Questions: How do climate conditions and the site's ecohydrological properties affect the age and size structure of natural Pinus sylvestris stands on pristine boreal mires? How do the long-term stand dynamics on mires proceed as stands age? Do the mire stands reach a balanced, old-growth stage?

Location: Boreal mire forests in southern and northern Finland.

Methods: Tree age and diameter distributions were analysed in 52 stands in two climate areas and in two mire site types with different ecohydrological properties. Temporal stand dynamics were examined by (1) comparing the graphs of the stands' mean tree ages by diameter at breast height (1.3 m) classes and (2) describing the changes in stand characteristics and stand age and size structures as a function of stand dominant age in a chronosequence.

Results: In the south, the DBH distributions were mostly unimodal and bell-shaped in both site type groups. Age distributions were multimodal and flat in fully-stocked sites but more uneven in sparsely forested composite sites. In the north, both the age and size distributions were clearly uneven in both site type groups. Tree age and size variation increased with stand age, but levelled out in the long term. Particularly in the south, the abundance of small trees decreased as stand age increased.

Conclusions: The pine stands on pristine boreal mires are more dynamic than anticipated and are generally not characterised by a balanced, self-perpetuating structure. Their dynamics reflect differences in climate and ecohydrology: on stocked sites in favourable boreal conditions, the stands showed structures typically resultant of inter-tree competition processes that control tree growth and regeneration, whereas in harsh boreal climates, the tree regeneration process is ongoing diversifying the stand structure.