Applegate's Milk-vetch

Astragalus applegatei

Description

Applegate's milk-vetch, Astragalus applegatei, is a tap-rooted perennial herb that grows to a height of about 1 ft (30.5 cm) and reproduces only by seed. The numerous tufted or trailing stems may be smooth or have sparse, stiff hairs. The flat leaves are 1.5-5 in (3.8-12.7 cm) long with 7 to 11 leaflets. The racemes are bear five to 20 pea-like flowers. The flowers are early spreading and ultimately declined; the calyx is hairy; the petals are whitish with a keel tip that is tinged a faint lilac. The pod is widely spreading or declined, stipitate, straight or nearly so, of 0.4-0.6 in (1-1.5 cm) long. The pod valves are green or faintly purple speckled or mottled; dehiscence (splitting) begins at the top of the pod and continues downward, producing up to 10 dark brown, dotted seeds with depressions. There are eight to 10 ovules.

Flowering and pod production in Applegate's milk-vetch occur in June and July; flowering typically commences in early June and continues to August. Fruits shed their seed shortly after flowering and exhibit no specialized mechanisms for long-distance seed dispersal. Although the species' historical occurrence on patches of bare soil may have allowed for some wind movement of seeds along the soil surface, today's dense coverage of the habitat by introduced grasses and weeds likely eliminates any significant post-dispersal, wind-blown seed movement. Some seed dispersal may take place through ingestion by rodents or jackrabbits, although this has not been documented. Localized seed dispersal is supported by field observations, which document that the majority of seedling establishment is immediately adjacent to mature plants. Following completion of flowering, aboveground portions of plants die back completely, succeeded in late fall by resprouting of short [0.5-2 in (1.3-5.1 cm)] stems bearing immature leaves, formed directly above the root crown.

Reproduction takes place exclusively by seeds. There is no evidence that the species is capable of asexual, vegetative reproduction. Pollination is probably mediated by the butterflies and polylectic bees observed visiting the species, although this milk-vetch is also capable of significant seed production through self fertilization. Self-fertilization is common in the genus Astragalus, facilitated by simultaneous ripening of anthers and stigmas. Self-compatibility is a typical reproductive strategy for rare, locally endemic species, as enough plants may not be flowering at any one time, or not in adequate concentration to reliably attract foraging pollinators.

Although Applegate's milk-vetch fruits typically contain eight to 10 ovules, production of greater than three seeds per pod is rare. It is unknown to what degree seed production may be limited by environmental, pollinator, and inherent genetic constraints. Reproductive output is further limited by pre-dispersal seed predation and inflorescence herbivory by butterfly larvae.

The seeds germinate readily in the greenhouse after scarification of the seed coat, usually within three to five days of imbibing water. Greenhouse seedlings develop rapidly, and can reach reproductive size within six months of seed germination. Timing and levels of seed germination in nature are unknown, as is seed longevity, extent of soil seed bank formation, and levels of post-dispersal seed mortality. Questions remain about other aspects of Applegate's milk-vetch's life history, including levels of seedling recruitment, natural rates of plant development, plant longevity, frequency and duration of plant dormancy, outcrossing rates, and to what degree parent and progeny fitness is related to self-versus cross-pollination.

Habitat

Applegate's milk-vetch occurs in flat, open, seasonally moist remnants of floodplain alkaline grasslands in the Klamath Basin, Oregon. The species is a member of the Poa nevadensis-Puccinellia lemmonii grassland community. This community is characterized as a bunchgrass flat, with about 10 to 20 percent exposed ground. The substrate is poorly drained, fine silt loam with an underlying hardpan about 20-40 in (50.8-101.6 cm) below the surface. Periodic flooding was probably a natural feature of this habitat type. The adjacent community is alkaline open shrubland dominated by Sarcobatus vermiculatus and Distichlis stricta. S. vermiculatus periodically is found in the grassland community.

