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Keywords:

  • Aspen regeneration;
  • Boreal forests;
  • Disturbance dynamics;
  • Fire history reconstruction;
  • Populus tremula ;
  • Salix caprea

Abstract

Questions

Is the current aspen or goat willow populations in boreal old-growth forest a legacy of the historical fires?

Location

A middle-boreal, old-growth forest landscape with a large proportion of wetlands in eastern Finland.

Method

Forest fire history of a 2100-ha area was reconstructed using fire scars, forest age estimates and the presence or absence of charred stumps and charcoal particles in the soil in 63 systematically located plots. Age and size structures of aspen and goat willow were studied in a subsample of 12 plots.

Results

During the past 400 yr, forest fires have been seven times more numerous in the study area than could be expected based on modern lightning fire statistics. The ages of mature aspens (average 177 and maximum 213 yr) and mature goat willows (average 111 and maximum 173 yr) were higher than earlier reported, strikingly so in goat willow. Aspen age structure clearly reflected the past fires. Most aspens originated from the beginning of the 19th century, during a period of frequent forest fires. The regeneration of aspen leading to mature trees has been very low since the cessation of forest fires in the late 19th century. In turn, goat willow has also regenerated in spite of the absence of fires.

Conclusions

The majority of the mature aspen trees were recruited at the time of the last fires, and there was a notable lack of middle-aged trees. This relationship to fires was not valid for goat willow. The main reason for the lack of young aspen cohorts in aspen populations in Fennoscandian old-growth stands is most likely the lack of forest fires. Small-scale gap dynamics cannot offset the lack of large-scale disturbances, such as forest fires, for the regeneration of aspen, possibly because of the prevailing herbivory and competitive effects. Hence, the population decline of aspen is likely to continue and to threaten a wealth of species specialized on aspen in boreal forests.