Ceriano, there is an issue with planting trees in lawns that you might want to consider:
Grass (roots) and trees (roots) require completely different watering methods. Subjecting trees, especially saplings, to the frequent and shallow watering that grass requires causes the roots to travel to the surface, making the trees less stable and less able to withstand periods of drought, and eventually causes the roots to become exposed and susceptible to lawnmower damage. To encourage the development of strong roots, young trees need regular (but less frequent than lawn) deep irrigation for their first year, at least.
That said, I'm a Westerner and I don't know what your rainfall is like ... do you even have to water your lawn? Does the rainfall soak the soil several feet down?