Numerous animals have been observed in association with Applegate's milk-vetch. Vertebrates, and potential herbivores include jackrabbits, Canada geese, and voles. Meadowlarks are also common (and pleasantly heard) occupants of Applegate's milk-vetch habitat. Applegate's milk-vetch is visited by numerous insects, with prominent visitors including bumblebees, other polylectic bees, bee-flies, and the butterflies Lycaeides argyrognomon and Plebejus melissa. The larvae of P. melissa also utilize Applegate's milk-vetch as a host plant, causing severe plant damage through herbivory. (These caterpillars are often tended and fiercely defended by ants which patrol plant stems and leaves.) Other associated insects include root weevils, click beetles, and long-horned beetles.

Distribution

Applegate's milk-vetch is restricted to the Lower Klamath Basin, Orgeon, about 15 mi (24.1 km) north of the Oregon-California border. The species is historically known from only four sites. Today, it is known to exist at only three sites, all situated approximately 4,100 ft (1,250 m) above sea level. The largest population contains an estimated 11,500 individuals and is located near Ewauna Lake at the southern edge of the city of Klamath Falls. This population currently consists of three distinct, locally aggregated patches of plants. Nearly 7 acres (2.8 hectares) of this population, where the greatest density of plants occurs, is owned and managed by The Nature Conservancy, while the remainder of the population is scattered on other private lands.

The second extant population occurs on Miller Island at the Klamath Wildlife Area, located about six miles southwest of Klamath Falls, near the town of Midland. This population includes four small patches of plants, cumulatively containing fewer than 500 individuals. This site is owned and managed by the Oregon Department of Fish and Wildlife.

The third extant population occurs within the vicinity of Wordon, approximately 3 mi (4.8 km) north of the California border. Three plants were discovered in May 1997.

Herbarium records indicate that Applegate's milk-vetch was once more widespread, occurring in a fourth area 2 mi (3.2 km) east of the town of Keno, Oregon, located about 10 mi (16.1 km) southwest of Klamath Falls. Efforts to relocate Applegate's milk-vetch in the Keno area have proven unsuccessful; widespread habitat conversion to fields and pastures has likely displaced the species in this portion of its historic range. Given the species' affinity for alkaline, floodplain habitat, and its current and documented historic distribution, it is probable that Applegate's milk-vetch once occurred along the fringes of the marshes and floodplain of Upper Klamath Lake and Ewauna Lake, and very likely other areas throughout the pre-settlement Lower Klamath Basin.

Threats

Habitat loss and modification. Applegate's milk-vetch was likely more abundant and widespread before intensive agricultural and urban development of the Klamath River floodplain, accompanied by extensive water control/land reclamation projects. Changes in land use have resulted in widespread depletion, fragmentation, and modification of Applegate's milk-vetch habitat, to the extent that even small parcels of truly undisturbed habitat are virtually nonexistent. Repeated efforts to relocate the species in its type locality near the town of Keno, Oregon, have proven unsuccessful, revealing only degraded remnants of potential habitat along fence rows and roadsides.

Beyond outright habitat loss, modification of remaining habitat also poses serious obstacles to the survival and recovery of the species. In addition to the ubiquitous proliferation of introduced weeds (discussed below), extensive construction of drainage ditches and dikes throughout the Lower Klamath Basin has altered the hydrologic character of Applegate's milk-vetch habitat. Drainage ditches carry away rainwater that may otherwise be retained in soils, and, in conjunction with dikes, reduce, if not eliminate entirely, the frequency and severity of flooding. These changes could result in lethally dry conditions for Applegate's milk-vetch, or may indirectly impact the species by allowing immigration of more drought-tolerant plants, including many exotic grasses.

Although there is no information on the frequency or intensity of natural fires predating European settlement of the Klamath River floodplain, natural fires may have played an important role in maintaining open habitat for Applegate's milk-vetch, creating plant litter and encroaching woody shrubs. Fires may also have promoted seed germination and controlled seed predator populations. Research is needed to determine the potential negative impacts of modem fire suppression, or the potential positive effects of fire reintroduction, on Applegate's milk-vetch. Residents observed that recent fires at the Miller Island population may have promoted reproduction and recruitment in the species.

