For dead trees- dried twigs & branches.
If you want larger areas covered with dried out grass and bare earth, try a cheaper grassmat and drybrush that with ochre and yellow (if you can't find one appropriately coloured allready). If you choose one that's paper backed, it's fairly easy to use e.g. a wet brush and rub that over the surface to remove the grass for a patchy look.
For larger areas that you want to look like cracked earth, take some air drying clay or premixed plaster and roll up a ball of alu foil. Try to pry that open and then compress it into a ball again until it looks wrinkled. Press that into the clay.
That's what I did on the base below (only that I used putty), though I didn't bother to make it look overly neat as it was supposed to be partly covered under snow for a frozen earth look.


Also thick bristle brushes will work for creating smaller, more subtle cracks if you press them into the clay lengthwise.
While the material is still workable, you can insert a few tufts of off cut bristles as well for some cheap dead reeds or longer grass.
For small shrubs- twisted strands of wire with a bit of putty on the lower parts. pull apart the wires to form branches. Paint greyish brown. Probably will look a bit more realistic, if you add a few dry leaves still hanging on a couple of branches.