Over vegetated land and during daytime, plants up-take carbon dioxide while modifying the exchange of water between the soil and atmosphere. Many factors contribute to this exchange such as solar radiation, turbulence, soil water availability and the state of the vegetation. By analysing observations of these variables from field campaigns but specially by using various models and parameterisations we attempt to assess old and/or derive new scaling laws for the evapotranspiration and net ecosystem exchange. Our final aim is to reveal relationships of the related underlying physical processes across different temporal scales (i.e. from seconds to 1-2 weeks) and spatial scales (i.e. leaf, canopy and landscape). We want to use the gained insights to investigate how clouds (through, for instance, modifications in radiation and turbulence) modify the plant exchange of CO2 and H2O and, how plants, in turn, influence cloud formation and maintenance.