Elm
Elm will last almost indefinitely when placed in water or below ground level (although it is not durable at ground level). This makes it ideal for the construction of fish ladders and sluice gates and other submerged structures
100% timber
Yes
Association of Scottish Hardwood Sawmills - www.ashs.co.uk
No
Depends on use and location
Limited - dwindling due to the effects of Dutch Elm disease