Eucalyptus trees on my layout are made from dead Manzanita branches and Petite Pines Northern tree matting material. If you live or can visit areas of the west where Manzanita grows, a few hours finding and picking the dead branches will end up being a lot of Eucalyptus trees for your layout.
Eucalyptus trees grow all over California, along railroad right-of-ways and used as wind breaks on the edges of farmers fields. Below are examples of Eucalyptus trees I had recently photographed in the Sacramento Valley near Corning, CA. Notice the shape of the Eucalyptus trees and the colors of the tree's bark.
Here is what to look for while out Manzanita branch hunting. Eucalyptus trees can grow easily over 80 feet tall, however modeling them that tall can dwarf the layout scenery, so I pick dead Manzanita branches 5-7 inches tall and not being too wide.
Petite Pines Northern tree matting material comes as a flat mat with green foam glued on. Cut roundish and uneven clumps from the matt then add a little dab of white glue to ends of the branch's and place on the matting material.
Work with the matting material over a newspaper so any falling green foam can be saved and reused. After the glue dries, I spray the clumps with a flat clear coat spray can and sprinkle on the remaining green foam.