Interest in growing mixed species stands has increased in recent years, and the cultivation of mixed stands is promoted in silvicultural guidelines and forest certification systems. Mixtures of tree species have often increased biomass production, but the effect of species mixture on productivity has been small in studies conducted in the Nordic countries. However, several previous studies are based on temporary plots selected according to predefined criteria, usually removing possible effects of damage affecting forest growth. Sample-based forest inventories describe the actual development of forest resources and provide an opportunity to analyse the factors affecting forest growth. Using the data of the Finnish National Forest Inventory (NFI), we analysed the species mixture-productivity relationship that take place in real environmental and management conditions. We analysed pure and mixed species stands dominated by spruce and pine in southern Finland and compared their volume increment, growing stock, and damage, and present results separately for age classes 21-40, 41-60, and > 60 years. Mixed stands were more common in the 1920 s because of historical forms of land use, but tree species composition in the current forests is quite different, and the stands are conifer-dominated. Slight and moderate broadleaved species mixture did not notably affect the volume growth of coniferous stands on average, while it varied somewhat from case to case. Instead, the growth of pine- and spruce-dominated stands was related to the amount of growing stock, site fertility, and stand age, regardless of the level of broadleaved species mixture. In addition, the results showed an increasing proportion of damaged stands with an increasing proportion of broadleaved trees. Our results at regional level confirm the results of previous Nordic studies conducted at stand level that slight and moderate mixtures of broadleaved trees in coniferous stands have no or only a low reducing effect on wood production.

Fire severity is increasing across the boreal forest biome as climate warms, and initial post-fire changes in tree demographic processes could be important determinants of long-term forest structure and carbon dynamics. To examine soil burn severity impacts on tree regeneration, we conducted experimental burns in summer 2012 that created a gradient of residual post-fire soil organic layer (SOL) depth within a mature, sparse-canopy Cajander larch (Larix cajanderi Mayr.) forest in the Eastern Siberian Arctic. Each fall from 2012 to 2016, we added larch seeds to plots along the burn severity gradient. We tracked density of new larch germinants and established seedlings (alive >= 1 year) during subsequent growing seasons, along with changes in seedbed conditions (permafrost thaw depth, moisture, and temperature). Over the study, a cumulative total of 17 and 18 new germinants m(-2) occurred in high and moderate severity treatments, respectively, while germinants were rare in unburned and low severity treatments ( 50%) germinated in summer 2017, following a mast event in fall 2016, suggesting safe sites for germination were not fully occupied in previous years despite seed additions. By 2017, established seedling density was similar to 5 times higher on moderate and high severity treatments compared to other treatments. Cumulative total density of new germinants and established seedlings increased linearly with decreasing residual SOL depth, as did thaw depth, soil moisture, and soil temperature. Our findings suggest that increased soil burn severity could improve seedbed conditions and increase larch recruitment, assuming seed sources are available. If these demographic changes persist as stands mature, a climate-driven increase in soil burn severity could shift forest structure from sparse-canopy stands, which dominate this region of the Siberian Arctic, to high density stands, with potential implications for carbon, energy, and water cycling.