Half a century of multiple anthropogenic stressors has altered northern forest understory plant communities

Abstract

The boreal forests constitute the largest forest biome in the northern hemisphere. These forests are under increasing anthropogenic impact from intensified forest management, eutrophication and climate change, which may change their ecosystem functions and the services they provide. Swedish forests cover a long climatic gradient, receive highly variable rates of nitrogen deposition, and have a long history of forest use. Extensive systematic long-term data on vegetation from the National Forest Inventories (NFI) make Sweden an ideal area to study how species composition and function of other, more pristine boreal forests, might change under increased anthropogenic impact. We used NFI-data to quantify changes in vegetation types between two periods, 1953-1962 and 2003-2012. Our results provide evidence of large-scale changes of the understory vegetation over the last half century. A majority of the vegetation types changed significantly in areal extent, with a general increase in types characterized by high nutrient acquisition and shade adaptation, e.g. high specific leaf area, and a decrease in types characterized by high nutrient conservatism and light demand, e.g. low specific leaf area. The mean cover of ericaceous dwarf-shrubs decreased dramatically. All these effects were most pronounced in areas with the highest anthropogenic impact, suggesting a link to drivers such as nitrogen deposition and land-use changes. Possible associated changes in ecosystem functions and services include effects on higher trophic levels and higher decomposition rates of plant litter in the expanding vegetation types, which can influence nutrient dynamics, with subsequent effects on ecosystem productivity and C sequestration.