"Instantaneous ecosystem water use efficiency (WUE), the ratio of gross primary production (GPP) to evapotranspiration (ET), is an important integral trait of plant and ecosystem productivity and a key variable for modelling and understanding the impact of climate change on terrestrial ecosystems. Unfortunately, the influence of climate change on ecosystem WUE is difficult to predict, because the underlying mechanisms that cause variation in ecosystem WUE are largely unknown. A major limitation so far in the research on ecosystem WUE is the current impossibility to measure WUE at sub-ecosystem (ie single tree or plant) level.The aim of this research proposal is to generate ET, GPP and WUE maps of extremely high resolution of three Australian forest biomes. The remote sensing imagery needed for this will be acquired by applying an unmanned aerial vehicle (UAV), equipped with thermal, visual and near infrared cameras. These maps will allow us to investigate how the ET, GPP and WUE of single plants and of entire ecosystems is affected by drought, and how species composition and vegetation structure affect this relation.Two innovative new algorithms to estimate ET and GPP will be developed. These algorithms benefit from two key advantages that UAV-remote sensing offers, namely the unprecedented spatial resolution and the possibility to construct highly precise 3D maps of the canopy, thanks to the large image overlap. Measurements will be performed in three representative Australian forests, which will be monitored regularly during an entire year. Spatial and temporal analyses will be performed to assess the influence of stand structure and species contribution on GPP, ET and WUE and how these factors evolve in time, particularly during drought periods."