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Despite increasing concern over wildfires in Fennoscandia, there are essentially no studies on the survivability of buildings within the wildland-urban interface of this region. We make use of four recent large-scale fires in Sweden to elucidate which factors are important for survival, using multiple logistic regression analysis of data collected at the sites. We obtained data on 187 buildings within the fire perimeters, nearly all with wood paneling and tile- or sheet metal roofing. 35 % of the buildings were lost or badly damaged. Results indicate that most buildings were approached by relatively low-intensity fire and that ignition primarily occurred through direct flame contact. The most important factor for survivability was the presence of a maintained lawn. The second most important was that no flammable material was present close to the building facade. Further, fire intensity often decreased close to buildings due to a larger portion of deciduous trees around gardens than in the surrounding forest. These factors were more important than specific features of the building itself, reflecting that the majority of buildings have combustible wooden facades. Our results suggest that the greatest potential for increasing building safety in the Swedish WUI is to keep the area immediately surrounding the building (similar to 5 m) free from tree litter and other flammable material. Also, since fire intensities are generally low, buildings can in most cases be defended with simple tools without compromising personal safety.

Aims Tropical tree lines are often associated with abrupt shifts in vegetation, soils and disturbance regimes, but the underlying mechanisms are poorly understood. We analysed the role of grazing, fuels and fire in maintaining a sharp tree line with flammable heathland above non-flammable forest. Location Bale Mountains, Ethiopia. Methods The study used grazing exclosures, repeated vegetation sampling, soil analyses and burning and sowing experiments along an altitudinal gradient withHagenia abyssinicaforest,Erica trimeraforest andEricaheathland; all were heavily grazed, the Erica heathland also burnt on short rotation. Results Contrary to expectation, livestock exclusion did not increase flammability in the forest, but instead resulted in a dense carpet of non-flammable herbs. In the heathland, livestock exclusion led to somewhat faster post-fire fuel recovery, but no major change in vegetation. Seeding of tree species resulted in some seedling establishment, but notablyHageniagrew poorly in the heathland, even when protected from livestock. A bioassay, as well as observations of outpost trees on atypical soil above the tree line, suggests that this poor growth is caused by the acidic soils, rather than harsh climate. Despite frequent fires, heathland soils had lower pH and higher organic matter content than forest soils. Below the tree line, tree seedling establishment was successful only in forest gaps, and if livestock was excluded. In both forest and heathland rapid vegetative regeneration in the ground flora after disturbance restricted major species shifts. Conclusions These results suggest that the contrasting fire potential between heathland and forest, and thus the sharp tree line would be maintained, or possibly even accentuated, in the absence of livestock grazing, and thatHageniacolonization upwards into the heathland is restricted not only by fire and grazing, but also the acidic soils, which is a legacy of centuries of dominance byErica.

Deepened knowledge on response of biota and ecological processes following fire is essential for a future with warmer climate and more disturbances. In 2014 the first mega-fire (13,100 ha) for at least a century in Scandinavia hit south-central Sweden, in a production forest landscape shaped by clearcutting forestry. Ecological dynamics is followed in >20 projects from universities, authorities and citizen science initiatives, rapidly accumulating substantial amounts of data. We outline projects and summarize their results during the first four years, demonstrating a rapid succession of fungi, lichens, vascular plants, birds, mammals, ticks, butterflies, beetles, and drastically altered carbon dynamics. We characterize forest operations including regeneration measures and point to patterns in pest and pathogen infestations. 8,000 ha is set aside for natural succession, with the rest harvested and managed for forest production, offering excellent opportunities for studies on salvage logging effects, already evident for birds. We demonstrate a strong regrowth of deciduous trees, and the protected part will in some decades likely develop into the largest deciduous-dominated area in boreal north Europe outside Russia. Continued studies of biodiversity and ecological processes are urgent for this unique area.

Massive tree-felling due to root damage is a common fire effect on burnt areas in Scandinavia, but has so far not been analyzed in detail. Here we explore if pre- and post-fire lidar data can be used to estimate the proportion of fallen trees. The study was carried out within a large (14,000 ha) area in central Sweden burnt in August 2014, where we had access to airborne lidar data from both 2011 and 2015. Three data-sets of predictor variables were tested: POST (post-fire lidar metrics), DIF (difference between post- and pre-fire lidar metrics) and combination of those two (POST_DIF). Fractional logistic regression was used to predict the proportion of fallen trees. Training data consisted of 61 plots, where the number of fallen and standing trees was calculated both in the field and with interpretation of drone images. The accuracy of the best model was tested based on 100 randomly selected validation plots with a size of 25 × 25 m. Our results showed that multi-temporal lidar together with field-collected training data can be used for quantifying post-fire tree felling over large areas. Several height-, density- and intensity metrics correlated with the proportion of fallen trees. The best model combined metrics from both datasets (POST_DIF), resulting in a RMSE of 0.11. Results were slightly poorer in the validation plots with RMSE of 0.18 using pixel size of 12.5 m and RMSE of 0.15 using pixel size of 6.25 m. Our model performed least well for stands that had been exposed to high-intensity crown fire. This was likely due to the low amount of echoes from the standing black tree skeletons. Wall-to-wall maps produced with this model can be used for landscape level analysis of fire effects and to explore the relationship between fallen trees and forest structure, soil type, fire intensity or topography.

Years with climatically mediated increases in boreal forest fire activity, referred to as large fire years (LFYs), contribute to a disproportionally large portion of the burned area over centuries, and are important drivers of ecosystem processes by affecting forest structure, biodiversity, and carbon balance at regional and continental scales. We analysed changes in LFY return intervals in northern Sweden (the area above 60 °N) over 1273–1960 using a network of 29 sites with dendrochronologically reconstructed fires, complemented by documentary records of fires available from forestry statistics. We observed large variability in return intervals of LFYs, an increase in LFY frequency during the 1800 s, and consistent associations between LFY occurrence and 500-hPa pressure anomalies over the European sub-continent over 1800xps2#1960. An increase in LFY frequency during the 1800 s might be climatically driven, and would thus long precede the period of likely human-induced climatic changes of the 1900 s. Long-term variability in climatically driven LFYs may present a challenge in partitioning the effects of human-related and human-independent components of climatic forcing upon forest fire activity.

