The carbon cycle of woody plants invaded dry calcareous pastures D. Vodnik1,*, M. Ferlan1,2, K. Eler1, G. Alberti3, A. Peressotti3, F. Miglietta4, P. Simončič2, F. Batič1 1
University of Ljubljana, Biotechnical faculty, Jamnikarjeva 101, 1000 Ljubljana, Slovenia; 2Slovenian Forestry Institute, Večna pot 2, 1000 Ljubljana, Slovenia; 3DISA - University of Udine, Italy; 4CNR-IBIMET – Firenze, Italy; * corresponding author (dominik.vodnik@bf.uni-lj.si)
Background and aims Spontaneous transition of grasslands to shrublands or forests is a near-global phenomenon that strongly affects carbon biogeochemical cycle and carbon balance of the ecosystems. In our research we studied carbon fluxes at calcareous submediterranean grasslands invaded by shrubs of early succession stages and also tree species of mid- and late succession. The aim was to analyze the seasonal changes in net ecosystem CO2 exchange (NEE) with respect to weather conditions and precipitation patterns and to assess the response of different components of carbon cycling to woody plants encroachment. Methods A paired eddy flux measurement design was used in order to asses NEE of two ecosystems: an extensively used dry pasture (grassland of Scorzoneretalia order) and proximate abandoned grassland with woody plants encroachment (Quercus pubescens) at Podgorski kras plain (SW Slovenia). Within the footprint area of one of two towers for Eddy flux measurement, we have also performed measurements of photosynthesis, litterfall analyses and decomposition studies. Soil respiration (Rs) measurements have been combined with the isotopic analyses. Key results Comparison of the yearly courses of NEE for 2009 clearly reveals the differences in growing season length and net production rates of both ecosystems. Invaded site showed one month time lag before becoming a net C sink in spring and continued to fix carbon for further two months in autumn in comparison to the grassland. On a yearly basis, succession was a net sink of carbon (NEE = -126 gC m-2 y-1) while grassland was a source of carbon (NEE = + 353 gC m-2 y-1). The results of Rs correspond well with the observed NEE values. In both cases, relatively high fluxes were observed. Conclusions Seasonal shifts of C balance are mainly governed by the activity of forest fragments. High CO2 fluxes can be partly contributed by inorganic and subterraneous sources of CO2.
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