Abstract. The process-based model 4C (FORESEE) has been developed over the past 20 years to study climate impacts on forests and is now freely available
as an open-source tool. The objective of this paper is to provide a comprehensive description of this 4C version (v2.2) for scientific users of the
model and to present an evaluation of 4C at four different forest sites across Europe. The evaluation focuses on forest growth as well as carbon
(net ecosystem exchange, gross primary production), water (actual evapotranspiration, soil water content), and heat fluxes (soil temperature) using
data from the PROFOUND database. We applied different evaluation metrics and compared the daily, monthly, and annual variability of observed and
simulated values. The ability to reproduce forest growth (stem diameter and biomass) differs from site to site and is best for a pine stand in
Germany (Peitz, model efficiency ME=0.98). 4C is able to reproduce soil temperature at different depths in Sorø and Hyytiälä
with good accuracy (for all soil depths ME > 0.8). The dynamics in simulating carbon and water fluxes are well captured on daily and
monthly timescales (0.51 < ME < 0.983) but less so on an annual timescale (ME < 0). This model–data mismatch is
possibly due to the accumulation of errors because of processes that are missing or represented in a very general way in 4C but not with enough specific
detail to cover strong, site-specific dependencies such as ground vegetation growth. These processes need to be further elaborated to improve the
projections of climate change on forests. We conclude that, despite shortcomings, 4C is widely applicable, reliable, and therefore ready to be
released to the scientific community to use and further develop the model.