Decadal and centennial changes of boreal forest vegetation and soil microbial communities. Natural and altered dynamics

Boreal forest form one of the largest land biomes and most notable carbon sinks. In Finland, forests cover 75% of land area. Humans and forests have always been in close association, but the use of forests and their value have changed in time. Recently the use of Finnish forests and their wellbeing has been discussed in national and international media. In addition to local land use, forest ecosystems are nowadays threatened by other anthropogenic drivers, like climate change. Together these human influences are called global change. One of the most important challenges of our time may be to understand how global change will affect forest systems.
Previous research verifies the poor state of Finnish forest ecosystems: forests are degraded to small patches, their structure is younger and monotonous, and the amount of dead wood has drastically decreased. These structural changes are evidently reflected to the forest biota. Nearly one third of threatened species live in forests. The main reason for this trend is the current use of forests.
Understory vegetation and soil have a huge impact on ecosystem properties and therefore largely regulate the habitat features for forest biota. The importance of linkages between above-ground and below-ground communities has only recently been understood. Changes in forest vegetation communities may therefore have important influences for ecosystem functioning and forest fauna. On one hand, forest vegetation is considered relatively resistant to changes, but on the other hand several studies show that climate change and land use may significantly influence them.
In my doctoral thesis, I studied natural and altered development of forest understory vegetation (vascular plants, bryophytes and lichens) using resurvey data. Resurvey provides a unique approach to detect changes over several decades and link them to environmental drivers, using large data sets. For example, the first resurvey data I used, was originally collected in 1961 and resurveyed in 1986 and 2013. History for this data was known since 1920’s. The second resurvey data consisted of 245 sites spread across the northern Finland. This data was originally sampled in 1960’s and resurveyed in 2019. The third data I used to study the development of vegetation and soil microbial communities during soil formation, was from a primary successional series created by post-glacial land uplift. As climate change and land use are considered the main threats for boreal forests, I also studied their impacts on fertile herb-rich forests and coniferous forests. I used forest management and reindeer husbandry as land use variables.
The results of my thesis showed forest vegetation and soil microbial communities to change in time. As expected, natural and altered changes led to partly different developmental pathways. In dry and nutrient poor forests, the development of above-ground and below-ground communities was associated to the accumulation of organic matter. In herb-rich forests and mesic coniferous forests, natural succession lead communities towards a spruce dominated state. In terms of herb-rich sites this development increases their homogeny to the surrounding vegetation and can reduce their biodiversity. In my data, climate change had no detectable influence. Forest management reduced biodiversity and altered vegetation community composition in herb-rich forests. Both in coniferous and herb-rich forests, increasing management intensity led to stronger responses. Forest management also strongly reduced the amount of dead wood.
The two most important findings of my thesis are the positive diversity impacts of reindeer on herb-rich forests and that some of the impacts of forestry practices were carried as legacy effects. This is of first studies showing reindeer grazing to maintain herb-rich forest vegetation by regulating vegetation height. These findings are of scientific and practical value when planning sustainable use of forests and maintaining forest biodiversity.