A former explosives testing site at Fort Belvoir is the only known home of a rare and endangered orchid in Fairfax County, according to a broad environmental study that illustrates the complex puzzle of construction and conservation.

The small whorled pogonia is a perennial flower listed as “threatened” federally and “endangered” byVirginia. At Fort Belvoir, a single such orchid on the Engineer Proving Ground’s western end was considered important enough to prompt the Army to move a planned parkway through the 800-acre parcel — if only by 8 feet.