Mats Niklasson

NUMBERS AND SIZES OF FIRES: LONG-TERM SPATIALLY EXPLICIT FIRE HISTORY IN A SWEDISH BOREAL LANDSCAPE

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.

Densities of large living and dead trees in old-growth temperate and boreal forests

We recorded and reviewed densities and basal areas of large living and dead trees in old-growth forest in Europe. Recorded densities were similar to those reported from old-growth forests in eastern North America, but lower than in northwestem North America. Based on our results we suggest that, 10-20 living trees per ha with dbh > 70 cm may have been typical values for many central European and south Scandinavian virgin forests. In boreal forests, it was probably common with at least 20 living trees per ha with dbh > 40 cm. Basal areas of living trees in mixed old-growth forests in central Europe and southern Sweden were 34-40 m 2 per ha on dry ground and about 60 m(2) per ha in wet alder-ash-spruce forests. Densities of large trees (dbh > 40 cm) were twice as high in the latter forest type than on dry ground in Bialowieza forest, Poland. Based on our results, we propose the following generalizations to be further tested in other old-growth temperate and boreal forests: 1. Among all standing trunks (including high stumps) about 10% are dead. but this proportion increases for the largest trees. The proportion of standing trees that are dead seem to be independent of total basal areas. Based on this, we suggest that the volume of dead wood is directly proportional to the productivity of old-growth forests. 2. Standing dead trees (snags) are on average larger than downed dead trees. Trees with dbh >40 cm often dominate the basal area and volume of standing dead trees and living trees. 3. About 30% (20-40%) of the basal area and volume of dead trees is standing in old-growth forests. This proportion seems to be independent of total volume of dead wood. Large disturbances by fire, strong winds and insects may temporarily change these proportions considerably in individual stands

Biodiversity and its assessment in boreal and nemoral forests

We review species richness in major organism groups, mainly using examples from northern Europe. A high proportion of these species is forest living, and large numbers are dependent on decaying wood. Biodiversity can be assessed at various scales using two different principles. One is to use features, such as ancient and dead trees, known to be important for a large number of species. The other method is to choose species or groups of species known to indicate high biodiversity or presence of many red-listed species. We argue that any serious biodiversity assessment method should include the most species rich organism groups, for example insects. In the present paper we point out the most important features for high biodiversity (old trees and large dead trees), and review the quantities of these features in near-virgin forests. The natural disturbance regime of a region should be the basis for defining a suitable scale and the appropriate features for biodiversity assessment. Possible indicator species for high biodiversity in northern Europe are suggested, based on previous investigations. Among epiphytic lichens and wood-living beetles there are many potentially useful species in addition to vascular plants in the nemoral forest. Among vertebrates, woodpeckers and grouses seem to be the most useful. Validation tests for indicator structures and species are largely lacking but urgently needed. The implications of possible delayed local extinctions are important to bear in mind when managing for sustainable forestry. The knowledge of forest history is useful when developing cost-efficient measures.

The influence of tree age and microhabitat quality on the occurrence of crustose lichens associated with old oaks

Abstract Questions: How do tree age, microhabitat characteristics and epiphytic competitors affect the occurrence of crustose lichens associated with old oaks? How do microhabitat characteristics and microclimate affect the cover of competitors (bryophytes and macrolichens)? How do microhabitat characteristics correlate with microclimatic variables? Location: Southeast Sweden. Methods: Eight crustose lichen species were surveyed on 165 Quercus robur trees, 17–478 years old, at three study sites. The occurrence patterns of these species were examined at two spatial scales: among trees and within trees. Occurrence patterns within trees were examined in 10 cm × 10 cm plots at all four cardinal aspects at different heights from 0.5 to 4.5 m above the ground. Results: At the tree level, age-related factors were the most important predictors of species occurrence. All species were more frequent on trees > 100 years than on younger trees. At the plot level, the frequency of occurrence increased with increasing bark crevice depth. The frequencies of all study species decreased with increasing cover of bryophytes. Bryophytes were in turn more frequent in plots that were exposed to rainwater and showed a low evaporation rate. Patches most exposed to rainwater were directed upwards, and the lowest evaporation rates occurred on the northern side of the trunks. Conclusions: For many crustose lichens the association with old oak trees seems at least partly to depend on their preference for the deep bark crevices that only occur on old trees. Trees represent epiphyte habitat patches that differ in size due to within-tree variability in habitat quality, such as bark crevice depth and microclimate. This study shows that variability at a finer scale, within habitat patches, contribute to explain species occurrence patterns at habitat patch level. Nomenclature: Santesson et al. (2004).