Background matching might lower the risk of seeds being eaten by seed predators that search visually. In aviary experiments, we analyzed the selection of diff erent-colored seeds by ground-feeding finches (Fringillacoelebs and F.montifringilla) against four naturally occurring forest soil substrates. The substrates were fresh burn (black), 6-year-old burn (brown), mineral soil (pale yellow) and Pleuroziumschreberi feather moss (green). We used color-sorted seeds of Pinussylvestris, a species with a large natural variation in seed color, ranging from pale yellow to black. Although seeds were scattered on the substrates at a density of only 91 seeds m−2, birds removed seeds effectively. Both bird species found more pale than dark seeds on the fresh burn substrate. F. montifringilla also recovered more pale than dark seeds on the old burn, and more dark than pale seeds on mineral soil. In moss, the birds found very few seeds compared to the other substrates, and there was no color discrimination. P.sylvestris is frequently regenerating after fire, suggesting that dark seeds would be favored under selection from visually searching predators. Fire-adapted conifers with serotinous cones, e.g., Pinuscontorta ssp. latifolia, which spread their seeds primarily on freshly burnt surfaces, produce uniformly black or dark brown seeds. However, regeneration of the non-serotinous P.sylvestris is often extended for several years after a fire, during which substrate color and structure change. This may have helped to maintain variation in seed color. When regeneration of a plant species occurs on a substrate of uniform color, we believe that selection by visually searching seed predators will result in the evolution of cryptic seed color.

Summary Shrubland ecosystems are often inherently flammable due to a canopy structure favourable for fire propagation. At the same time, the fuel bed is not spatially uniform, but a complex mix of shrubs and herbaceous vegetation that will change with time since fire. These patterns are further influenced by megaherbivores capable of consuming large quantities of biomass that otherwise would enter the fuel bed, but the net effects for temporal thresholds of flammability are poorly known. We quantified post-fire fuel succession and effects of free-ranging cattle in high-elevation Erica shrublands in Ethiopia where traditional fire management is still practised above the treeline at 3500 m, despite being challenged by authorities. We found a near-linear accumulation of canopy fuel at 2·6 Mg ha−1 year−1, but stands 5 years. But if stands escape fire for several decades, flammability again decreases as canopy fuels become increasingly separated from the ground. This may be the ultimate reason for sharp treelines on many tropical mountains where fire is confined mainly to higher elevations. Synthesis and applications. In shrublands where dominant plants can outgrow consumption by large herbivores, provision of good cattle habitat typically requires fire. We found that fire and cattle interact to maintain a relatively stable system, where fuel limitation in early succession creates fire breaks that prevent landscape-wide wildfires. The same negative feedback protects the Erica from degradation by too frequent fires. As shrublands can have widely different compositions, the response to variation in fire frequency and herbivore pressure is likely to differ, but for sustainable management fire and grazing have to be treated in consort.

Fire ephemerals are few in the boreal forest despite a long history of recurrent fires, which suggests such a life-history pose problems here. We analysed the fate of recruiting populations of two rare and fire-dependent annual Geranium species at burnt forest sites in South-eastern Sweden, to extract vital information on their life-history. Seedlings emerged from the soil seed bank only in the year of fire but spread over several weeks. At sites that burnt early in the season, some seedlings exhibited a summer-annual life-cycle, but those were less successful than plants at the same sites that delayed reproduction until the following year (winter-annuals). Herbivory was frequent in the fire year and until the following spring, but later almost absent, and thus hit seed production in summer-annuals badly. Winter mortality was highly variable for rosette-stage winter-annuals, with some populations nearly obliterated. Reproductive success varied greatly between populations mainly due to pre-reproductive mortality, with a return of 0.2–395 (average 79) seeds per seedling. The vast majority of seeds (92–100 %) were produced by the primary generation, emerging from the seed bank. Out of this first seed crop, 0.2–2.5 % germinated within the study period, resulting in secondary generations. Plants in these later generations were small and produced few seeds, showing that the opportunity for high reproductive success is essentially restricted to one year only. This makes populations highly vulnerable to local near-complete reproductive failure due to winter mortality and herbivory and may be the ultimate reason why strict fire ephemerals are so few in northern forests.

Several large Swedish wildfires during recent decades were caused by forestry machinery in operation, fires for which there is still no characterisation. We combined 18 years of data on dispatches, weather and fire danger and interviewed forestry workers to understand the spatial, temporal and weather distributions of these fires, and their underlying mechanisms. We estimate the average annual number of ignitions from forestry machinery in Sweden at 330–480 (2.0±0.4 ignitions per 1000ha clear-felling) of which 34.5 led to firefighter dispatches, constituting 2.2% of all forest fire dispatches and 40% of area burnt. Soil scarification causes the most ignitions and the main mechanism is likely high-inertia contact between discs and large stones, causing sparks igniting dry humus or moss, countering reports suggesting that such metal fragments cannot fulfil ignition requirements. We found a spatial relationship between forestry machine ignitions and abundance of large stones, represented by a Boulder Index generated from a nationwide dataset. Further, 75% of the dispatches occurred on days with relative humidity 26 and Fire Weather Index >12. 75% of the area burned when Fire Weather Index was >20. Results suggest machine-caused forest fires can be largely avoided by cancelling operations in stony terrain during high-risk weather.

We analyzed critical life-history variables for two rare fire-dependent annual Geranium species in southern Sweden, which are today threatened because of effective fire suppression. At recently burned sites with abundant recruitment, seedlings occurred only where the humus layer had been completely removed by smoldering fire. Emergence depths ranged 1–6 cm in the mineral soil. Soil sampling at four sites revealed that in unburned soil Geranium seeds were located only in the mineral soil. Surprisingly, residual seeds were still present where fire had burned away the humus layer. An experiment showed that both species deposit seeds relatively evenly within a radius of 5–6 m, through ballistic dispersal. Repeated sampling in the field over a 2-year period after seed dispersal at one site indicated a low rate of seed depletion, corroborated by an indoor incubation of seeds. Our results show that successful management of these species depend on deep-burning prescribed fire, which can only result after severe drought. On the other hand, the seed bank is extremely long-lived, as viable seeds were present at a site last burned 200 years ago. This well-protected seed bank will likely buffer against both ill-timed fires and occasional failure in the recruiting seedling populations.

