Soil salinity is a problem in many parts of Australia. Mainly in pastoral areas however as this is a tree site not a farming site I'll stay focused on the role trees play.
In a diagramatic way of showing this I found and excellent site and link.
NSW SoE 2000: Land Backgrounder Figure 1
In a nut shell the removal of trees for crops and grazing means saline ground water rises to the surface as tree roots aren't there to transpire the water. Trees act like big pumps and the water is transpired thru the trees leaves.
Also, increased watering and irrigation for shallow rooted crops means an increase in the water table ... more water going into the soil profile raising the ground water table again.
Doesn't happen everywhere but when it does very hard to reverse if at all possible.
Anyway, I've attached the diagram from that page just in case they pull it down ... nothing worse than linking to things that become moved or deleted.
Some conflict from a reliable source, grass was the dominant native vegetation removed.
See
post 9 below 