Interactions between mechanical and environmental stresses on timber structures can significantly alter their mechanical behaviour, leading to cracking. This can complicate their implementation and compromise their service life. A study was therefore conducted to assess the open-mode cracking properties of two tropical species from the Gabonese forest and one temperate species, namely Okoume (Aucoumea klaineana Pierre), Padouk (Pterocarpus soyauxii) and White Fir (Abies alba Mill). Various 3-point bending tests were carried out at a relative humidity (RH) varying from 75 to 45%, at constant temperature and mechanical load. The samples tested were beams notched at both ends and measuring 10 × 6 × 160 mm. The crack parameters, crack length and crack opening were measured using a microscope. The results showed the evolution of the critical energy release rate in mode I as a function of the cracking increment induced by the specimens over the three tests carried out ( \({\text{TE}}_{3}^{1}\) , \({\text{TE}}_{4}^{1}\) and \({\text{TE}}_{1}^{2}\) ). This mode I energy release rate (GI) was calculated using the imposed displacement compliance. The results show that the GI values for White fir are slightly higher than those for Okoume, and that the GI values for these two species are lower than those for padouk. This means that the amount of energy required to induce crack propagation is much higher for Padouk than for White fir. This seems to be justified by their higher densities than Okoume.