Seasonal and inter-annual variability of the net ecosystem CO2 exchange of a temperate mountain grassland:: Effects of weather and management

The role and relative importance of weather and cutting for the seasonal and inter-annual variability of the net ecosystem CO2 exchange (NEE) of a temperate mountain grassland was investigated. Eddy covariance CO2 flux data and associated measurements of the green plant area index and the major environmental driving forces acquired during 2001-2006 at the study site Neustift (Austria) were analyzed. Driven by three cutting events per year which kept the investigated grassland in a stage of vigorous growth, the seasonal variability of NEE was primarily modulated by gross primary productivity (GPP). The role of environmental parameters in modulating the seasonal variability of NEE was obscured by the strong response of GPP to changes in the amount of green plant area, as well as the cutting-mediated decoupling of phenological development and the seasonal course of environmental drivers. None of the environmental and management metrics examined was able to explain the inter-annual variability of annual NEE. This is thought to result from (1) a high covariance between GPP and ecosystem respiration (R-eco) at the annual timescale which results in a comparatively small inter-annual variation of NEE, (2) compensating effects between carbon exchange during and outside the management period, and (3) changes in the biotic response to rather than the environmental variables per se. GPP was more important in modulating inter-annual variations in NEE in spring and before the first and second cut, while Reco explained a larger fraction of the inter-annual variability of NEE during the remaining periods, in particular the post-cut periods.