Douglas also made contributions as a forestry consultant, working with Frederick Law Olmsted on several projects, including the Biltmore Estate.

Douglas enjoyed traveling and made a trip to the Pacific Ocean during the California Gold Rush via the Humboldt River route.

There he found that the native Pseudotsuga menziesii was adaptable and could be cultivated in the Midwestern United States.

[1][2] Douglas also identified the timber potential for the western catalpa and the hardiness of the blue spruce.

Douglas was also a forestry consultant and frequently associated with landscape architect Frederick Law Olmsted.