Agricultural and Forest Meteorology 151 (2011) 595–606 Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet The effects of drought and timing of precipitation on the inter-annual variation in ecosystem-atmosphere exchange in a Mediterranean grassland Marjan Jongen a,∗ , João Santos Pereira a , Luis Miguel Igreja Aires b , Casimiro Adrião Pio c a Department of Forestry, Instituto Superior de Agronomia, Technical University of Lisbon, Tapada da Ajuda, 1349-017 Lisbon, Portugal b Department of Environmental Engineering, School of Technology and Management, Polytechnic Institute of Leiria, 2411-901 Leiria, Portugal c CESAM & Department of Environment, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal article info Article history: Received 19 May 2010 Received in revised form 10 October 2010 Accepted 10 January 2011 Keywords: Ecosystem respiration Eddy-covariance Gross primary productivity Mediterranean grassland Net ecosystem carbon exchange Precipitation Radiation-use efficiency Inherent water-use efficiency abstract We studied the seasonal and inter-annual variation in carbon, water and energy fluxes over a Mediter- ranean 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 pre- cipitation 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 (E) fluxes was associated with changes in soil water content, coinciding with plant emergence and senescence. The ecosystem was E-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 (r 2 = 0.78). However, timing of precipitation influenced NEE, with individual rain events in the dry season resulting in large pulses of ecosystem respiration (R eco ). 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 H 2 O. 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Accurately predicting the future feedback of the biosphere on atmospheric CO 2 partly depends on a better understanding of the carbon balance in major land-use classes. On the other hand, water resources are likely to be the major factor affecting humanity in this century. Therefore, long-term measurements of CO 2 and H 2 O fluxes between the earth’s surface and the atmosphere are important to understand the processes controlling carbon cycling across the globe (Baldocchi et al., 2001). Grassland ecosystems cover approxi- mately 40% of the earth’s terrestrial area. Yet, large uncertainties remain in resolving whether these ecosystems function as CO 2 sources or sinks (Baldocchi et al., 2001). This uncertainty is primar- ∗ Corresponding author. Tel.: +351 21 365 3515; fax: +351 21 365 5000. E-mail address: fay@ireland.com (M. Jongen). ily attributable to the sensitivity of grasslands to climate variability, especially variation in annual precipitation (Flanagan et al., 2002; Knapp and Smith, 2001; Sala et al., 1988) and an incomplete under- standing of the regulation of ecosystem respiration (Flanagan and Johnson, 2005). Climate change may impact strongly on grasslands, as most future scenarios predict an increase in climate variability, with a concomitant higher frequency of drought periods. Paredes et al. (2006) showed that precipitation during March in western Iberia (including Portugal) has been decreasing since the early 1960s, and the PDSI drought index illustrates a clear trend for increasing drought in the same period (Pires, pers. comm.). Increasing drought may affect the potential of grasslands to act as carbon sinks, as droughts are the main source of inter-annual variation in terrestrial carbon sequestration, reducing both productivity and net ecosys- tem carbon exchange (Ciais et al., 2005). For example, the drought affecting Europe in 2003 reduced the sink capacity of a grassland in 0168-1923/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2011.01.008