We use high-quality pressure-treated rather than pre-treated timber. This means that it's permeated with wood preservative. This deep treatment protects the wood against penetration by dampness or water. It does not deteriorate as quickly as wood that is not pressure-treated, and so it does not need to be replaced as often.
This timber is also covered by an 'envelope' that protects it from insects and fungal attack. That's why pressure-treated timber is excellent for outdoor projects such as fencing panels, garden furniture and sheds. You save money because the timber lasts a long time.
Note that round timber and, to a lesser extent, squared timber post and frames may crack because of solar radiation. The cracking is likely to close again, depending on the weather. It is also natural for timber to warp a little in extreme temperatures. Similarly, when timber dries out, little knots may fall out.