Wood behaviour is characterized by high sensibility to humidity and strongly anisotropic properties. The drying shrinkage along the fibres, usually small due to the reinforcing action of cellulosic microfibrils, is surprisingly high in the so called tension wood, produced by trees to respond to strong reorientation requirements. In this study, nitrogen adsorption-desorption isotherms of supercritically-dried tension wood and normal wood show that tension wood cell wall has a gel-like structure characterised by a pore surface more than 30 times higher than in normal wood. Syneresis of the tension wood gel explains its paradoxical drying shrinkage. This result could help to reduce technological problems during drying. Potential applications in biomechanics and biomimetics are worth investigating, considering that, in living trees, tension wood produces tensile growth stresses 10 times higher than normal wood.