## 1 Introduction  
Wood has been one of the most widely used hygroscopic materials since ancient times, both for buildings and works of art, due to its strength and hardness properties. Resistance and elasticity are strictly related to the shape and chemical composition of the plant cell walls, making wood's tensile strength value (850 kg/cm\({}^{2}\)) the highest after those of steel and cast iron [1]. Wood tends to be at thermodynamic equilibrium with surrounding environmental conditions by absorbing and desorbing moisture. These processes can cause mechanical degradation due to the swelling and shrinkage of fibers and can also lead to the formation of cracks. Since wood is an orthotropic material, the main anatomical directions of wood--longitudinal, radial, and tangential--are characterized by different water diffusion coefficients [2]. Wood is the main material studied in the framework of the international project \"SyMBoL--Sustainable Management of heritage Buildings in a Long-term perspective\" (2018-2021) [3]. The SyMBoL project focused on searching for adequate environmental conditions for the conservation of collections and the interiors of the most significant Norwegian historic wooden buildings, the Medieval _Stave Churches_, which can potentially be affected by climate change, mass tourism, and internal heating issues [4]. The SyMBoLproject focused on a better understanding of the amplitude and duration of indoor climate change, which may induce mechanical decay in the wood and polychromes of these preserved churches. In the SyMBoL framework, the study of the mechanical properties of pine wood, i.e., Scots pine (_Pinus sylvestris_ L.), is mainly performed by means of the acoustic emission non-destructive technique. Pine is a softwood with a density ranging between 365 kg/m\({}^{3}\) and 590 kg/m\({}^{3}\). Specifically, Scots pine shows a density that ranges between 505 kg/m\({}^{3}\) and 540 kg/m\({}^{3}\) when the moisture content is varied between 12% and 12.4% [5]. Indeed, recently, [PERSON] et al. demonstrated that moisture content has a direct effect on the mechanical properties of wood by modifying orthotropic viscoelastic performance during a water loss process [6], and other recent studies assessed that this change in the mechanical behaviour of wood leads to differences in energy release for acoustic emission monitoring during fracture when moisture content changes [7; 8]. Indeed, fluctuations in ambient relative humidity (RH) are considered the main factor that contributes to the deterioration of wooden objects and acoustic emissions are able to detect the stress induced by such climate variability [9]. Acoustic emissions are the energy released by a structure that is undergoing deformation due to micro and macro fractures. The energy propagates through the material as ultrasound and sound waves, and can be detected at the surface using piezoelectric transducers (AE sensors), which convert the surface vibrations into an electrical signal [10]. The acoustic waves propagate inside wooden material in different ways according to the type of wood [11] or the directionality of wood (radial, tangential, or longitudinal) within the same wood species [10]. Therefore, the acoustic emission monitoring has a wide variety of applications: it is an early warning technique that is useful for preventive detection of the stability of a structure before it can suffer damage from earthquakes or landslides [12]; it can be useful to detect the type and direction of grains in a wood [13]; and it can estimate moisture content in wooden objects [14; 15] pinpointing its response to environmental variability [10; 16]. For the above reasons, the combined use of acoustic emission (AE) and digital image correlation (DIC), both non-destructive techniques (NDTs), were set up in a laboratory to detect the occurrence and progression of fractures of wooden samples at different moisture content conditions during tensile tests, performed using a universal testing machine (UTM).