Mammalian herbivores have the potential to alter the competitive relations of woody species, if consumption is unevenly distributed between species. At elevations above 3500 m in the southern Ethiopian highlands, vegetation is dominated by Erica arborea and Erica trimera. Both species can potentially grow into short trees, but are burnt on a rotation of 6 to 10 years, and regenerate by re-sprouting from belowground lignotubers. The regenerating scrub is heavily browsed by cattle. We set up browsing exclosures at three burnt sites to quantify the impact of browsing over a three-year period. When protected from browsing, E. trimera had similar or better height growth than Erica arborea, but in browsed vegetation, Erica arborea instead grew taller. Browsing was more intense on E. trimera in the first years after fire, indicating a difference in palatability between the species. We checked if browse quality differed, by analysing shoot contents of acid detergent fibre, protein, phenolics and tannins. Contrary to expectations the preferred E. trimera contained more acid detergent fibre, less protein and had a higher tannin activity than E. arborea. Although the vegetative growth of E. arborea is favoured relative to E. trimera under high browsing pressure, rapid change in abundance would not be expected, since short-interval fire will repeatedly eradicate any gains in vegetative growth. However, within the typical fire return interval of less than 10 years, E. trimera barely reach a reproductive state, whereas E. arborea flower profusely. Under the current regime of fire and browsing this may in the long run be more important than differences in height growth, leading to a gradual increase in the proportion of E. arborea.

The seasonal distribution of fires is one fire regime variable which has received little attention with regard to its effects on plants. For species with a short life-span that recruits after fire, the seasonal timing of a fire can be expected to be important due to effects on potential growth period and reproduction. We observed phenology and reproductive output in two annual and fire-dependent Geranium spp. in the southern part of the European boreal forest. In a garden experiment with the two species under two levels of nutrition, we established cohorts of seedlings at several dates over three summers. Time from germination to flowering and first mature seed differed little between the two species and levels of nutrition; i.e. plant size or level of nutrition had almost no effect on phenology. However, emergence time controlled the timing of reproduction. Most plants emerging before the second week of July in the garden experiment bolted the same year. Plants emerging later behaved as winter-annuals and started to flower in June the following year. A similar dichotomy was observed for populations of Geranium spp. at a number of burnt forest sites that differed in date of fire. This response is likely controlled by photoperiod. Nevertheless, at sites that burnt early some plants did not bolt in the same season; probably an effect of variable seedling emergence dates in the populations. In both the field and garden experiment, there were plants entering reproduction too late to produce mature seeds. Our results indicate that management fires should be conducted either very early, or during July and August to achieve a high seed production in these rare forest plants.

Fire can potentially have a large direct impact on tree roots and, thus, contribute to reduced vitality. Tree canopy status after fire should have an impact on the postfire production of fine roots, further affecting root function. We analyzed the standing crop of live and dead roots in Pinus sylvestris L. with varying degrees of crown scorch, 1 year after fire in northern Sweden. On the burned sites, total Pinus live fine-root biomass was 74% of that at the control sites, and it was only 19% of the control for roots

We report an analysis of both the long- and short-term drivers of the carbon (C) isotope composition (δ¹³C) values of current year needles of Pinus sylvestris L. linked to changing atmospheric carbon dioxide (CO₂) concentrations (ca) and climate using data from a uniquely long-term nitrogen (N) fertilization experiment in the north of Sweden (consisting of three N dosage levels and a control treatment) from 1970 until 2002. N loading produced trees with less negative δ¹³C of foliage, by around 0.45[per thousand] on average, with the difference in δ¹³C between control and N treatments not dependant upon N dosage. The average δ¹³C values decreased at a rate of around 0.03[per thousand] yr⁻¹, even after accounting for the Suess effect (the decrease in the atmospheric CO₂δ¹³C due to anthropogenic emissions of isotopically light CO₂). This decrease is large enough to cause a significant, progressive change in the δ¹³C down through a soil profile. Modelled values of plant intrinsic water use efficiency (WUEi) and the ratio of leaf internal to external [CO₂] (ci/ca) showed that this was the result of ci increasing in parallel with ca (while ci/ca increased), thus causing little change in WUEi over the 32 years of study. The residuals from the relationships between year and δ¹³C were used to examine the impact of climate on the interannual variation of C isotope composition of needles. This included the use of a fire hazard index (FHI) model, which integrates climatic factors known to influence plant stomatal conductance and hence δ¹³C. The FHI produced the best fit with δ¹³C values when climate data were averaged over the whole growth season (for control plots) and for July for all the N treatments, explaining ca. 60% of the total interannual variation in δ¹³C. Further, trees from the N treatments appeared more susceptible to air-humidity-based climate parameters, as seen from higher correlation coefficients, than were control trees. Thus, our data suggest the possibility of increased susceptibility to drought conditions in ecosystems with moderate to high N deposition rates. Also, there is the possibility that, because there was no apparent change in WUEi of P. sylvestris in this ecosystem over the last 32 years, the rate of sequestration of C into boreal ecosystems may not increase with ca, as has been predicted.

