The estimation of the received signal strength inside forest environments is very dependent on the vegetation density. However, most published empirical models only provide calculation of the path loss as a function of distance and frequency. The importance of these models is their simplicity. This paper presents a model that was developed from an extensive measurement campaign carried out for different vegetation densities and types of trees. Noting the typical decaying behaviour of signal propagation, the objective was to estimate the main parameters of the log-normal model that fits the measured data. Since in many applications both the transmitter and the receiver are inside the forest, the propagation path is mainly characterized by tree trunks. In this case, it was found that the signal attenuation is dependent on the vegetation density, defined by the product of the tree density and the average diameter of the trunks. In addition to the measurements used to develop the model, other experiments were conducted to test it. The study also included a comparison with other methods in order to evaluate the performance of the model.