Thanks for the help. Just for anyone else looking at this, it's alpha offset that needs adjusting, not alpha gain (the latter is a multiplicative factor)
The map I have is just a basic one made using the displacement thing in the tools menu... I'm new to zbrush, so I can't say more than that.
anyway, here's the result of my work
the treetrunk was formerly very smooth, but now has nice displacement lumps and detail. The leaves still need work, I have some odd corners that shouldn't be there if anyone has any iedea where they come from?