Say you’re a growing country club in a nicely-treed community, and you need to enlarge your parking lot. And perhaps you want to lower its grade. The lot has some mature oak trees in it, and they add a certain je ne sais quoi to the scene, so you decide to save the trees by keeping the grade as is around the base of their trunks. You retain the roots and soil with a mortared stone wall. Voila!
Good idea – but woops! The minimum standard for root preservation is to keep 10 inches of root mass diameter per caliper inch of tree. For these trees, that would spell at least 360-inch diameter root masses. Because the trees are so close together, their roots overlap significantly — but still, 360 inches is thirty feet of diameter.
This 18-footish enclosure takes a tad too much root; the country club will almost certainly be watching these trees decline and die over the next few years (and they may well drop dead branches onto the parking lot, or cars in it, in the process).
The idea of saving a mature tree is a good one, as long as the tree’s actual requirements for continued healthy life are met. Now that we have the tools to see how large a tree’s root mass really is, it’s much easier to see how big the unimpeded area around it has to be for the tree to survive happily and to thrive.