In the European boreal forest, early successional tree species in the genera Salix and Populus are among the most favoured by browsers, often causing poor regeneration. It is frequently assumed that the fast height growth rate of these species on favourable sites can increase their chances to outgrow browsers. Spatial or temporal variation in browsing pressure could be important mechanisms for escape too, but there are few examples of this. In 1999 a large area of old-growth mixed Pinus sylvestris L. and Picea abies L. Karst. forest burned in Tyresta National Park in central Sweden. In the following year, an abundance of Populus tremula L. and Salix caprea L. seedlings regenerated naturally. Four years later we analyzed seedling height and browsing history for the tallest seedlings inside and outside natural aggregations of dead wood formed by windthrow of fire-killed trees. All seedlings outside the aggregations had been browsed (on average three times) and average height was 60 ± 9 cm for P. tremula and 54 ± 12 cm for S. caprea . Inside aggregations, only 33% of the tallest seedlings had any evidence of browsing, and in most cases only from one episode. Average height for P. tremula was 153 ± 41 cm and for S. caprea 167 ± 27 cm. Stem base diameter was also wider for seedlings growing in dead wood aggregations than in open areas. Droppings of moose, roe deer, and hare were abundant in the area, but judging from bite marks, most browsing damage was due to the ungulates. For seedlings inside dead wood aggregations, backward regression analysis showed that both the structure of the aggregations and the spatial position of seedlings influenced seedling height: for P. tremula seedlings, there was a significant association with increasing height of the dead wood aggregation, increasing distance to the edge of the aggregation, and decreasing distance to the nearest dead wood stem. For S. caprea seedlings there was a significant association only with the height of the aggregation. The results show that the legacy of the pre-fire tree stand can allow seedlings of palatable early successional trees to escape browsers. The effectiveness of this mechanism depends on both the quality of the pre-fire stand and on the fire behaviour generating the dead wood. A fire regime of long fire-intervals (allowing for wide-crowned trees to develop) followed by a stand-replacing fire (allowing for large aggregates of dead wood) would be optimal.

We examined moss-dominated surface fuel moisture in a southern boreal fire environment by carrying out destructive moisture sampling on 47 days in Pinus sylvestris L. stands aged 0, 15, 30, 45, and in Picea abies (L.) Karst stands aged 0, 40, and 60 years since regeneration cutting. The moss layer was found to be consistently moister under dense canopy cover, formed by either of the studied tree species, than in the respective open stands. In the most exposed stand type (clear-felled Pinus ), the frequency distribution of daily surface fuel moisture was strongly skewed towards the low end, with 37% of the observations falling below 50% MC. For 45-year-old Pinus , the corresponding figure was only 7%. Moisture difference between open and closed stands increased during the latter half of fire season, probably due to increased canopy-interception of radiation with lowering sun-angle. Average surface fuel moisture correlated with effective leaf area index and canopy openness with ρ ranging from −0.68 to −0.84 ( p R 2 : 0.56–0.98). Surface fuels reached a postulated ignition threshold moisture content of 30% and below in Pinus clear-cuts at an FFI value of 3.6, and in closed Pinus sylvestris stands at FFI 5.6, but never went below 40% in the closed P. abies stands. The results show that stand structure has a remarkable impact on surface fuel moisture regimes. Integrating spatially explicit stand data such as LAI with weather-derived fire risk indices would greatly improve moisture content predictions to the benefit of various fire management operations.

Forest fire ignition potential was investigated in Picea abies (L.) Karst. (Norway spruce) and Pinus sylvestris L. (Scots pine) dominated stands of 0, 15, 30–45, and 40–60 years of age. A series of small-scale (2) ignition tests were carried out in experimental plots on 61 different days in June, July, and August. Ignition success percentages were analyzed in relation to stand structural properties, preclassified stand types, and the output of the Canadian Fire Weather Index system. In addition, the number of average stand-type-specific fire days was estimated based on weather data (June–August) for southern Finland for the years 1991–2002. Factors in stand structure that significantly correlated with the ignition success percentage were canopy depth and leaf area index, the correlation coefficients being –0.575 (p < 0.005) and –0.582 (p < 0.005), respectively. In Pinus sylvestris dominated stands, ignition tests produced self-sustained surface fires in 32.0%, 24.0%, and 19.3% of cases in 0-, 15-, and 30- to 45-year age classes, respectively. In Picea abies dominated sites conditions were favorable for fire in 12.0% and 4.6% of trials in the 0- and 40- to 60-year age classes, respectively. The output of the FWI-system correlated well with the ignition success in June and July but poorly in August. Based on the 12-year time series analysis, there were on average per year 27, 18, and 14 potential fire days in 0-, 15-, and 30- to 45-year-old Pinus sylvestris stands, and 10 and 4 potential fire days in 0- and 40- to 60-year-old Picea abies stands, respectively. We concluded that the dominance of Picea abies or Pinus sylvestris and stand age appear to modify the ignition conditions significantly and should be used as threshold indicators in fire occurrence predictions.

To analyze the effect of landscape structure (viz. amount of wetlands) on the past forest fire regime in boreal Sweden, we reconstructed detailed fire histories by cross-dating fire scars in living and dead Scots pine (Pinus sylvestris L.) in two different landscape types: mire-free landscapes with a low proportion (1%–2%) of mires and mire-rich landscapes with a high proportion (21%–33%) of mires. Two localities were selected and at each one, adjacent mire-free and mire-rich areas of 256–601 ha were sampled. Over the studied 650-year period, the two landscape types differed primarily in the fire intervals and sizes of fires. In the mire-rich landscapes, fires had frequently stopped against mire elements. The net effect was significantly longer fire intervals in the mire-rich than in the mire-free landscape (on average, 32 versus 56 years). The mire-rich areas also had a tail of very long fire intervals lacking in the mire-free areas (maximal interval 292 years). We conclude that mires can have a profound effect on both spatial and temporal patterns of forest fires in the boreal forest, but only when they are effective fuel breaks (i.e., they are wet enough) at the time the fires burn and if they truly dissect the nonmire portion of the forest landscape.

Response of plant communities to fire in an Acacia woodland and a dry Afromontane forest, southern Ethiopia

The aim of this investigation was to survey the reactions taking place during heat-induced yellowing of a TCF-bleached birch kraft pulp bleached according to the sequence O Q P Q (PO). This was don ...

