For prototyping or small test series vacuum molding is the way to go.
It uses silicone casting moulds which can be easily split into several parts to feature undercuts. The moulds themselves can be cast from 3D printed parts.
From one mould several up to tens of pieces can be cast before it has to be replaced.
Injection moulding on the contrary is suitable for high output numbers from thousands to hundred thousands.
Furthermore massive parts like the top of your trees can't be injection moulded easily, because of the effects if thermal expansion. They won't keep their desired shape during cool down.
If you really want to produce a larger numbers of your meadow, you can resort to thermoplastic foam injection moulding. It is much better in keeping shape of thick parts.
update based on your comments
Some more words on injection moulding.
A tool for injection molding for a mentioned size of 100 * 80 * 20 mm may cost 5000,- € even with low quality surfaces. But for each tree you need probably two sliders to form the undercuts. A sliders may cost something between 500,- € and 3000,- € depending on size and complexity.
Be aware that as Russel already pointed out in his post a certain combination of trees on that meadow may be unmoldable at all because of trees being in the way of sliders needed for another tree.
Especially the room between a group of more than two trees is prone to be inaccessible to sliders.
IMHO having more than 5 trees on your plate brings the probability that this can be injection moulded close to zero.
Tooling can be extremely expensive with injection moulding. And the restrictions are somewhat complex.
Also note that when scaling a part you have to make a new tooling casting tens of thousands again.
What can you do instead of injection moulding?
You may think of using elastomeres instead of thermoplastic materials. They can be dispensed, which makes the tools much cheaper.
You should also think of producing the trees separately. You can laser weld them to the base plate which makes it nearly as solid as moulded in one piece.