Tracing diurnal variations of carbon and water cycle tracers over a tropical and temperate forest

Diurnal temperature and carbon dioxide ranges are key metrics to quantify the impact of regional climate changes in forests. These ranges depend on biophysical processes, surface heat, water and carbon exchange, and boundary-layer dynamics. A crucial and elusive process is the entrainment of air from the free troposphere and residual air layers into the atmospheric boundary layer. Here we provide observational constraints on entrainment for two contrasting measurement sites: the Amazon Tall Tower Observatory (ATTO) in central Amazonia and the Loobos flux tower (NL-Loo) in a temperate forest in the Netherlands. We used radio soundings, air samples from tall towers and aircraft data in combination with surface air measurements and ecophysiological data. Fluxes and concentrations were measured for biophysical-process tracers  CO₂, O₂/N₂, δ¹³C, δ¹⁸O (in CO₂) and δ¹⁸O (in water). These novel tracers are proposed to partition gross carbon and water fluxes and for estimating plant properties and we present a unique dataset with our interpretation. Our analysis enables us to unravel the role of entrainment on the diurnal ranges and how this is controlled by surface and entrainment fluxes. By means of a coupled forest-atmosphere model constrained by the comprehensive observations, we perform a sensitivity study on the surface flux partitioning (photosynthesis versus soil respiration; soil evaporation versus plant transpiration, sensible versus heat flux) under a wide range of leaf traits, surface and boundary-layer dynamic conditions. Our results are useful to assess the performance of carbon-climate models in tropical and temperate forests.