Forest fires are one of the main disturbance agents in boreal and temperate ecosystems. To decipher large-scale temporal and spatial patterns of past fire activity in Scandinavia, we analysed the synchronicity of dendrochoronologically reconstructed fire events in a large network of sites (n=62; 3296 samples, 392 individual fire years) covering a wide geographical gradient (56.5-67.0 degrees N and 9.3-20.5 degrees E) over AD 1400-1900. We identified large fire years (LFY) as years with regionally increased forest fire activity and located the geographical centres of climatic anomalies associated with synchronous LFY occurrence across the region, termed LFY centroids. The spatial pattern of LFY centroids indicated the presence of two regions with climatically mediated synchronicity of fire occurrence, located south and north from 60 degrees N. The return intervals of LFYs in Scandinavia followed a Weibull distribution in both regions. Intervals, however, differed: a period of 40years would carry a 0.93 probability of LFY occurrence in the southern region, but only a 0.48 probability of LFY occurrence in the northern region. Over 1420-1759, the northern region was characterized by significantly higher temporal variability in LFY occurrence than the southern region. Temporal correlation of LFYs with reconstructed average summer temperature and total precipitation was evident mainly for the northern region. LFYs in this region were associated with positive temperature and negative precipitation anomalies over Scandinavia and with colder and wetter conditions in more southern parts of the European subcontinent. Synthesis. Historical patterns of the occurrence of large fire years (LFY) in Scandinavia point towards the presence of two well-defined zones with characteristic fire activity, with the geographical division at approximately 60 degrees N. The northern and mid-boreal forests, although exhibiting lower LFY frequencies, appeared to be more sensitive to past summer climate, as compared to the southern boreal forests. This would imply that fire regimes across Scandinavia may show an asynchronous response to future climate changes.

Past fire regimes may indicate under what conditions species have been able to exist in the past, but they do not provide clear and easy answers as to how the boreal landscape should be managed today. First, fire regimes have changed considerably over time through the effects of a changing climate, and sometimes even more dramatically through the influence of humans. Therefore it is unlikely that a close quantitative link exists between fire regime and biodiversity. Alternatively, if there is such a link, then biodiversity should have been highly variable in the past, even at the landscape scale. Second, it is impossible to re-enact all fire variables within forestry, particularly those operating at the landscape scale. An alternative to the superficial ''emulation'' of the past, which is so often claimed in today's forest management, would be to identify critical processes and habitats at the stand and landscape scale and find ways to maintain these at sufficient levels. This requires detailed knowledge ...

The purpose of this investigation was to identify the most important variables influencing the thermal yellowing of TCF-bleached kraft pulps. The variables studied were: hexenuronic acid content, dry content, temperature, time, pH, and iron content. All these variables influence the process of colour reversion. It was shown that there is a strong relationship between hexenuronic acid content and the magnitude of the colour reversion. Of the conditions investigated, long time (6 days), high temperature (80 degreesC), slightly acidic conditions (pH 4.5) and a moist environment (92% dry content) caused the greatest brightness reversion. At 99% dry content the discoloration was insignificant. Impregnation of handsheets has shown that ferrous ions are more detrimental than ferric ions to the colour formation. It is suggested that the hydrolysis of hexenuronic acid into the known acidic degradation products, 2-furancarboxylic acid and 5-formyl-2-furancarboxylic acid can not be the rate limiting reaction in the colour formation. It is also suggested that new aromatic structures are formed during the thermal yellowing process. Finally it was shown that the discoloration can be partly washed away with water.

Summary 1. Forest management involving live tree retention and natural regeneration after cutting is currently increasing in boreal areas. This calls for further analysis of the optimal conditions for seedling establishment from seed following stand disturbances due to logging. 2. We studied post-dispersal predation on seeds and juvenile seedlings of Pinus sylvestris over 3 years in 32 north Swedish boreal forest stands with different levels of stand disturbance by logging. The aims were to identify the most important predator species and to quantify the damage inflicted upon seeds and seedlings in relation to disturbance. 3. In most stands and years, seed predation resulted in 70% mortality in 10 cases and unlogged forest > clear-cut. 6. The most important predator on juvenile seedlings was the slug Arion subfuscus, which attacked seedlings during the first weeks after germination. Pitfall trap catches of Hylobius abietis, which commonly damage planted (1–3-year-old) conifer seedlings, were not related to the levels of seedling predation. 7. Seedling predation was negatively related to stand disturbance, with the highest predation levels by slugs in unlogged forests and the lowest in clear-cuts. Seedling predation was higher in wet than in dry summers, probably because slugs are moisture-limited. 8. There was a large between-site variation in both seed and seedling predation, but predation was not strongly related to forest site types. The fact that predation was strongly affected by logging operations indicates that there may be opportunities to reduce damage through modification of the silvicultural practices.

The spatial display of fire over time on the landscape is ecologically important, and spatially explicit analyses offer a possibility of revealing anthropogenic influence on fire regimes. Nonetheless few such analyses have been attempted for longer time frames. We identified past fires in a northern Swedish boreal landscape using fire scars on Pinus sylvestris trees. Within a 19 × 32 km area, local fire chronologies were established at 203 points by cross-dating fire scars on 1133 wood samples, the earliest dating back to the 1100s. A total of 349 separate fires were identified to location and size. The estimated number of fires per unit area and time (after correcting for varying sample density over time) was relatively constant at 0.095 fires·(104 ha)−1·yr−1 from 1350 to 1650. It increased gradually thereafter, except for a low period in the early 1700s, peaked at 1.17 fires·(104 ha)−1·yr−1 in the mid-1800s, and then dropped dramatically after 1860. The proportion of the area burned per unit time also increased after 1650, in parallel with the increase in the number of fires (although much less strongly due to a counteracting trend in fire size), from an annual rate of 0.8% prior to 1650 to 2.8% in the mid-1800s. Prior to 1650, 90% of the total burned area was due to fires larger than 1000 ha, compared to 55% after 1650. This decrease in fire size with increasing number of fires may be an intrinsic property of the system: a negative feedback caused by lack of fuel in early succession. Fire intervals shorter than 15 yr were rare, and there was an increase in the hazard of burning during the first 3–5 decades after fire, suggesting an effect of fuel accumulation. Thus, the proportion of the area burned per unit time does not increase linearly with the number of fires in the landscape, because the probability that fires will stop at boundaries with recently burned areas increases over fires. The changes in the number of fires per unit time mirror changes in the cultural use of the land, i.e., the gradual expansion of permanent settlements in the area after the late 1600s. They are not explained by changes in climate records. This suggests that the increase in fire numbers from the second half of the 1600s represents an increase in anthropogenic fires. Before 1650, the number of fires detected per unit area and time was only marginally different from the present-day density of lightning ignitions in the region (∼ 0.1 fires·(104 ha)−1·yr−1), whereas during the mid 1800s it was 11.7 times higher. These results show that large alterations in the fire regime can occur without substantial changes in the proportion of area burned per unit time, as exemplified by the trend after 1650, when there were concurrent changes in the number of fires and in average fire size. Therefore, the number of fire events per unit area and time should be an important variable in the analysis of fire history and its underlying causes.

