Plenty more educated people than me on the site, but most wooden engine bearers of WW-1 types were I think, simply the extension/reinforcement of the fuselage longerons, so would normally have been made of the same spruce-type woods.

Some aircraft might have used ash, this being heavier sometimes came in handy to get the centre of gravity into the right position!

By the end of WW1, some aeroplanes had already moved towards tubular steel mounts, although the DH60 Moth persevered with wooden mounts into the 1930s. A weakness of the DH60 was that in a heavy landing, the engine mount would break, taking the forward fuselage with it!!

Some interesting pics HERE (from ‘the other side!’)