Growth and community responses of alpine dwarf shrubs to in situ CO2 enrichment and soil warming

Background/Question/Methods

Rising CO₂ concentrations and the associated global warming are expected to have strong impacts on alpine and arctic ecosystems, yet long-term multi-factor experiments in these environments are rare. Dwarf shrubs are a major component of plant communities in these regions, and shifts in their growth and distribution are likely to have important ecological consequences. In this study, we observed how nine years of free air CO₂ enrichment (FACE) and three years of soil warming affected 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 larch and pine trees at treeline in the Swiss Alps. We measured shoot growth and leaf morphology during individual years of the study and used total plant height to represent the cumulative growth response over several years. To explore if growth responses were associated with changes in N availability, we measured leaf N concentration and soil inorganic N. We recorded the number of vascular plant, moss and lichen species in each experimental plot to study treatments effects on species richness. We aimed to understand (1) the growth responses of individual dwarf shrub species to CO₂ enrichment, soil warming, and the combined treatments and (2) how the treatments influenced species richness in the experimental plots.

Results/Conclusions

Vaccinium myrtillus was the only species that showed a clear positive CO₂ effect on growth, with no decline over time in the annual shoot growth response (12% mean stimulation). The CO₂ effect on each dwarf shrub species was consistent over several years of enrichment, providing a clear indication that these species differ in their responsiveness irrespective of annual variations in climate. Soil warming stimulated V. myrtillus growth even more than elevated CO₂ (31% mean stimulation), while  V. gaultherioides and E. hermaphroditum growth were not responsive. Warming increased plant-available soil N and leaf N concentrations, so it is possible that enhanced nutrient availability during these first years of warming contributed to the growth stimulation observed for V. myrtillus. Vascular plant species richness declined in elevated CO₂ plots with larch, while the number of moss and lichen species decreased under warming. Our findings indicate that ongoing environmental change could lead to less diverse plant communities and increased dominance of the particularly responsive V. myrtillus in the studied alpine treeline. These changes are largely the consequence of independent CO₂ and warming effects.