Conifer seedlings are succulent and vulnerable to animal depredation in the first few weeks after germination. Several animal taxa are reported to feed on seedlings, and field observations indicate varying rates of losses in boreal vegetation. Almost nothing is known about factors regulating seedling predation levels. Here, we test the hypothesis that the risk of predation is affected by the moisture content of the forest floor. The underlying assumption is that slugs are important seedling predators and that their activity depends on the substrate moisture. In a relatively open coniferous forest in northern Sweden, 1-week-old seedlings of Scots pine (Pinus sylvestris L.) were placed out in plots where moisture conditions were experimentally manipulated. There was a significant effect of forest floor moisture on predation. The average predation level for the five experimental runs was over three times higher in watered than in rain-protected plots (68 versus 22%). In unmanipulated plots, average predation was intermediate (49%) but with large variation, and more seedlings were killed during wet than dry periods. The slug Arion subfuscus (Drap.) was the only important seedling predator, with a population density of approximately 3 m n2 . Our results show that slug predation can be a quantitatively important mortality factor, acting mainly when conditions otherwise favor seedling establishment, i.e., when the forest floor is moist. Resume : Durant les premires semaines qui suivent la germination, les semis de conifres sont succulents et vulnOrables ‡ liattaque par les animaux. On rapporte que plusieurs taxons dianimaux se nourrissent de semis et les observations sur le terrain rOvlent diffOrents niveaux de pertes parmi la vOgOtation borOale. On ne sait pratiquement rien des facteurs qui rOgissent le niveau de prOdation. Nous avons testO lihypothse selon laquelle le risque de prOdation est fonction du contenu en humiditO du parterre forestier. Lihypothse sous-jascente est que les limaces sont diimportants prOdateurs pour les semis et que leur activitO dOpend du degrO dihumiditO du substrat. Dans une forŒt de conifres relativement ouverte du nord de la Sude, des semis de Pinus sylvestris L. ‚gOs diune semaine ont OtO placOs dans des parcelles oo les conditions dihumiditO furent modifiOes expOrimentalement. LihumiditO du parterre forestier a eu un effet significatif sur le niveau de prOdation. Le niveau moyen de prOdation pour les cinq essais Otait plus de trois fois plus OlevO dans les parcelles irriguOes que dans les parcelles oo les prOcipitations Otaient exclues (68 vs 22%). Dans les parcelles tOmoins, le niveau moyen de prOdation Otait intermOdiaire (49%) mais il y avait une forte variation et plus de semis ont OtO tuOs durant les pOriodes humides que durant les pOriodes sches. La limace Arion subfuscus (Drap.) Otait le seul prOdateur important pour les semis et avait une densitO de population de approx. 3 m n2 . Nos rOsultats montrent que la prOdation par les limaces peut, quantitativement, Œtre un important facteur de mortalitO qui se manifeste lorsque les conditions favorisent par ailleurs liOtablissement des semis; ciest-‡-dire lorsque le parterre forestier est humide. (Traduit par la ROdaction)

A comparative study of seed germination was performed on 25 species of trees, shrubs, woody climbers and herbs from the dry Afromontane forest zone of Ethiopia. Despite the restricted geographical range of the species tested, the optimum constant temperature for germination varied considerably. For 17 of the species studied, it was between 20 and 25 °C while for three, germination was highest at 30 °C Germination was virtually inhibited (c. 3 mg. When these were incubated under leaf-filtered light, germination was in most cases even more effectively suppressed. Most of these species are known to have large soil seed banks. Mechanical scarification improved germination over the control in nine species. Five of these also reponded to heat or sulphuric acid scarification. Circumstantial evidence suggests that the species showing heat-induced germination may be fire adapted, although fire is probably a rare natural phenomenon in the Afromontane forest proper. Only one species, Bersama abyssinica, showed a classic recalcitrant behaviour. Germination dropped from 88% when the seeds were fresh to less than 20% after 1 mo of dry storage and 0% after 3 mo. Germinability of seeds of Ekebergia capensis also declined significantly with increasing storage time, although less rapidly; after 24 mo of dry storage, germination was 4%. In contrast, germinability of Juniperus procera was not significantly altered during 54 mo of storage. The results indicate that dormancy plays a major role in regulating germination in dry Afromontane species. Out of the 25 species studied, 15 (60%) showed some degree of initial dormancy. In most of the species with initially non-dormant seeds, dormancy would evidently be induced if the seeds are dispersed under a closed forest canopy or buried in the soil. These patterns of dormancy may have evolved in response to a highly unpredictable climate and small-scale disturbances, and should make the Afromontane forest flora relatively resilent also to anthropogenic disturbances.

The fate of seeds of eight tree species was followed during 4 y of storage in the soil of an Afromontane forest at Gara Ades in the eastern highlands of Ethiopia. Seeds were enclosed in nylon mesh bags and buried at 5 cm soil depth. The bags were exhumed at intervals and the viability of the seeds was assessed by germination and cutting tests. Seeds ofBersama abyssinicaandEkebergia capensisgerminated in the soil almost completely within a year after burial. The seeds ofJuniperus procera, Olea europaeaandPodocarpus falcatusalso germinated to a substantial degree in the soil but with a distribution over several years, and some seeds of these species remained viable at the end of the 4-y period. Germination in the soil was very low in seeds ofAcacia abyssinicaandCroton macrostachyusthroughout the whole burial period and the seeds kept their viability. InC. macrostachyusfresh seeds were highly dormant, but after 3 y or more in the soil they germinated readily in the laboratory suggesting an altered dormancy with time in the soil. Dormancy in seeds ofA. abyssinicaandIndigofera rothiiwas not altered throughout the study period as evidenced by marginal or no germination during incubation in the laboratory. The differential seed behaviour observed during storage in the soil can be an indicator of the regeneration strategy of the species studied.B. abyssinica, E. capensis, J. procera, O. europaeaandP. falcatusform seedling banks on the forest floor and lack persistent soil seed reserves in contrast toA. abyssinica, C. macrostachyusandI. rothiiwhich accumulate reserves of long-lived seeds in the soil. The generally high levels of dormancy and somewhat extended viability in the soil, even in several of the species producing seedlings in undisturbed forest, may have been selected for under a climate of seasonal drought and unreliable rainfall that characterizes this region.

