The Plant Press

From The Plant Press, Vol. 24, No. 2, April 2021.

A botanical cliff-hanger

By Nancy Khan and Warren Wagner

Dramatic cliffs along the northern coast of Guam rise over 190 m to form a broad limestone plateau that encompasses the most diverse native plant communities of this small Micronesian island. Until recently the biological diversity of Micronesia was not fully understood, but it is now thought that this region contains a higher percentage of endemic plant species per square kilometer land area than any other hotspot of biodiversity. With an area of about 541 km² Guam is the largest and southernmost of the Mariana Islands, but exhibits the lowest percent of endemism of the archipelago (Micronesica 43: 51–100; 2012). This bleak account inspired further investigation of an intriguing, yet unidentified specimen collected in 1994 as part of a floristic inventory for an environmental assessment of Andersen Air Force Base sponsored by the U.S. Fish and Wildlife Service (USFWS).

Most of the Guam National Wildlife Refuge (GNWR), about 22,456 acres, is an "overlay refuge" (land that is under the primary jurisdiction of one Federal agency and the refuge purpose is superimposed as a secondary interest) formed by two units: Andersen Air Force Overlay and the Navy Overlay Unit. Although the military mission comes first on these overlay lands, the USFWS assists in protecting native species and habitats. Together with the 1,217 acre Ritidian Unit which is managed solely by the USFWS they comprise the GNWR which was established in 1993 when the area was determined to be critical habitat for six species of plants and animals listed as endangered pursuant to the Endangered Species Preservation Act of 1966.

A previous collection made within the GNWR by Derral Herbst (USFWS) in 1982 guided botanists Steve Perlman and Ken Wood (National Tropical Botanical Garden) to the unusual plant. The pair searched sharp, jagged cliffs along the coast, repeatedly rappelling down vertical escarpments, eventually locating five subpopulations totaling 113 individuals. Field identification tentatively likened it to patchouli, Pogostemon cablin (Lamiaceae), however it lacked the aromatic oils characteristic of the patchouli plant. Six new collections were made by Wood and Perlman to document the distribution and morphological variation of the subpopulations. Subsequent expansion of the military installation and development of cooperative but parallel conservation plans by the USFWS and the U.S. Air Force hindered opportunities to re-visit the site for further exploration.

The six specimens seemed likely to remain an enigma until Smithsonian Department of Botany lab technician Gabe Johnson became involved. He applied molecular techniques to determine the identity of the plant. Small fragments were removed from four of the specimens collected in 1994. Initial extraction indicated that the DNA was highly degraded, but the application of a precise and intricate methodology yielded two samples with 2 regions each of sequenceable DNA that could be compared to known sequences obtained from GenBank (Molecular Phylogenetics and Evolution 98: 184–200; 2016). A phylogenetic reconstruction of the genus Pogostemon revealed that while there is superficial resemblance to Pogostemon cablin, the mystery plant was a new species native to Guam and more closely related to three species with origins on the Indian subcontinent.

In 2020 the species was published as Pogostemon guamensis Lorence & W.L. Wagner (PhytoKeys 169: 61–73), a rare endemic considered by the authors to be critically endangered primarily due to the limited population size, imperiled habitat, and high levels of alien herbivory and seed predation. As noted by the authors, “in many of the specimens examined flowers and nutlets had been eaten by herbivorous insects in the field”. An accompanying illustration by Alice Tangerini deftly captures the range of diagnostic characters despite the impediment posed by such deterioration across the limited number of herbarium specimens.

Prior to this discovery the only federally listed plant species on Guam was the critically endangered tree, Serianthes nelsonii (Fabaceae). A specimen of this tree was first collected in the late 1800s but not described until 1919 albeit from a holotype that was destroyed during a World War II bombing raid in Manila. Of the six mature trees ever known to exist on the island, the population has been reduced to one remaining individual at Ritidian Point. 

Thanks to the persistent effort of the many boots on the ground that have contributed to its identification and assessment over the preceding 28 years and those who will continue to serve as its stewards in the future, it is hopeful that this 12^th endemic to be added to the native flora of Guam can persist in its precarious cliffside location.