Greger Hörnberg

HABITAT QUALITIES VERSUS LONG-TERM CONTINUITY AS DETERMINANTS OF BIODIVERSITY IN BOREAL OLD-GROWTH SWAMP FORESTS

Abstract Natural swamp forests are extremely rare in Sweden because of extensive drainage and logging activities. This study reports a total of 517 species (148 vascular plants, 131 mosses, 64 hepatics, 142 lichens and 32 wood-inhabiting fungi) from 10 small remnants of boreal old-growth swamp forests. The 195 bryophytes found represent 33% of the total number of bryophyte species in the Swedish boreal flora. As the average area of the forests was only 2ha, this figure clearly illustrates the important role of the swamp forests as centres of bryophyte biodiversity. There was no relationship between the occurrence of 33 lichen and fungi species considered to indicate natural forests with long stand continuity and the known long-term continuity of the forests. The forests most affected by earlier natural and man-made fires harboured more of these species than forests less affected by fire disturbance. The most important variable explaining biodiversity as well as presence of ‘continuity indicator species’ was the amount of dead wood present. A more critical and careful use of the concept of forest continuity is needed.

Stand dynamics, regeneration patterns and long-term continuity in boreal old-growth Picea abies swamp-forests

. The occurrence of macrofossil charcoal (long axis > 0.5 mm) and Picea abies (Norway spruce) pollen in peat stratigraphies, in combination with size and age data from 2976 P. abies trees were used to analyse ecosystem continuity and stand-structure in ten old-growth swamp-forests in northern Sweden. All stands were dominated by P. abies, a species whose abundance increased westwards in Sweden between 3000 and 2000 yr B.P. In three stands no macrofossil charcoal was found and the maximum age of the peat, determined by 14C dating, varied from 1800 to 3600 yr B.P. In the other seven stands the number of levels containing charcoal varied from 1 to 23, but only between 1 and 7 levels were found after the appearance of spruce. Here the maximum age of the peat varied from 400 to 7900 yr B.P. The ten stands had an all-sized stand structure and a stand continuity of ca. 300 yr. The shape of the age structure was similar to an inverse J-curve. This indicates a continuous recruitment over time in a self-perpetuating ecosystem. In a short-term perspective (< 300 yr), the swamp-forests are characterized by individual trees continually emerging while others are dying. it is suggested that internal dynamics of continuous small-scale disturbances in combination with local site-specific factors determine the structure of these forests. in a long-term perspective, some of the present spruce swamp-forests within the northern boreal zone have functioned as true fire-free refugia since the establishment of P. abies populations while others have been affected by recurring fires, although not as frequently as forests on surrounding drier sites. The hypothesis that Scandinavian spruce swamp-forests in general have functioned as true longterm fire-free refugia is thus modified by the present results.

Long-term fire frequency not linked to prehistoric occupations in northern Swedish boreal forest

Knowledge of past fire regimes is crucial for understanding the changes in fire frequency that are likely to occur during the coming decades as a result of global warming and land-use change. This is a key issue for the sustainable management of forest biodiversity because fire regimes may be controlled by vegetation, human activities, and/or climate. The present paper aims to reconstruct the pattern of fire frequency over the Holocene at three sites located in the same region in the northern Swedish boreal forest. The fire regime is reconstructed from sedimentary charcoal analysis of small lakes or ponds. This method allows fire events to be characterized, after detrending the charcoal influx series, and allows estimation of the time elapsed between fires. The long-term fire regime, in terms of fire-free intervals, can thus be elucidated. At the three sites, the mean fire-free intervals through the Holocene were long and of similar magnitude (approximately 320 years). This similarity suggests that the ecological processes controlling fire ignition and spread were the same. At the three sites, the intervals were shorter before 8600 cal yr BP (calibrated years before present), between 7500 and 4500 cal yr BP, and after 2500 cal yr BP. Geomorphological and vegetation factors cannot explain the observed change, because the three sites are located in the same large ecological region characterized by Pinus sylvestris-Ericaceae mesic forests, established on morainic deposits at the same elevation. Archaeological chronologies also do not match the fire chronologies. A climatic interpretation is therefore the most likely explanation of the long-term regional pattern of fire. Although recent human activities between the 18th and the 20th centuries have clearly affected the fire regime, the dominant factor controlling it for 10000 years in northern Sweden has probably been climatic.

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.