We studied plant survival and colonization over an experimental gradient, from fire lightly scorching the soil to fire consuming most of the organic soil layer, at two forest sites in northern Sweden. The gradient was achieved by adding different amounts of fuel to small plots that were burned in 1988 and 1989. Temperature was recorded at four soil strata during burning. We analyzed survival of seeds and rhizomes in the soil immediately after fire, and followed vegetation cover and seedling establishment until 1993. During fire, there was a steep decline in maximum temperature with increasing depth below the char, irrespective of the depth of burn in the mor layer, indicating that burn depth can be used as a general indicator of heat impact below ground. Lethal temperature was not recorded deeper than 20-30 mm under the burn boundary. Plant survival was determined both by depth of burn and by depth distribution of regenerative structures in the soil. Three rhizomatous species, the dwarf shrubs Vaccinium myrtillus and Vaccinium vitis-idaea and the grass Deschampsia fiexuosa, were dominant in the prefire vegetation. For all three species, the bulk of the soil bud bank was located within the mor layer, but was more superficial for D. flexuosa. Initial mortality in the bud bank was progressively higher with increasing depth of burn, and this determined the regrowth over the following years. After fires that consumed only the moss layer, cover of the Vaccinium species returned to prefire levels within 2-4 yr, and D. fiexuosa showed a dramatic increase in cover as well as in fruiting. Fires that burned slightly deeper nearly eliminated D. flexuosa, and the deepest burning fires also eliminated Vaccinium s-pp. In contrast to regrowth from rhizomes, col- onization from seed was better after relatively deep-burning fire, both for species with a soil seed bank and for species dispersing seed onto the burnt soil. However, after fires consuming most of the organic soil layer, seed bank species were also badly affected, whereas dispersers showed progressively better establishment with increasing depth of burn. Differences between treatments were still great after 5 yr, indicating that variation in depth of burn will have a long-lasting impact on the vegetation. These results from ex- perimentally burned plots were corroborated by an analysis of depth distribution of viable plant rhizomes and seeds, and the initial colonization at a site newly burned in a wildfire. The precise response patterns of boreal vegetation to variation in burn depth will depend on characteristics of the species present. However, we assume that these results have a high degree of generality, since, in podzolized soils, most rhizomatous species are predominantly located in the mor layer, since the dormant seed bank typically is concentrated at the interface of mor and mineral soil, and also since a thick organic soil layer is a poor seedbed for incoming seeds. The results indicate that in boreal forest, depth of burn is a more important variable than fire front intensity for the understory vegetation, in contrast to the situation in ecosystems with little accumulation of organic material on the mineral soil.

The soil seed bank was investigated in four dry Afromontane forests of Ethiopia. At least 167 plant species were identified in the 0–9 cm soil layer with total densities ranging between 12 300 and 24 000 seeds/m2. Herbs were represented with the largest numbers of species and seeds in the seed bank, while the contribution of tree species was generally low. The overall vertical distribution of seeds was similar at all sites with the highest densities occurring in the upper three cm of soil and gradually decreasing densities with increasing depth. Relatively high densities also occurred in the litter layer. There were large differences in depth distribution between species, suggesting differences in seed longevity. A large number of species in dry Afromontane forests evidently store quantities of seeds in the soil and this is in contrast to the situation in most tropical rain forests, dry lowland forests and savannas, where both the number of seeds and the number of species are relatively small. It is possible that the strongly seasonal and unpredictable climate of this region may have selected for high levels of dormancy, and that herb regeneration is associated with small scale disturbance. The fact that most of the dominant tree species do not accumulate seeds in the soil suggests that their regeneration from seed would be unlikely if mature individuals disappeared. Most tree species have relatively large seeds and poor long-distance dispersal; this implies that restoration of Afromontane forests after destruction would be difficult. Since there is a diverse seed bank of the ground flora, this component of the vegetation would have a better chance of reestablishing. However, because most cleared forest land is used for agricultural crop production, it is probable that the seed bank will be depleted in only a few years. Therefore, the future of the Afromontane forest flora seems to depend on the successful conservation of the few fragments of remaining natural forest.

Analysis of fire statistics revealed that there are steep gradients in the distribution of lightning-caused fire ignitions in Sweden. The highest ignition density was found in the southeastern provinces of Kalmar and Osterg6tland, ca. 0.23/10 000 ha/yr. From there, densities generally declined both to the north and to the west, with a density averaging ca. 0.05 in the six northernmost provinces, and an equally low density in the southwestern province of Halland. For both northern and southern Sweden, lightning ignitions peaked in early July, but in the south the season for ignitions started 2 - 3 weeks earlier and ended 2 - 3 weeks later. The geographical gradients in lightning ignition density correspond to the aver- age precipitation during summer. The patterns of lightning ignition densities may also indicate gradients in natural fire frequencies. This hypothesis is supported by the distribution of certain fire-adapted plant species.

Fire, being both a natural and cultural phenomenon, presents problems in disentangling the historical effect of humans from that of climate change. Here, we investigate the potential impact of humans on boreal fire regimes from a perspective of fuels, ignitions and culture. Two ways for a low technology culture to impact the fire regime are as follows: (i) by altering the number of ignitions and their spatial distribution and timing and (ii) by hindering fire spread. Different cultures should be expected to have quite different impacts on the fire regimes. In northern Fennoscandia, there is evidence for fire regime changes associated with the following: a reindeer herding culture associated with few ignitions above the natural; an era of cattle husbandry with dramatically increased ignitions and somewhat higher fire frequency; and a timber exploitation era with decreasing fire sizes and diminishing fire frequency. In other regions of the boreal zone, such schemes can look quite different, but we suggest that a close look at the resource extraction and land use of different cultures should be part of any analysis of past fire regimes.

