Current issue: 53(3)
In an earlier paper of the author it was established that sporal regeneration of bracken fern (Pteridium aquilinum (L.) Kuhn.) is almost entirely connected with fire, and that the size of the bracken fern clones, distinguished by their characteristics, was related to the time of the fire. In previous studies has been found that also sporal regeneration of ground pine (Lycopodium complanatum L., now Diphasiastrum complanatum) is rare. The occurrence of several plant species were studied in relation to bracken fern in the earlier investigation. This paper reports findings concerning ground pine.
When the size-age problem of bracken fern was solved, parallel measurements of ground pine stand on the same site led to the solution of the size-age problem and sporal regeneration of ground pine. The linkage is also valid when the size of ground pine stands was compared to the dates of fires. The ground pine stands are very long-lasting. The stands are fragmented more easily than bracken fern by environmental factors, such as fires, and tend to form large patch clusters with time. Large individual stands reveal the rarity of sporal regeneration of ground pine. The resemblance with bracken fern clones indicate a common factor of regeneration, fire, and a very even spreading rate. Though considerable variation of the colour and structure of ground pine was observed, the circular stands were identical patch by patch.
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In an earlier paper of the author it was established that bracken fern (Pteridium aquilinum (L.) Kuhn.) that the sporal regeneration of the species is almost entirely connected with fire, and that the size of the bracken fern clones, distinguished by their characteristics, was related to the time of the fire. The time of the fire was determined by the samples taken with auger from the trees on the sample plots. A large data on the occurrence of bracken fern was collected around Finland, but only part of it could be used in the previous study, as the trees of the site provided no means to find out it the site had been burned earlier.
In earlier studies, a mass occurrence of sporelings of bracken fern in England has been connected to cities bombed and burned during the World War II. In this study the data of the clones is connected to historical sources and the time of wars in Finland. The comparisons in this study have confirmed the results achieved by comparing the dimensions of the bracken clones to the fire dates. It was found that the wars have caused an increase in the regeneration frequency of bracken fern.
The objective of the present study was to obtain information on the regeneration by spores of bracken fern (Pteridium aquilinum (L.) Kuhn.) as well as the permanence and vegetative spreading of the clones. Prevention of bracken fern, which can hinder forest regeneration, is possible only when the basic facts of the life cycle is known. It would be useful to know if it is probable that bracken fern can become prevalent by spore regeneration, and to what extent it invades forest land by vegetative spreading.
No bracken fern sporelings were found, and the experiments of regeneration by spores were unsuccessful, therefore, the studies were concentrated on the clones. Due to the fire tolerance of the plant, it was expected that the clone material would show gradation according to the periods of fire. The first part of the work concerns individual variation in bracken, the second part concentrates on the data collected from sample plots, which latest fire could be dated from the trees.
The results of the studied clones confirm the correlation between the stands interpreted as clones and the length of the post-fire period. The stands were delimited to clones by their individual characteristics. The spreading on average sites (Vaccinium and Myrtillus type forests) was in average 35.8±2 cm annually. It is concluded that bracken fern regeneration by spores is very rare in Finland and that it has been rarer during the last 50 years compared to the preceding period. This is probably caused by the end of slash-and-burn cultivation and decrease of forest fires in Finland. Bracken fern clones often survive the fires through their rhizomes, and fragments of old clones originating before the fire could be found in the study areas. These clones may be even 300 years old. Especially in its northern areas of distribution, bracken fern is connected with settlement and old slash and burn sites. Regeneration by spores in the northern distribution areas seem to be less than in the southern areas.