The aim of this study was to experimentally test the most favourable seeding times for Scots pine (Pinus sylvestris L.), and to compare Scots pine and lodgepole pine (Pinus contorta Douglas ex Loudon) in respect to suitable seeding times and site factors. The experimental area was located in Sodankylä, Central Finnish Lapland. Generalized linear mixed effects models with binomial distribution assumption were applied to model the presence or absence of a seedling at a seeding point. The study shows that in addition to spring and early summer, direct seeding of Scots pine can also be carried out during late autumn or even early winter (October and November) in northern Fennoscandia. On the contrary, seeding in late summer and early autumn (August and September) is much less successful and cannot be recommended as such, but can be used if the amount of seeding material is increased to compensate the loss. There is more flexibility for lodgepole pine for which the proper seeding period seems to be from spring through to late autumn. Whether lodgepole pine seeds were stratified or not had no statistically significant effect on regeneration success. Our results clearly indicate that lodgepole pine is less susceptible to unfavourable site and soil factors than Scots pine. Our results give support to extend the period of direct seeding from the present early summer to another period in late autumn and even early winter.
The boreal timber- and tree-line forests grow in harsh environmental conditions in their outermost distribution limit. Here even small environmental changes may cause dramatic changes in the distribution of tree species. We examined changes of the forest lines of Norway spruce (Picea abies (L.) H. Karst.) and Scots pine (Pinus sylvestris L.) in Finnish Lapland five times during 1983–2009. We monitored the number of stems and the volume of the growing stock in thirteen different locations in forest-line areas. The linear temporal trends and the variations of these response variables were used as indicators of a possible change during the study period. Spruce showed a significant increase both in the volume of the growing stock (up to 40% increase) and in the total stem number (up to 100% increase). A significant increase in the volume of the growing stock was observed in the pine data as well (up to 70% increase), whereas the stem number stagnated or even decreased. The results suggest that spruce needs favourable conditions to have an abundant regeneration, but after the establishment the seedlings seem to be more resistant against biotic and abiotic disturbances than pine seedlings. The increasing stand volume might result in a climate-related northward and upward extension of forests in the future. However, our results show that responses in the boreal forest line are species and location specific and a more favourable climate does not necessarily lead to an advance of the coniferous forest line.