Eight years of continuous carbon fluxes measurements in a Portuguese eucalypt stand under two main events: Drought and felling

This paper reports on results from eddy covariance measurements of carbon uptake and evapotranspiration in the eucalypt site of Espirra in Southern Portugal (38°38′N, 8°36′W). This site was included in the “Carboeurope” European network and is part of a 300 ha eucalypt forest, with about 1100 trees ha⁻¹, intensively managed as a coppice for pulp production and characterized by a 12-month annual growing period. The climate is of Mediterranean type with a long term (1961-1990) annual average precipitation of 709 mm and an annual average air temperature of 15.90 °C. During the measurement period (2002-2009) two main events took place, which changed the annual sink pattern of the forest: a drought period of two years (2004-2005) and a tree felling (October and November 2006). We analyzed the daily, seasonal and inter-annual variation of carbon uptake and evapotranspiration, and their relationships with the events and the variability of the main meteorological variables. Before the felling, annual net ecosystem exchange (NEE) increased from −865.56 g C m⁻² in 2002 to −356.64 g C m⁻² in 2005 together with a deep decrease in rainfall from 748 mm in 2002 to 378.58 mm and 396.64 mm in 2004 and 2005, respectively. For the same period, seasonal patterns of carbon uptake showed maximum values in April and decreased in July-August. The eucalypt stand recovered its carbon sink ability since June 2007 and had a NEE of −209.01 g C m⁻² in 2009. After the felling, the carbon uptake occurred from mid-February to mid-October, following an almost opposite pattern than that of the trees in the term of their productive cycle. A quantitative approach using generalized estimating equations (GEEs) was made for the period before the felling to relate monthly NEE and GPP with accumulated photosynthetic active radiation, water vapour pressure and precipitation. In conclusion, our study showed the relevant effects of water stress and anthropogenic interventions in the daily, seasonal and annual patterns of carbon uptake, under a context of good environmental conditions for carbon sequestration.