How is an area/square footage assigned to a tree?

Hi Tricia, Good question. The WaterSense tool calculates areas of planted zones with similar watering and maintenance needs. If your trees all have similar watering and maintenance needs, they should be grouped together in a zone, which is measured in square feet. Assuming that the plaza is hardscaped, your zone would be the sum of the landscaped areas within each tree grate, and not include the hardscaped areas. Note that the reference guide reads: "Use the tool for vegetated areas only. Do not include hardscapes (whether pervious or impervious) or unvegetated softscapes, such as mulched paths and playgrounds." Therefore the simple answer is: from the information you provided here, the square footage isn't based on the tree's caliper or canopy, but rather the soil it's planted in. Hope this helps!