Biomass shifts in treeline vegetation after nine years of CO2 enrichment and six years of soil warming

Background/Question/Methods

As boundary ecosystems between mountain forests and alpine tundra, high-elevation treelines are expected to be especially sensitive to global change. In particular, increases in atmospheric CO₂ concentration and warming could alter the productivity and species composition of trees and low-stature plants, with consequences for ecosystem structure and functions. Knowledge about how lowland forests will respond to environmental change has advanced rapidly in recent years. However, it remains unclear if these patterns apply to alpine treeline ecosystems in the temperate zone, environments characterized by long snow cover duration, low growing season temperatures and low nutrient availability. We used an in situ experiment within a c. 40-year-old at treeline in the Swiss Alps (Stillberg, Davos) to investigate changes in treeline vegetation biomass after nine years of free air CO₂ enrichment (+200 ppm; 2001-2009) and six years of soil warming (+4°C; 2007-2012). The study contained two key alpine treeline species, Larix decidua and Pinus uncinata, growing in heath vegetation dominated by dwarf shrubs. In 2012, we conducted a complete harvest and measured biomass for all trees (including root systems), above-ground understorey vegetation (separated by individual species or functional group), and fine roots (all species combined, sampled using soil cores).

Results/Conclusions

Overall, warming only soil for six years had clearer effects on plant biomass than nine years of CO₂ enrichment, and there was no evidence of interactive effects between the treatments. Total plant biomass per unit land area increased in warmed plots containing Pinus but not in those with Larix. This response was driven by changes in tree aboveground and coarse root mass, which when combined contributed an average of 75% (5.7 kg m^-2) of total biomass. Total understory aboveground biomass was not altered by soil warming or elevated CO₂, yet significant shifts occurred within this layer. Vaccinium myrtillus biomass increased with both treatments, whereas the other major dwarf shrub species V. gaultherioides and Empetrum hermaphroditum showed no response. Grass biomass declined with soil warming, and forb and nonvascular plant (moss and lichen) biomass decreased with both soil warming and CO₂ enrichment. Fine roots showed a substantial reduction under soil warming, possibly due to drier conditions at shallow soil depths, but no change under CO₂ enrichment. Our findings suggest that enhanced overall productivity is likely to occur at the treeline, particularly with global warming, but that individual species will respond differently to environmental changes.