To analyse the potential reaction to firegenerated heat pulses, seeds of 12 species of plants and rhizomes of three species were exposed to elevated temperatures for 10 min. The tested material split into three groups with respect to heat tolerance: (1) the rhizomes, for which the lethal temperatures were in the range 55–59° C; (2) the seeds of most of the species tested, for which the lethal temperatures were in the range 65–75° C; (3) The seeds of two species of Leguminosae and three species of Geranium for which the lethal temperatures were around 100° C. For all three Geranium species and for one of the legume species, Anthyllis vulneraria, exposure temperatures above ca. 45° C resulted in dormancy release, and maximum germination occurred above 60–65° C. Speed of germination was little affected for most species, except after exposure to nearlethal temperatures, where it slowed down dramatically, although the seedlings emerging were healthy. We conclude that due to sharp temperature gradients in the soil during fire, differences in heat tolerance between species in most cases are not large enough to be a decisive factor in their post-fire colonising success. There are exceptions: the seeds of certain taxa that are impermeable to water in the dormant state, some of which have heat triggered germination.

Fire behaviour characteristics were studied in managed Pinus sylvestris L. and Picea abies L. Karst stands in a series of field burning experiments. Stand characteristics, surface fuel moisture content, mid-flame wind speed, rates of spread, flame heights, and torching were recorded. The Canadian Forest Fire Weather Index System (FWI System) and Finnish Fire Risk Index (FFI) were used to evaluate burning conditions and analyse the observed fire behaviour. Mid-flame wind speed was a good predictor (R2 = 0.96 for exponential curve) of the fire spread rates. Torching formed the strongest correlation with the height of the dead branch limit. An increase in predicted fire weather hazard from FWI 4 to FWI 20 (FWI = the FWI code of the FWI System) increased burn coverage remarkably in 15–45-year-old Pinus stands and to a lesser extent in Pinus and Picea clear-cuts, but did not affect 40–60-year-old Picea stands. The FFI was unable to predict burn coverage or any other fire behaviour characteristics.

Seeds of Betulapubescens Ehrh. and Betulaverrucosa Ehrh. were sown onto the forest floor at sites of different vegetation types in mature north Swedish coniferous forests. Subsamples of the seeded soil were taken each spring and fall and analysed for viable seeds. During the 1st year after seeding, the population of viable birch seeds in the soil declined to approximately 6% of the original. During the 2nd year, the rate of depletion was much smaller and the population of viable seeds decreased to approximately 3% of the original. Part of the material was followed over 3 years and the rate of decline was similar during the 3rd year to that during the 2nd year. There were no significant differences when comparing the proportion of seeds remaining viable in different vegetation types, nor were there any significant differences when comparing the two species. It is inferred that seeds produced in a forest stand and stored in the soil at the time of clear-cutting could give rise to Betula seedlings. These results indicate that the risk would be strongly reduced if seed-producing birch trees are removed a few years ahead of clear-cutting, when birch regeneration is undesirable.

The viable seed content of soil samples from five coniferous forest stands in northern Sweden, aged 16–169 years, was determined by means of germination trials. The soil samples were separated into five fractions: three organic horizons and two mineral soil horizons. Seedlings of 15 phanerogam species emerged, representing densities of 239–763 seeds/m2 in the soils from the different stands. The depth distributions of the seeds varied with both plant species and forest stand. Most seeds were found at various depths in the humus layer, but in one stand an appreciable seed density of Luzula pilosa was present in the mineral soil. Most of the seedlings belonged to plant species present in the vegetation or with good means of dispersal. The depth distribution data, however, suggest that Luzula pilosa in particular may have a persistent seed bank. The role of persistent seeds in the vegetational composition of the boreal forest is discussed.

The rotation length is a key component of even-aged forest management systems. Using Fennoscandian forestry as a case, we review the socioecological implications of modifying rotation lengths relative to current practice by evaluating effects on a range of ecosystem services and on biodiversity conservation. The effects of shortening rotations on provisioning services are expected to be mostly negative to neutral (e.g. production of wood, bilberries, reindeer forage), while those of extending rotations would be more varied. Shortening rotations may help limit damage by some of today's major damaging agents (e.g. root rot, cambium-feeding insects), but may also increase other damage types (e.g. regeneration pests) and impede climate mitigation. Supporting (water, soil nutrients) and cultural (aesthetics, cultural heritage) ecosystem services would generally be affected negatively by shortened rotations and positively by extended rotations, as would most biodiversity indicators. Several effect modifiers, such as changes to thinning regimes, could alter these patterns. Supplement: 2 Future Forests

(1) Seeds of fourteen species were placed in the mor layer of a north Swedish coniferous forest in autumn 1979. Samples were excavated and tested for viability at intervals until autumn 1984. (2) Three species showed no innate dormancy and germinated to a large extent in the soil: Epilobium angustifolium and Pinus sylvestris were eliminated within one year, but a small fraction of the sample of Deschampsiaflexuosa remained viable for five years. (3) Three others had a strong innate dormancy which was gradually lifted within a few years, resulting in a high degree of germination in the soil: Prunus padus was eliminated within three years but a fraction of Sorbus aucuparia and Trientalis europaea remained viable after five years. (4) For nine species no germination in situ was evident, but viability was generally well maintained over five years. For three of these (Calluna vulgaris, Rubus idaeus and Rumex acetosella) germination was 80-100% after 5 years. (5) For Rubus idaeus and Rumex acetosella there was an increase with storage time in the soil in the proportion of seeds germinating in the laboratory during the first few days after excavation, indicating gradual changes in dormancy characteristics over several years. (6) A proportion of the seeds of Betula pendula, Sorbus aucuparia, Vaccinium myrtillus and V. vitis-idaea had their seed coats partly degraded after five years in the soil, although this had not affected the viability of the seeds. (7) The patterns of depletion observed in the seed populations did not indicate a constant rate of depletion, suggesting that the long-term survival of seeds in boreal forest soils may be very different from that found in many agricultural soils.