Effects of drought ? altered seasonality and low rainfall ? in net ecosystem carbon exchange of three contrasting Mediterranean ecosystems

International audienceDroughts cause reductions in gross primary production (GPP) and also in net ecosystem exchange (NEE), contributing to most of the inter-annual variability in terrestrial carbon sequestration. In seasonally dry climates (Mediterranean) droughts result from reductions in annual rainfall and from changes in rain seasonality. In western Iberia, the hydrological-year (i.e., from October to September) of 2004?2005 was extremely dry, with precipitation 50% below the long-term mean (691 mm in 1961?1990), but 2005?2006 was normal. We compared the carbon fluxes measured by the eddy covariance technique from three contrasting ecosystems in southern Portugal: an evergreen oak woodland (savannah-like) with ca. 21% tree cover; a Mediterranean C3/C4 grassland; and a coppiced eucalyptus plantation. During the dry hydrological-year of 2004?2005, NEE was lowest, the highest sink strength was in the eucalypt plantation (NEE = ?399 g C m ?2 year?1) as compared to the oak woodland (NEE = ?88 g C m ?2 year?1), and the grassland (NEE = +49 g C m ?2 year ?1). The latter was a source of carbon dioxide. The NEE values of the dry year were, however, much lower than those for wetter years, e.g. NEE = ?861 g C m?2 year ?1 in 2002?2003 in the eucalypt plantation. The NEE of the grassland and the oak savannah in the 2005?2006 hydrological-year, with annual precipitation above the long term mean, were ?190 and ?120 g C m ?2 year?1, respectively. All ecosystems studied increased their rain-use efficiency (GPP per unit of rain volume) increased in dry years. In the case of annual vegetation ? grassland and low tree density woodland, however &ndash, rain-use efficiency decreased with severe drought. However, this was more pronounced in the eucalypt plantation due to greater GPP and the use of deep soil water resources. Although both calendar years of 2004 and 2005 had equally low rainfall, the effect of drought on the eucalypt plantation was delayed until the second dry year. This suggests that the effects of water deficits on Mediterranean forests are exacerbated by prolonged droughts when long-term soil water reserves are depleted. The grassland, however, was more vulnerable and responded faster to water deficits. This effect of drought was less pronounced in the oak woodland due to the sparse tree cover