Anders Granström

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.

Fire Severity and Vegetation Response in the Boreal Swedish Forest

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.

Spatial and temporal variation in lightning ignitions in Sweden

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.

Soil seed banks in dry Afromontane forests of Ethiopia

. 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.

Heat effects on seeds and rhizomes of a selection of boreal forest plants and potential reaction to fire.

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.

Seed viability of fourteen species during five years of storage in a forest soil.

(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.

Seed banks at six open and afforested heathland sites in southern Sweden

Although modern hunting is different from traditional hunting, it remains a controversial topic. A large number of scholars in the world have studied the effects of hunting on wild animals from an ecological, ethological, genetic and economic aspect. This paper reviewed the role of controlled hunting in wildlife production from population dynamics, behavior, genetic and a phenotypic level, and by integrating a large number of domestic and foreign literatures. Many studies have shown that regulated hunting is an efficient approach in managing wildlife populations, which could be beneficial to the recovery and possibly even growth of wildlife populations. Meanwhile, over-exploitation or inappropriate hunting could affect the sex, birth and mortality ratios of wildlife populations, change foraging behavior and socio-spatial behavior and generate artificial selection of their genotype and phenotype. To apply modern hunting properly to wildlife management, China could learn from successful hunting programs implemented in many other countries, which are based on ecological and economic principles to formulate scientifically determined hunting quotas and set up an effective system to regulate and manage the hunting of wildlife populations.(1) Germinable seed banks in the soil were determined for six sites in southern Sweden: a grazed heathland, four first generation Picea abies plantations aged 30-73 years and one recently clear-felled area previously covered with an 85-year-old first generation P. abies plantation. (2) On the open heathland site, the top 6 cm of soil was estimated to contain twenty-two species with a total of 45 200 (S.E. 4200) seeds m-2, 88% being of Calluna vulgarism (3) The depth distributions of the predominant species down to 9 cm in the mineral soil varied greatly, although all species occurred through the whole sampled section, suggesting that many seeds were probably not recently deposited. (4) The four P. abies plantations had fairly similar soil seed floras. Twenty-four species were found, of which fifteen had probably been stored in the soil since before afforestation. Most seeds were of Calluna vulgaris (up to 26 600 (S.E. 2000) seeds m-2 down to 6 cm in the mineral soil for the different sites), Carexpilulifera (up to 2400 (S.E. 410) seeds m-2) and Juncus spp. (up to 780 (S.E. 250) seeds m-2). There was no trend in seed densities with stand age. (5) On the clear-felled area, dense stands of C. vulgaris and C. pilulifera were present, but only in scarified patches of soil, showing that the seeds of certain heathland species may survive an entire forest rotation in the soil in numbers sufficient to be important for the revegetation after felling if the humus layer is disturbed.

Post-dispersal predation on Pinus sylvestris seeds by Fringilla spp: ground substrate affects selection for seed color

Abstract 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.

Seed viability of Afromontane tree species in forest soils.

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 en-closed in nylon mesh bags and buried at - cm soil depth. The bags were exhumed at intervals an1d the viability of the seeds was assessed by germination and cutting tests. Seeds of Bersama abissinzica aind Ekebergia capensis germinated in the soil almost completely within a year after burial. The seeds of Junliperus procera, Olea europaea and Podocapus falcatuis also germinated to a substan-tial 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 of Acacia abyssinica and Crotoii rnacrostacIpyus throughout the whole burial period and the seeds kept their viability. In C. macrostaclzvus fresh seeds were highly dormant, but after 3 y or more in the soil they germinatecl readily in the laboratory suggesting an altered dormancy with time in the soil. Doimancy in seeds of A. abyssinica and Indigofera rothii was not altered throughout the study period as evideinced 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, 0. europaea and P. falcatuts form seeclling banks on the forest floor and lack peisistent soil seed reserves in contrast to A. abissinica, C. rnacrostachvyzs and I. rothii whiclh accumulate reserves of long-lived seeds in the soil. The generally high levels of dormn- ancy and somnewlhat 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 un-reliable rainfall that characterizes this region.

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.