OpenTreeMap is a wiki-inspired, web-based geographic tree inventory application that enables individuals, organizations, and governments to work together and collaboratively map the urban forest. OpenTreeMap is a project of Azavea and Urban Ecos. The code is currently being used to create PhillyTreeMap.org, UrbanForestMap.org, GreenprintMaps.org, SanDiegoTreeMap.org, and the Grand Rapids Urban Forest Project tree map (treemap.urbanforestproject.com). The code to create an iOS app version of OpenTreeMap is available at https://github.com/azavea/OpenTreeMap-iOS. A default skin for the iOS app is available at https://github.com/azavea/OpenTreeMap-iOS-skin.