Two kinds of timber is commonly used by bridge engineers. Softwood and Hardwood.
Softwood is more economical. It comes form Scandinavia and the Baltic States. It is farmed there on a massive scale and absorbes huge amounts of carbon.
The two most common species used are Douglas Fir and Redwood. Douglas Fir is stronger and has a natural preservative in it. Redwood is used for the parts which come into contact with the user as it planes up better than the Douglas Fir.
All our softwood is stress grades C24 and it is all tanalised after machining to maximise its life.
Hardwood is expensive. It is also durable and long life. It comes from tropical countries. Although it is now FSC controlled it has a high carbon footprint due to the shipping.
We generally use Ekki. It come from The Gabon in West Africa.