Competition with exotic weeds. Applegate's milk-vetch once occupied patches of bare soil between sparse bunchgrasses and occasional shrubs. Today the species' habitat is replete with aggressive, introduced weeds, an invasion that could severely affect Applegate's milk-vetch. Foremost among these may be displacement of Applegate's milk-vetch through competitive exclusion. The competitive ability of the milk-vetch against exotic weeds is unknown. Although established individuals may be large enough to persist under weedy conditions, competition may inhibit seedling establishment, which lack the stout taproots, microbial affiliations, and energy reserves of mature plants. Given the potentially dire consequences of inhibited population regeneration, further research on the impacts of competition on Applegate's milk-vetch is urgently needed. Habitat colonization by weeds, especially thatch-forming grasses, could promote greater densities of voles and other potential plant herbivores and granivores, through provision of increased cover and protection from predators.

Herbivory and seed predation. Severe damage to Applegate's milk-vetch plants has been observed due to herbivory by caterpillars, identified as the larval stage of P. melissa, which is also a potential pollinator of the milk-vetch. As many as seven caterpillars have been collected from a single Apple-gate's milk-vetch individual, with plants commonly suffering complete defoliation. Similar damage, although generally less severe, has been observed on nearby sweet clover and alfalfa plants. Both Apple-gate's milk-vetch populations suffer from herbivory, which may threaten the plants at the smaller population at Miller Island. Whereas occasional herbivory may result in only short-term setbacks to individuals, herbivory over consecutive years may weaken or kill plants, contribute to depletion of soil seed banks, and impact population recruitment.

Seed Dispersal and Production. Applegate's milk-vetch also suffers seed loss due to predispersal seed predation. Seed predation studies indicate per-plant seed losses of nearly 30 percent. The significance of even low levels of seed predation is enhanced by the already limited number of seeds produced by the species. Insect larvae responsible for seed loss in the species have not been identified, but are almost certainly a beetle or weevil. Further research is needed to understand the extent, impacts, and possible control of herbivory and predation.

Although flowers of Applegate's milk-vetch have eight to 10 ovules that can mature into seeds, only rarely do more than three do so. Low seed sets among irrigated, fertilized, greenhouse-grown, and hand-pollinated plants suggest that seed set is not a result of water or macronutrient deficiency, but may be limited by inbreeding depression or other resource constraints. As with seed predation and inflorescence herbivory, low seed production may adversely affect population dynamics and generation of soil seed banks.

Conservation and Recovery

Extensive, but not exhaustive, inventories have been conducted for Applegate's milk-vetch throughout most portions of its presumed historic range. This work resulted in the discovery of the three known populations. The species had previously been believed to be extinct. Surveys have been conducted by the Oregon Natural Heritage Program and Oregon Department of Agriculture personnel. Biological inventories have also been conducted by the Oregon Department of Transportation on state highway right-of-ways before initiating potentially destructive land actions. Inventories, in addition to discovering populations, have confirmed the extreme rarity of Applegate's milk-vetch and improved our understanding of remaining habitat availability in the Lower Klamath Basin.

Various research projects have been, and are currently being conducted to increase our knowledge about Applegate's milk-vetch. Areas of research include: habitat analysis, population monitoring, reproductive and pollination biology, propagation and transplantation, seed predation, mycorrhizal and other microbial studies, and experimental habitat management treatments.

Nearly 7 acres (2.8 hectares) of the largest extant Applegate's milk-vetch population near Ewauna Lake in Klamath Falls was purchased by TNC, providing urgently needed security against habitat loss to development.

The Applegate's milk-vetch population at Miller Island is managed by the Oregon Department of Fish and Wildlife, and is afforded protection from grazing, vehicular traffic, development, and other potentially destructive activities. The Nature Conservancy has fenced its portion of the Ewauna Flat population, and is experimenting with prescribed burning, herbicide application, and mowing in management of Applegate's milk-vetch habitat.

Contact

U.S. Fish and Wildlife Service
Regional Office
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
http://pacific.fws.gov/

References

U.S. Fish and Wildlife Service. 28 July 1983. "Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for Astragalus applegatei (Applegate's milk-vetch)." Federal Register 58(143): 40547-40551.

U. S. Fish and Wildlife Service. 10 April 1998. "Recovery Plan for the Applegate's Milk-vetch." U.S. Fish and Wildlife Service, Portland. 52 pp.