Alpine ecosystems are highly sensitive to climate change because of their cold-adapted vegetation and short growing seasons. This mesocosm study from Mont Blanc (Italy) assessed short-term responses of an alpine grassland to projected future climates. The experiment aimed to evaluate how combined changes in temperature, precipitation, humidity, radiation and elevated CO2 would affect carbon fluxes, biomass allocation and plant functional traits under scenarios approximating RCP 4.5 (moderate) and RCP 8.5 (severe).
Researchers used Ecotron mesocosms containing intact ecosystem monoliths collected from the alpine grassland. Treatments combined current atmospheric CO2 (~420 ppm) and elevated CO2 concentrations (~550 and ~800 ppm) with corresponding climate conditions for RCP 4.5 and RCP 8.5, altering temperature and other microclimatic variables. Throughout a single growing season the team measured CO2 fluxes—net ecosystem production (NEP), gross primary productivity (GPP) and ecosystem respiration (Reco)—as well as soil chemistry, vegetation cover, above- and belowground biomass, and plant functional traits.
Under the severe warming scenario (RCP 8.5) GPP increased relative to current conditions, indicating enhanced photosynthetic activity. However, Reco also rose under RCP 8.5, and daytime NEP remained effectively stable because increased CO2 uptake during the day was offset by higher respiration. On a diel basis the pattern showed that higher daytime photosynthesis was counterbalanced by enhanced respiration, producing a trajectory that suggests a potential shift toward net carbon loss if sustained over longer timescales. Under the moderate warming scenario (RCP 4.5), the ecosystem fluxes did not show major destabilization within the single season observed.
The severe warming treatment led to increased vegetation cover and greater total aboveground biomass. Importantly, there was a shift in biomass allocation toward aboveground growth with reduced belowground investment. This reallocation under RCP 8.5 contrasts with the relative stability observed under RCP 4.5. The change toward aboveground-dominated biomass potentially affects carbon storage dynamics because belowground biomass often contributes disproportionately to longer-term soil carbon pools.
Species-specific responses were evident. Salix herbacea in particular showed functional trait adjustments under the severe warming treatment, including higher leaf mass per area (LMA), elevated sodium (Na) concentration and increased δ15N. The authors interpret these shifts as consistent with greater leaf longevity, altered water-use efficiency, and changes in nitrogen metabolism. Such trait changes indicate physiological acclimation by some species and imply potential reorganization of community composition if differential responses persist.
During the course of the short-term mesocosm experiment the authors reported no detectable limitations from water or nutrients that would have constrained the increased productivity under RCP 8.5. This suggests that, in the conditions of the experiment and over a single growing season, resource availability did not prevent the observed increases in GPP and biomass allocation shifts.
The combination of higher photosynthesis and substantially higher ecosystem respiration under severe warming produces a complex outcome: daytime NEP remained stable because increased assimilation was balanced by elevated respiration, but diel patterns and the respiration increase point toward a risk of net carbon loss at the ecosystem scale. The observed shift from belowground to aboveground allocation reduces investment in soil-root carbon pools, which could, if maintained long-term, diminish soil carbon sequestration and tip the system from a carbon sink to a carbon source. Moreover, species-specific trait changes imply uneven acclimation and possible community reorganization, which would further influence long-term carbon dynamics.
Over a single growing season, the alpine grassland showed resilience to moderate warming (RCP 4.5). Under severe warming (RCP 8.5), the system exhibited increased GPP, vegetation cover and aboveground biomass alongside elevated Reco and a shift in biomass allocation toward aboveground tissues. These changes, together with species-specific trait adjustments (notably in Salix herbacea), suggest that sustained severe warming could alter the carbon balance of alpine grasslands and potentially convert them from sinks into sources. The source reports only short-term mesocosm results for one growing season; long-term ecosystem trajectories and broader landscape-level outcomes were not reported in the source and therefore remain uncertain.