than today. The Fagus forest area has decreased considerably due to cultural activities, mainly during the eighteenth and nineteenth centuries. The pollen records of two small adjacent forest hollows (lying c. 50 m apart) in the Bocksten area in central Halland indicate that the pre-Fagus forest was of a rich memoral type with many woody taxa present. Quercus, Tilia, Alnus, and Corylus were important components of this forest type. Fagus became established in the area around 1450 14C years BP, and rapidly became dominant in the forest. The expansion of Fagus was remarkably rapid, and probably facilitated by slight human disturbance. The rather pure Fagus stands found today in the area are a product of recent human activities. The high pollen percentages of Tilia at one of the sites (site A) until 200 BP is remarkable, and stands in contrast to the picture seen in regional-scale pollen diagrams from Sweden. At this site, Tilia grew in a mixture with Fagus, Quercus and Corylus for almost 1200 years until the stand was suddenly cleared c. 200 BP. At the other site (B), Tilia shows a gradual decrease beginning before the establishment of Fagus, which is in agreement with the regional picture. It is evident that stand-level differences in vegetation is not possible to detect with conventional pollen diagrams based on regional-scale sites. Different grazing regimes, or human influence, may have caused these stand-scale differences in vegetation. The boreo-nemoral forest type found today in the studied area is domi nated by intensively managed Picea plantations and Fagus stands. This forest type has little resemblance to the vegetation that existed during earlier periods.

The role of human disturbance in the local Late Holocene establishment of Fagus and Picea forests at Flahult, Western Småland, Southern Sweden

A pollen record from a small alder carr located in the centre of aFagus stand near the hamlet of Flahult in southern Småland has shown thatFagus became established in a semi open cultural landscape about 900 B.P. Human disturbance seems to have controlled the local establishment ofFagus at this site through an expansion of pastoral farming. TheFagus dominance in the present stand seems to be of recent origin, asFagus pollen percentages and influx values have increased considerably only during the last 50 years. The modern composition and structure of theFagus stand are probably an effect of changes in land-use and decreased human activity at the end of the last century. Today, only occasionalPicea individuals occur in the studied stand, andPicea does not appear to have been more abundant in the recent past. The regional expansion ofPicea has probably occurred during this century and has been favoured by modern forestry during the last 50 years.

The role of human disturbance in Late Holocene vegetation changes on Kullaberg, southern Sweden

Abstract. A pollen record from a small peatland located on Kullaberg in northwestern Skåne, southern Sweden, revealed that grazing pressure might have been the major disturbance agent controlling the vegetation, at least from ca. 1500 B. C. until the latter part of the nineteenth century. The pollen data also indicate a step-by-step increase in grazing pressure, expressed as a marked increase in non-arboreal pollen representation at ca. 1500 B. C., A. D. 650 and A. D. 1650. The increase at A. D. 650 probably indicates a more regular and intensive use of the area, resulting in a forest structure that was much more open than earlier, together with an expansion of Fagus, which rapidly replaced Quercus as the local forest dominant. At about this time the first patches of heath vegetation originated, but they probably only covered a small part of Kullaberg. Larger areas with an intensively grazed Calluna heath, as shown by eighteenth century maps, probably evolved around ca. A. D. 1650, when much of Kullaberg seems to have been deliberately deforested.

A PALEOECOLOGICAL RECONSTRUCTION OF THE LATE GLACIAL AND HOLOCENE BASED ON MULTIDISCIPLINARY STUDIES AT STEREGOIU SITE (GUTÂI MTS., NW ROMANIA)

High resolution analyses of pollen, mineral magnetic properties, loss of ignition, lithostratigraphy and AMS 14 C measurements of lake sediments and peat deposits accumulated in the former crater lake of Steregoiu (Gutâiului Mts., NW Romania), gave new and important information about vegetation and climate changes from the period GS-2 to the present. During the Lateglacial, three cold events were recorded: before 14,700 cal. years BP (GS-2), 14,050- 13,800 cal. years BP (GI-1d), 12,900-11,500 cal. years BP (GS-1), and a warm climatic event between 13,800-12,950 cal. years BP (GI-1c to GL-1a). The Late Glacial/Holocene transition around 11,500 cal. years BP, was determined by an expansion of Betula, Alnus and Picea, followed by a rapid and strong expansion of Ulmus. At 10,700 cal. years BP, dense and highly diverse forests with Ulmus, Quercus, Tilia, Fraxinus and a few Acer and Corylus individuals dominated the area. Corylus and Picea were the dominant species in the forests from 10,150 to 8,500 cal. years BP. The first occurrence of single Fagus pollen grains was around 8,000 cal years BP. Only at 4,700 cal year BP Fagus and Carpinus became widespread and established trees in the local woodlands.