Functional changes in the control of carbon fluxes after 3 years of increased drought in a Mediterranean evergreen forest?

Abstract

Our objective was to test how a long‐term increased water limitation affects structural and functional properties of a Mediterranean ecosystem, and how these changes modify the response of the main carbon fluxes to climatic controls. In 2003, a 27% throughfall exclusion experiment was installed in a Quercus ilex L. forest in France. Gross primary production (GPP), ecosystem respiration ( R ECO ) and net ecosystem exchange (NEE) were estimated in a control and a dry treatment. Decreasing throughfall decreased GPP by 14% and had a smaller effect on R ECO (−12%), especially soil respiration R S (−11%). Interannual variability of GPP (29%) was higher than for R ECO (12%). Error propagation was used to estimates uncertainties in the NEE fluxes, which ranged from 3% to 10% in the control treatment but up to 167% for NEE in the dry treatment because more steps and data types were involved in the scaling. After 3 years of throughfall exclusion, we found no acclimation of R S to climatic drivers. Functional properties of the response of R S to soil water, temperature and rain pulse remained similar in the control and the dry treatments. A diurnal clockwise hysteresis in R S was probably controlled by canopy photosynthesis with a 3 h lag. The proportion of diurnal variation of respiration due to photosynthesis was similar in all treatments (4–5%). Because of the characteristic of rain in Mediterranean climates, a continuous decrease of water input in these environments have an effect on topsoil water and consequently on R S only during short periods when rainfall is characterized by infrequent and small events that does not allow the topsoil to reach field capacity and does not allow to dry completely. However, in the longer term, we expect a stronger decrease in R S in the dry treatment driven by the decrease in GPP.

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