Mikael Ohlson

Interpretation of the charcoal record in forest soils : forest fires and their production and deposition of macroscopic charcoal

Traps were used to quantify charcoal production and transport during three experimental forest fires in Boreal Scandinavia. The traps were spatially arranged to collect charcoal particles inside burn areas, and outside burn areas at different distances (0.1–100 m) from the fire edge. The number of inside and outside traps was 280 and 424, respectively. Trap area was 48 cm2. After the burn, trap content was sorted and sieved in two size-classes of charcoal particles, namely small (0.5–2.0 mm) and large (. 2.0 mm), and number and mass of particles were determined. The production and distribution of charcoal were highly variable at fine spatial scales inside burn areas. On average, inside traps contained 12.1 small and 10.1 large particles, and the average charcoal mass was 0.112 g per trap (corresponding to 235 kg ha-1). The largest size-class made up 94% of the mass. Outside traps contained 0.3 small and 0.1 large particles per trap, and 45% of the outside particles were distributed, 1 m from the fire edge. It is concluded that the occurrence of macroscopic charcoal (

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

0.5 mm) in forest soils provides a solid evidence for local fire influence, and that the presence of large charcoal particles can be used to distinguish between fire-prone and fire-free areas with high spatial precision. Absence of large particles must, however, be more carefully interpreted as 14% of the inside traps lacked macroscopic charcoal. We argue that the charcoal in Boreal forest soils should be less persistent than previously suggested because documented fire-return intervals result in an unrealistic charcoal accumulation presupposing high persistence.

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

Towards a global assessment of pyrogenic carbon from vegetation fires

. 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.

Growth of epiphytic old forest lichens across climatic and successional gradients

The production of pyrogenic carbon (PyC; a continuum of organic carbon (C) ranging from partially charred biomass and charcoal to soot) is a widely acknowledged C sink, with the latest estimates indicating that ~50% of the PyC produced by vegetation fires potentially sequesters C over centuries. Nevertheless, the quantitative importance of PyC in the global C balance remains contentious, and therefore, PyC is rarely considered in global C cycle and climate studies. Here we examine the robustness of existing evidence and identify the main research gaps in the production, fluxes and fate of PyC from vegetation fires. Much of the previous work on PyC production has focused on selected components of total PyC generated in vegetation fires, likely leading to underestimates. We suggest that global PyC production could be in the range of 116-385 Tg C yr(-1) , that is ~0.2-0.6% of the annual terrestrial net primary production. According to our estimations, atmospheric emissions of soot/black C might be a smaller fraction of total PyC (<2%) than previously reported. Research on the fate of PyC in the environment has mainly focused on its degradation pathways, and its accumulation and resilience either in situ (surface soils) or in ultimate sinks (marine sediments). Off-site transport, transformation and PyC storage in intermediate pools are often overlooked, which could explain the fate of a substantial fraction of the PyC mobilized annually. We propose new research directions addressing gaps in the global PyC cycle to fully understand the importance of the products of burning in global C cycle dynamics.

Effects of an exotic plant and habitat disturbance on pollinator visitation and reproduction in a boreal forest herb

This paper aims to assess the influence of canopy cover on lichen growth in boreal forests along a regional forest gradient. Biomass and area gain, and some acclimation traits, were assessed in the ...

BOREAL SWAMP FORESTS : BIODIVERSITY HOTSPOTS IN AN IMPOVERISHED FOREST LANDSCAPE

The invasion of exotic species into natural habitats is considered to be a major threat to biodiversity, and many studies have examined how exotic plants directly affect native plant species through competitive interactions for abiotic resources. However, although exotics can have potentially great ecological and evolutionary consequences, very few researchers have studied the effect of exotics on the interactions between plants and their mutualistic partners, such as pollinators, and none have reported on such impacts in logged and undisturbed boreal forest ecosystems. Here we show how experimental introductions of an exotic plant species (Phacelia tanacetifolia Bentham) affect pollinator visitation and female reproductive success of a native plant (Melampyrum pratense L.) in recently disturbed (i.e., logged) and in undisturbed boreal forest habitats. The presence of Phacelia significantly increased the number of bumble bees entering plots in both habitat types. However, the exotic species had a strong negative impact on the visitation rate to the native species in both habitat types. Despite this negative impact on pollinator visitation, the exotic had no effect on female reproductive success of the native species in any habitat. Our results show that seed production may be more robust than pollinator visitation to exotic invasion, irrespective of habitat disturbance history.

SIZE-DEPENDENT REPRODUCTIVE EFFORT IN THREE POPULATIONS OF SAXIFRAGA HIRCULUS IN SWEDEN

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

SPATIAL VARIATION IN RATES OF CARBON AND NITROGEN ACCUMULATION IN A BOREAL BOG

SUMMARY (1) The perennial herbaceous mire plant Saxifraga hirculus was studied in five localities, chosen to represent a wide range of habitats occupied by this species in Sweden. Reproductive effort, estimated as seeds produced g- I vegetative biomass, was determined once a year for three years. (2) Size-dependent patterns in reproductive effort occurred within four of the five habitats studied. Small ramets had a reproductive effort that was up to four times as high as the effort in large ramets. (3) Mean reproductive effort, determined from ramets with matched size distributions, was highly variable both between habitats and years. In one habitat, the between-year variation was more than two-fold. In total, the reproductive effort varied by a factor of eight, ranging from 312 to 2538 seeds produced g- I vegetative biomass. The minimum was obtained under low population density (c. 3 fertile ramets m - 2) in an intermediate fen, and the maximum under high population density (c. 120 fertile ramets m-2) in a rich fen.

Seasonal trends in the biomass and structure of bryophyte-associated fungal communities explored by 454 pyrosequencing

Although previous studies hint at the occurrence of substantial spatial variation in the accumulation rates of C and N in bogs, the extent to which rates may vary on high-resolution spatial and temporal scales is not known. A main reason for the lack of knowledge is that it is problematic to determine the precise age of peat at a given depth. We determined rates of carbon and nitrogen accumulation in the uppermost decimeters of a bog ecosystem using the pine method, which enables accurate dating of surface peat layers. We combined accumulation data with numerical and geostatistical analyses of the recent vegetation to establish the relationship between bog vegetation and rate of peat accumulation. Use of a laser technique for spatial positioning of 151 age-determined peat cores within a 20 × 20 m plot made it possible for us to give the first fine-scaled account of spatial and temporal variation in rates of mass, carbon, and nitrogen accumulation during the last century. Rates of C and N accumulation were...