Each tree is hand-crafted to our very high standards of quality. Believe us, if we could make trees more quickly without compromising quality we would. Drill a hole in your scenery and plug-in the tree. If you use rigid extruded foam scenery you can just push our trees' spikes into the scenery. We don't recommend using glue to install trees as this prevents reusing trees on a different layout or moving them to get at track for maintenance.
Trees tend to grow near other trees.
That isn't to say that you never find trees by themselves. If you're making a model forest you'll definitely need more than a few trees. When planting large trees for an old growth forest 10 trees per square foot. With medium trees you'll likely need 15 to 25 to make a dense forest. With small trees you'll need a lot more.