Thanks for writing. One solution that will cut down mowing time AND help your trees at the same time is to get rid of the grass around the trees and spread a 2- or 3-inch-deep layer of mulch.
The mulch will help on several fronts --- first of all, it cuts down on competition for water and nutrients from the grass. It also helps the soil hold moisture longer during dry periods, and your trees will definitely appreciate that. It can also protect your trees from accidentally having their bark nicked by a lawn mower.
In other areas, you could look at replacing the lawn with a meadow of wildflowers; this is becoming more and more popular these days. Or if the naturalistic look is too wild for you, you might look at adding low-maintenance planting beds of perennials and shrubs.
---Justin, Senior Garden Editor, BHG.com