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 9ￂﾠyears of free air CO2 enrichment (+200ￂﾠppm; 2001￢ﾀﾓ2009) and 6ￂﾠyears of soil warming (+4ￂﾠￂﾰC; 2007￢ﾀﾓ2012). The study contained two key tree line species, Larix decidua and Pinus uncinata, both approximately 40ￂﾠyears old, growing in heath vegetation dominated by dwarf shrubs. In 2012, we harvested and measured biomass of all trees (including root systems), above￢ﾀﾐ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% (5.7ￂﾠkgￂﾠm￢ﾈﾒ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￢ﾀﾐ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 (￢ﾈﾒ40% for all roots <2ￂﾠmm in diameter at 0￢ﾀﾓ20ￂﾠcm 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.