Consequences of tree species mixture for growth and damage based on large-scale inventory data in Finland

Abstract

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.