I was taught the same thing about deciduous tree root span, but have never found that to be true. Perhaps in rich soils with consistent moisture. The reality is I can be digging out in the middle of the yard a long way from trees, and hit some fairly large roots questing for moisture and nutrients. Trees like Oak, Maple, and Ash.
Then there are root spreading trees like Aspen and Sumac. I can find roots from Sumac in New Zealand from my yard probably. Those quest everywhere, but are typically quite shallow. I find them fairly thick in my raised bed gardens and the kid's sandbox after only one season.
