Abstract. The understanding of biochemical feedback mechanisms in the climate system is lacking knowledge in relation to bi-directional ammonia (NH₃) exchange between natural ecosystems and the atmosphere. We therefore study the atmospheric NH₃ fluxes during a 25-day period during autumn 2010 (21 October to 15 November) for the Danish beech forest Lille Bøgeskov to address the hypothesis that NH₃ emissions occur from deciduous forests in relation to leaf fall. This is accomplished by using observations of vegetation status, NH₃ fluxes and model calculations. Vegetation status was observed using plant area index (PAI) and leaf area index (LAI). NH₃ fluxes were measured using the relaxed eddy accumulation (REA) method. The REA-based NH₃ concentrations were compared to NH₃ denuder measurements. Model calculations of the atmospheric NH₃ concentration were obtained with the Danish Ammonia MOdelling System (DAMOS). The relative contribution from the forest components to the atmospheric NH₃ flux was assessed using a simple two-layer bi-directional canopy compensation point model. A total of 57.7% of the fluxes measured showed emission and 19.5% showed deposition. A clear tendency of the flux going from deposition of −0.25 ± 0.30 μg NH₃-N m⁻² s⁻¹ to emission of up to 0.67 ± 0.28 μg NH₃-N m⁻² s⁻¹ throughout the measurement period was found. In the leaf fall period (23 October to 8 November), an increase in the atmospheric NH₃ concentrations was related to the increasing forest NH₃ flux. Following leaf fall, the magnitude and temporal structure of the measured NH₃ emission fluxes could be adequately reproduced with the bi-directional resistance model; it suggested the forest ground layer (soil and litter) to be the main contributing component to the NH₃ emissions. The modelled concentration from DAMOS fits well the measured concentrations before leaf fall, but during and after leaf fall, the modelled concentrations are too low. The results indicate that the missing contribution to atmospheric NH₃ concentration from vegetative surfaces related to leaf fall are of a relatively large magnitude. We therefore conclude that emissions from deciduous forests are important to include in model calculations of atmospheric NH₃ for forest ecosystems. Finally, diurnal variations in the measured NH₃ concentrations were related to meteorological conditions, forest phenology and the spatial distribution of local anthropogenic NH₃ sources. This suggests that an accurate description of ammonia fluxes over forest ecosystems requires a dynamic description of atmospheric and vegetation processes.