Grand Canyon National Park takes steps to recover the endangered sentry milk-vetch.

Article by Allyson Mathis

Grand Canyon’s rarest plant, sentry milk-vetch (Astragalus cremnophylax var. cremnophylax), grows at only three locations on the South Rim.

This tiny member of the pea family with minute pale purple flowers favors very specific habitats at the canyon's edge. It grows in openings of the pinyon-juniper woodland, in shallow soil pockets atop the highly porous Kaibab Limestone. Listed as endangered in 1990, sentry milk-vetch is at risk of extinction because the plant exists in just three populations in very small numbers.

In 2009, the vegetation program of Grand Canyon National Park’s Division of Science and Resource Management took a number of important steps in the recovery of this endemic species that “watches over the gorge,” as its scientific name states.

The 2006 Sentry Milk-vetch Recovery Plan prepared by the U.S. Fish and Wildlife Service called for a number of major actions to be implemented in order to protect the species, including establishing a refuge greenhouse (ex situ) population, conducting ecological studies of the plant’s habitat requirements, surveying potential habitat to see if other populations exist, and establishing new populations to meet recovery goals.

The species will be considered recovered when there are eight stable populations of 1,000 plants each.

The largest population of sentry milk-vetch is located near Maricopa Point on Hermit Road. The area has been enclosed by a fence since 1990 to protect plants and their habitat from people trampling them while walking off trail.

In 2008, Grand Canyon National Park rehabilitated the historic Hermit Road to improve visitor safety and experience and to protect natural resources along the roadway.

An important part of the road construction project was the removal of the Maricopa Point parking lot adjacent to the existing population in order to provide more habitat for sentry milk-vetch. Most of the former Maricopa Point parking lot was restored with native shrubs and grasses, but a portion of it was set aside for a pilot project to restore sentry milk-vetch habitat and test reintroduction methods for the plant.

The re-introduction of sentry milk-vetch there will be a multi-faceted project, including recreating the unique soil characteristics that will support these plants, planting companion species such as the Tusayan flameflower, and ultimately planting both sentry milk-vetch seeds and plants that were propagated by the park in an ex situ population.

In 2009, park staff undertook important steps to prepare for the initiation of sentry milk-vetch re-introduction trials at Maricopa Point. Park horticulturalist Janice Busco has coordinated and supervised all the complex components of this major restoration and protection effort.

Utilizing funds from the U.S. Fish and Wildlife Service, and in partnership with Grand Canyon Association, Grand Canyon National Park’s official fund-raising partner, Grand Canyon National Park’s Division of Facility Management constructed a 200 square foot passive solar greenhouse to house the park’s ex situ sentry milk-vetch population.

Busco explained, “Having a greenhouse dedicated to sentry milk-vetch will really help us establish the ex situ population. Sentry milk-vetch is hard to grow in a greenhouse setting, and with the new greenhouse, we will be able to fine tune it to meet the plants’ needs with lower humidity and bright sunlight in order to optimize sentry milk-vetch growth and seed production. It’s important to get the greenhouse population established as it is needed to provide seed and plants for the pilot project within the former Maricopa Point parking lot area.”

Building on seed germination studies began in 1989 by the Arboretum at Flagstaff, Grand Canyon National Park vegetation program staff initiated seeding trials in 2009.

Using seeds carefully hand-collected from the three wild populations, Student Conservation Association (SCA) Resource Assistant Emily Douglas conducted a seeding trial in which sentry milk-vetch seeds received two different treatments prior to planting. One set of seeds was soaked in hot water prior to planting, and the other set was “poked” prior to soaking and planting—that is, rubbed, or scarified, with sandpaper. Busco said, “There was a distinct difference in the germination success of the two sets of seeds. The ‘poked’ seeds released a colored substance during soaking, which was mostly likely a germination inhibitor.

Having this information will help us maximize germination success as we establish the greenhouse population, and we may test the ‘poke’ method in seeding trials at Maricopa Point. There is still a lot to learn about the needs of sentry milk-vetch, and Emily’s work in the greenhouse last summer really added to our knowledge base.”

Douglas said, “As an SCA intern, I feel honored and humbled to be working with such a rare species.

All the time I have spent with sentry milk-vetch has really put into perspective the invasiveness of our own species and the need for greater action in preservation everywhere.

People ask me what the point is in saving one little plant. I believe it has intrinsic value. After all, the plants were here before us and I was raised to respect my elders.”

In 2010, Grand Canyon National Park plans to begin a pilot planting and seeding trial at Maricopa Point after restoration of the disturbed land at the former Maricopa Point parking lot.

"This planting is made possible only by seed production from the ex situ population," Busco said. “The availability of sentry milk-vetch seed is the limiting factor in increasing the number of plants in the wild and meeting recovery plan goals.

Emily's greenhouse planting trials and the establishment of our ex situ population will provide both the seeds and additional knowledge to make sentry milk-vetch recovery possible.

By spring 2010, seedlings in the greenhouse population of sentry milk-vetch should have matured sufficiently to begin to flower and produce seed. Each tiny flower will be carefully hand pollinated to maximize seed set.”

Vegetation Program Manager Lori Makarick said, “National Park Service policies guide managers to preserve, protect, and restore native ecosystems, and to specifically strive to recover all native species, such as sentry milk-vetch, that are listed under the Endangered Species Act. This is an especially complex project because very little is known about this plant – not even what pollinates the flowers. It takes a team of dedicated specialists to unveil its mysteries. This project has been extremely successful due to the support of the network of partners, and the excellent leadership of Jan Busco, who serves as the steward for sentry milk-vetch.”

Although still critically endangered, the actions Grand Canyon National Park took in 2009 made important progress in implementing the recovery plan for this Grand Canyon endemic, and ensuring that sentry milk-vetch plants continue to watch over the gorge from its perches on the South Rim.