Alongside his better-known career in ichthyology, Herre carried on decades of work as a lichenologist, beginning with his Stanford training in botany and culminating in a sustained focus on North American lichens. After earning his doctorate at Stanford in 1908 from research on the lichens of California's Santa Cruz Peninsula, he continued collecting and studying lichens during and after his museum and fisheries appointments, often gathering specimens on the same trips that supported his zoological work. In retirement he collected actively in
Washington and
northern California, and records compiled from
herbarium databases attribute hundreds of
Usnea ("beard lichen") specimens to him, reflecting a long-running commitment to documenting the genus in the field and in collections. In the late 1950s and early 1960s, Herre concentrated on the difficult genus
Usnea, working toward a comprehensive treatment of the North American species at a time when identification in the group was widely regarded as challenging and disagreements over names were common. He secured
National Science Foundation support for a "Monograph of the Genus
Usnea in North America" and, at age 90, undertook a months-long "herbarium crawl" across the United States to examine vouchers firsthand, correct misidentifications, and test his draft descriptions and
identification keys against real collections. Contemporary accounts described the
monograph as nearly complete by 1960, with a large typescript,
photographic plates, and an extensive key, but publication stalled and Herre died in 1962 during the final stages of the effort. The monograph itself has not been located in major archives despite repeated searches, although related keys associated with his project circulated later among lichenologists and were recast for use decades afterward. ==Legacy==