It may seem logical that fallen leaves would be a free source of nutrients for your soil and lawn, and it is true to an extent. However, they also provide a breeding ground and refuge for many insects that can do more harm to your yard than the benefits derived from leached nutrients. Have fallen leaves removed to protect your lawn.
Care to guess the average life span of a typical oak tree? Although actual life expectancy can vary based on location and other factors, it isn't unusual for an oak tree to survive 200 years or more. Even 500 year old oak trees aren't unheard of.