The growing conditions in northern boreal forests have remained similar for millennia. However, amplified climate change may cause higher mean annual temperatures and precipitation sums, longer growing seasons, along with increased occurrence of extreme weather events (drought, heavy rain, or summertime frost) in the region. The relationship that forest vegetation has with soil nutrients and the exchange of carbon dioxide (CO2) between the forest and atmosphere may change. This dissertation focuses on quantifying the baseline status of northern boreal forests from these aspects, to be able to predict the upcoming changes more precisely. Soil total phosphorus (P) and nitrogen (N) contents were important factors in explaining the community composition of understory vegetation in the study site. The site was located in a region near a phosphate ore, where soil nutrient contents are highly variable. The number of herb, grass, and sedge species increased with N and P contents in the humus, especially with P. The increasing P content, on the other hand, positively correlated with downy birch (Betula pubescens Ehrh.), which was the dominant tree species of the research plot.
The understory vegetation had an important role in the CO2 exchange rates of a northern boreal Scots pine (Pinus sylvestris L.) forest site. The annual CO2 dynamics varied between the canopy and understory, so that when the canopy began photosynthesizing in the spring, the understory was still under snow cover. The cumulative temperature sum had a higher positive correlation with photosynthesis than the total ecosystem respiration (TER) rate of the pine site. Overall, the pine site was a weak carbon sink during the growing season, although it temporarily turned into a carbon source during a cold and rainy summer.
Extreme weather events, and their effects on the CO2 dynamics of forests, were studied on a Scots pine site and a Norway spruce (Picea abies (L.) Karst.) site. Both sites had experienced extreme summers during the studied times, but the CO2 flux rates in the Norway spruce site responded more clearly to them. The TER rates of the Norway spruce forest declined when it was warm and dry. This likely happened because of decreased decomposition of organic matter. The decline was, however, only temporary, and TER returned to normal when the temperature and precipitation returned to their average levels. Thus, the studied forest sites seemed to, so far, be rather resilient towards extreme weather events.
Several studies have found that N availability will increase because of warmer temperatures, which speeds up decomposition and nutrient mineralization. However, decomposition may potentially slow down in some spruce forests due to heat. Local variation may thus be high when it comes to the availability of nutrients or to the CO2 dynamics of forests. While modeling studies are important for predicting the responses of northern forests to climate change on the large scale, our research reminds that local-scale studies are also inevitable for gaining a more precise picture.