Fuzzy logic (FL) was applied to simulate latent heat fluxes of six coniferous forests. The goal was to test the application possibilities of fuzzy logic and to evaluate the information that could be derived from it. The data set consisted of five forests of the Euroflux project and a set of the Douglas fir Speulderbos, 1 year before and 1 year after a thinning. A set of models was compared to their performance and interpretability: a general FL model, FL models for the 2 years of the Speulderbos, and a forest growth model parameterized for Douglas fir. The performance of the FL models was close to optimal. The rules of the FL models made objective model comparison possible and showed a change in the sensitivity to vapor pressure deficit of the Speulderbos. Fuzzy logic proved to be a very promising method for top down modeling of forest gas exchange, systematic model evaluation, and model simplification. ¿ 2000 American Geophysical Union