I have a living trust that designates a successor trustee to manage the trust in case I become incapacitated. Do I also need a durable power of attorney?
You should still have a durable power of attorney for finances. Think of your successor trustee as reigning over a limited kingdom -- your living trust, with whatever property you have put into it. If you become incapacitated, your trustee will have power over all this property and be able to use it for your needs -- but that's where the power ends. Your successor trustee has no power over property outside the kingdom walls. And most people transfer into a living trust only assets that are too expensive to put through probate, such as real estate and valuable securities; few transfer all their property to a living trust.
Personal checking accounts, for example, are rarely transferred to a living trust -- and most people want someone to be able to make deposits and pay bills from these accounts. You can grant this power to an "attorney-in-fact," or agent, using a durable power of attorney for finances. Also, under a durable power of attorney, you can give your attorney-in-fact the authority to handle tasks such as collecting government benefits, filing tax returns, handling legal actions, and dealing with many other matters that are also outside the boundaries of your living trust.
You may even want to empower your attorney-in-fact to transfer into your living trust any property that becomes yours after you become incapacitated. Only a durable power of attorney for finances can grant that authority.