The Takoma Park Tree Selection Guide presents the following definition of native species at the outset (pages 2-3) because it helps explain why native species are critical to biodiversity.

native:  a plant or animal that has evolved in a given place over a period of time sufficient to develop complex and essential relationships with the physical environment and other organisms in a given ecological community.

Rick Darke & Doug Tallamy
The Living Landscape

However, this is not a practical definition that allows one to decide if, say, the Shumard oak (Quercus shumardii) is native to Takoma Park. For that purpose, the guide developed a second definition,

“native to Maryland and with a significant population in Montgomery County.”

This practical definition is explained below and on page 11 in the guide. Basically, the tree species in this guide are native to Maryland and Montgomery County.

The Maryland Plant Atlas identifies all species in this guide as native to Maryland. Recent legislation establishing the Maryland Native Plants Program cites this atlas as the definitive source for vascular plants native to Maryland. The guide therefore uses the identical source.

To define which species are native to Montgomery County, we relied on data provided in The Biota of North America Program (BONAP): North American Plant Atlas.

The Bonap website has a Maryland page listing the genera native to the state. Clicking on any genus listed produces a page with a map for each species in that genus and where it is native (light green counties).

Example: Below is a species map for Amelanchier canadensis, which Bonap indicates is native to Montgomery County.

The procedure just described therefore identifies tree species that have likely evolved in Montgomery County over a period of time sufficient to develop complex and essential relationships with their ecosystem.