Two decades of experimental manipulations of heaths and forest understory in the subarctic

Abstract

Current atmospheric warming due to increase of greenhouse gases will have severe consequences for the structure and functioning of arctic ecosystems with changes that, in turn, may feed back on the global-scale composition of the atmosphere. During more than two decades, environmental controls on biological and biogeochemical processes and possible atmospheric feedbacks have been intensely investigated at Abisko, Sweden, by long-term ecosystem manipulations. The research has addressed questions like environmental regulation of plant and microbial community structure and biomass, carbon and nutrient pools and element cycling, including exchange of greenhouse gases and volatile organic compounds, with focus on fundamental processes in the interface between plants, soil and root-associated and free-living soil microorganisms. The ultimate goal has been to infer from these multi-decadal experiments how subarctic and arctic ecosystems will respond to likely environmental changes in the future. Here we give an overview of some of the experiments and main results.

Fig. 1

Ecosystem manipulations near Abisko, Northern Sweden. a Mesic Cassiope tetragona–Empetrum hermaphroditum–Betula nana heath near the treeline, with manipulations of light (dome-shaped greenhouses with shading cloth), temperature (open-top plastic greenhouses), and nutrient availability (NPK fertilizer). b Mixed, wet dwarf shrub heath with open-top greenhouses and mountain birch litter addition plots in a fully factorial design. c Measurement of greenhouse gas exchange in species removal experiments in the understory of a mountain birch birch (Betula pubescens ssp. czerepanovii) forest. Photos by Anders Michelsen

Fig. 2

Plant coverage at mesic treeline heath near Abisko, N. Sweden after 20 years of summer warming with open-top plastic greenhouses. Mean coverage (%) of different plant functional types (± SE, n = 6) in the control and warming treatment in analysis of 100 pin-points in July 2008. Results of one-way ANOVA’s for different plant functional groups are indicated; (*) P < 0.10, * P < 0.05, ** P < 0.01, *** P < 0.001. Modified from Sorensen et al. (2012)

Fig. 3

Long-term effects of warming, litter addition and fertilizer addition on nitrogen fixation measured with the acetylene reduction method at ecosystem level (20 × 20 cm heath squares), and in plugs with the moss Hylocomium splendens. Data are from the wet heath after 10 years of treatment, and from the mesic heath after 20 years of treatment. Data are means across growing season, adapted from Sorensen and Michelsen (2011) and Sorensen et al. (2012). Significant differences between treatments investigated with repeated measures mixed model ANOVA with Tukey adjusted means comparison. Treatments influenced means in all cases, P < 0.05. Within site or type, bars with the same letters are not significantly different (± SE, n = 6)

Fig. 4

Microbial biomass C in mesic shrub heath with open-top chambers and fertilizer addition plots in a fully factorial design. Treatments were initiated in 1989. The figure shows the relative change in microbial biomass C compared to control plots (± SE, n = 6). Data modified from Jonasson et al. (1999), Ruess et al. (1999), S. Jonasson and A. Michelsen (unpublished), Rinnan et al. (2007a) and Sorensen et al. (2008a, b). Microbial biomass C differed between years (P < 0.001; repeated measures two-way ANOVA) and was generally significantly higher in fertilized plots (P < 0.01) but with a tendency towards interaction between year and fertilizer treatment (P < 0.1)

Fig. 5

Volatile organic compound emissions from mixed, wet dwarf shrub heath with warming by open-top chambers and litter addition plots. The figure shows average emissions (± SE, n = 6) from July 2006 and 2007, after 8–9 years of treatment. Data modified from Tiiva et al. (2008), and Faubert et al. (2010). Percentage of isoprene, monoterpene and sesquiterpene emissions of the total BVOC emission in each treatment is shown