title: Soil warming and CO2 enrichment induce biomass shifts in alpine tree line vegetation.
abstract: Responses of alpine tree line ecosystems to increasing atmospheric CO2 concentrations and global warming are poorly understood. We used an experiment at the Swiss tree line to investigate changes in vegetation biomass after 9 years of free air CO2 enrichment (+200 ppm; 2001-2009) and 6 years of soil warming (+4 °C; 2007-2012). The study contained two key tree line species, Larix decidua and Pinus uncinata, both approximately 40 years old, growing in heath vegetation dominated by dwarf shrubs. In 2012, we harvested and measured biomass of all trees (including root systems), above-ground understorey vegetation and fine roots. Overall, soil warming had clearer effects on plant biomass than CO2 enrichment, and there were no interactive effects between treatments. Total plant biomass increased in warmed plots containing Pinus but not in those with Larix. This response was driven by changes in tree mass (+50%), which contributed an average of 84% (5.7 kg m(-2) ) of total plant mass. Pinus coarse root mass was especially enhanced by warming (+100%), yielding an increased root mass fraction. Elevated CO2 led to an increased relative growth rate of Larix stem basal area but no change in the final biomass of either tree species. Total understorey above-ground mass was not altered by soil warming or elevated CO2 . However, Vaccinium myrtillus mass increased with both treatments, graminoid mass declined with warming, and forb and nonvascular plant (moss and lichen) mass decreased with both treatments. Fine roots showed a substantial reduction under soil warming (-40% for all roots <2 mm in diameter at 0-20 cm soil depth) but no change with CO2 enrichment. Our findings suggest that enhanced overall productivity and shifts in biomass allocation will occur at the tree line, particularly with global warming. However, individual species and functional groups will respond differently to these environmental changes, with consequences for ecosystem structure and functioning.
authors.affiliation: WSL Institute for Snow and Avalanche Research - SLF, Flüelastrasse 11, CH-7260, Davos Dorf, Switzerland; Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstrasse 111, CH-8903, Birmensdorf, Switzerland.
authors.forename: Melissa A
authors.initials: MA
authors.lastname: Dawes
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Philipson
authors.forename: Patrick
authors.initials: P
authors.lastname: Fonti
authors.forename: Peter
authors.initials: P
authors.lastname: Bebi
authors.forename: Stephan
authors.initials: S
authors.lastname: Hättenschwiler
authors.forename: Frank
authors.initials: F
authors.lastname: Hagedorn
authors.forename: Christian
authors.initials: C
authors.lastname: Rixen
mesh_terms: D018533:Biomass; D002245:Carbon Dioxide / Q000032:analysis* / Q000494:pharmacology; D057232:Global Warming; D028221:Larix / Q000254:growth & development*; D015233:Models, Statistical; D028223:Pinus / Q000254:growth & development*; D012987:Soil; D013045:Species Specificity; D013557:Switzerland; D013696:Temperature; D065929:Tundra
chemical_list: D012987:Soil; D002245:Carbon Dioxide
keywords: European larch; Larix decidua; Pinus uncinata; dwarf shrub; free air CO2 enrichment; global change; mountain pine