Comparing carbon fluxes between different stages of secondary succession of a karst grassland

Abandonment of marginal agricultural areas with subsequent secondary succession is a widespread type of land use change in Mediterranean and mountain areas of Europe, leading to important environmental consequences such as change in the water balance, carbon cycling, and regional climate. Paired eddy flux measurement design with grassland site and tree/shrub encroached site has been set-up in the Slovenian Karst (submediterranean climate region) to investigate the effects of secondary succession on ecosystem carbon cycling. The invasion of woody plant species was found to significantly change carbon balance shifting annual NEE from source to an evident sink. According to one year of data succession site stored −126 ± 14 g C m−2 y−1 while grassland site emitted 353 ± 72 g C m−2 y−1. In addition, the seasonal course of CO2 exchange differed between both succession stages, which can be related to differences in phenology, i.e. activity of prevailing plant species, and modified environmental conditions within forest fragments of the invaded site. Negligible effect of instrument heating was observed which proves the Burba correction in our ecosystems unnecessary. Unexpectedly high CO2 emissions and large disagreement with soil respiration especially on the grassland site in late autumn indicate additional sources of carbon which cannot be biologically processes, such as degassing of soil pores and caves after rain events.