Examples of stormwater credits for urban trees
Link to this table
Municipality Credit Website
Pine Lake, GA Saved trees contribute to site runoff requirements: < 12 inches DBH = 10 gal/in; > 12 inches DBH = 20 gal/in http://pinelakega.com/environment-green-space/waterfirst-plan/
Sacramento, CA A portion of impervious cover underneath the canopy of new or existing interceptor trees from approved species list may be subtracted from the site’s overall impervious cover:

New deciduous = 100 ft2; New evergreen = 200 ft2

Existing = ½ existing canopy
2007 Stormwater Quality Design Manual: http://www.sacramentostormwater.org/SSQP/documents/DesignManual/SWQ_DesignManual_May07_062107.pdf
Portland, OR A portion of impervious cover underneath the canopy of new or existing trees from approved species list and within 25 feet of impervious surfaces may be subtracted from the site’s overall impervious cover. 2004 Stormwater Management Manual: http://www.portlandonline.com/BES/index.cfm?c=35122
Indianapolis, IN 100 ft2 impervious area reduction for each new tree within 20 feet of impervious surfaces and from approved species list. http://www.indygov.org/eGov/City/DPW/Pages/Supplemental%20Green%20Infrastructure%20Document%20(Draft).aspx Adapted from

Adapted from the Center for Watershed Protection Stormwater Credits