Many evergreens will die if you prune them severely in the fall. On any tree, pruning wounds can draw away moisture during the winter. Mulching around trees helps them retain moisture during harsh winter weather. But be careful to keep mulch at least six inches away from the trunk. Many people mound mulch up around the trunks of their trees.
Moisture retained by mulch can cause the bark to rot, girdling and killing the tree.
Rodents also can burrow under the mulch and chew off the bark. If you planted trees or perennials this fall, continue watering them regularly until the ground is frozen. However, be careful not to keep the soil saturated. This will kill the developing root system. Finally, if you haven't had your soil tested in a while, now is an excellent time.