In case anyone didn't read the above link. Here's the Science synopsis.
"During heat treatment changes occur within the cellulose, hemicellulose and lignin components of the wood. Cellulose undergoes some reduction in molecular weight, but cellulose crystallinity can increase. Hemicelluloses are the most thermally reactive cell wall constituents having lower thermal stability due to their lack of crystallinity, and during thermal
treatment carbonic acids, mainly acetic acid, will be formed as a result of cleavage of the acetyl groups. The formation of acetic acid further catalyses carbohydrates cleavage, causing a reduction of the degree of polymerisation. Acid catalysed degradation results in the formation of formaldehyde, furfural and other aldehydes as well as some lignin cleavage. The decomposition of hemicellluloses results in the reduction of the number of available hydroxyl groups and a lower concentration of reactive sites will therefore decrease the equilibrium moisture content of heat-treated wood and thus improve the dimensional stability. In addition, the acetic acid that has formed depolymerises the cellulose microfibrils in the amorphous area. Changes within the lignin are equally complex. During the treatment bonds between phenylpropane units are partly broken but condensation reactions occur also. The longer the autohydrolysis time is, the more condensation reactions occur. Auto-condensation of lignin is believed to occur through the formation of methylene bridges connecting aromatic rings. The extent of these reactions is mild but they lead to an increase in cross-linking with consequent improvement in its dimensional stability and decreased hygroscopicity of wood. In summary heat-treated wood has reduced hygroscopicity and improved dimensional stability because the cellulose microfibrils are surrounded by a more firm and inelastic network due to cross-linking within the lignin complex. The microfibrils have decreased expansion possibility and less capacity to absorb water between cellulose chains. This results in a lower fibre saturation point and a higher resistance to biological decay. The cell wall hemicellulose is transformed into a more hydrophobic network.
Timber species and applications Northern European softwoods including Scots pine, maritime pine, the spruces and white fir have been the predominant species used in the process and have found applications in cladding, decking, flooring, door and window frames and garden furniture. In many of these applications the warm brown colour, imparted by the process is seen as an initial aesthetic advantage. Poplar is probably the most important hardwood to receive attention and has found application as cladding/siding. Other hardwoods with a history of treatment are beech, which it is claimed will match the durability of teak after treatment, and ash and birch. Juvenile teak has also been heat treated to improve stability for furniture applications, thereby improving the utility of small diameter plantation grown materials."