Long-Term Human Impact and Vegetation Changes in a Boreal Forest Reserve: Implications for the Use of Protected Areas as Ecological References

Northern boreal forest reserves that display no signs of modern forest exploitation are often regarded as pristine and are frequently used as ecological reference areas for conservation and restoration. However, the long-term effects of human utilization of such forests are rarely investigated. Therefore, using both paleoecological and archaeological methods, we analyzed temporal and spatial gradients of long-term human impact in a large old-growth forest reserve in the far north of Sweden, comparing vegetational changes during the last millennium at three sites with different land use histories. Large parts of the forest displayed no visible signs of past human land use, and in an area with no recognized history of human land use the vegetation composition appears to have been relatively stable throughout the studied period. However, at two locations effects of previous land use could be distinguished extending at least four centuries back in time. Long-term, but low-intensity, human land use, including cultivation, reindeer herding and tree cutting, has clearly generated an open forest structure with altered species composition in the field layer at settlement sites and in the surrounding forest. Our analysis shows that past human land use created a persistent legacy that is still visible in the present forest ecosystem. This study highlights the necessity for ecologists to incorporate a historical approach to discern underlying factors that have caused vegetational changes, including past human activity. It also indicates that the intensity and spatial distribution of human land use within the landscape matrices of any forests should be assessed before using them as ecological references. The nomenclature of vascular plants follows Krok and Almquist (Svensk flora. Fanerogamer och ormbunksväxter, 2001).

Isostatic land uplift and Mesolithic landscapes: lake-tilting, a key to the discovery of Mesolithic sites in the interior of Northern Sweden

Until recently only a few Mesolithic sites were known from the interior of N. Sweden, although extensive archaeological surveys have been carried out since the 1950s. The lack of archaeological data made every attempt to interpret the process of pioneer colonization quite fruitless. In this paper we present a model of non-uniform glacio-isostatic uplift and lake-tilting used to identify potential areas of Mesolithic habitation. By reconstructing shoreline displacement of ancient lakes, archaeological, palaeoecological and geological studies have resulted in the discovery of a significant number of Mesolithic sites and of an early post-glacial landscape previously unknown.

A thousand years of human impact in the northern Scandinavian mountain range: Long-lasting effects on forest lines and vegetation:

The vegetation history has been examined by pollen, microscopic and macroscopic charcoal analyses in the vicinity of two alpine Stállo settlement sites and one forested ‘Reference area’ with no settlement remains in northwestern Sweden in the northern part of the Scandinavian mountain range. The acquired data provide indications of the effects of human activities on local vegetation during the settlement phase, and on ecological processes in both long- and short-term perspectives. The results show that one settlement site, Gieddeålge, was established just above the forest line in the Betula treeline zone, and the other, Varenodjukke, in the Betula forest. At Gieddeålge an increase in herbs and graminoids appears to have occurred c. 700 cal. BP (during the thirteenth century) suggesting that the vegetation was altered by nutrient addition, most likely related to human activities associated with the settlement. At Varenodjukke, the Betula trees around the settlement were cut down and the site developed into an alpine heath dominated by Betula nana, which still characterizes the site today. In the Reference area evidence of changes to the vegetation probably caused by human impact was also found from c. 3500 to 700 cal. BP. In conclusion, the vegetation currently present at Gieddeålge and Varenodjukke has been strongly influenced by previous human activities, which have clearly had profound effects, both short and long term, on the composition of the vegetation and the properties of the ecosystems in these areas. Further, when studying processes, such as anticipated global warming that will influence future patterns of vegetation in these forest-line areas, legacies of land use must be taken into consideration.

Vegetation dynamics and disturbance history in three deciduous forests in boreal Sweden

Abstract Remaining deciduous forests in the Fennoscandian boreal landscape have high ecological value, and are considered as key components of the forest landscape as well as remnants of a former natural forest type. To improve our understanding of the formation of deciduous forests, we studied past disturbance regimes and vegetation dynamics in three deciduous forests in boreal Sweden using dendro-ecology, pollen analysis and charcoal analysis. We identified three stages in the development of the studied stands. Firstly, the coniferous period (pre 1800), a long-lasting period characterized by frequent fires, livestock grazing and extensive agriculture during which Pinus sylvestris was dominant. Secondly, the transformation period (1800 - 1900), when logging removed most pines from the sites while fire and grazing continued. At the time of the last fire, the sites lacked a local seed source of pines, resulting in a post-fire succession dominated by deciduous species with the capacity to disperse over long distances. Thirdly, the deciduous period (1900 - present), with little or no disturbance from fire, grazing or logging. Thus, the present deciduous stands have their origins in a complex interaction between changes in fire regime, extensive land use patterns and logging, contrary to earlier simplified explanations. We conclude that the complexity of historical patterns of land use, vegetation dynamics and disturbance should be acknowledged in the future when selecting areas for nature conservation and developing models for ecologically oriented forestry. Nomenclature: Lid (1985).

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.