The effects of drought and timing of precipitation on the inter-annual variation in ecosystem-atmosphere exchange in a Mediterranean grassland

We studied the seasonal and inter-annual variation in carbon, water and energy fluxes over a Mediterranean grassland ecosystem in Portugal, dominated by annual species, using the eddy-covariance technique. The study period, from 2004 to 2008, was characterised by high intra- and inter-annual precipitation variation: the hydrological years 2004–2005 and 2007–2008 had precipitation below average (‘dry’ years) whereas the other two hydrological years had precipitation above average (‘wet’ years). The variation in energy partitioning into sensible heat (H) and latent heat (λE) fluxes was associated with changes in soil water content, coinciding with plant emergence and senescence. The ecosystem was λE-dominated in those periods with soil water content above 15%, and became H-dominated when soil water content fell below 15%. Annually integrated net ecosystem carbon exchange (NEE) was highly sensitive to drought, varying from a carbon sink in ‘wet’ years (e.g., 2005–2006, −190 g C m−2 y−1), to a carbon source in ‘dry’ years (e.g., 2004–2005, +49 g C m−2 y−1). There was a negative correlation between annually integrated NEE and annual precipitation (r2 = 0.78). However, timing of precipitation influenced NEE, with individual rain events in the dry season resulting in large pulses of ecosystem respiration (Reco). Low precipitation at the peak of the growing season, as in the spring of 2007, decreased carbon sequestration. The best descriptor for inter-annual variation in NEE was leaf area duration (LAD), which explained 96% of the variance. Average radiation-use efficiency (RUE) was 1.59 g MJ−1 and inherent water-use efficiency (IWUE) was 24.6 g C hPa kg−1 H2O. RUE and IWUE had similar trends in inter-annual variation, reflecting differences in productivity and timing of precipitation. Our study showed that carbon and energy fluxes in this Mediterranean environment depended strongly upon water availability.