A tree ordinance is part of a local code that regulates removing trees during development. Most set a size above which a tree is protected, measured as diameter at breast height (DBH), the trunk's width about four and a half feet above the ground. The thresholds, the species covered and the names differ from place to place.
The largest trees often get the strongest protection. In the Charleston area, for example, they are called grand trees, and taking one down can require approval at a public hearing rather than a staff permit. Elsewhere the same idea appears as heritage, landmark or specimen trees.
Protection reaches beyond the trunk. Codes usually protect a zone around each tree, its critical root zone, where grading, paving and building are limited; a common rule sets its radius at one foot for each inch of trunk diameter. A cluster of large trees can therefore take a real share of a lot out of use.
When removal is allowed it often comes with replacement planting or a payment into a tree fund, scaled to the size of the trees removed. A tree survey, which locates and measures every tree above the threshold, is usually required with a site plan, and it is one of the first documents worth reading when judging what a lot can hold.