Carbon and water vapor fluxes over four forests in two contrasting climatic zones

The inter- and seasonal patterns of water vapor and canopy carbon fluxes were compared for four forest ecosystems in two contrasting climatic zones in Europe. The eddy covariance and ancillary data were taken from the Carboeurope and FLUXNET databases and a linear modeling statistical analysis was made. The four sites were a high-density poplar (Populus spp.) short rotation coppice plantation (in Lochristi, Belgium) and a mature Scots pine (Pinus sylvestris) forest (in Brasschaat, Belgium) in the Temperate climate versus a fast-growing Eucalypt (Eucalyptus) plantation (in Espirra, Portugal) and a Holm oak (Quercus ilex) forest (in Puechabon, France) in the Mediterranean climate. The Eucalypt stand showed an efficient stomatal control in response to changes in vapor pressure deficit (VPD), suggesting an ideal adaptation of this species to the severe Mediterranean climate. The fast-growing poplar stand did not show a similar stomatal control under conditions of moderate water stress. But during an intensive dry period a decrease in the development of the leaf area index (LAI) was observed. The Holm oak stand showed a low GPP, which is typical for a low productive species with a long rotation cycle. The GPP showed low diurnal variability, even under high solar radiation. This behavior suggested a strong stomatal control caused by the severe water stress, a mechanism that allowed this stand to cope with diurnal and seasonal water deficits. The mature Scots pine forest in the Temperate climate showed no variation in the GPP - radiation relationship. In this forest no water stress was observed, probably because the trees always had access to the water table. Irrespective of the climate the evapotranspiration of the Scots pine forest presented a tight coupling with the atmosphere, i.e. a low decoupling factor, Omega, comparable with the Holm oak and the Eucalypt forests. The high Omega values of the young poplar plantation were not typical for forest canopies. These values confirmed the strong influence of solar radiation and available energy on evapotranspiration and on the dynamics of this fast-developing canopy. At all four sites the forests showed their capacity to react to the environmental drivers, characteristic from their respective climatic types. However, drastic climatic changes - such as heat waves or long drought spells - may compromise the productivity of fast-growing plantations such as the Eucalypt and poplar stands. The response of the poplars to these events is mainly achieved through LAI control in contrast to the stomatal control in the Eucalypts. (C) 2013 Elsevier B.V. All rights reserved.