Atmospheric CO2 concentrations are predicted to reach 730–1020 ppm by the year 2100 and, as one consequence of this change, the global mean surface air temperature is expected to increase by 1.8–4.0°C during the same period (Meehl et al., 2007). Elevated CO2 concentrations might enhance carbon uptake by plants through a direct stimulation of photosynthesis, and responses might be particularly strong at high elevation where atmospheric pressure, and therefore CO2 partial pressure, are lower (Körner, 2003). Similarly, ongoing global warming is already documented to have particularly large ecological impacts on high-latitude and high-elevation regions where plants grow close to their low temperature limit (Walther, 2003; Dorrepaal et al., 2009). For these reasons, research on environmental change in alpine and arctic ecosystems has increased substantially in recent years, and has included both manipulation experiments (see review by Dormann & Woodin, 2002) and observations of natural vegetation change (e.g. Wilson & Nilsson, 2009; Hill & Henry, 2011).
Elevated CO2 concentrations have been experimentally applied to high-latitude and high-elevation vegetation in only a few studies. Field experiments in a late successional alpine grassland in the Swiss Central Alps (Körner et al., 1997), in a forest heath ecosystem in subarctic Sweden (Gwynn-Jones et al., 1997) and in the wet tussock tundra in Alaska (Tissue & Oechel, 1987) indicate that responses of plant growth and biomass to rising CO2 concentrations are generally small or nonexistent but that co-occurring species often vary in their responses (Dormann & Woodin, 2002). More broadly, CO2 enrichment studies of various plant types conducted in relatively natural growth conditions have revealed that other variables, such as climate and availability of nutrients or water, can influence CO2 effects (e.g. Niklaus & Körner, 2004; Dawes et al., 2011) and that any initial growth stimulation often declines after a few years (Körner, 2006). In ecosystems where nitrogen (N) availability is low, this temporal response pattern can be caused by increasing N limitation over time as a result of increased N immobilization in plant biomass and long-lived soil organic matter and of enhanced microbial activity (Luo et al., 2004). Studies lasting several years are clearly important for determining temporal dynamics of plant responses to CO2 enrichment and interactions with other environmental factors.
In contrast to the small number of CO2 manipulation experiments, many relatively long-term warming experiments have been completed in (sub)arctic (Chapin & Shaver, 1985; Parsons et al., 1994; Shevtsova et al., 1997; Aerts et al., 2009) and alpine (Harte & Shaw, 1995; Kudo & Suzuki, 2003; Kudernatsch et al., 2008) ecosystems. Increased plant growth and biomass production with warming were observed in several experiments, although responses were often small or transient and varied across species and study sites (Rustad et al., 2001; Walker et al., 2006). Many high-latitude and high-elevation environments are characterized by low availability of soil nutrients, particularly N, as a result of low-temperature constraints on decomposition and mineralization (Nadelhoffer et al., 1992; Körner, 2003), and higher soil temperatures tend to accelerate these processes in systems that are not water limited (Cornelissen et al., 2007; Kammer et al., 2009). Therefore, plant growth responses to warming might be caused by enhanced nutrient availability in addition to a direct effect of increased rates of photosynthesis at higher temperatures. Similar to experimental CO2 enrichment, warming experiments lasting several years are needed to elucidate responses at the individual species, community and ecosystem scales, especially as shifts in competitive interactions and plant–soil feedback processes can alter responses over time (Wookey et al., 2009).
Despite clear predictions that rising CO2 concentrations will be accompanied by increased temperatures, relatively few studies combining CO2 enrichment and experimental warming have been conducted within intact systems (Beier, 2004). In a glasshouse study conducted in tussock tundra vegetation (Toolik Lake, AK, USA), CO2-induced stimulation of net primary productivity (NPP) lasted only one season when applied alone but was sustained over three growing seasons when combined with a 4°C temperature increase and unaltered water availability (Oechel et al., 1994). However, most field studies of natural vegetation combining elevated CO2 and warming have been conducted in grassland systems and have yielded conflicting results regarding how the combined changes influence plant productivity, community composition and N cycling (Shaw et al., 2002; Hovenden et al., 2008; Engel et al., 2009; Dijkstra et al., 2010). While experiments combining CO2 enrichment and nutrient addition have been conducted in alpine environments (Körner et al., 1997; N. Inauen, pers. comm.), to our knowledge no previous in situ studies of high-elevation systems have simultaneously manipulated CO2 concentration and temperature.
Dwarf shrubs are a major component of arctic and alpine plant communities, and shifts in the growth and reproductive output, abundance and/or distribution of these species under environmental change are likely to have important ecological consequences. In this study, we determined the effects of 9 yr of free air CO2 enrichment (FACE) and 3 yr of soil warming on three dominant ericaceous dwarf shrub species, deciduous Vaccinium myrtillus (bilberry) and Vaccinium gaultherioides (northern bilberry) and evergreen Empetrum hermaphroditum (crowberry), growing in the understorey of treeline trees in the Swiss Alps. Studying three co-occurring species allowed us to explore whether individual dwarf shrub species respond differently to the manipulations. We hypothesized that: (1) any initial stimulation of above-ground growth of these dwarf shrub species in response to CO2 enrichment would decline over time; (2) soil warming would lead to enhanced dwarf shrub growth, reflecting a direct stimulation and/or an indirect benefit from increased soil N availability; (3) soil warming would alleviate temperature or nutrient constraints on the growth response to elevated CO2, yielding a positive interactive effect of the two experimental treatments on dwarf shrub growth; and (4) species-specific responses of understorey vegetation to the experimental treatments would lead to shifts in species composition in the experimental plots.