Net ecosystem production in a Little Ice Age moraine: the role of plant functional traits

Abstract. Current glacier retreat allows vast mountain ranges available for vegetation establishment and growth. Little is known about the effective carbon (C) budget of these new ecosystems and how the presence of different vegetation communities, characterized by their specific physiology and life forms influences C fluxes. In this study, using a comparative analysis of the C fluxes of two contrasting vegetation types, we intend to evaluate if the different physiologies of the main species have an effect on Ecosystem Respiration (R_eco), Gross Primary Production (GPP), annual cumulated Net Ecosystem Exchange (NEE), and long-term carbon accumulation in soil.

The NEE of two plant communities present on a Little Ice Age moraine in the Matsch glacier forefield (Alps, Italy) was measured over two growing seasons. They are a typical C3 grassland, dominated by Festuca halleri All. and a community dominated by CAM rosettes Sempervivum montanum L. on rocky soils. Using transparent and opaque chambers, we extrapolated the ecophysiological responses to the main environmental drivers and performed the partition of NEE into R_eco and GPP. Soil samples were collected from the same site to measure long-term C accumulation in the ecosystem.

The two communities showed contrasting GPP but similar R_eco patterns and as a result significantly different in NEE. The grassland acted mainly as a carbon sink with a total cumulated value of −46.4 ± 35.5 g C m⁻² NEE while the plots dominated by the CAM rosettes acted as a source with 31.9 ± 22.4 g C m⁻². In spite of the NEE being different in the two plant communities, soil analysis did not reveal significant differences in carbon accumulation. Grasslands showed 1.76 ± 0.12 kg C m⁻², while CAM rosettes showed 2.06 ± 0.23 kg C m⁻².

This study demonstrates that carbon dynamics of two vegetation communities can be distinct even though the growing environment is similar. The physiological traits of the dominant species determine large differences in the carbon cycle. Therefore, to analyze NEE of any glacier forefield ecosystem, different functional traits of the vegetation communities must be taken into consideration. Moreover, to assess the net ecosystem carbon balance it is necessary to consider the lateral fluxes of carbon via animal consumption, winter respiration, and in a broader temporal perspective, the different stages characterizing the primary succession.