Is local distribution of the epiphytic lichen Lobaria pulmonaria limited by dispersal capacity or habitat quality

By surveying and re-surveying 12 forest sites in southern Sweden for the epiphytic lichen Lobaria pulmonaria with a 9-year interval, and measuring tree-related habitat quality variables, we have investigated whether the local distribution of the lichen is limited by poor dispersal capacity or by habitat quality. Dispersal distances were measured indirectly as the distances between colonised trees and the nearest trees occupied by L. pulmonaria in both 1992 and 2001. To compare habitat quality between trees occupied by L. pulmonaria and neighbouring control trees, we recorded tree species and measured age and growth rate of trees, light conditions, bark structure and bryophyte cover. The estimated mean dispersal distance was 35 m, with a recorded maximum of 75 m. Occupied trees were larger and had a larger cover of bryophytes than unoccupied trees of similar size. The results indicate that dispersal capacity probably is the most important factor in limiting the local distribution of L. pulmonaria, but habitat-quality factors may be important on a smaller spatial scale.

A 600-year tree-ring fire history from Norra Kvills National Park, southern Sweden: implications for conservation strategies in the hemiboreal zone

The fire history of Norra Kvills National Park in southeastern hemiboreal Sweden was reconstructed for the last 600 years by cross-dating of fire scars in trees, stumps and snags of Scots pine (Pinus sylvestris). Between 1401 and 1770 forty different fires were recorded; fire frequency at point scale was 20 years, which is 3–4 times more frequent than in northern Sweden. Since 1770, no large fires have been recorded. This has had considerable consequences for the forest structure and fauna in that it has resulted in a shift from a Scots pine-dominated ecosystem to a fire-sensitive Norway spruce (Picea abies) system over large parts of the park. For rare late-successional species associated with Norway spruce (e.g. Fomitopsis rosea, a polyporous fungus and Ceruchus chrysomelinus, a beetle) this has been advantageous, while fire-, light- and thermophilous species like the saproxylic beetle Tragosoma depsarium may be increasingly threatened if the process of spruce invasion and suppression of fires is allowed to continue. We discuss the importance and problems of incorporating fire in management plans of reserves and in forestry practices, with the aim of preserving the large number of species that are directly or indirectly dependent on fire in southern Sweden.

A comparison of three age determination methods for suppressed Norway spruce: implications for age structure analysis

Abstract Three methods for age determination of suppressed Norway spruce were compared: (1) tree-ring counting in core-like strips; (2) pith node counting and (3) a method based on dendrochronological cross-dating (named within-tree cross-dating). The methods were tested on 20 suppressed spruces (age 100–140 years) from a boreal forest in Northern Sweden with known disturbance history. In all studied trees, the oldest pith dates were found below the root collar, the point which is normally aimed for when coring trees in field, thus, ring counts in the root collar level gave on average 20–26 years younger ages than pith node counting and within-tree cross-dating. Pith node counting and within-tree cross-dating gave largely identical pith dates but when top breaks had occurred at the seething stage, within-tree cross-dating gave better results. Cross-dating was achieved between short stem sections from the base of the tree with the aid of conspicuous rings that were followed down through the lower stem, section by section. This enabled more accurate dating of the pith in the lower portions of the stem where many rings are missing in the outer parts due to early bending and formation of adventitious roots which allocate stern growth above the point of origin. The results have wide implications for studies of Norway spruce stand age structures. When analysing tree age in Norway spruce stands and stands of other species that are capable to extremely slow juvenile growth (leading to initiation of adventitious roots and missing rings), ages may be systematically underestimated, and short periods of successful regeneration may “disappear” in age–class diagrams when age data are derived from counted tree rings in cores extracted at root collar level. The within-tree cross-dating method and pith node counting are two methods that (1) provide a precise age determination for small suppressed Norway spruce and (2) can give a better age estimation of the dating error for bigger trees where the oldest part (juvenile wood) of the tree is not possible to date accurately with normal coring and ring counting.

Long-time record of fire and open canopy in a high biodiversity forest in southeast Sweden

We studied a ca 200 ha large forest site that holds an exceptionally large number of red-listed saproxylic beetles. We conducted pollen, charcoal and dendroecological analyses to reveal the long-term stand-level history of the site. We also investigated which of the red-listed saproxylic beetles recorded at the site are probably dependent on forest fires or structures that are created by fires. Pollen from Pinus and Betula dominate the pollen diagram, and the pollen record shows that the canopy is more closed today than ever during the last 2500 years. Large amounts of charcoal fragments were found throughout the core except during three shorter periods, one of which is the latest ca 200 years. The dendroecological investigation revealed 11 different fires, the earliest dated to AD 1586 and the last dated to AD 1868. Of the 105 red-listed saproxylic beetles recorded at the site, at least 12 are associated with open forests with Pinus sylvestris or Birch spp. trees. We conclude that the fires, as recorded both by the charcoal and dendroecological analysis, kept the site largely open in the past, and this is likely one important explanation for the high conservation value of the site today. Other important factors could be that the site is rich in boulders, the relatively warm summer climate in the region, that Picea abies has not been planted in the surroundings until recently, and finally the presence of many old trees. The study supports the theory that fire could have been an important factor for keeping some forest types open before the large human impact on the northwestern European forests started.

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.

Potentials and limitations for human control over historic fire regimes in the boreal forest.

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.