The long plastic tube trunk was cut down and a new shorter trunk made from florist's wire wrapped with thread was added. The exposed edges were sealed with superglue prior to gluing the tree to the MDF base.
The base and roots were built-up and sculpted from DAS modelling clay. I enjoy sculpting the roots, adding small sausages of DAS and sculpting the surface with a plastic sculpting tool as well as scoring the trunk with the blunt edge of a scalpel.
I just have to add some additional ground cover/texture before painting the base and trunk.